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AbstractThis paper presents the development of a benchmark for predicting thermal striping through simulation. This work utilized large eddy simulation and will be used to benchmark future models. The testing domain was created using both the STRUCT and the Reynolds-averaged Navier-Stokes turbulence models and is based on an earlier design of the General Atomics Fast Modular Reactor upper plenum. The plenum features two adjacent, identical hexagonal bundles each with a center-placed axial rod drive, with a hot left coolant stream and a cold right coolant stream. The simulation solves the nondimensional Navier-Stokes equations, with temperature accounted as a passive scalar. First- and second-order flow statistics were obtained after 600 convective time units of averaging. The first-order statistics reveal that the hot jet is damped by a recirculatory flow from the near wall. At the same location, the second-order statistics show strong oscillations both in velocity and temperature. The power spectral density was utilized to determine that a low-frequency oscillation occurs here that is within the range of interest for thermal striping. Furthermore, proper orthogonal decomposition was used to identify coherent structures that confirm the oscillatory behavior, indicative of thermal striping. Overall, this benchmark can aid in the development of future models for predicting thermal striping in nuclear reactors, potentially leading to improved reactor safety and performance. [ABSTRACT FROM AUTHOR]

Large Eddy Simulations (LES) are a powerful tool to analyze the flow in Rotating Detonation Engines, but their implementation is not straightforward. Many groups use simplifications (perfect premixing, geometrical 2D representations of the chamber) and numerical high-fidelity analysis comparing mixing assumptions in full scale configurations are not commonly found in literature. In this work a 3D LES of a full RDE tested at TU Berlin is performed and the influence of mixing assumptions (fully premixed vs. non-premixed) and sub-grid scale models on the detonation propagation are investigated. Simulations show that premixed RDEs burn more fuel in a detonation regime, resulting in a faster detonation wave than in the non-premixed case, while a change in the sub-grid scale model does not lead to significant changes. LES reveals that all cases lose a high amount of fuel to non-detonative combustion. These results show, that mixing plays a significant role in the LES of RDEs and has to be included to increase the accuracy of the simulations. The importance of deflagration in the overall RDE combustor implies, that chemistry models need to account for deflagration properties as well as for detonation to capture the efficiency of RDEs. • Fully compressible 3D LES of a Rotating detonation engine including the plenums are performed. • Two different mixing approaches and two sub-grid scale models are tested. • A detonation index for obtaining a detonation efficiency is introduced. • Mixing quality significantly impacts the detonation speed and the detonation efficiency. [ABSTRACT FROM AUTHOR]

This study investigated the flow and flame structure of a bluff-body stabilised CH4-H2 flame using the large eddy simulation (LES) and the flamelet progress variable (FPV) model. The performance of three subgrid-scale (SGS) LES models, i.e. the standard Smagorinsky model (SSM), the wall-adapting local eddy viscosity (WALE) model, and the dynamic Smagorinsky model (DSM) in predicting turbulent and reactive flow characteristics was assessed. Parameters, namely y+, Pope index (M), vorticity field, and resolved turbulent kinetic energy (kt), were analysed to determine the predictive capability of the three models. The results demonstrated that DSM and WALE successfully resolved 80% of kt for chosen grid configurations, whereas SSM exhibited discrepancies. The overall trends captured by all three models matched well with the experimental trend with DSM and WALE outperforming SSM in predicting reacting flow fields in the recirculation zone, while SSM produced better predictions in the regions away from the recirculating zone. [ABSTRACT FROM AUTHOR]

The response of the oceanic surface layer to atmospheric shallow convection is explored using realistic atmospheric large eddy simulations coupled with an oceanic 1D model with high vertical resolution. The effects of cloud shading, rain, and enhanced heat loss due to gust fronts on the edge of cold pools and their interactions are investigated in a case study of the Cooperative Indian Ocean Experiment on Intraseasonal Variability/Dynamics of the Madden–Julian Oscillation experiment in the tropical Indian Ocean, during a suppressed phase of the Madden–Julian Oscillation. Conditions of low surface wind and strong solar heating result in diurnal warming of the oceanic surface of 2 °C over a depth of 1 m. Analysis of specific periods covering the diurnal cycle shows the contrasting effects of cloud shading, rain, and turbulent heat fluxes under the cold pools on the sea temperature at the surface and below. On the one hand, decreasing the solar radiation (cloud shading) results in slight cooling extended horizontally and penetrating down to 1–2 m depth, depending on the time of the day. On the other hand, turbulent heat fluxes enhanced up to 300 W·$$ \cdotp $$m −2$$ {}^{-2} $$ by gusts and freshwater lenses due to rain act together and more locally. They isolate and strongly cool a thin inner layer at the surface, which eventually destabilizes the surface layer and propagates the cooling downward. The exact relative part and efficiency of these processes depend on the time evolution of the thermal stratification and vertical turbulent mixing in the oceanic upper layer. Surface cooling of up to −$$ - $$0.5 °C may occur in a few tens of minutes and last for several hours, significantly mitigating the effects of diurnal warming over large extents. [ABSTRACT FROM AUTHOR]

We introduce an analytical model that describes the vertical structure of Ekman boundary layer flows coupled to the Monin-Obukhov Similarity Theory (MOST) surface layer representation, which is valid for conventionally neutral (CNBL) and stable (SBL) atmospheric conditions. The model is based on a self-similar profile of horizontal stress for both CNBL and SBL flows that merges the classic 3/2 power law profile with a MOST-consistent stress profile in the surface layer. The velocity profiles are then obtained from the Ekman momentum balance equation. The same stress model is used to derive a new self-consistent Geostrophic Drag Law (GDL). We determine the ABL height (h) using an equilibrium boundary layer height model and parameterize the surface heat flux for quasi-steady SBL flows as a function of a prescribed surface temperature cooling rate. The ABL height and GDL equations can then be solved together to obtain the friction velocity (u ∗) and the cross-isobaric angle ( α 0 ) as a function of known input parameters such as the Geostrophic wind speed and surface roughness (z 0) . We show that the model predictions agree well with simulation data from the literature and newly generated Large Eddy Simulations (LES). These results indicate that the proposed model provides an efficient and relatively accurate self-consistent approach for predicting the mean wind velocity distribution in CNBL and SBL flows. [ABSTRACT FROM AUTHOR]

Shallow convection plays an important role in transporting heat and moisture from the near-surface to higher altitudes, yet its parameterization in numerical models remains a great challenge, partly due to the lack of high-resolution observations. This study describes a large eddy simulation (LES) dataset for four shallow convection cases that differ primarily in inversion strength, which can be used as a surrogate for real data. To reduce the uncertainty in LES modeling, three different large eddy models were used, including SAM (System for Atmospheric Modeling), WRF (Weather Research and Forecasting model), and UCLA-LES. Results show that the different models generally exhibit similar behavior for each shallow convection case, despite some differences in the details of the convective structure. In addition to grid-averaged fields, conditionally sampled variables, such as in-cloud moisture and vertical velocity, are also provided, which are indispensable for calculation of the entrainment/detrainment rate. Considering the essentiality of the entraining/detraining process in the parameterization of cumulus convection, the dataset presented in this study is potentially useful for validation and improvement of the parameterization of shallow convection. [ABSTRACT FROM AUTHOR]

With air traffic expected to grow 5% annually until the year 2030, alternative fuels such as hydrogen are being investigated in order to tackle the current environmental crisis. Due to safety concerns, future hydrogen combustion chambers will require new designs of injection systems and are expected to operate under multimode combustion regimes. From a large-eddy-simulation (LES) perspective, a prerequisite for the shift toward new hydrogen combustion chamber technologies is a robust turbulent combustion model capable of functioning in non-premixed conditions. Turbulent combustion modeling using flame front filtering is a well-developed strategy in premixed combustion (filtered-tabulated chemistry for large-Eddy-simulation (F-TACLES)). This approach has been extended to non-premixed flames however, it suffers from high flame filter size sensitivity. Moreover, thin hydrogen flame fronts will result in lower resolution on the LES grid, potentially amplifying this issue. In order to address the feasibility of the non-premixed F-TACLES model applied to hydrogen fuel, simple one-dimensional and two-dimensional laminar counterflow diffusion flames are computed. The model is then tested on the three-dimensional Sandia hydrogen jet flame with a Reynolds number of 10,000. Simulations and a priori tests show that tabulated subgrid-scale correction terms are stiff and can result in nonphysical results, however the model is capable of correctly reproducing non-premixed flame structures for small filter sizes. [ABSTRACT FROM AUTHOR]

Flow circulation and heat removal through shield and reflector assemblies can have major impacts on safety in long transients for sodium fast reactors (SFRs). These transients are typically categorized by reduced flow rates and large-scale organized flow patterns, including potential intra-assembly circulation. Such low-flow cases can provide challenges for experiments because of complications in measuring the flow rates and temperatures with high accuracy in different areas. This consequently also raises the uncertainty of many modeling approaches for these phenomena. In an effort to address some of these issues, high-fidelity large eddy simulations are performed using the highly parallel solver NekRS. A 19-pin configuration of a tight-lattice wire-wrapped hexagonal bundle (pitch-to-diameter ratio = 1.07), representing a prototypical internal configuration of a shield assembly, was investigated. The sodium flow was set at a bundle Reynolds number of 2000, with simulations being performed for modified Richardson numbers of 0.0 (i.e., no buoyancy), 0.01, and 0.04, where mixed-convection effects are anticipated. The flow and temperature fields for these cases are discussed in detail. The high-fidelity data should prove useful as reference data for expanding and improving on various reduced-resolution approaches. A basic framework for combining subchannel and computational fluid dynamics methodologies in SFRs is also presented, with preliminary results from simulations of light water reactor bundles and a discussion of changes that need to be made for potential application to SFRs. [ABSTRACT FROM AUTHOR]

In the present work, two large eddy simulations (LESs) of single isothermal jets discharging into large enclosure facilities are proposed. The geometries and tested flow conditions correspond to scaled experiments of the upper plenum of high-temperature gas-cooled reactors. More specifically, two reference experiments were conducted at Texas A&M University and Michigan University. The objective of the present work is to validate these simulations with their corresponding reference experiments. The proposed LES models are performed with NekRS, a spectral element code with graphics processing unit capabilities developed at Argonne National Laboratory. These simulations were performed on the Summit supercomputer at Oak Ridge National Laboratory. For validation purposes, first- and second-order statistics from the computational fluid dynamics (CFD) calculation are compared with measurements obtained from the experiments. The models proved to be accurate, as these results are in good agreement. Additionally, flow visualization is provided showing that these models are able to retrieve similar effects to what are described in the literature for this type of flow configuration. Finally, the proposed models are part of a broader effort under the current Integrated Research Project of Nuclear Energy Advanced Modeling and Simulation 1.1, whose main objective is to deliver fast-running models to accurately predict complex physical phenomena, including for instance, turbulent mixing and thermal stratification. In this regard, the CFD models proposed here will be used to generate a high-fidelity data set to be applied in conjunction with data-driven methods to improve turbulence modeling closures. [ABSTRACT FROM AUTHOR]

In view of the complexity of flow induced vibration mechanism of steel gate and the frequent occurrence of flow induced vibration of steel gate in engineering practice, taking two-dimensional flat steel gate as an example and based on ADINA finite element analysis software, the flow field finite element model and solid finite element model under different opening degrees were established respectively, the dynamic response law of hydraulic steel gate with different opening degrees was explored, and the difference of dynamic response of steel gate under one-way and two-way fluid structure coupling was clarified, then the mechanism of flow induced vibration of hydraulic steel gate was revealed. The results show that a) with the increase of opening degrees, the fluctuating pressure at different positions of the flow field is decreased gradually, and the frequency of vortex shedding near the steel gate is decreased gradually. b) For hydraulic steel gate, one-way fluid structure coupling can be used instead of two-way fluid structure coupling. c) The mechanism of flow-induced vibration of hydraulic steel gate is that the vortex shedding frequency of the flow field around the steel gate is consistent with or close to the first-order natural vibration frequency of the steel gate structure, resulting in the resonance of the steel gate. [ABSTRACT FROM AUTHOR]

The high-frequency pulse flow, equivalent to the natural frequency of rocks, is generated by a self-excited oscillating cavity to achieve resonance rock-breaking. The flow field and oscillating mechanism of the self-excited oscillating cavity were simulated using the large eddy simulation method of Computational Fluid Dynamics (CFD). A field-scale testing apparatus was developed to investigate the impulse characteristics and verify the simulation results. The results show that the fluid at the outlet at the tool is deflected due to the pulse oscillation of the fluid. The size and shape of low-pressure vortices constantly change, leading to periodic changes in fluid impedance within the oscillating cavity. The impulse frequency reaches its highest point when the length–diameter ratio is 0.67. As the length–diameter ratio increases, the tool pressure loss also increases. Regarding the cavity thickness, the impulse frequency of the oscillating cavity initially decreases, then increases, and finally decreases again. Moreover, both the impulse frequency and pressure loss increase with an increase in displacement. The numerical simulation findings align with the experimental results, thus confirming the validity of the theoretical model. This research provides theoretical guidance for the practical application of resonance rock-breaking technology. [ABSTRACT FROM AUTHOR]

This work numerically investigates longitudinal and azimuthal thermo-acoustic instabilities in the swirl-stabilised can-type industrial SGT-100 gas turbine combustor operated at elevated pressures of 3 and 6 bar. Previous experiments have shown that the combustor is susceptible to self-excited flame oscillations sustained by a thermo-acoustic feedback loop at specific operating conditions. In order to gain a better understanding of this feedback loop, a fully compressible large eddy simulation method is applied. The unknown sub-grid scale turbulence-chemistry interactions are modelled via a transported probability density function approach solved by the Eulerian stochastic fields method. First, the reaction zones and global flame topology at both operating pressures are analysed and compared to experimental images providing good qualitative agreement. Radial profiles of time-averaged and root-mean-square quantities furthermore demonstrate good quantitative agreement with the available measurement data. The applied simulation approach is capable of successfully reproducing self-excited thermo-acoustic instabilities in the longitudinal direction. The fundamental frequency of the predicted limit-cycle oscillation matches the experimentally measured frequency with high accuracy. Similar to the experimental observations, the fluctuation amplitudes of the pressure and global heat release rate increase significantly upon increasing the mean operating pressure from 3 to 6 bar. In addition to the dominant longitudinal mode, a high-frequency, low-amplitude azimuthal mode is also identified at both pressures. This azimuthal mode is periodically amplified and attenuated by the superposed longitudinal mode and induces small asymmetric (around the burner circumference) fluctuations of the local fuel and total mixture mass flow rates entering the flame region. [ABSTRACT FROM AUTHOR]

The physics and the dynamics of static fires were studied numerically and experimentally by burning single Douglas fir trees. The mass loss rate was recorded, as well as the radiative heat flux and temperature at different positions. The numerical simulations were carried out using a fully physical CFD code (FireStar3D), and two levels of description were considered: in the first one, the vegetation was represented using a single fuel-type; in the second, four fuel-types were considered. The numerical results were compared to the experimental data for fuel moisture content of 14% and 50%. The results clearly show the advantage of considering several fuel-types to represent the tree; nevertheless, compared to the experiments, faster tree burning dynamics is obtained by simulation. The role played by the ignition process was also analysed numerically, highlighting the sensitivity of the tree burning dynamics to the ignition protocol. [ABSTRACT FROM AUTHOR]

Distributed combustion systems have shown the potential to reduce emissions as well as increase load and fuel flexibility. A characteristic feature of such systems is a reacting jet in crossflow, which exhibits complex vortical structures. In this paper, a generic combustion chamber with elliptic reacting jets in crossflow is examined, operating under lean-premixed conditions at elevated pressure and exhibiting high-frequency transverse mode shapes. It can be seen that depending on the orientation of the elliptical shape of the jet to the crossflow, thermoacoustic modes can be suppressed. A multidimensional fast Fourier transform shows that low aspect ratios (major axis of the jet aligned with the crossflow) result in the mixed 1L1T mode of first longitudinal and first transverse structure, while this mode disappears at high aspect ratios. To get a more detailed insight into the different vortex systems of the various aspect ratios, dynamic mode decomposition is applied. This modal decomposition technique reveals for low aspect ratios a shear layer mode that oscillates at a frequency close to the acoustic mixed mode. For this configuration, a mode representing a flapping motion is also identified. For high aspect ratios, the shear layer vortex increases its frequency and a higher-frequent mode appears in the acoustic spectrum. [ABSTRACT FROM AUTHOR]

The variations of the frontogenetic trend of a cold filament induced by the cross-filament wind and wave fields are studied by a non-hydrostatic large eddy simulation. Five cases with different strengths of wind and wave fields are studied. The results show that the intense wind and wave fields further break the symmetries of submesoscale flow fields and suppress the levels of filament frontogenesis. The changes of secondary circulation directions—that is, the conversion between the convergence and divergence of the surface cross-filament currents with the downwelling and upwelling jets in the filament center—are associated with the inertial oscillation. The filament frontogenesis and frontolysis caused by the changes of secondary circulation directions may periodically sharpen and smooth the gradient of submesoscale flow fields. The lifecycle of the cold filament may include multiple stages of filament frontogenesis and frontolysis. [ABSTRACT FROM AUTHOR]

AbstractThe large Eddy Simulation approach is applied to simulate turbulent mixed convection of nanofluid through a vertical channel. Nanofluid consists of nano-encapsulated phase change material (NEPCM) nanofluid (5% n-octadecane/water) and hybrid nanofluid (1% CuO + 5% n-octadecane/water). The wall-adapting local eddy-viscosity model is used. The computations are performed at a friction Reynolds number of 180. The turbulence statistics quantities including normalized mean velocity and temperature, root mean square of velocity and temperature fluctuations, Reynolds shear stress and turbulent heat flux are estimated. The agreement between computed mean velocity and temperature profiles and turbulence statistics by direct numerical simulation data is good. The present numerical results show that addition of nanoparticles into pure water increases normalized mean stream-wise velocity, fluctuations of velocity and temperature, Reynolds shear stress and turbulent heat flux. By adding NEPCM and CuO nanoparticles into pure water normalized velocity fluctuations grow in three directions, as a result of which normalized Reynolds shear stress augments. Enhancement of normalized root mean square of velocity fluctuations can cause a strengthening exchange of mass and momentum, and leads to augmentation of heat transfer. The average decrease in normalized mean temperature is 8.5% for NEPCM nanofluid and 12% for hybrid nanofluid. The results indicate that hybrid nanofluid containing CuO and NEPCM nanoparticles shows a better thermal performance than nanofluid containing only NEPCM nanoparticles and pure water. This makes hybrid nanofluid in presence of NEPCM particles to be a possible choice in cooling process applications. [ABSTRACT FROM AUTHOR]

The frequency characteristics of lung sounds have great significance for noninvasive diagnosis of respiratory diseases. The rales in the lower respiratory tract region that can provide rich information about symptoms of respiratory diseases are not clear. In this paper, a three-dimensional idealized bifurcated lower respiratory tract geometric model, which contains 3rd to 13th generation (G3–G13) bronchi is constructed, where Re ∼ 10 1 - 10 3 , and then the large eddy simulation and volume of fluid are used to study the fluid flow characteristics. Ffowcs Williams and Hawkings model are subsequently used to study the frequency characteristics of rale of different generations of bronchi. The results showed that bronchial blockage and sputum movement will enhance the turbulence intensity and vortex shedding intensity of flow. The dominant frequency and highest value of sound pressure level (SPL) of rhonchi/moist crackles decrease with the increase of bronchial generation. The change rates of dominant frequency of rhonchi / moist crackles in adjacent generations were 5.0 ± 0.1 ~ 9.1 ± 0.2% and 3.1 ± 0.1 ~ 11.9 ± 0.3%, respectively, which is concentrated in 290 ~ 420 Hz and 200 ~ 300 Hz, respectively. The change rates of SPL of rhonchi/moist crackles were 8.8 ± 0.1 ~ 15.7 ± 0.1% and 7.1 ± 0.1 ~ 19.5 ± 0.2%, respectively, which is concentrated in 28 ~ 50 dB and 16 ~ 32 dB, respectively. In the same generation of bronchus (e.g., G8, G9) with the same degree of initial blockage, the dominant frequency and SPL of moist crackles can be 3.7 ± 0.2% and 4.5 ± 0.3% slightly higher than that of rhonchi, respectively. This research is conducive to the establishment of a rapid and accurate noninvasive diagnosis system for respiratory diseases. [ABSTRACT FROM AUTHOR]

A novel formulation of the vortex particle method (VPM) is developed for large-eddy simulation (LES) in a meshless scheme that is numerically stable. A new set of VPM governing equations are derived from the LES-filtered Navier-Stokes equations. The new equations reinforce the conservation of angular momentum by resizing vortex elements subject to vortex stretching. In addition to the VPM reformulation, a new anisotropic dynamic model of subfilter-scale (SFS) vortex stretching is developed. This SFS model is well suited for turbulent flows with coherent vortical structures, where the predominant cascade mechanism is vortex stretching. The mean and fluctuating components of turbulent flow and Reynolds stresses are validated through the simulation of a turbulent round jet. The computational efficiency of the scheme is showcased in the simulation of an aircraft rotor in hover, showing our meshless LES to be 100 times faster than a mesh-based LES with similar fidelity. The implementation of our meshless LES scheme is released as open-source software, called FLOWVPM. [ABSTRACT FROM AUTHOR]

Large-eddy simulation of subsonic round jet impinging on a flat circular plate is performed at Mach number 0.9 and Reynolds number 25,000, based on the diameter and centerline velocity of the jet for two impingement distances h of 6r0 and 8r0 using high-order compact finite difference scheme. The complex flow phenomena associated with the impinging jet flowfield are studied. The power spectral density (PSD) of instantaneous pressure reveals a discrete impingement tone. Dynamic mode decomposition (DMD) of the velocity and pressure is also done for both cases to extract the most dominant modes of the flow. The fundamental impingement tone frequency obtained from the PSD of pressure is identical to that of the first dominant pressure mode obtained from DMD. Both axisymmetric and semihelical modes are present for the h=8r0 case, but only axisymmetric modes are there in the h=6r0 case, and it is noted that the modes corresponding to the fundamental tone have an axisymmetric structure. Also, one dominant mode showing the wall jet structures denoting the liftoff of fluid from the wall after impingement at some radial distance away from the stagnation region is present in both impingement distance cases. [ABSTRACT FROM AUTHOR]

Mesoscale‐to‐microscale coupling is an important tool for conducting turbulence‐resolving multiscale simulations of realistic atmospheric flows, which are crucial for applications ranging from wind energy to wildfire spread studies. Different techniques are used to facilitate the development of realistic turbulence in the large‐eddy simulation (LES) domain while minimizing computational cost. Here, we explore the impact of a simple and computationally efficient Stochastic Cell Perturbation method using momentum perturbation (SCPM‐M) to accelerate turbulence generation in boundary‐coupled LES simulations using the Weather Research and Forecasting model. We simulate a convective boundary layer (CBL) to characterize the production and dissipation of turbulent kinetic energy (TKE) and the variation of TKE budget terms. Furthermore, we evaluate the impact of applying momentum perturbations of three magnitudes below, up to, and above the CBL on the TKE budget terms. Momentum perturbations greatly reduce the fetch associated with turbulence generation. When applied to half the vertical extent of the boundary layer, momentum perturbations produce an adequate amount of turbulence. However, when applied above the CBL, additional structures are generated at the top of the CBL, near the inversion layer. The magnitudes of the TKE budgets produced by SCPM‐M when applied at varying heights and with different perturbation amplitudes are always higher near the surface and inversion layer than those produced by No‐SCPM, as are their contributions to the TKE. This study provides a better understanding of how SCPM‐M reduces computational costs and how different budget terms contribute to TKE in a boundary‐coupled LES simulation. Plain Language Summary: Grid nesting is a technique in atmospheric modeling where smaller model domains with finer resolutions are embedded in larger domains with coarser resolutions, allowing us to simulate processes for which a range of resolutions is important. When nesting a micro‐scale domain, which resolves atmospheric turbulence, within a mesoscale domain, where turbulence is parameterized instead, the micro‐scale domain needs to generate turbulence. This can take a long distance, reducing the area within the micro‐scale domain where turbulence is fully developed. Turbulence generation methods can speed up this process, leading to more efficient multi‐scale simulations. Here, we take a close look at one such turbulence generation method, which applies random perturbations to the momentum field along the boundaries of the micro‐scale domain. We test several different configurations of momentum perturbations and evaluate their impacts on the turbulent kinetic energy budget within the micro‐scale domain. Our results can be used as guidance on how to apply momentum perturbations most efficiently in grid‐nested simulations. Key Points: The fetch is reduced after applying momentum perturbation and the reduction is directly proportional to the amplitude of the perturbationsApplying momentum perturbation up to half the boundary layer height produces an adequate amount of turbulence without additional entrainmentMomentum perturbations result in turbulent kinetic energy budget terms of higher magnitude than simulations without perturbations [ABSTRACT FROM AUTHOR]

The effect of the injection pressure on the dynamic flow characteristics of a lean direct injection gas turbine combustor under nonreactive conditions during liquid fuel atomization was investigated. The location of the internal recirculation core, size of the vortex breakdown bubble, and shape of the precessing vortex core and shear layer were confirmed by increasing the fuel injection pressure. It was found that as the fuel injection pressure increased, the momentum of the droplets increased. The tangential momentum of the constant main air swirl flow transferred to the droplets increased as the pressure increased, causing the swirl strength to decrease. It was confirmed that, as the swirl intensity decreased with increasing injection pressure, the shape of the precessing vortex core generated in front of the combustor became shorter, and the size of the central recirculation area also decreased. [ABSTRACT FROM AUTHOR]

The lack of observational data in Taklamakan Desert makes it very difficult to study its unique boundary layer structure. As a common means of supplementing observational data, the mesoscale boundary layer parameterization scheme in the numerical model method is difficult to capture small-scale turbulent processes, which may lead to large deviations in simulation. In order to obtain more accurate simulation data of desert atmospheric boundary layer, nested LES into WRF (WRF-LES) was configured to simulate the seasonal variations in Taklamakan Desert. By comparing LES with the conventional boundary layer parameterization scheme, the error characteristics between the two schemes are analyzed. The results show that LES exhibits superior performance in solving key atmospheric features such as small-scale processes and low-level jet streams. The simulation results in winter and summer have great uncertainty due to the boundary condition errors, respectively. LES also shows the maximum and minimum optimization degree in summer and winter, respectively, while the simulation results in spring and autumn are relatively stable. In the analysis of turbulence parameters, there are clear seasonal differences in turbulence characteristics, and the intensity of turbulence in summer is significantly higher than that in other seasons. When turbulent activity is strong, the difference in potential temperature and horizontal wind speed simulated between the two schemes is closely related to intense turbulent kinetic energy in LES. More accurate turbulence reproduced in LES leads to the better potential temperature and horizontal wind speed simulations in summer. In addition, large-scale cloud systems can lead to considerable simulation bias. Neither scheme can accurately simulate the cloud emergence process, and large differences between the two schemes occur at this point. [ABSTRACT FROM AUTHOR]

Non-zonal methods of hybrid Reynolds Averaged Navier–Stokes (RANS) equations-Large Eddy Simulation (LES) with subfilter transport equations enable to simulate turbulent flows out of spectral equilibrium on relatively coarse grid resolution. They can operate from RANS to LES depending on a control parameter linked to the grid step size. Variational analysis gives a useful framework to specify the functional dependence of this parameter to the grid size. This is the case for the partially integrated transport modelling method originally established in spectral space for homogeneous turbulence. The partitioning control function then monitors the ratio of the subfilter part to the total turbulent energy and the same process is developed in the present work for the thermal or transported scalar variance as well. So, we demonstrate here that this control function mechanism can be derived by variational calculus from a mathematical physics formalism developed both for first-order and second-order moment closures. We show that the result is entirely consistent with the spectral model derivation made in previous papers and that this approach can be transposed to almost any subfilter transport model developed in hybrid RANS-LES methodology in full generality. As a result, it will be evidenced also that resolved turbulent diffusion terms play a significant role in the acting mechanisms of turbulence and cannot be therefore neglected, even if these terms do not explicitly appear in LES subfilter closure because they are computed by the simulation itself and not modelled by the subfilter model. In addition, numerical simulations have been performed for illustrating the theoretical results of the variational analysis. [ABSTRACT FROM AUTHOR]

This research endeavors to study the kinetic behavior of hydrogen explosion flames as they pass through confined spaces with varying obstacle passage lengths using Large Eddy Simulation (LES). The model of premixed flame was customized adopting the OpenFOAM library built upon the evolution of progress variables. By considering the turbulence-induced wrinkling and stretching effects, this study accounts for the resulting increase in flame velocity. The numerical findings managed to correctly match the experimental data. Longer obstacle passages were found to give the flame more acceleration time, resulting in a higher peak propagation velocity and pressure rise rate, according to the study. The first and second peak velocity ratios were 1.09 and 1.46 for the ducts with passage lengths of 3.0 H and 0.5 H, respectively, and the proportion of the pressure rise rate was 1.26. The analysis of vortex dynamics reveals that the vorticity distribution range and intensity are proportional to the length of the obstacle channel, where the baroclinic torque and gas expansion effects are much higher than the contribution of viscous dissipation to vorticity. The study uncovered that during obstacle-induced flame acceleration and propagation, the flame is affected by the coupling effect of RayleighTaylor instability (RTI) and Kelvin-Helmholtz instability (KHI) mechanisms. [ABSTRACT FROM AUTHOR]

Turbulence-radiation interaction (TRI) arises from the highly non-linear coupling between fluctuations of radiative intensity and fluctuations of temperature and chemical composition of the medium. It is well-recognized that neglecting turbulent fluctuations of the latter scalars when computing the mean radiation field, as is standard in Reynolds-Averaged Navier-Stokes (RANS) simulations, leads to mispredictions of the radiative heat fluxes and the radiant fraction in as much as one order of magnitude. Conversely, the importance of TRI in large eddy simulations (LES)--i.e., the effect of subfilter-scale (SFS) fluctuations on the filtered radiation field--has only started to be investigated in the last decade. The present text reviews the knowledge gathered so far on the subject. The instantaneous and the spatially filtered forms of the radiative transfer equation (RTE) are presented, and the unresolved correlations resulting from the filtering operation are discussed. Models for accounting for SFS-TRI are surveyed, with the most popular ones (namely, probability density function-based methods and empirical approximations) reviewed in detail. An appraisal of the literature on the importance and characteristics of SFS-TRI is performed, with special attention given to turbulent jet diffusion flames and pool fires, which are the two configurations for which SFS-TRI has been studied the most. The main findings of these studies are summarized, and recommendations are given on directions of future works on the subject. [ABSTRACT FROM AUTHOR]

Flow around a cylinder is an important research problem in fluid mechanics. However, studies on traveling wave wall (TWW) flow control for the wake generated by the flow around a 3D cylinder remain limited. This study performs large eddy simulation (LES) to investigate the wake characteristics of flow around a three-dimensional (3D) circular cylinder with the TWW. The influence of the main control parameters of TWW on the aerodynamic forces and wake of the 3D cylinder is analyzed. The wake characteristics of the 3D cylinder at Re = 4 × 104 are obtained, and the traveling wave propagating downstream is achieved on the rear surface of the 3D cylinder using dynamic mesh. The control effects of wave amplitude, number of waves, and wave velocity of the traveling wave on the aerodynamic forces of the cylinder are analyzed, and the optimal control parameter combination is determined. The results demonstrate that the TWW control method completely eliminates the spanwise 3D flow characteristics of the cylinder when the amplitude ratio, number of waves, and velocity ratio are 0.02, 4, and 1.5, respectively. Moreover, it effectively suppresses the flow separation on the cylinder surface, eliminates the wake vortex, and suppresses vortex-induced vibration (VIV). [ABSTRACT FROM AUTHOR]

In this study, the generality and prediction accuracy of a generalised series model for the large eddy simulation of premixed and non-premixed turbulent combustion is explored. The model is based on the Taylor series expansion of the chemical source term in scalar space and implemented into OpenFOAM. The mathematical model does not depend on combustion regimes and has the correct limiting behaviour. The numerical error sources are also outlined and analysed. The model is first applied to a piloted methane/air non-premixed jet flame (Sandia Flame D). The statistical (time-averaged and RMS) results agree well with the experimental measurements, particularly with regard to the mixture fraction, velocity, temperature, and concentrations of major species CH4, CO2, H2O, and O2. However, the concentrations of the intermediates CO and H2 are over-predicted, due to the limitations of the reduced reaction mechanism employed. Then, a Bunsen-piloted flame is simulated. Most of the statistical properties of both the reactive species and progress variables are well reproduced. The only major discrepancy evident is in the temperature, which is probably attributed to the experimental uncertainties of temperature fields in the pilot stream. These findings demonstrate the model's generality for both a premixed and non-premixed combustion simulation, as well as the accuracy of prediction of reactive species distribution. [ABSTRACT FROM AUTHOR]

As modern aeroengine combustors advance towards high temperatures, afterburners are inevitably affected by diminished oxygen content in incoming flows, thus affecting combustion efficiency, instability, and flammability limits. In this study, the dynamic combustion characteristics of V-shaped bluff body-stabilised diffusion flames were investigated using a large eddy simulation method with an oxygen mass fraction range of 14–23% and temperatures ranging from 900 to 1100 K. The results show the significant effects of oxygen content and inflow temperature on the flame/flow behaviours downstream of the bluff-body flame holder. In a separated shear layer, two distinct modes of flow/flame shedding are observed when varying the oxygen content and inflow temperature. The results show that BVK instability governs the far-field wake flow/flame features, whereas the oxygen concentration and temperature significantly affect their oscillation amplitudes. In addition, variations in the incoming oxygen content and temperature shift the axial position of the transition from KH instability to BVK instability. Finally, a spectral analysis is conducted to investigate the characteristics of pressure and heat release pulsations under different scenarios. This study highlights the importance of oxygen content on the combustion dynamics of bluff body-stabilised diffusion flames at various temperatures, which is essential for optimising combustion efficiency and stability in practical applications. [ABSTRACT FROM AUTHOR]

The present study is aimed to investigate the effect of temperature on the flow field. In this study, LES (large eddy simulation) and HWA (hot wire anemometry) are used to investigate the time-averaged and instantaneous flow field in the cyclone at different temperatures. The numerical results agree well with the experiment data. The results show that temperature has an impact on both the time-averaged flow field and the fluctuations of the flow field. To investigate the effect of temperature on flow field fluctuations, we first reveal the mechanism of flow field fluctuations. The effect of temperature on vortex motion and tangential velocity fluctuation is analyzed, and a mathematical model for predicting the freqeuncy of vortex core precession at different temperatures is proposed, f T / f 0 = k (T 0 / T) 0.125 (1 + m) . The Q criterion is used to identify the vortex in the cyclone. When the temperature is high, the vortex core undergoes a typical vortex breakdown phenomenon at the lower part of the cyclone separator: the vortex core breaks up into a double helix shape. This is the reason for the disappearance of the negative pressure region, the decrease in tangential velocity and the weak fluctuation of tangential velocity in the dipleg at high temperature. • LES model is used to simulate the flow at different temperatures. • HWA is used to measure the instantaneous tangential velocity. • Q criterion is applied to analyze vortex motion in the cyclone. [ABSTRACT FROM AUTHOR]

Sand disasters can adversely impact on the operational safety of railway projects. To investigate eolian sand movement characteristics in the wind–sand environment around bridge–subgrade transition sections, in this study, we examine vortex and particle motion characteristics in eolian sand flow fields based on the large eddy simulation (LES) technique and discrete phase model (DPM) while proposing corresponding protection measures for this particular railway section. The results reveal that these methods can be employed to simulate turbulent flow fields. The deposition morphology of particles resembles a wavy ribbon, and the presence of horseshoe–shaped and conical vortices in the wind field affects the deposition of particles. Moving particles are deflected toward the side of the passageway under the bridge. Horizontal and vertical retaining walls are designed to control the wind speed and sand movement. The former obstructs particles, creating a low-velocity zone around the wall, while the latter prevents particle deflection. These protection measures can reduce the saltation distance of sand particles by up to 42.9%. The particle velocity is largely reduced to 0 ~ 2.5 m/s, and only a few small-sized particles (d < 0.35 mm) are transported in suspension. The designed T-shaped retaining wall can effectively prevent sand particles from deviating, achieving a protection rate of 99.85%, with only 0.81% of the sand particles entering the area of the railway operation. [ABSTRACT FROM AUTHOR]

In this paper, a large eddy simulation (LES) method was used to conduct a study on single and tandem jets in a crossflow, focusing particularly on their trajectory, penetration, and interaction. The numerical model was validated with an experimental test campaign. Examination of the time-averaged flow field allowed both the velocity and the tangential angle of the jet trajectories to be examined. In addition, the penetration depth of the jet based on a scalar transport model was analyzed. The unsteady flow characteristics around the trajectories were studied using both the power spectral density (PSD) function and a spectral proper orthogonal decomposition (SPOD). The results show that the upstream jet's trajectory changes little as a function of spacing, while the downstream jet deflects as a result of the influence of the counterrotating vortex pair. In addition, the curve height of the tandem jet trajectories is significantly higher than that of the single jet. The height of the trajectory formed by the tandem jets can reach four times that of the single jet, and the penetration depth of the tandem jets can be 2.8 times that of the single jet. Meanwhile, when the spacing between the two jets is small, the coherent structures tend toward the upstream jet distribution, and the fluctuation frequency after mixing is dominated by the upstream jet. With the increase of spacing, the fluctuation frequency after mixing is greatly affected by the downstream jet, and the frequency decreases. Furthermore, when the dimensionless spacing D′ is 5.67, the frequency difference between both jets is minimal and the coherent structures are significantly reduced, indicating that flow mixing is optimal and stable. [ABSTRACT FROM AUTHOR]

A numerical simulation is conducted to study the flow of a three-dimensional incompressible wall jet. The study aims to determine the jet structure and to compare the propagation mechanisms of turbulent and laminar wall jets. The numerical solution of the Navier–Stokes equations in the turbulent case is obtained using the wall-resolved large eddy simulation. The simulation results are compared to experimental data. [ABSTRACT FROM AUTHOR]

A numerical simulation is conducted to study the flow of a three-dimensional incompressible wall jet. The study aims to determine the jet structure and to compare the propagation mechanisms of turbulent and laminar wall jets. The numerical solution of the Navier–Stokes equations in the turbulent case is obtained using the wall-resolved large eddy simulation. The simulation results are compared to experimental data. [ABSTRACT FROM AUTHOR]

Air-core vortices are a ubiquitous phenomenon in the intakes of hydropower stations. Due to the transient and instability of two-phase vorticial flow, the prediction of air-core vortex formation is challenging, and understanding of the instability mechanism remains elusive. In this study, the large eddy simulation (LES) method and a coupled level-set and volume-of-fluid (CLSVOF) method are performed to study air-core vortex formation in a benchmark reservoir with a horizontal intake pipe. The process of air-core vortex formation can be classified into an inception stage, an instability stage, and a stability stage. In the instability stage, the surface vortex repeatedly goes through the process of inception, enhancement, attenuation, and extinction. The movement of the counterrotating secondary vortices and water surface level fluctuation plays a negative role in air-core vortex formation. The additional motion generated by the counterrotating pair drives the pair to the back wall. The main vortex undergoes attenuation due to the stretching/tilting effects induced by the secondary vortex. Water level fluctuations briefly increase the submergence depth, which in turn reduces the vertical velocity gradient and vertical vorticity, destabilizing the vortex. The perturbation of the air-core vortex by water level fluctuations is present only at the beginning of the instability stage. [ABSTRACT FROM AUTHOR]

The wellbore hole simulator as a high-temperature high-pressure thick-walled container, is an experimental device used to simulate the high-temperature high-pressure downhole environment of oilfield. Based on thermodynamics and the large eddy simulation (LES) theory, a physical equation was established. The projection method was applied to solve the temperature field governing equation, and the trapezoidal-rule numerical integration was used to solve the thermal stress governing equation. The discrete scheme for the governing equation was given. The fluid-structure-interaction heat transfer was solved with the virtual density method, and the thermal stress, the pressure stress and their coupling effect of the wellbore hole simulator were numerically analyzed under the principle of stress superimposition. The research results indicate that, the wellbore hole simulator with a minimum wall thickness of 0.18 m could meet the strength requirements for high-temperature high-pressure experiments with 400 ℃ and 220 MPa working parameters. The experiments prove the correctness of the established mathematical model and the numerical solution methods, providing a theoretical basis for the design of thick-walled cylinder containers under high-temperature high-pressure conditions. [ABSTRACT FROM AUTHOR]

The flow around cylinders is one of the most fundamental phenomena in extracting wave energy from ocean waves. Compared with flows around a single cylinder, the investigation of flows around multiple cylinders is still limited and requires further studies to reveal flow characteristics. To this end, large eddy simulations are conducted to investigate the flow around double cylinders with crosswise and streamwise arrangements. Systematic studies on the influence of the number of mesh cells, the first near-wall mesh size, and the transient time step are carried out to achieve accurate and efficient simulations. The drag coefficient, flow separation, and flow pattern for different arrangements under various cylinder spacings are analyzed according to simulation results. For the crosswise arrangement, the flow pattern switches from the single-body regime to the synchronized vortex-shedding regime as the spacing increases. For the streamwise arrangement, the flow pattern develops from the reattachment regime to the vortex-shedding regime as the spacing increases. [ABSTRACT FROM AUTHOR]

A large-eddy simulation (LES) of wake vortices in ground effect is conducted under the influence of airport hangar buildings to investigate the airport-specific behavior of wake vortices. We consider a landing at Tokyo/Haneda International Airport in Japan, where a wake of hangar buildings covers a landing path and touchdown zone under particular wind conditions. We combine an LES code and the Weather Research and Forecasting numerical weather prediction model to retrieve realistic wind conditions around the airport. The behavior of wake vortices is then investigated in the retrieved wind field using the adaptive mesh capability of the LES code for capturing the tiny vortex core of the wake vortex. At the crosswind on the order of a vortex descent speed, the hangar wake reduces the crosswind near the runway, enhancing the windward vortex's rebound. The effect is alleviated by doubling the crosswind to twice the vortex descent speed. On the other hand, the wind fluctuations caused by the hangar buildings only slightly affect the wake vortex decay. [ABSTRACT FROM AUTHOR]

We evaluate three subgrid-scale (SGS) models in large eddy simulations (LES) of compressible mixing layers up to convective Mach number ( $ M_c $ M c ) 2.0. The initial momentum-thickness based Reynolds number is 1000 to match recent high-resolution simulations of the same configuration. The simulations employ the Localised Artificial Diffusivity (LAD) shock-capturing scheme and evaluate constant-coefficient and dynamic versions of Anisotropic Minimum Dissipation (AMD), Sigma, and Modulated Gradient (MGM) SGS models. The results show that the LAD scheme is crucial for stability at higher convective Mach numbers, for $ M_c \gt 1.2 $ M c > 1.2. The SGS models improve the accuracy of the LES predictions, particularly for dissipation and Reynolds stresses. Dynamic versions of all three SGS models give accurate results. The AMD kernel is found to be most sensitive to the flow conditions since the dynamically obtained model coefficient varies the least with $ M_c $ M c . Dynamic versions of the SGS models enhance the agreement with reference results compared to the constant-coefficient versions, particularly for the eddy-viscosity AMD and Sigma models, and to a lesser extent for the MGM model. The results also emphasise the importance of retaining a non-zero artificial-shear-viscosity term, even in the presence of explicit SGS models. [ABSTRACT FROM AUTHOR]

Small shallow cumulus clouds (<1 km) over the tropical oceans appear to possess the ability to self‐organize into mesoscale (10–100 km) patterns. To better understand the processes leading to such self‐organized convection, we present Cloud Botany, an ensemble of 103 large‐eddy simulations on domains of 150 km, produced by the Dutch Atmospheric Large Eddy Simulation model on supercomputer Fugaku. Each simulation is run in an idealized, fixed, larger‐scale environment, controlled by six free parameters. We vary these over characteristic ranges for the winter trades, including parameter combinations observed during the EUREC4A (Elucidating the role of clouds–circulation coupling in climate) field campaign. In contrast to simulation setups striving for maximum realism, Cloud Botany provides a platform for studying idealized, and therefore more clearly interpretable causal relationships between conditions in the larger‐scale environment and patterns in mesoscale, self‐organized shallow convection. We find that any simulation that supports cumulus clouds eventually develops mesoscale patterns in their cloud fields. We also find a rich variety in these patterns as our control parameters change, including cold pools lined by cloudy arcs, bands of cross‐wind clouds and aggregated patches, sometimes topped by thin anvils. Many of these features are similar to cloud patterns found in nature. The published data set consists of raw simulation output on full 3D grids and 2D cross‐sections, as well as post‐processed quantities aggregated over the vertical (2D), horizontal (1D) and all spatial dimensions (time‐series). The data set is directly accessible from Python through the use of the EUREC4A intake catalog. Plain Language Summary: The organization of shallow cumulus clouds over the tropical ocean has recently received a lot of attention. This type of organization is potentially important for how the clouds are affected by a changing climate and also for how they modulate further warming. We present a collection of 103 detailed simulations of shallow cumulus clouds in idealized atmospheric environments. These environments are described by six parameters, and our collection is formed by systematically simulating different parameter combinations. This way an ensemble is created that spans up a multidimensional phase space of environmental conditions typical for the wintertime subtropical Atlantic Ocean. This approach allows us to form a picture of how the environmental conditions relate to the cloud organization that develops in the simulations. At a glance, most simulations evolve similarly: They quickly form small cumulus clouds, which then grow in size and organize into patterns. Often this leads to rainfall, which then causes further heterogeneity and pattern formation. The data is openly available online, and will serve future studies of cumulus clouds, their organization, and how they interact with the climate. Key Points: We present Cloud Botany, an ensemble of idealized large‐eddy simulations of the winter trade wind regions, controlled by six varied parametersThe parameter ranges are chosen to match the climatology of the trade wind regionThe simulations show a variety of cloud organization patterns: small cumulus, stripes, cold pools, cloud arcs, and anvils [ABSTRACT FROM AUTHOR]

To explore the influence of openings on wind loads and wind speeds in high-rise buildings, the wind flow around three-dimensional (3D) square cylinders with a breadth/height aspect ratio of 1:6 was numerically simulated using the large eddy simulation (LES) method via the Fluent 15.0 platform. The opening measures in the X-direction, Y-direction and both directions were all taken into consideration. Firstly, the inflow turbulence synthesis method and parameter settings for LES were verified by comparing the simulation results of standard square cylinders with those of wind tunnel experiments, and the optimal boundary conditions were determined. Then, the wind speed was extracted and compared with the mean wind speed of incoming flow at the same height to analyze the influence of different opening measures on the wind speed of incoming flow by setting monitoring points in the open holes. Finally, the mechanism underlying the effect of the opening form on wind loads and wind speeds was analyzed from the perspective of time-averaged and transient flow field. The results show that the X-direction openings affect the magnitude and distribution of the surface wind pressures by changing the flow separation and flow reattachment. The narrow tube effect can significantly increase the wind speed, while the Y-direction openings have no obvious improvement effect on the surface wind pressures of the structure. The wind speeds in the open holes are greatly reduced due to the shielding effect, and the wind pressures are also reduced for the Y-direction openings. In the X-direction opening holes, the wind speed at the monitoring point increases, while it decreases in the crosswind open holes. In general, the measure of openings in the X-direction can greatly improve the wind load of the structure compared to openings in the Y-direction, and it can provide a good reference for wind power generation in high-rise buildings. [ABSTRACT FROM AUTHOR]

The present work aims to study the tonal and broadband noise associated with a supersonic planar jet, of Mach number 1.28 and Reynolds number 50000, impinging on a flat plate. Five different cases are considered corresponding to different plate distances and angles of impingement. The near-field noise is analyzed using Large Eddy Simulation (LES). For this, we employ a low dispersive and dissipative Finite Volume solver using AUSM for inviscid fluxes and third-order Runge–Kutta method for temporal discretization. The far-field noise is calculated using a hybrid approach in which the acoustic sources and mean flow are provided by the fluid solver while the far-field acoustic pressure is calculated by an acoustic solver. It solves Acoustic Perturbation Equations using a spectral/hp element method with a Discontinuous Galerkin projection. The present work highlights many important acoustics studies of impinging jets. Firstly, the far-field sound with the effect of non-uniform base fields is obtained which is important as the far-field acoustic pressure in most of the literature is reported either without any base fields or by the application of uniform base fields. Secondly, a correlation between the different modes of oscillations of the impinging jet and the production of impinging tones is studied using Modal Decomposition techniques. This analysis also sheds some light on the number of concurrent cycles for the different nozzle-to-plate distances. Lastly, the relation between the phase lag term and the distance between the plate and the nozzle exit is highlighted using a vortex tracking method. These observations can serve to provide a better understanding of the sound generation process. [ABSTRACT FROM AUTHOR]

Combined with infrared thermography measurements, large eddy simulation was used to study the square-wave pulsating film cooling on turbine vane at the time-averaged blowing ratios of 0.5 and 1.5. On suction surface, the introduction of coolant pulsation results in the decrease of cooling effectiveness. On pressure surface, coolant pulsation at high blowing ratio can improve coolant coverage. Starting vortex is the typical structure in pulsating film cooling. However, on suction surface, due to fully-turbulent mainstream, plenty of small-scale vortices play a dominating role in flow fields, and the effect of starting vortex is unobvious. The statistic characteristics and power spectrum density (PSD) of velocity fluctuation were also analyzed in detail. The results show that the disturbance of coolant jet on suction-side mainstream boundary layer is very weak, However, on pressure side, coolant pulsation causes notable increase of PSD and turbulent kinetic energy. [ABSTRACT FROM AUTHOR]

Head-flow HQ curves for a Fontan cavopulmonary assist device (CPAD) were measured using a blood surrogate in a mock circulatory loop and simulated with various computational fluid dynamics (CFD) models. The tests benchmarked the CFD tools for further enhancement of the CPAD design. Recommended Reynolds-Averaged Navier-Stokes (RANS) CFD approaches for the development of conventional ventricular assist devices (VAD) were found to have shortcomings when applied to the Fontan CPAD, which is designed to neutralize off-condition obstruction risks that could contribute to a major adverse event. The no-obstruction condition is achieved with a von Karman pump, utilizing large clearances and small blade heights, which challenge conventional VAD RANS-based CFD hemodynamic simulations. High-fidelity large eddy simulation (LES) is always recommended; however, this may be cost-inhibitive for optimization studies in commercial settings, thus the reliance on RANS models. This study compares head and power predictions of various RANS turbulence models, employing experimental measurements and LES results as a basis for comparison. The models include standard k-ϵ ⁠, re-normalization group k-ϵ ⁠, realizable k-ϵ ⁠, shear stress transport (SST) k-ω ⁠, SST with transitional turbulence, and Generalized k-ω ⁠. For the pressure head predictions, it was observed that the standard k-ϵ model provided far better agreement with experiment. For the rotor torque, k-ϵ predictions were 30% lower than LES, while the SST and LES torque values were near identical. For the Fontan CPAD, the findings support using LES for the final design simulations, k-ϵ model for head and general flow simulation, and SST for power, shear stress, hemolysis, and thrombogenicity predictions. [ABSTRACT FROM AUTHOR]

Understanding ice shelf water (ISW) structure is crucial for studying the basal melting of ice shelves. In this study, we performed large‐eddy simulation experiments to assess ISW structure and basal melt patterns under different current velocity scenarios observed in the Larsen C ice shelf, Antarctica. The LES results revealed that the thickness of ISW is primarily determined by the meridional velocity (perpendicular to the grounding line), while the zonal velocity influences the potential temperature and salinity of ISW. We found that a key parameter determining the basal melt rate was northward meltwater advection which originates from variances in meltwater generation. This advection, in turn, leads to the tilted isopycnals and the potential for thermohaline interleaving in a diffusive convection regime. The different slopes of isopycnals induce distinct heat fluxes, resulting in different basal melt rates far from and near the grounding line (∼0.44 and 1.59 m yr−1, respectively). Plain Language Summary: The loss of ice mass from the Antarctic ice sheet is accelerating, posing a threat to human lives through global sea level rise. Understanding ice shelf water (ISW), which refers to seawater below freezing temperature, is crucial as it directly or indirectly influences basal ice melting. However, direct observations are extremely challenging, leaving this understanding unclear. To tackle this issue, we utilized a numerical model to gain insight into the fundamental characteristics of ISW. We demonstrated that the direction and magnitude of ocean currents beneath the ice shelf play a significant role in determining the thickness and properties of ISW. Moreover, the key factor in basal melting was the northward movement of meltwater from intense ice melting regions near the grounding line. This movement determined the spatial distribution of ocean temperature and salinity. The horizontal gradient of ocean temperature and salinity induces mixing and horizontal intrusion. Interestingly, these mixing and intrusion phenomena occur in opposite directions, resulting in a wiggling pattern in the velocity profile. The main findings of our study will contribute to the formulation of a parameterization for basal melting, which can be incorporated into large‐scale ocean models or ice sheet dynamics models. Key Points: Direction and magnitude of ocean currents beneath an Larsen C ice shelf affect the ice shelf water thickness and propertiesNorthward meltwater advection causes 0.052°C difference of thermal drivings with different melt rates far from and near the grounding lineOcean heat intrusion to ice shelf base is induced by Ekman dynamics and thermohaline interleaving by tilted isopycnals [ABSTRACT FROM AUTHOR]

An internal unsteady compressible flowfield of a pulsed jet actuator (PJA) is simulated by unsteady Reynolds-averaged Navier-Stokes (URANS) and large-eddy simulation (LES) frameworks. The PJA's feedback loops are short enough to have non-negligible jet switching time compared to the oscillation period. The results show that within a range of inlet pressure (which is the only control parameter) where the jet goes from subsonic to supersonic regime, the oscillation frequencies and the exit velocity waveform predicted by LES are in better agreement with the measured data than those predicted by URANS. Although URANS is able to capture the flowfield inside the feedback loops, it fails to predict the exit jet dynamics, unlike LES. This indicates the importance of a proper resolution of turbulence in the interaction region where jet switching occurs. Using LES data, it is found that the total jet switching time remains constant as the inlet pressure changes. It is also found that only the part of the jet switching period that ends at the first moment when the jet attaches to the opposite wall is involved in the oscillation period. Therefore, a new lumped model is proposed to estimate the oscillation frequency, which also explains the trend observed for the low inlet pressure values. [ABSTRACT FROM AUTHOR]

The meso‐scale variability in cloudiness of the marine trade‐wind layer is explored with large‐eddy simulations of regional extent and validated against observations of the EUREC4A field campaign. 41 days of realistically forced simulations present a representative, statistical view on shallow convection in the winter North Atlantic trades that includes a wide range of meso‐scale variability including the four recently identified patterns of spatial organization: Sugar, Gravel, Flowers and Fish. The results show that cloud cover is on average captured well but with discrepancies in its vertical and spatial distribution. Cloudiness at the lifting condensation level depends on the model resolution with the finer one producing on average a more realistic cloud profile. Independent of the resolution, the variability in cloudiness below the trade inversion is not captured, leading to a lack of stratiform cloudiness with implications on the detectability of meso‐scale patterns whose cloud patches are characterized by stratiform clouds. The simulations tend to precipitate more frequently than observed, with a narrower distribution of echo intensities. The observed co‐variability between cloudiness and environmental conditions is well captured. Plain Language Summary: Clouds generally cool the planet due to their ability to reflect sunlight efficiently. To estimate this cooling in a future climate, the processes leading to cloud formation need to be understood. A process that current climate simulations struggle to capture due to their coarse resolution is the variability and patterning of cloudiness on scales on the order of 10–100 km. In this study we ran higher resolved simulations at hm‐resolutions by limiting the region to the downstream North Atlantic trades where the patterning of shallow clouds is common. Coinciding our simulations with the measurements of the EUREC4A field campaign and being able to run them for over a month allowed us to pin‐point current deficits that these higher resolved simulations have. These are in particular the vertical cloud distribution with too little stratiform cloud amount and too much precipitation that hardly changes with the patterning in cloudiness. Nevertheless, the simulations do a good job in capturing the day‐to‐day variability in total cloud cover and its co‐variability with environmental conditions justifying a further study of the phenomenon with these kind of simulations and ultimately improving the climate simulations on this aspect. Key Points: A 41 days, hm‐resolution simulation is run to quantify the ability of ICON‐LEM to reproduce the observed cloudinessThe simulated cloudiness co‐varies with precipitable water, temperature and wind speed mimicking the observationsThe vertical distribution of cloudiness remains challenging at the cost of representing mesoscale patterns with stratiform cloud amount [ABSTRACT FROM AUTHOR]

To study the mechanism by which an increase in the number of obstacles affects the propagation of hydrogen-air premixed gas explosions under a constant overall volume of obstacles, a large eddy simulation method was used to carry out numerically simulate configurations with different distribution modes of combined obstacles. The study focused on the flame structure, evolution process of overpressure dynamics, and flame-flow coupling relationship. The results showed that the flame propagation velocity and flame front area are increased during the conversion of the combined obstacles from 1-30 mm to 4–7.5 mm, while the flame front area logarithmically depends on the number of obstacles. The flames gradually develop from "corrugated flamelets" to "thin reaction zones" in different distribution modes. In addition, the results showed that although increasing dispersion increases the explosion overpressure, a critical number of obstacles likely exist. Beyond the critical point, explosion overpressure peak no longer strongly varies with the number of obstacles. Furthermore, for working configurations with different numbers of obstacles, an increase in the overall number of obstacles before reaching the same number of obstacles weakly affects the flame shape and flow rate of the flame front. This study provides theoretical guidelines for safety designs to prevent hydrogen-air premixed gas explosion in obstructed spaces. • The accuracy of some premixed combustion simulation results is verified. • Flame modes for different combined obstacles are elucidated. • The coupling relationship between the flame and flow field is analyzed. [ABSTRACT FROM AUTHOR]

An OpenFOAM-based solver for spray combustion simulation with the large eddy simulation (LES) and the flamelet generated manifold (FGM) method, was developed. A simple reduction of the temperature was employed to account for the evaporative heat loss. The solver was firstly validated against the Sydney piloted ethanol spray flame benchmark EtF7. The predicted mean gas temperature and droplet statistics correspond well with the experimental data and have similar accuracy to the spray flamelet model. The turbulence-chemistry interaction modeling may have a larger influence on the simulation accuracy. Then a realistic gas turbine slinger combustor was simulated with 2 sets of operating conditions. The simulation results reveal different flame characteristics of the 2 working conditions. The predicted total pressure losses are close to the measured values. [ABSTRACT FROM AUTHOR]

The atmospheric boundary layer flow downstream of an abrupt rough-to-smooth surface roughness transition is studied using large eddy simulations (LES) for a range of surface roughness ratios. Standard wall models assume horizontal homogeneity and are inapplicable for heterogeneous surfaces. Two heterogeneous-surface wall models are evaluated, one based on a local application of similarity theory using a twice-filtered velocity field (BZ model) and another based on a local friction-velocity obtained by blending the upstream and downstream profiles (APA model). The wall shear stress and the turbulence intensity (TI) are sensitive to the wall model while the mean streamwise velocity and the total shear stress (TSS) are less sensitive. The APA model is more accurate than the BZ model on comparison to previous experiments. The wall shear stress obtained using the APA wall model is sensitive to the ratio of the equilibrium and the internal boundary layer (IBL) heights, while other statistics are not. The IBL height is insensitive to the turbulent quantity (TSS or TI) on which it is based. Several analytical relations for the IBL height are evaluated using the LES data. Two models are found to be accurate for different roughness ratios while one model is reasonable over the full range investigated. A phenomenological model is developed for the TI downstream of the roughness jump using a weighted average of the upstream and far-downstream profiles. The model yields reasonable predictions for all roughness ratios investigated. [ABSTRACT FROM AUTHOR]

The question of at what resolution the large eddy simulations (LESs) of a tropical cyclone (TC) high wind area may converge largely remains unanswered. To address this issue, LESs with five resolutions of 300 m, 100 m, 60 m, 33 m, and 20 m are performed in this study to simulate a high wind area near the radius of maximum wind of Typhoon Chanthu (2021) using the Weather Research and Forecasting (WRF) model. The results show that, for a limited area LES, model grid resolution may alter the local turbulence structure to generate significantly different extreme values of temperature, moisture, and winds, but it only has a marginal impact on the median values of these variables throughout the vertical column. All simulations are able to capture the turbulent roll vortices in the TC boundary layer, but the structure and intensity of the rolls vary substantially in different resolution simulations. Local hectometer-scale eddies with vertical velocities exceeding 10 m s−1 are only observed in the 20 m resolution simulation but not in the coarser resolution simulations. The ratio of the resolved turbulent momentum fluxes and turbulent kinetic energies (TKEs) to the total momentum fluxes and TKEs appears to show some convergence of LESs when the grid resolution reaches 100 m or finer, suggesting that it is an acceptable grid resolution for LES applications in TC simulations. [ABSTRACT FROM AUTHOR]