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Abstract Two different wdf (wall-distance-free) approaches are devised using local wall-distance and dissipation based representations to obtain consistency formulations for blending functions associated with the SST model. The strain-dependent coefficient C ̄ μ assists the SST model in constructing the functions such as to benefit complex flows with non-equilibrium turbulence, while retaining the predictive quality of unmodified SST closure. Naturally, the proposed methods allow the original SST model to be modified accurately and alleviate the free-stream dependency. Comparisons with the original SST k - ω model demonstrate that the newly developed alternatives to actual wall-distance usually improve the response of SST model to non-equilibrium effects. [ABSTRACT FROM AUTHOR]

Backward-facing step (BFS) constitutes a canonical configuration to study wall-bounded flows subject to massive expansions produced by abrupt changes in geometry. Recirculation flow regions are common in this type of flow, driving the separated flow to its downstream reattachment. Consequently, strong adverse pressure gradients arise through this process, feeding flow instabilities. Therefore, both phenomena are strongly correlated as the recirculation bubble shape defines how the flow is expanded, and how the pressure rises. In an incompressible flow, this shape depends on the Reynolds value and the expansion ratio. The influence of these two variables on the bubble length is widely studied, presenting an asymptotic behaviour when both parameters are beyond a certain threshold. This is the usual operating point of many practical applications, such as in aeronautical and environmental engineering. Several numerical and experimental studies have been carried out regarding this topic. The existing simulations considering cases beyond the above-mentioned threshold have only been achieved through turbulence modelling, whereas direct numerical simulations (DNS) have been performed only at low Reynolds numbers. Hence, despite the great importance of achieving this threshold, there is a lack of reliable numerical data to assess the accuracy of turbulence models. In this context, a DNS of an incompressible flow over a BFS is presented in this paper, considering a friction Reynolds number ($Re_{\unicode[STIX]{x1D70F}}$) of 395 at the inflow and an expansion ratio 2. Finally, the elongation of the Kelvin–Helmholtz instabilities along the shear layer is also studied. [ABSTRACT FROM AUTHOR]

Parameterizations are developed for each of the subgrid turbulence interaction classes in fully three-dimensional global atmospheric flows over topography, typical of January and July climate states. Stochastic and deterministic parameterizations are developed for the eddy–eddy interactions and deterministic parameterizations for eddy–mean field, eddy–topographic, mean field–mean field, and mean field–topographic interactions. All parameterizations are calculated from the statistics of higher-resolution reference direct numerical simulations (DNSs) truncated into resolved and subgrid scales and employed without tuning coefficients. This parameterization framework is validated by performing large-eddy simulations (LESs) that closely agree with the reference DNSs in terms of time-averaged kinetic energy spectra, zonal jet structure, and nonzonal streamfunction fields. Both the DNSs and LESs are formulated in such a way that the usual problem of a long artificial dissipation range does not occur. Successful LESs are produced with truncation wavenumbers 31 and 15, using, respectively, only 11.9% and 1.3% of the DNS computational effort at truncation wavenumber 63. The lower-resolution LESs show that the parameterizations are successful even when the energy injection due to baroclinic instability is not completely resolved. The contribution of each of the parameterized interaction classes to the quality of the LES is identified. The best agreement is achieved when all subgrid components are included. There is a very high level of agreement between the LESs and DNSs with typical streamfunction pattern correlations of r = 0.962 for the nonzonal components and r = 0.999 for the total fields when the complete sets of parameterizations are used. [ABSTRACT FROM AUTHOR]

Highlights • A new physical scenario of the influence of turbulence on reaction waves is studied. • A relevant theory is developed. • The theory is validated using direct numerical simulation data. • The theory explains recent experimental and direct numerical simulation data. Abstract A big database (23 cases characterized by Damköhler number less than unity) created recently in 3D Direct Numerical Simulation (DNS) of propagation of a statistically one-dimensional and planar, dynamically passive reaction wave in statistically stationary, homogeneous, isotropic turbulence is analyzed. On the one hand, the DNS data well support the classical Damköhler expression, i.e., square-root dependence of a ratio of turbulent and laminar consumption velocities on the turbulent Reynolds number. On the other hand, contrary to the common interpretation of the Damköhler theory and, in particular, to the concept of distributed burning, the DNS data show that the reaction is still localized to thin zones even at Da as low as 0.01, with the aforementioned ratio of the consumption velocities being mainly controlled by the reaction-zone-surface area. To reconcile these apparently inconsistent numerical findings, an alternative regime of propagation of reaction waves in a highly turbulent medium is analyzed, i.e., propagation of an infinitely thin reaction sheet is theoretically studied, with molecular mixing of the reactant and product being allowed in wide layers. In this limiting case, an increase in the consumption velocity by turbulence is solely controlled by an increase in the reaction-sheet area. Based on physical reasoning and estimates, the area is hypothesized to be close to the mean area of an inert iso-scalar surface at the same turbulent Reynolds number. This hypothesis leads to the aforementioned square-root dependence. Thus, both the DNS data and the developed theory show that a widely accepted hypothesis on penetration of small-scale turbulent eddies into reaction zones is not necessary to obtain the classical Damköhler scaling for turbulent consumption velocity. [ABSTRACT FROM AUTHOR]

Abstract A wind measurement system was established on Mount Wangye, Fujian, China. Multi-level three-dimensional wind measurements of two Typhoon cyclones have been recorded and analyzed to find the characteristics of the near-surface boundary layer. This paper presents wind speed profiles and related parameters, turbulence intensities, turbulence integral length scales, gust factors, and the characteristics of the fluctuating wind speed power spectra in Typhoon conditions. We found a region-specific wind speed power spectrum and its associated spectrum parameters that can be used for the design of wind-resistant structures in Typhoon prone coastal region in southeast of China. Highlights • In this research, a wind measurement system was established on Mount Wangye, Fujian, China. • Multi-level three-dimensional wind measurements of two Typhoon cyclones have been recorded and analyzed. • Based on real measured data, wind parameters in Typhoon conditions are analyzed. [ABSTRACT FROM AUTHOR]

Summary: The analysis and optimization of complex multiphysics systems presents a series of challenges that limit the practical use of computational tools. Specifically, the optimization of such systems involves multiple interconnected components with competing quantities of interest and high‐dimensional spaces and necessitates the use of costly high‐fidelity solvers to accurately simulate the coupled multiphysics. In this paper, we put forth a data‐driven framework to address these challenges leveraging recent advances in machine learning. We combine multifidelity Gaussian process regression and Bayesian optimization to construct probabilistic surrogate models for given quantities of interest and explore high‐dimensional design spaces in a cost‐effective manner. The synergistic use of these computational tools gives rise to a tractable and general framework for tackling realistic multidisciplinary optimization problems. To demonstrate the specific merits of our approach, we have chosen a challenging large‐scale application involving the hydrostructural optimization of three‐dimensional supercavitating hydrofoils. To this end, we have developed an automated workflow for performing multiresolution simulations of turbulent multiphase flows and multifidelity structural mechanics (combining three‐dimensional and one‐dimensional finite element results), the results of which drive our machine learning analysis in pursuit of the optimal hydrofoil shape. [ABSTRACT FROM AUTHOR]

Abstract Serrated jet nozzles are considered to be an efficient and practical passive control approach for jet noise. However, some fundamental mechanisms of serration effects on jet noise are not fully understood, especially in terms of the sound source. In this paper, a high-fidelity simulation framework using large-eddy simulation (LES) is demonstrated to predict near-field turbulence and far-field acoustics from an ultra-high-bypass-ratio engine with round and serrated nozzles. Far-field sound is predicted using Ffowcs Willams–Hawkings (FWH) integration. The results show that the serrated nozzle increases mixing near the nozzle and hence the turbulence decay rate, reducing the turbulence level downstream. The serrations shift the energy from the low frequencies to the high frequencies and decrease overall sound pressure levels by about 3 dB over the low-frequency range. Sound sources are analysed based on fourth-order space–time correlations. There are six major source components (R 1111 , R 2222 , R 3333 , R 1313 , R 1212 , and R 2323) inside the jet shear layers. The serrations are able to reduce the amplitude of these source terms, causing them to decay rapidly to a level below the round nozzle jet within 2 D downstream of the nozzle. [ABSTRACT FROM AUTHOR]

It is known that certain geometrical-optics predictions often agree well with optical turbulence field observations even though theoretical constraints for ignoring diffraction may be violated. Geometrical optics assumptions can simplify analyses, and ray optics can significantly reduce simulation computation time. Here, an investigation into angle-of-arrival fluctuations is presented involving wave optics and geometrical (ray) optics computer simulations of a plane wave of visible light propagating through a turbulent refractiveindex field. The simulation and Rytov-based theory results for the variances of aperture-filtered angle-of-arrival fluctuations generally agree well for weak scattering (Rytov variance, s² R ≲ 0.2), but for increasing Rytov variance, the simulation results demonstrate a positive slope that can be significantly shallower than that predicted by the theory. For weak-to-moderate scattering regimes (s2 R ≲ 2.67), a comparison of the ray and wave results show they match for aperture diameters greater than about two Fresnel lengths. This result is consistent with a previous theoretical analysis by Cheon and Muschinski. For the strongest scattering case studied (s² R = 26.7), the wave and ray simulations match for aperture diameters greater than about 10 Fresnel lengths. For smaller apertures, we attribute the disparity between the wave and ray simulation results to a Fresnel filtering effect. [ABSTRACT FROM AUTHOR]

A kinematic simulation with an approximate deconvolution (KSAD) hybrid model is proposed to predict the Lagrangian relative dispersion of fluid particles in a large eddy simulation (LES) of isotropic turbulent flows. In the model, a physical connection between the resolved and subgrid scales is established through the energy flux rate at the filter width scale. Due to the lack of subgrid-scale (SGS) turbulent structures and SGS model errors, the LES cannot accurately predict the two- and multi-point Lagrangian statistics of the fluid particles. To improve the predictive capability of the LES, we use an approximate deconvolution model to improve the resolved scales near the filter width and a kinematic simulation to recover the missing velocity fluctuations beneath the subgrid scales. To validate the proposed hybrid model, we compare the Lagrangian statistics of two- and four-particle dispersion with the corresponding results from the direct numerical simulation and the conventional LES. It is found that a significant improvement in the prediction of the Lagrangian statistics of fluid particles is achieved through the KSAD hybrid model. Furthermore, a parametric study regarding the wavenumbers and orientation wavevectors is conducted to reduce the computational cost. Good results can be obtained using a small number of wavenumber modes and orientation wavevectors. Thus, we can improve the prediction of the Lagrangian dispersion of fluid particles in the LES by applying the KSAD hybrid model at an acceptable computational cost. [ABSTRACT FROM AUTHOR]

In this paper, the flow around a main circular cylinder with two small control cylinders is numerically investigated at a subcritical Reynolds number of 3900. Two small control cylinders with diameter ratio of d/D = 0.04 (d and D are the diameters of the control cylinders and the main cylinder, respectively) are symmetrically placed inside the separated shear layers emanating from the main cylinder. The computation is performed with highly resolved direct numerical simulations employing a high-order spectral/hp element method. For validation purposes, the results of the flow over a single cylinder are presented at first, and the current simulation is in good agreement with the existing experiments and large eddy simulations. For the main cylinder with the two control cylinders, the dynamics of shear layers is found to be modified strongly owing to the disturbance from the small control cylinders. As a consequence, the recirculation length is decreased by 89% as compared to that of a single cylinder. The presence of the small control cylinders further leads to a significant increase in the fluctuating lift and the mean drag exerting on the main cylinder. Furthermore, the statistical analysis and turbulent wake visualization imply that the turbulent intensity is amplified in the near wake region and becomes weaker further downstream. This study then suggests that the small control cylinders located in the near wake region of the main cylinder may work differently in the turbulent wake against that in the laminar wake. [ABSTRACT FROM AUTHOR]

We present a study of a large-scale energy spectrum and integral invariants in temporally developing grid turbulence at mesh Reynolds numbers of ReM = 10 000 and 20 000 by employing direct numerical simulations (DNSs) in a periodic box. The simulations are initialized with a velocity field that approximates the wakes induced by the bars of conventional square grids. The turbulence statistics obtained in the temporal DNS agree well with those of the previous experiments in both the production and decay regions. The temporally developing grid turbulence also has a so-called non-equilibrium region, which is consistent with its spatially developing counterpart, where the normalized dissipation rate of turbulence kinetic energy (TKE), Cε, increases as the turbulence decays. The decay exponent n of TKE is n = 1.22 at ReM = 20 000 and n = 1.35 at ReM = 10 000, which are close to the values for the Saffman turbulence [i.e., 6/5 for ReM = 20 000 and 6(1 + p)/5 ≈ 1.36 for ReM = 10 000 with p ≈ 0.13 obtained by Cε ∼ tp at large t]. The longitudinal integral length scale and the TKE dissipation rate also exhibit temporal evolutions consistent with the Saffman turbulence for both ReM. The Saffman integral directly evaluated in the grid turbulence tends to be time-independent after the turbulence evolves for about 200 times of characteristic time scale defined by mesh size divided by the mean velocity of a fluid passing the grid. A direct evaluation of the TKE spectrum E(k) shows that E(k) ≈ Lk2/4π2 is valid for a finite range of low wavenumbers. [ABSTRACT FROM AUTHOR]

The vertically averaged velocity of the 3D Rayleigh–Bénard problem is analyzed and numerically simulated. This vertically averaged velocity satisfies a 2D incompressible Navier–Stokes system with a body force involving the 3D velocity. A time average of this force is estimated through time averages of the 3D velocity. Relations similar to those from 2D turbulence are then derived. Direct numerical simulation of the 3D Rayleigh–Bé nard is carried out to test how prominent the features of 2D turbulence are for this Navier–Stokes system. [ABSTRACT FROM AUTHOR]

It is known that wind turbines actually operate in the lower part of the atmospheric boundary layer (ABL), the modelling of this ABL flow is an important precondition for the simulations of wind turbine wakes. So, the capabilities of various inlet boundary conditions with related modelling methodologies in constructing equilibrium ABL are assessed firstly through cases of ABL flows over uniformly rough terrain, to ascertain that there are no substantial changes in the prescribed profiles throughout the whole computational domain. In this process, six popular turbulence inflow profiles, including four uniform and two non-uniform ones, are considered and investigated. Then, sensitivity studies on inflow profiles for predicting wind turbine wake development are carried out. Through comparing with the Sexbierum field experimental data, in terms of wake velocity and turbulence intensity along the cross-wind direction at several downstream positions, this study finds out that the shape and magnitude of wake velocity and wake turbulence profiles are significantly affected by different inflow profiles. Possible reasons for this sensibility are discussed, and accordingly, some suggestions are given to improve the accuracy of wind turbine wake simulation. [ABSTRACT FROM AUTHOR]

The heat transfer enhancement (HTE) from buoyancy occurs in upward and downward flows at supercritical pressures when the buoyancy parameter, Bo*, is above 8 × 10 −6 . Numerical simulations of experiments on buoyancy-enhanced heat transfer of supercritical CO 2 in a heated vertical tube with an inner diameter of 2.0 mm at an inlet Reynolds number, Re in , of 1970 were performed using low Reynolds number turbulence models. The Myong and Kasagi (MK) model quantitatively predicted the buoyancy-enhanced heat transfer. The trade-off between buoyancy effect on the viscous length scale and the redistribution of velocity profiles resulted in similar heat transfer results although the mechanisms on the two aspects were different for upward and downward flows. Heat transfer deterioration (HTD) occurred in upward flow as Re in increased. The supercritical heat transfer in upward flows for various Re in were compared to obtain a better understanding of the buoyancy effect under HTE and HTD conditions. [ABSTRACT FROM AUTHOR]

Abstract: ClusColl, an economical simulation method for droplet motions and collisions in turbulent flows, has been developed, implemented, tested, and applied. In the Linear Eddy Model, permutations called triplet maps representing individual turbulent eddies implement turbulent advection of fluid in 1‐D. This captures flow processes down to the smallest turbulent eddy (Kolmogorov microscale), but the inertial response of small Stokes number droplets to turbulence has important features at scales down to the droplet radius, notably sub‐Kolmogorov‐scale clustering of finite‐inertia droplets that can increase collision rates significantly. Additionally, shear due to the smallest scales of turbulence increases collision rates of zero‐inertia droplets. In ClusColl, a 3‐D triplet map for droplets captures both effects. We implemented collision detection, enabling simulation of droplet collisions and coalescence, and a sedimentation treatment in ClusColl. Published direct numerical simulations (DNSs) of monodispersions were used to tune parameters. For sedimenting droplets in turbulence, ClusColl's turbulent enhancement of bidisperse collision kernels agrees reasonably well with published DNS results. We compared ClusColl and DNS coalescence growth results. For weak turbulence (ε≤100 cm2/s3), ClusColl's turbulent enhancement of coalescence growth closely matches that of the DNS. For ε≥200 cm2/s3, lack of accurate collision efficiencies precludes definitive quantitative evaluation of ClusColl's coalescence growth. In a comparison of coalescence growth dependence on the droplet size distribution width and on turbulent enhancement, ClusColl indicates that the latter dramatically accelerates cloud droplet conversion into raindrops, while the former has significantly less impact. [ABSTRACT FROM AUTHOR]

Wall-bounded turbulence, where it occurs in engineering or nature, is commonly subjected to spatial variations in wall shear stress. A prime example is spatially varying roughness. Here, we investigate the configuration where the wall shear stress varies only in the lateral direction. The investigation is idealised in order to focus on one aspect, namely, the similarity and structure of turbulent inertial motion over an imposed scale of stress variation. To this end, we analyse data from direct numerical simulation (DNS) of pressure-driven turbulent flow through a channel bounded by walls of laterally alternating patches of high and low wall shear stress. The wall shear stress is imposed as a Neumann boundary condition such that the wall shear stress ratio is fixed at 3 while the lateral spacing s of the uniform-stress patches is varied from 0.39 to 6.28 of the half-channel height δ. We find that global outer-layer similarity is maintained when s is less than approximately 0.39δ while local outer-layer similarity is recovered when s is greater than approximately 6.28δ. However, the transition between the two regimes through s/δ is not monotonic owing to the presence of secondary roll motions that extend across the whole cross-section of the flow. Importantly, these secondary roll motions are associated with an amplified skin-friction coefficient relative to both the small- and large-s/δ limits. It is found that the relationship between the secondary roll motions and the mean isovels is reversed through this transition from low longitudinal velocity over low stress at small s/δ to high longitudinal velocity over low stress at large s/δ. [ABSTRACT FROM AUTHOR]

We use a direct numerical simulations (DNS) database for turbulent flow in a square duct up to bulk Reynolds number Reb D 40 000 to quantitatively analyse the role of secondary motions on the mean flow structure. For that purpose we derive a generalized form of the identity of Fukagata, Iwamoto and Kasagi (FIK), which allows one to quantify the effect of cross-stream convection on the mean streamwise velocity, wall shear stress and bulk friction coefficient. Secondary motions are found to contribute approximately 6% of the total friction, and to act as a self-regulating mechanism of turbulence whereby wall shear stress non-uniformities induced by corners are equalized, and universality of the wall-normal velocity profiles is established. We also carry out numerical experiments whereby the secondary motions are artificially suppressed, in which case their equalizing role is partially taken by the turbulent stresses. [ABSTRACT FROM AUTHOR]

In this study, macro-rough flows over beds with different permeability values are simulated using the large-eddy simulation, and the results are analysed by applying the double-averaging (DA) methodology. Spheres of different sizes and arrangements were used to form the beds, which are deemed to be permeable granular beds. The influence of bed permeability on the turbulence dynamics and structure is investigated. It was observed that the scales of the spanwise vortical structures over more permeable beds are larger than those over less permeable beds. This is attributed to large-scale spanwise-alternate strips of varying Reynolds shear stress (RSS), emerging from the surface of macro-rough elements for the permeable beds. The DA stress balance suggests that the time-averaged spanwise vortical structure leads to a damping in DA RSS and an unusual peak of the form-induced stress in the main flow. In the streamwise direction, both large turbulent structures that originate from the Kelvin--Helmholtz-type instability and small turbulent structures that are associated with the turbulent transport across the gaps of the roughness elements are more prevalent over highly permeable beds. Near the bed, the relative magnitude of turbulent events shows a transition from a ejections-dominating to sweeps-dominating zone with vertical distance. Further, several hydrodynamic characteristics normalized by inner scales (kinematic viscosity to shear velocity ratio) show a greater dependency on permeability Reynolds number than those normalized by sediment size. The study provides an insight into the mechanism of mass transfer near the fluid--particle interface, which is vital to benthic and aquatic ecology. [ABSTRACT FROM AUTHOR]

A family of cases each containing a small separation bubble is treated by direct numerical simulation (DNS), varying two parameters: the severity of the pressure gradients, generated by suction and blowing across the opposite boundary, and the Reynolds number. Each flow contains a well-developed entry region with essentially zero pressure gradient, and all are adjusted to have the same value for the momentum thickness, extrapolated from the entry region to the centre of the separation bubble. Combined with fully defined boundary conditions this will make comparisons with other simulations and turbulence models rigorous; we present results for a set of eight Reynolds-averaged Navier--Stokes turbulence models. Even though the largest Reynolds number is approximately 5.5 times higher than in a similar DNS study we presented in 1997, the models have difficulties matching the DNS skin friction very closely even in the zero pressure gradient, which complicates their assessment. In the rest of the domain, the separation location per se is not particularly difficult to predict, and the most definite disagreement between DNS and models is near reattachment. Curiously, the better models tend to cluster together in their predictions of pressure and skin friction even when they deviate from the DNS, although their eddy-viscosity levels are widely different in the outer region near the bubble (or they do not rely on an eddy viscosity). Stratford's square-root law is satisfied by the velocity profiles, both at separation and reattachment. The Reynolds-number range covers a factor of two, with the Reynolds number based on the extrapolated momentum thickness equal to approximately 1500 and 3000. This allows tentative estimates of the improvements that even higher values will bring to the model comparisons. The solutions are used to assess models through pressure, skin friction and other measures; the flow fields are also used to produce effective eddy-viscosity targets for the models, thus guiding turbulence-modelling work in each region of the flow. [ABSTRACT FROM AUTHOR]

Many turbulent flows are characterized by complex scale interactions and vorticity generation caused by compressibility and variable-density effects. In the large-eddy simulation of variable-density flows, these processes manifest themselves as subgridscale (SGS) terms that interact with the resolved-scale flow. This paper studies the effect of the variable-density SGS terms and quantifies their relative importance. We consider the SGS terms appearing in the density-weighted Favre-filtered equations and in the unweighted Reynolds-filtered equations. The conventional form of the Reynolds-filtered momentum equation is complicated by a temporal SGS term; therefore, we derive a new form of the Reynolds-filtered governing equations that does not contain this term and has only double-correlation SGS terms. The new form of the filtered equations has terms that represent the SGS mass flux, pressure-gradient acceleration and velocity-dilatation correlation. To evaluate the dynamical significance of the variable-density SGS effects, we carry out direct numerical simulations of compressible decaying turbulence at a turbulent Mach number of 0.3. Two different initial thermodynamic conditions are investigated: homentropic and a thermally inhomogeneous gas with regions of differing densities. The simulated flow fields are explicitly filtered to evaluate the SGS terms. The importance of the variable-density SGS terms is quantified relative to the SGS specific stress, which is the only SGS term active in incompressible constant-density turbulence. It is found that while the variable-density SGS terms in the homentropic case are negligible, they are dynamically significant in the thermally inhomogeneous flows. Investigation of the variable-density SGS terms is therefore important, not only to develop variable-density closures but also to improve the understanding of scale interactions in variable-density flows. [ABSTRACT FROM AUTHOR]

The thermal non-equilibrium effect of the small-scale structures in the canonical two-dimensional turbulence is studied. Comparative studies of Unified Gas Kinetic Scheme (UGKS) and GKS-Navier-Stokes (NS) for Taylor-Green flow with initial Ma = 1, Kn = 0.01 and decaying isotropic turbulence with initial , show that the discrepancy exists both in small and large scales, even beyond the dissipation range to 10 with accuracy to 8% in the SGS energy transfer of the decaying isotropic turbulence, illustrating the necessity for resolving the kinetic scales even at moderated . [ABSTRACT FROM AUTHOR]

In the present study, a swirl diffuser with a novel design is proposed, in which three geometrical parameters, the angle of the guide vanes φ , the height of the convergent chamber h , and the radius of the diffuser outlet r , can be varied independently. Large eddy simulations were conducted to examine the airflow characteristics downstream of the swirl diffuser. Grid independence was obtained for the grid number 4000,000, and this result was validated by comparison with an experiment. The vorticities, the instantaneous and averaged velocities, and the evolution of the age of air were investigated. The turbulent kinetic energy budget and the momentum balance in the time-averaged Navier–Stokes equations were then studied in an attempt to clarify the mechanism of the swirl diffuser's high diffusing performance. With the increasing swirling strength, the pressure gradient force could not balance the centrifugal force in the radial direction. As a result, the vortex core expanded, generating turbulence. A non-dimensional swirl number parameter was suggested which can be used for determining the structure of the swirl diffuser airflow. To validate this non-dimensional parameter, different combinations of φ , h , and r were examined. It was found that only if the swirl number is the same, will the airflow structure be similar. The results of our study are likely to be useful for obtaining deeper insights about the swirl diffuser airflow, and for better understanding of the effect of the controlling parameter on the swirl diffuser's performance. [ABSTRACT FROM AUTHOR]

A hybrid procedure consisting of a high order continuum (HOC) model and the direct simulation Monte-Carlo (DSMC) solver is proposed in this paper, as it represents a promising approach for seamless computation of hypersonic flows in all regimes. This approach also allows the effects of thermophysics (thermal and chemical non-equilibrium) and turbulence to be included so that gas interactions can be modeled much more easily than in other approaches. Such hybrid procedures can also be developed into robust and efficient parallel computing tools for practical 3D computations. The main idea behind the proposed HOC/DSMC methodology consists of incorporating the physically realizable and computationally stable version of the Burnett equations into hypersonic codes that have the capability for calculating non-equilibrium chemistry and temperature. We explore the feasibility of simplified, yet accurate and numerically stable, versions of the Burnett equations. We discuss such a model in detail, providing an analysis of its stability and performance for Alsmeyer's shock wave problem and hypersonic flow over a sphere. We also report on the performance of the DSMC component of the proposed hybrid scheme. [ABSTRACT FROM AUTHOR]

The corner separation flow in a highly loaded prescribed velocity distribution (PVD) cascade has been studied by using DDES method based on SST k – ω model (DDES-SST). The numerical simulation results of DDES and RANS have been compared with experiment. The DDES-SST method could provide accurate predictions on corner separation region and RANS-SST method overestimate the corner separation flow in the cascade. The development and structure of vortices in the corner separation flow have been carefully studied. The velocity histogram of the monitor points in mainstream and corner region are collected and the velocity “bimodal” distribution in corner separation region, which was once found in the experiment, has been approved with numerical result by DDES method. Turbulence anisotropy has been discussed with Lumley anisotropy invariant map (AIM) theory and the high anisotropy region is found corresponding to high shear stress region. Other turbulence characteristics, like turbulence energy spectra, turbulence helicity density and velocity skewness have also been discussed and analyzed carefully to help understand the different aspects of turbulence behaviors of the three-dimensional (3D) corner separation flow in the cascade. [ABSTRACT FROM AUTHOR]

Thermal mixing phenomena of a coaxial jet with perforated obstacles are analyzed both experimentally and numerically. Perforated obstacles are used in front of a coaxial jet that has different temperatures of the fluid to control the thermal mixing behavior. An experimental set-up was constructed to perform several cases and all cases are simulated by using large eddy simulation (LES) turbulence model. Results of the study presented that inserting perforated obstacle affects the mixing performance positively and the best mixing performance is obtained in the case that the obstacle has the highest permeability. Experimental and computational results were compared and a good agreement was obtained. [ABSTRACT FROM AUTHOR]

In this paper we present scaling results of a FFT library, FFTK, and a pseudospectral code, Tarang, on grid resolutions up to 819 2 3 grid using 65536 cores of Blue Gene/P and 196608 cores of Cray XC40 supercomputers. We observe that communication dominates computation, more so on the Cray XC40. The computation time scales as T comp ∼ p − 1 , and the communication time as T comm ∼ n − γ 2 with γ 2 ranging from 0.7 to 0.9 for Blue Gene/P, and from 0.43 to 0.73 for Cray XC40. FFTK, and the fluid and convection solvers of Tarang exhibit weak as well as strong scaling nearly up to 196608 cores of Cray XC40. We perform a comparative study of the performance on the Blue Gene/P and Cray XC40 clusters. [ABSTRACT FROM AUTHOR]

This paper proposes a resolution to the conundrum of the roles of convective and absolute instability in transition of the rotating-disk boundary layer. It also draws some comparison with swept-wing flows. Direct numerical simulations based on the incompressible Navier-Stokes equations of the flow over the surface of a rotating disk with modelled roughness elements are presented. The rotating-disk flow has been of particular interest for stability and transition research since the work by Lingwood (J. Fluid Mech., vol. 299, 1995, pp. 17-33) where an absolute instability was found. Here stationary disturbances develop from roughness elements on the disk and are followed from the linear stage, growing to saturation and finally transitioning to turbulence. Several simulations are presented with varying disturbance amplitudes. The lowest amplitude corresponds approximately to the experiment by Imayama et al. (J. Fluid Mech., vol. 745, 2014a, pp. 132-163). For all cases, the primary instability was found to be convectively unstable, and secondary modes were found to be triggered spontaneously while the flow was developing. The secondary modes further stayed within the domain, and an explanation for this is a proposed globally unstable secondary instability. For the low-amplitude roughness cases, the disturbances propagate beyond the threshold for secondary global instability before becoming turbulent, and for the high-amplitude roughness cases the transition scenario gives a turbulent flow directly at the critical Reynolds number for the secondary global instability. These results correspond to the theory of Pier (J. Engng Maths, vol. 57, 2007, pp. 237-251) predicting a secondary absolute instability. In our simulations, high temporal frequencies were found to grow with a large amplification rate where the secondary global instability occurred. For smaller radial positions, low-frequency secondary instabilities were observed, tripped by the global instability. [ABSTRACT FROM AUTHOR]

We study the effect of large-scale spectral forcing on the scale-dependent anisotropy of the velocity field in direct numerical simulations of homogeneous turbulence. ABC-type forcing and helical or non-helical Euler-type forcing are considered. We propose a scale-dependent characterisation of anisotropy based on a modal decomposition of the two-point velocity tensor spectrum. This produces direction-dependent spectra of energy, helicity and polarisation. We examine the conditions that allow anisotropy to develop in the small scales due to forcing and we show that the theoretically expected isotropy is not exactly obtained, even in the smallest scales, for ABC and helical Euler forcings. When adding rotation, the anisotropy level in ABC-forced simulations is similar to that of lower Rossby number Euler-forced runs. Moreover, even at low rotation rate, the natural anisotropy induced by the Coriolis force is visible at all scales, and two distinct wavenumber ranges appear from our fine-grained characterisation, not separated by the Zeman scale but by a scale where rotation and dissipation are balanced. [ABSTRACT FROM PUBLISHER]

This paper presents the influence of circular front and back shroud fences on the performance of a backward swept centrifugal fan impeller handling air. The analysis is carried out to determine the main characteristics of the fan i.e. the variation of head and theoretical efficiency for various flow coefficients. The circular fence size is optimized parametrically by varying its diameter and the location on the shroud surface. The effect of the shroud fences on the flow field was analyzed for the fences placed on both front and back shroud surfaces of the impeller separately. The numerical simulation is carried out using an appropriate k - ε turbulence model. The relative flow inside the impeller passages is modeled using the standard sliding mesh technique. The numerical results for the base model (i.e. without the fence) are validated against experimental results obtained from the test rig built specially for validation study and are found to have good agreement. It is found from the analysis that the average percentage increase in head coefficient is significant and is about 2 % higher as compared to the base model for the optimized geometrical configuration of diameter ratio and radial fence location as determined in the study. The optimized geometrical configuration also yields a higher theoretical efficiency of about 2.3 % corroborating the improvement in head coefficient with respect to the base model. Hence the efficacy of optimally placed circular fence on the performance of centrifugal fan is established in this numerical study. [ABSTRACT FROM AUTHOR]

The use of double-unit trains is becoming a common means of increasing passenger capacity for a rail network. However, their expanded usage may create additional aerodynamic challenges. The present work obtains the characteristics of the slipstream caused by single and double unit trains using the detached eddy simulation (DES) method for 1/20th scaled models. The numerical results are verified by full-scale experiments. The slipstream velocities and pressures obtained by the two train models at different distances from the center of track (COT) and the top of rail (TOR) are compared. The coupling structure of the double-unit train model is found to produce a velocity peak that is much greater than that of the single-unit train model in the same position. At the area away from the COT and close to the TOR, the velocity of the far wake region is larger for the double-unit train model. The coupling structure also leads to a positive pressure change in the coupling region, and its value is comparable to or even much greater than that caused by the nose. It is considerable that the subsequent pressure criteria could take the positive pressure on the coupling region into account for the double-unit train. [ABSTRACT FROM AUTHOR]

The paper presents aeroacoustic results for the flow around finite-length circular cylinders at Reynolds number 84 770 for various length-to-diameter (L/D) ratios (= 3, 9, 20, 25, 30, and 35). The incompressible Navier-Stokes equations are solved using the large eddy simulation model of turbulence followed by acoustic predictions in the far field using Ffwocs Williams and Hawkings method. The comparisons of numerical and anechoic wind tunnel measurements show good agreement in terms of the aerodynamic forces and acoustic parameters such as tonal frequency, tonal sound pressure level, and overall sound pressure level. The cylinder L/D ratio was observed to be a significant parameter that controls vortex shedding and consequently the flow induced sound generation. [ABSTRACT FROM AUTHOR]

Purpose This paper aims to investigate the mechanisms lying behind the cycloidal rotor under hovering status.Design/methodology/approach Experiments were conducted to validate the numerical simulation results. The simulations were based on unsteady Reynolds-averaged Navier–Stokes (URANS) equations solver and the sliding mesh technique was used to model the blade motion. 2D and 2.5D simulations were made to investigate the 3D effects of turbulence. The effects of pressure and viscosity were compared to study the significance of the blade motion on force generation.Findings The 2.5D numerical simulation cannot produce more accurate results than the 2D counterpart. The pitching motion of the blade results in dynamic stall. The dynamic stall vortices induce parallel blade vortex interaction (BVI) upon downstream blades. The interactions between the blades delay the stall of the blade which is beneficial to the thrust generation. The blade pitching motion is the dominant contributor to the force generation and the turbulence is the secondary. Strong downwash in the rotor cage varied the inflow velocity as well as the effective angle of attack (AOA) of the blade.Practical implications Cycloidal rotor is a propulsion device that can provide omni-directional vectored thrust with high efficiency and low noise. To understand the mechanisms lying behind the cycloidal rotor helps the authors to design efficient cycloidal rotors for aircraft.Originality/value The authors discovered that the blade pitching motion plays primary role in force generation. The effects of the dynamic stall and BVI were studied. The reason why cycloidal rotor can be more efficient was discussed. [ABSTRACT FROM AUTHOR]

The eddy-covariance method is an important technique for investigating the exchange of energy and substances between the atmosphere and an ecosystem. However, an inappropriate averaging period leads to inaccurate fluxes and a low energy-balance ratio (EBR). The effects of various averaging periods on fluxes and the EBR are analyzed using flux data from the entire growth stage of maize measured with an eddy-covariance system in northern China. We find that the relative error of the flux between an averaging period of 10-60 min and the commonly used averaging period of 30 min is within 3%. When the averaging period is between 10 and 60 min, the magnitudes of fluxes increase with the length of the averaging period at various growth stages. For averaging periods exceeding 60 min, the magnitudes of fluxes vary significantly, particularly for periods longer than 120 min. In general, $${ EBR} > 0.8$$ in the maize field, tending to increase within periods of 10-60 min, but decreasing rapidly at various growth stages for averaging periods longer than 120 min. Ogive functions indicate an optimal averaging period of the seedling-shooting and shooting-heading stages is approximately 10-30 min, and that of the heading-filling and filling-maturity stages is 30-60 min. [ABSTRACT FROM AUTHOR]

Sensitivity tests using the 'Consortium for Small Scale Modeling' model in large-eddy simulation mode with a grid spacing of 100 m are performed to investigate the impact of the resolution of soil- and vegetation-related parameters on a cloud-topped boundary layer in a real-data environment. The reference simulation uses the highest land-surface parameter resolution available for operational purposes (300 m). The sensitivity experiments were conducted using spatial averaging of about $$2.5\,\hbox {km}\times 2.5\,\hbox {km}$$ and $$10\,\hbox {km} \times 10\,\hbox {km}$$ for the land-surface parameters and a completely homogeneous distribution for the whole model domain of about $$70\,\hbox {km} \times 70\,\hbox {km}$$ . Additionally, one experiment with a higher mean soil moisture and another with six mesoscale patches of enhanced or reduced soil moisture are performed. Boundary-layer clouds developed in all simulations. To assess the deviations of cloud cover on different scales within the model domain, we calculated the root-mean-square deviation ( RMSD) between the sensitivity experiments and the reference simulation. The RMSD depends strongly on the spatial resolution at which cloud fields are compared. Different spatial resolutions of the cloud fields were generated by applying a low-pass filter. For all sensitivity experiments, large RMSD values occur for cut-off wavelengths $${<}1$$ km, reflecting the stochastic nature of convection, but they decrease rapidly for wavelengths between 1 and 5 km. For cut-off wavelengths $${>}5\,\hbox {km}$$ , the RMSD is still pronounced for the simulation with higher mean soil moisture. Additionally, for cut-off wavelengths between 5 and 30 km, considerable differences can be found for the experiment with mesoscale patches and for that with homogeneous land-surface parameters. Spatial averaging of land-surface parameters for areas of $$2.5\,\hbox {km} \times 2.5\,\hbox {km}$$ and $$10\,\hbox {km} \times 10\,\hbox {km}$$ results in larger patch sizes but simultaneously in reduced amplitudes of land-surface parameter anomalies and shows the lowest RMSD for all cut-off wavelengths. [ABSTRACT FROM AUTHOR]

Large-Eddy Simulations (LESs) and experiments were employed to study the influence ofwater depth on the hydrodynamics in the wake of a conical island for emergent, shallow, and deeply submerged conditions. The Reynolds numbers based on the island's base diameter for these conditions range from 6500 to 8125. LES results from the two shallower conditions were validated against experimental measurements from an open channel flume and captured the characteristic flow structures around the cone, including the attached recirculation region, vortex shedding, and separated shear layers. The wake was impacted by the transition from emergent to shallow submerged flow conditions with more subtle changes in time-averaged velocity and instantaneous flow structures when the submergence increases further. Despite differences in the breakdown of the separated shear layers, vortex shedding, and the upward flow region on the leeward face (once the cone's apex is submerged), similar flow structures to cylinder flow were observed. These include an arch vortex tilted in the downstream direction and von Karman vortices in the far-wake. Spectra of velocity time series and the drag coefficient indicated that the vortex shedding was constrained by the overtopping flow layer, and thus the shedding frequency decreased as the cone's apex became submerged. Finally, the generalised flow structures in the wake of a submerged conical body are outlined. [ABSTRACT FROM AUTHOR]

A new approach to turbulence simulation, based on a combination of large eddy simulation (LES) for the whole flow and an array of non-space-filling quasi-direct numerical simulations (QDNS), which sample the response of near-wall turbulence to large-scale forcing, is proposed and evaluated. The technique overcomes some of the cost limitations of turbulence simulation, since the main flow is treated with a coarse-grid LES, with the equivalent of wall functions supplied by the near-wall sampled QDNS. Two cases are tested, at friction Reynolds number Reτ=4200 and 20000. The total grid point count for the first case is less than half a million and less than 2 million for the second case, with the calculations only requiring a desktop computer. A good agreement with published direct numerical simulation (DNS) is found at Reτ=4200, both in the mean velocity profile and the streamwise velocity fluctuation statistics, which correctly show a substantial increase in near-wall turbulence levels due to a modulation of near-wall streaks by large-scale structures. The trend continues at Reτ=20000, in agreement with experiment, which represents one of the major achievements of the new approach. A number of detailed aspects of the model, including numerical resolution, LES-QDNS coupling strategy and subgrid model are explored. A low level of grid sensitivity is demonstrated for both the QDNS and LES aspects. Since the method does not assume a law of the wall, it can in principle be applied to flows that are out of equilibrium. [ABSTRACT FROM AUTHOR]

We perform numerical simulations of a turbulent channel flow over an hyper-elastic wall. In the fluid region the flow is governed by the incompressible Navier-Stokes (NS) equations, while the solid is a neo-Hookean material satisfying the incompressible Mooney-Rivlin law. The multiphase flow is solved with a one-continuum formulation, using a monolithic velocity field for both the fluid and solid phase, which allows the use of a fully Eulerian formulation. The simulations are carried out at Reynolds bulk Re = 2800 and examine the effect of different elasticity and viscosity of the deformable wall. We show that the skin friction increases monotonically with the material elastic modulus. The turbulent flow in the channel is affected by the moving wall even at low values of elasticity since non-zero fluctuations of vertical velocity at the interface influence the flow dynamics. The near-wall streaks and the associated quasi-streamwise vortices are strongly reduced near a highly elastic wall while the flow becomes more correlated in the spanwise direction, similarly to what happens for flows over rough and porous walls. As a consequence, the mean velocity profile in wall units is shifted downwards when shown in logarithmic scale, and the slope of the inertial range increases in comparison to that for the flow over a rigid wall. We propose a correlation between the downward shift of the inertial range, its slope and the wall-normal velocity fluctuations at the wall, extending results for the flow over rough walls. We finally show that the interface deformation is determined by the fluid fluctuations when the viscosity of the elastic layer is low, while when this is high the deformation is limited by the solid properties. [ABSTRACT FROM AUTHOR]

It is becoming increasingly clear that the strong spatial and temporal fluctuations observed in a narrow Reynolds number regime around the laminar-turbulent transition in shear flows can best be understood using the concepts and techniques from a seemingly unrelated discipline - statistical mechanics. During the last few years, a consensus has begun to emerge that these phenomena reflect an underlying non-equilibrium phase transition exhibited by a model of interacting particles on a crystalline lattice, directed percolation, that seems very far from fluid mechanics. Now, Chantry et al. (J. Fluid Mech., vol. 824, 2017, R1) have developed a truncated-mode computation of a model shear flow, capable of simulating systems far larger and longer than any previous study and have for the first time generated enough statistical data that a high-precision test of theory is feasible. The results broadly confirm the theory, extending the class of flows for which the directed percolation scenario holds and removing any remaining doubts that non-equilibrium statistical mechanical critical phenomena can be exhibited by the Navier-Stokes equations. [ABSTRACT FROM AUTHOR]

Thermoacoustic phenomena can be modeled by means of analytical closed-forms and numerical models. The analytical solution can be obtained with a minimum computational cost, whereas the numerical approach is computationally expensive. The former, however, provides an average solution along the engine, while within the numerical simulations non-stationary phenomena can be observed. This paper includes results related to the modeling of the operation of a thermoacoustic engine both by means of the analytical approach and numerical models. Some computational details and assumptions adopted in both models are discussed. The main focus is placed on the numerical approach, where the model size (number of pores analyzed) is taken into account. The results show differences between a single-pore symmetrical model and multiple-pore ones. The object of the comparisons is to determine the reliability of the modeling of the operation of a thermoacoustic engine and to evaluate sources of discrepancy between models, both numerical ones, as well as the numerical and analytical approaches. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of the incompressible Navier-Stokes equations are not feasible yet for most practical turbulent flows. Therefore, dynamically less complex mathematical formulations are necessary for coarse-grained simulations. In this regard, eddy-viscosity models for Large-Eddy Simulation (LES) are probably the most popular example thereof. This type of models requires the calculation of a subgrid characteristic length which is usually associated with the local grid size. For isotropic grids, this is equal to the mesh step. However, for anisotropic or unstructured grids, such as the pancake-like meshes that are often used to resolve near-wall turbulence or shear layers, a consensus on defining the subgrid characteristic length has not been reached yet despite the fact that it can strongly affect the performance of LES models. In this context, a new definition of the subgrid characteristic length is presented in this work. This flow-dependent length scale is based on the turbulent, or subgrid stress, tensor and its representations on different grids. The simplicity and mathematical properties suggest that it can be a robust definition that minimizes the effects of mesh anisotropies on simulation results. The performance of the proposed subgrid characteristic length is successfully tested for decaying isotropic turbulence and a turbulent channel flow using artificially refined grids. Finally, a simple extension of the method for unstructured meshes is proposed and tested for a turbulent flow around a square cylinder. Comparisons with existing subgrid characteristic length scales show that the proposed definition is much more robust with respect to mesh anisotropies and has a great potential to be used in complex geometries where highly skewed (unstructured) meshes are present. [ABSTRACT FROM AUTHOR]

We performed direct numerical simulations of decaying isotropic turbulence laden with finite-size particles using an immersed boundary method combined with a soft-sphere collision model. The particle diameter is about 25 times the Kolmogorov length scale, and the particle-to-fluid density ratio ranges from 10 to 1000. The turbulence is enhanced by heavy particles but suppressed by light particles. The radial and spectral distributions of the main quantities are examined. The enstrophy budget is analyzed to reveal how the particles enhance the dissipation rate. The dependence of the two-way coupling term on the particle Reynolds number is loosely predicted. The relation between the particle-induced dissipation rate and particle Reynolds number is also estimated. The results indicate that the work done by particles outweighs the particle-induced dissipation at high particle Reynolds number, thereby enhancing the turbulence. Thus the particle Reynolds number should be a key parameter for turbulence modulation by finite-size and high-inertia particles. [ABSTRACT FROM AUTHOR]

In some degraded situations of heat-exchangers, tubes may be loosely supported while subjected to intense cross-flow generating both turbulence and fluid-elastic forces, and so have vibro-impacting responses. This paper aims at providing a better understanding of the dynamics of such systems in which the conjunction of stops with gaps, broadband random excitation, and fluid-elastic coupling forces, either stabilizing or destabilizing, produces some rather amazing effects linked to a general capacity for the system to be auto-adjusted by the impacts. It is especially shown, (i) how a non-linear gap-system “escapes from instability” by reinforcing the sequence of impacts, (ii) the almost perfect time-history mirror symmetry between the works of turbulence and fluid-elastic forces in the energy balance; (iii) the dissipative aspect of turbulence in some cases. Moreover the paper deals with the relative weight of turbulence and fluid-elastic forces in the tube response that notably depends on the gap size. For this, turbulence is considered as a perturbation of limit cycles obtained when the fluid-elastic forces are the only acting fluid-forces. The study is based on configurations mainly 1-DOF, like the one that has been experimentally tested, but extrapolations for real tube bundle are evoked in conclusion. [ABSTRACT FROM AUTHOR]

Computations of three-dimensional turbulent vertical up-flow of supercritical fluid in the subchannel of a heated rod bundle were made using the Computational Fluid Dynamics codes ANSYS CFX and ANSYS Fluent. Results for a total of six cases from three different sets of experiments are presented. For all six cases, steady-state predictions of fluid velocity, pressure, and temperature were made using six versions of the two-equation Reynolds-averaged Navier-Stokes turbulence models with accompanying wall treatments. A conjugate heat transfer model was used that also predicted the temperature distribution in an adjacent solid region representing a heater. In the model of one experiment, the solid region also included cladding and insulation. The k-epsilon turbulence model, implemented using CFX and Scalable Wall Functions, provided the numerical results that have the smallest overall deviation from experimental results for three of the six cases, and predicted the experimental data of the remaining four cases reasonably well, unlike other turbulence models that sometimes severely over-predict the experimental data for wall surface temperature. [ABSTRACT FROM AUTHOR]

A novel method for the hybridization of RANS and LES is presented tailored for the utilization in supersonic combustor ducts. The intention is to treat the largest portion of the boundary layers with RANS in order to reduce the computational requirements of LES in near-wall regions. The method is comprised of two components: Component A accounts for boundary layer growth in supersonic flow regions with thin boundary layers. Component B is designed to identify shock/boundary layer interaction regions and places the RANS/LES interface such that these interaction regions are treated with RANS. The method is not tied to specific models for RANS or LES, instead it represents a procedure to couple an eddy viscosity type RANS turbulence model with an arbitrary LES model. The hybrid RANS/LES approach is applied to the simulation of a two-staged injection supersonic combustor and the findings are compared to URANS simulations conducted for the same flow conditions. [ABSTRACT FROM AUTHOR]