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The present work investigates the influence of rectangular deflectors on the performance of a Savonius turbine mounted in an L-shaped channel, which represents a geometry like that found in one oscillating water column (OWC) device. It also performs a geometric investigation of the entrance region of the channel. More precisely, it investigates the effect of the height/length ratio (H1/L1) of the entering region of the channel on the system performance for three different configurations: (1) without the use of deflectors, (2) with just one deflector upstream the turbine, and (3) with one deflector upstream and another downstream the turbine. The geometric investigation is performed based on the constructal design method, and the entering channel area (A1) is the problem constraint. The performance indicators are the mechanical power in the Savonius turbine and the available power in the device. For all cases, it is considered turbulent airflow in the domain, being solved by the unsteady Reynolds Averaged Navier–Stokes mass and momentum equations. The numerical solution was obtained with the finite-volume method using the Ansys FLUENT software (version 2021 R1). The k-ω shear stress transport turbulence closure model is used. The results demonstrated that the mechanical and available powers depend on the H1/L1 ratio, regardless of the usage of deflectors. For instance, differences of up to 16.35% in mechanical power and 7.25% in available power were observed between the best and worst performance configurations in the case without deflectors. The use of deflectors resulted in increases of two and three times in available and mechanical powers, respectively, when the cases with one and two deflectors are compared with those without deflectors. This demonstrates that the enclosed domain and the insertion of the deflectors can enhance the performance of the Savonius turbine. [ABSTRACT FROM AUTHOR]

The article "Structure and energy transfer in homogeneous turbulence below a free surface" delves into the dynamics of turbulence beneath a free surface, emphasizing energy transport across scales. The research uncovers alterations in energy distribution and velocity gradients near the surface, impacted by surface deformation and boundary conditions. The findings corroborate previous studies and theoretical expectations, shedding light on the influence of the free surface on inter-scale energy transfer and turbulence dynamics. The study offers valuable insights into the intricate dynamics of free-surface flows and turbulent energy distribution, contributing to a deeper understanding of turbulence near a free surface. [Extracted from the article]

The article in the Journal of Fluid Mechanics explores the linear stability analysis of turbulent mean flows around an airfoil, specifically focusing on predicting three-dimensional stall cells. By using global stability analyses of turbulent mean flows computed with Reynolds-averaged Navier-Stokes equations, the study investigates corrections to turbulence models to enhance mean flow estimation. The research highlights the significance of data assimilation in improving predictions of flow separation and recirculation regions near the airfoil's trailing edge, ultimately leading to a more precise representation of flow behavior and the onset of stall cells. The study underscores the importance of selecting the appropriate correction for turbulence models in stability analyses, with correction ¯f1 demonstrating the most accurate predictions of stall cell onset. [Extracted from the article]

The article delves into the turbulent flow of non-Newtonian fluids in rough channels, specifically focusing on Herschel-Bulkley fluids. Through direct numerical simulations, the study reveals that surface roughness affects HB fluids differently than Newtonian fluids, slightly increasing viscosity in troughs and reducing the bulk Reynolds number. The research also uncovers unique flow structures and energy spectra of non-Newtonian fluids near the wall, shedding light on the impact of surface roughness on turbulence statistics. The document encompasses a range of research articles exploring various aspects of fluid dynamics, utilizing numerical simulations to deepen understanding of fluid behavior in different flow conditions. [Extracted from the article]

Direct numerical simulations of the turbulence of a Herschel–Bulkley (HB) fluid in a rough channel are performed at a shear Reynolds number $Re_{\tau } \approx 300$ and a Bingham number ${Bn} \approx 0.9$. For the type of rough surface used in this study, the results indicate that Townsend's wall similarity hypothesis also holds for HB fluids. However, there are notable differences compared with the effect of roughness on Newtonian fluids. More specifically, the effect of roughness appears to be slightly stronger for HB fluids, in the sense that the bulk Reynolds number, based on the viscosity at the wall, is reduced further due to the increase in viscosity in the troughs of the roughness surface induced by the low shear. At the same time, for the simulated rough surface, the contribution of form drag to the total pressure drop is reduced from 1/4 to about 1/5 due to the persistence of viscous shear in the boundary layer, reducing its shielding effect. As for the friction factor, due to the nonlinearity of the HB constitutive relation, its use with the wall shear rate from the mean wall shear stress underpredicts the minimum viscosity at the wall by up to 18 %. This inevitably leads to uncertainties in the prediction of the friction factor. Finally, it is observed that the rough surface is unable to break the peculiar near-wall flow structure of HB fluids, which consists of long persistent low-speed streaks occupying the entire domain. This means that the small-scale energy is significantly reduced for HB fluids, even in rough channels, with the energy more concentrated in the lower wavenumber range, implying an increase in the slope of the power spectrum to $-7/2$ in the inertial range, as shown by Mitishita et al. (J. Non-Newtonian Fluid Mech. , vol. 293, 2021, 104570). [ABSTRACT FROM AUTHOR]

The article delves into the detuned instability of wall streaks in turbulent channel flow, revealing an unstable branch in the eigenspectra for high friction Reynolds numbers. It highlights the impact of base flow streaks' characteristics on the growth rates of leading unstable modes, suggesting that near-wall streak instability can generate large-scale motions in turbulent flows. The text also includes a compilation of research articles exploring the stability and dynamics of turbulent flows in different fluid systems, offering valuable insights into the mechanics of organized waves, coherent structure generation, and spectral scaling in turbulent shear flow. These studies contribute to a deeper understanding of fluid dynamics and turbulence in engineering and natural systems. [Extracted from the article]

The article in the Journal of Fluid Mechanics delves into flow patterns and energy spectra in forced quasi-two-dimensional turbulence, examining how system size and damping rate influence these phenomena. It uncovers that larger system sizes display streamers while smaller sizes exhibit coherent vortices, and the damping rate impacts the energy spectrum transition. The study proposes the dimensionless Taylor microscale as a diagnostic tool for spectral transition. Overall, the research sheds light on the intricate dynamics of turbulence and challenges current theories in the field. [Extracted from the article]

The article "An analytical model for the slip velocity of particles in turbulence" in the Journal of Fluid Mechanics introduces a model to predict the slip velocity of spherical particles in turbulent flows. The model, based on the inertial filtering framework, offers equations to calculate slip velocity magnitude based on specific parameters. Validation against experimental and numerical data shows agreement across various particle properties and flow conditions, emphasizing the importance of accurate slip velocity estimation for understanding particle dynamics in turbulent environments. The document also includes references to studies on particle behavior in turbulent flows, covering topics such as particle clustering, settling, dispersion models, and bubble dynamics in high-Reynolds-number flows, providing valuable insights into the complex interactions between particles and turbulent fluid flows. [Extracted from the article]

A laboratory study was conducted using particle image velocimetry (PIV) to measure flow velocity distributions over two-dimensional smooth and rough fixed dunes. It comprised 28 tests, each yielding 146 velocity profiles over one complete dune length. Two kinds of double-averaged velocity profiles were computed, one based on all the 146 lines of data (called global average), and the others from only some of them (called partial average). The results show that the global average velocity distribution is generally close to the partial average profile derived from evenly-distributed three or five lines along one dune length. Furthermore, the global average velocity profile can also be reasonably approximated using a single profile, measured at the representative line in this paper. The representative line is found to locate near the reattachment point. This result would be helpful to simplify measurements of general velocity distribution for a flow over dunes. The paper also applies the concept of representative line to the description of distributions of turbulence characteristics. [ABSTRACT FROM AUTHOR]

This study investigates the accuracy of dynamic models in predicting model constants within inviscid flow fields, such as those used in wind farm flow analysis, from the perspectives of identity and turbulent energy conservation accuracy. Specifically, results are compared between tensor-level and vector-level identities, the latter of which includes the calculation of model constants taking into account the errors of differential approximation. The subgrid-scale models employed include the Smagorinsky model and the coherent structure model. The analysis focuses on inviscid flow fields within a three-dimensional periodic domain. Fourth- or second-order spatial accuracy was applied to a coarse computational grid. The results yielded values of a model constant that compensated the resulting energy conservation errors to zero. The statistics of the velocity fluctuation derivatives in the flow fields where the energy conservation errors were compensated were examined. Dynamic model predictions for both identities were then computed for the Smagorinsky and coherent ctructure models and compared with the correct values. The results show that the dynamic model predictions are largely independent of the energy conservation errors, and that the predictions based on the tensor-level identity deviate significantly more from the correct values than those based on the vector-level identity.

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Assessing drag force and Reynolds stresses in turbulent flows is crucial for evaluating the stability and longevity of hydraulic structures. Yet, this task is challenging due to the complex nature of turbulent flows. To address this, physical models are often employed. Nonetheless, this practice is associated with difficulties, especially in the case of high sampling frequency where the inherent randomness of velocity fluctuations becomes mixed with the measurement noise. This study introduces a stochastic approach, which aims to mitigate bias from measurement errors and provide a probabilistic estimate of extreme stress values. To accomplish this, a simple experimental setup with a hydraulic jump was employed to acquire long-duration velocity measurements. Subsequently, a modified first-order autoregressive model was applied through ensemble simulations, demonstrating the benefits of the stochastic approach. The analysis highlights its effectiveness in estimating the uncertainty of extreme events frequency and minimizing the bias induced by the noise in the high-magnitude velocity measurements and by the limited length of observations. These findings contribute to advancing our understanding of turbulent flow analysis and have implications for the design and assessment of hydraulic structures. [ABSTRACT FROM AUTHOR]

Tom et al. (J. Fluid Mech., vol. 947, 2022, p. A7) investigated the impact of two-way coupling (2WC) on particle settling velocities in turbulence. For the limited parameter choices explored, it was found that (i) 2WC substantially enhances particle settling compared with the one-way coupled case, even at low mass loading Φm and (ii) preferential sweeping remains the mechanism responsible for the particles settling faster than the Stokes settling velocity in 2WC flows. However, significant alterations to the flow structure that can occur at higher mass loadings mean that the conclusions from Tom et al. (J. Fluid Mech., vol. 947, 2022, p. A7) may not generalise. Indeed, even under very low mass loadings, the influence of 2WC on particle settling might persist, challenging the conventional assumption. We therefore explore a much broader portion of the parameter space, with simulations covering cases where the impact of 2WC on the global fluid statistics ranges from negligible to strong. We find that, even for Φm = 7.5 × 10-3, 2WC can noticeably increase the settling for some choices of the Stokes and Froude numbers. When Φm is large enough for the global fluid statistics to be strongly affected, we show that preferential sweeping continues to be the mechanism that enhances particle settling rates. Finally, we compare our results with previous numerical and experimental studies. While in some cases there is reasonable agreement, discrepancies exist even between different numerical studies and between different experiments. Future studies must seek to understand this before the discrepancies between numerical and experimental results can be adequately addressed. [ABSTRACT FROM AUTHOR]

The present work investigates the influence of rectangular deflectors on the performance of a Savonius turbine mounted in an L-shaped channel, which represents a geometry like that found in one oscillating water column (OWC) device. It also performs a geometric investigation of the entrance region of the channel. More precisely, it investigates the effect of the height/length ratio (H1/L1) of the entering region of the channel on the system performance for three different configurations: (1) without the use of deflectors, (2) with just one deflector upstream the turbine, and (3) with one deflector upstream and another downstream the turbine. The geometric investigation is performed based on the constructal design method, and the entering channel area (A1) is the problem constraint. The performance indicators are the mechanical power in the Savonius turbine and the available power in the device. For all cases, it is considered turbulent airflow in the domain, being solved by the unsteady Reynolds Averaged Navier–Stokes mass and momentum equations. The numerical solution was obtained with the finite-volume method using the Ansys FLUENT software (version 2021 R1). The k-ω shear stress transport turbulence closure model is used. The results demonstrated that the mechanical and available powers depend on the H1/L1 ratio, regardless of the usage of deflectors. For instance, differences of up to 16.35% in mechanical power and 7.25% in available power were observed between the best and worst performance configurations in the case without deflectors. The use of deflectors resulted in increases of two and three times in available and mechanical powers, respectively, when the cases with one and two deflectors are compared with those without deflectors. This demonstrates that the enclosed domain and the insertion of the deflectors can enhance the performance of the Savonius turbine.

Many turbulent flows exhibit time-periodic statistics. These include turbomachinery flows, flows with external harmonic forcing and the wakes of bluff bodies. Many existing techniques for identifying turbulent coherent structures, however, assume the statistics are statistically stationary. In this paper, we leverage cyclostationary analysis, an extension of the statistically stationary framework to processes with periodically varying statistics, to generalize the spectral proper orthogonal decomposition (SPOD) to the cyclostationary case. The resulting properties of the cyclostationary SPOD (CS-SPOD for short) are explored, a theoretical connection between CS-SPOD and the harmonic resolvent analysis is provided, simplifications for the low and high forcing frequency limits are discussed, and an efficient algorithm to compute CS-SPOD with SPOD-like cost is presented. We illustrate the utility of CS-SPOD using two example problems: a modified complex linearized Ginzburg–Landau model and a high-Reynolds-number turbulent jet. [ABSTRACT FROM AUTHOR]

The high cost associated with high-fidelity computational fluid dynamics (CFD) is one of the main challenges that inhibit the design and optimisation of new fluid-flow systems. In this study, we explore the feasibility of a physics-informed deep learning approach to predict turbulence evolution and mixing without requiring a classical CFD solver. The deep learning architecture was inspired by integrating U-Net with inception modules for capturing the multi-scale nature of turbulent flows. In addition, a physics-constrained loss function was designed to enforce the mass and pressure conservation of the predicted solution. After trained, the optimised model was validated in the large eddy simulation (LES) of a forced turbulent mixing layer at two distinct Reynolds numbers ( $\mathrm {Re} =3000$ and 30000). The results demonstrate that the proposed approach achieves a promising solution accuracy and extrapolation ability with a significant reduction in computing time when compared to those obtained using a classical LES flow solver. The success in developing such a physics-informed deep learning approach not only justifies the potential of ML-based surrogate solvers for fast prototyping and design of generic fluid-flow systems but also highlights the key challenges arising from data-driven surrogate solver development for turbulence modelling.

The purpose of this study is to numerically analyze the effect of vortex generators that are shaped like vanes in enhancing the mixing of subsonic and sonic jet and to determine the best design which yields maximum reduction in jet potential core length and minimum thrust loss at the nozzle exit. Four different nozzle designs namely, models A, B, C and D are designed and compared with a base nozzle which is a plain circular nozzle without any vanes. The simulation is performed in ANSYS Fluent using the S-A turbulence model. The centerline pressure decay and radial pressure decay from models A to D are compared with that of the base nozzle to determine the ability of the vane to enhance the jet mixing characteristics. To evaluate the thrust loss, the total pressure at the exit plane of models A to D is measured and compared with that of the base nozzle. When comparing all the designs, it is observed that Model B produces the highest reduction in potential core length which is 66.4% at Mach no. 1 and Model D produces minimum total pressure loss which is 0.47% at Mach no. 0.4. In contrast to the conventional method, this design introduces a novel approach by placing the vanes parallel to the flow instead of the usual perpendicular arrangement. This unique configuration allows the vanes to redirect the flow rather than hinder it, resulting in a total pressure loss of less than 3%.

In this work, a comparison between three different configurations of deflectors inserted inside the duct of an oscillating water column device (OWC), where is attached a rotating Savonius turbine in the same domain, is done. In the first configuration, the numerical model is composed only by the Savonius turbine inside the OWC duct. In the second configuration, just one deflector is inserted upstream of the turbine region. In the third case, a second deflector is inserted, together with the first one, but downstream of the turbine. For every studied model, the geometry of the hydropneumatic chamber was varied by three values of the ratio H1/L1 (the height times the length). This geometric variation was performed according to the Constructal Design methodology. The purpose of this comparison was the evaluation of the OWC pneumatic power, the power coefficient, and the Savonius turbine power. The results indicated that for all the studied configurations, the ratio H1/L1 as well as the deflectors presence inside the OWC duct had an influence in terms of the performance indicators, especially regarding the pneumatic power, where variations up to 100 W were noticed. [ABSTRACT FROM AUTHOR]

The purpose of this study is to numerically analyze the effect of vortex generators that are shaped like vanes in enhancing the mixing of subsonic and sonic jet and to determine the best design which yields maximum reduction in jet potential core length and minimum thrust loss at the nozzle exit. Four different nozzle designs namely, models A, B, C and D are designed and compared with a base nozzle which is a plain circular nozzle without any vanes. The simulation is performed in ANSYS Fluent using the S-A turbulence model. The centerline pressure decay and radial pressure decay from models A to D are compared with that of the base nozzle to determine the ability of the vane to enhance the jet mixing characteristics. To evaluate the thrust loss, the total pressure at the exit plane of models A to D is measured and compared with that of the base nozzle. When comparing all the designs, it is observed that Model B produces the highest reduction in potential core length which is 66.4% at Mach no. 1 and Model D produces minimum total pressure loss which is 0.47% at Mach no. 0.4. In contrast to the conventional method, this design introduces a novel approach by placing the vanes parallel to the flow instead of the usual perpendicular arrangement. This unique configuration allows the vanes to redirect the flow rather than hinder it, resulting in a total pressure loss of less than 3%. [ABSTRACT FROM AUTHOR]

Due to their long-lived nature, vortex rings are highly promising for the non-contact transportation of colloidal microparticles. However, because of the high complexity of the structures, their description using rigorous, closed-form mathematical expressions is challenging, particularly in the presence of strongly inhomogeneous colloidal suspensions. In this work, we comprehensively study this phenomenon, placing special emphasis on a quantitative description of the ability of vortex rings to move the particles suspended in a liquid over distances significantly exceeding the ring's dimensions. Moreover, within the study, we present straightforward analytical approximations extracted by using the fitting of the experimental and numerical simulation observations that reveal the dynamics of vortex rings transporting the microparticles. It includes both the dependence of the concentration on the distance traveled by the vortex ring and coefficients describing the evolution of vortex ring shape in time, which were not presented in the literature before. It turns out that despite the fact that 2D modeling is a simplification of the full 3D problem solution and is unable to capture some of the minor effects of real behavior, it has demonstrated a good consistency with the results obtained via experiments regarding the process of particles transportation. [ABSTRACT FROM AUTHOR]

Kinetic solvers for incompressible fluid simulation were designed to run efficiently on massively parallel architectures such as GPUs. While these lattice Boltzmann solvers have recently proven much faster and more accurate than the macroscopic Navier-Stokes-based solvers traditionally used in graphics, it systematically comes at the price of a very large memory requirement: a mesoscopic discretization of statistical mechanics requires over an order of magnitude more variables per grid node than most fluid solvers in graphics. In order to open up kinetic simulation to gaming and simulation software packages on commodity hardware, we propose a HighOrder Moment-Encoded Lattice-Boltzmann-Method solver which we coined HOME-LBM, requiring only the storage of a few moments per grid node, with little to no loss of accuracy in the typical simulation scenarios encountered in graphics. We show that our lightweight and lightspeed fluid solver requires three times less memory and runs ten times faster than state-of-the-art kinetic solvers, for a nearly-identical visual output. [ABSTRACT FROM AUTHOR]

The passive control of jets using vanes as vortex generators is studied by numerical simulation in this paper. The vanes are positioned inside the nozzle near the exit, inclined to the flow with the longitudinal direction of the jet. Two configurations namely, straight vanes (k = 0 mm−1) and curved vanes (k = 0.05 mm−1) are considered. Curvature k is defined as the reciprocal of the radius of the vanes. The blockage due to the presence of the vanes is 0.5%. The total pressure variation along the jet centreline and along the radial distance is determined from nozzle exit at a Mach number of 0.4, 0.6 and 0.8. It is found that the vanes cause faster decay of the jet, both in the near field and far field compared to the base nozzle (plain circular nozzle) and the curved vanes perform better than the straight vanes in promoting the jet mixing. A maximum of 54% reduction in jet potential core length is achieved by the curved vanes and the jet becomes asymmetrical due to the presence of the vanes inside the nozzle, as observed in the radial pressure decay plots and Mach number contours. [ABSTRACT FROM AUTHOR]

This paper attempts to develop a Dynamic Mode Decomposition (DMD)-based Reduced Order Model (ROMs) that can quickly but accurately predict the forces and moments experienced by a road vehicle such that they be used by an on-board controller to determine the vehicle’s trajectory. DMD can linearize a large dataset of high-dimensional measurements by decomposing them into low-dimensional coherent structures and associated time dynamics. This ROM can then also be applied to predict the future state of the fluid flow. Existing literature on DMD is limited to low Reynolds number applications. This paper presents DMD analyses of the flow around an idealized road vehicle, called the Ahmed body, at a Reynolds number of 2.7×106. The high-dimensional dataset used in this paper was collected from a computational fluid dynamics (CFD) simulation performed using the Menter’s Shear Stress Transport (SST) turbulence model within the context of Improved Delayed Detached Eddy Simulations (IDDES). The DMD algorithm, as available in the literature, was found to suffer nonphysical dampening of the medium-to-high frequency modes. Enhancements to the existing algorithm were explored, and a modified DMD approach is presented in this paper, which includes: (a) a requirement of higher sampling rate to obtain a higher resolution of data, and (b) a custom filtration process to remove spurious modes. The modified DMD algorithm thus developed was applied to the high-Reynolds-number, separation-dominated flow past the idealized ground vehicle. The effectiveness of the modified algorithm was tested by comparing future predictions of force and moment coefficients as predicted by the DMD-based ROM to the reference CFD simulation data, and they were found to offer significant improvement.

Assessing drag force and Reynolds stresses in turbulent flows is crucial for evaluating the stability and longevity of hydraulic structures. Yet, this task is challenging due to the complex nature of turbulent flows. To address this, physical models are often employed. Nonetheless, this practice is associated with difficulties, especially in the case of high sampling frequency where the inherent randomness of velocity fluctuations becomes mixed with the measurement noise. This study introduces a stochastic approach, which aims to mitigate bias from measurement errors and provide a probabilistic estimate of extreme stress values. To accomplish this, a simple experimental setup with a hydraulic jump was employed to acquire long-duration velocity measurements. Subsequently, a modified first-order autoregressive model was applied through ensemble simulations, demonstrating the benefits of the stochastic approach. The analysis highlights its effectiveness in estimating the uncertainty of extreme events frequency and minimizing the bias induced by the noise in the high-magnitude velocity measurements and by the limited length of observations. These findings contribute to advancing our understanding of turbulent flow analysis and have implications for the design and assessment of hydraulic structures.

We consider the three-dimensional inviscid Boussinesq–Voigt system which is a regularization model for the inviscid Boussinesq equations. We prove a regularity criterion for the weak solutions (in particular, for the time-derivative of the velocity), in L p -spaces, involving first derivatives of the pressure. [ABSTRACT FROM AUTHOR]

Incipient motion has been a topic of investigation by researchers, engineers and scientists for more than a century. The main approach for studying sediment entrainment has been the static approach that uses temporal and spatial averaged flow parameters like bed shear stress and stream power to link them indirectly to sediment entrainment. Recent research outputs have shed light on the important role of turbulent fluctuations in the sediment transport process. It is suggested that the approach of using temporal and spatial averaged parameters fails to account for the dynamic and probabilistic nature of the entrainment process, as inherited by flow turbulence. This has led to the introduction of the only dynamic criteria in the literature for studying sediment entrainment, namely the impulse and energy criteria. These criteria take into account both the magnitude and duration of the turbulent flow event used for assessing the conditions that can result in sediment entrainment. In light of this, this work aims to assess whether there is a trend in terms of the type of flow structures that occur in sequence before and after the occurrences of the flow impulses that have resulted in the coarse particle's entrainment. To achieve this, we conducted a well-controlled laboratory experiment to investigate the incipient motion of a 7 cm diameter instrumented particle. Five runs of the experiment were performed at flowrates close to the threshold of motion. The instrumented particle was equipped with micro-electro-mechanical sensors (MEMS) to accurately measure its inertial dynamics and detect motion. The sensors recorded entrainment events, and these events were stochastically linked to the impulses occurring for the tested flow conditions. Quadrant analysis was used to investigate the type of flow structures that occurred before, during and after the occurrence of quadrant events with an impulse above the critical impulse. The findings herein associate coarse particle entrainments with energetic impulses linked primarily to sweep events (Q4) and secondarily, sequence of sweeps (Q4) and ejections (Q1). [ABSTRACT FROM AUTHOR]

Flow separation caused by steep topography remains a significant obstacle in accurately predicting turbulent boundary-layer flows over complex terrain, despite the utilization of sophisticated numerical models. The addition of atmospheric thermal stability, in conjunction with steep topography, further complicates the determination of disrupted turbulent wind patterns. The turbulent separated flows over a two-dimensional (2D) steep hill under thermal stratification has not been extensively addressed in previous experimental studies. Such measurements are crucial for enhancing our comprehension of flow physics and validating numerical models. We measured the turbulent wind flows over a 2D steep hill immersed in a stable boundary layer (of the bulk Richardson Number Ri b = 0.256) in a thermally-stratified boundary-layer wind tunnel. The flow separation, re-circulation zone and flow reattachment were characterized by the planar particle image velocimetry technique. Vertical profiles of mean air temperature and its fluctuations are also quantified at representative locations above the 2D steep hill and in the near wake region. Results indicate that the separated shear layer, initiated near the crest of the 2D steep hill, dominates the physical process leading to high turbulence levels and the turbulent kinetic energy production in the wake region for both stable and neutral thermal stability. Although the stable boundary layer does not dramatically change the turbulent flow pattern around the hill, the mean separation bubble is elongated by 13%, and its vertical extent is decreased by approximately 20%. Furthermore, the reduced turbulence intensities and turbulent kinetic energy of the near wake flow are attributed to the relatively low turbulence intensity and low momentum of the stable boundary layer due to buoyancy damping, compared to the neutral boundary layer. Additionally, a distinct low-temperature region—a cold pool—is extended beyond the separation bubble, reflecting the significant sheltering effect of the 2D steep hill on the downwind flow and temperature field. [ABSTRACT FROM AUTHOR]

We use experiments to explore the effect of surfactants on bubble-induced turbulence (BIT) at different scales, considering how the bubbles affect the flow kinetic energy, anisotropy and extreme events. To this end, high-resolution particle shadow velocimetry measurements are carried out in a bubble column in which the flow is generated by bubble swarms rising in water for two different bubble diameters (3 and 4 mm) and moderate gas volume fractions (0.5 %–1.3 %). We use tap water as the base liquid and add 1-Pentanol as an additional surfactant with varying bulk concentration, leading to different bubble shapes and surface boundary conditions. The results reveal that with increasing surfactant concentration, the BIT generated increases in strength, even though bubbles of a given size rise more slowly with surfactants. We also find that the level of anisotropy in the flow is enhanced with increasing surfactant concentration for bubbles of the same size, and that for the same surfactant concentration, smaller bubbles generate stronger anisotropy in the flow. Concerning the intermittency quantified by the normalized probability density functions of the fluid velocity increments, our results indicate that extreme values in the velocity increments become more probable with decreasing surfactant concentration for cases with smaller bubbles and low gas void fraction, while the effect of the surfactant is much weaker for cases with larger bubble and higher void fractions. [ABSTRACT FROM AUTHOR]

This work aims at studying the mechanisms behind the occurrence of extreme dissipation events in a channel flow, identifying nonlinear optimal perturbations as potential precursors of these events. Nonlinear optimal perturbations with respect to a generic turbulent instantaneous snapshot are computed for the first time using a direct-adjoint algorithm in the channel flow at $Re_{\tau }\approx 180$. The resulting initial perturbation displays the upstream tilting characteristic of Orr's mechanism and is positioned along the interfaces between two opposite-sign velocity streaks of the pre-existing turbulent field. Such a perturbation induces a sudden breakdown of the pre-existing structures and a heavier tail in the dissipation probability density function distribution. Different mechanisms are at play during this process: the high shear present at the interface between coherent low- and high-momentum regions is exploited to break down the larger structures and drive energy to small scales. This energy cascade is fed by an enhanced lift-up effect that produces intense streaks near the wall. It is found that the optimal perturbation grows exponentially during the first phase of its evolution reflecting the existence of a secondary modal instability of the streaks. To corroborate the results, the conditional spatiotemporal proper orthogonal decomposition (POD) analysis of Hack & Schimdt (J. Fluid Mech. , vol. 907, 2021, A9) is performed both in the perturbed and in the unperturbed flow, showing a clear agreement between the two cases and with the reference study. Thus, the optimal perturbation at initial time can be considered as a precursor of extreme events. [ABSTRACT FROM AUTHOR]

In this paper a mechanism is formulated and a principle is proposed that makes it possible to explain and, in some cases, to predict the shape of secondary flows of Prandtl’s second kind that arise in turbulent flows in straight pipes of non-circular cross-section. The effectiveness of the proposed principle is demonstrated by a number of known examples from the literature. The results of this work provide a rational basis for understanding the reasons for the formation and prediction of the shape of secondary flows of Prandtl’s second kind in straight pipes of non-circular cross-section.

For large Reynolds number flows, it is typically necessary to perform simulations that are under-resolved with respect to the underlying flow physics. For nodal discontinuous spectral element approximations of these under-resolved flows, the collocation projection of the nonlinear flux can introduce aliasing errors which can result in numerical instabilities. In Dzanic and Witherden (J. Comput. Phys., 468, 2022), an entropy-based adaptive filtering approach was introduced as a robust, parameter-free shock-capturing method for discontinuous spectral element methods. This work explores the ability of entropy filtering for mitigating aliasing-driven instabilities in the simulation of under-resolved turbulent flows through high-order implicit large eddy simulations of a NACA0021 airfoil in deep stall at a Reynolds number of 270,000. It was observed that entropy filtering can adequately mitigate aliasing-driven instabilities without degrading the accuracy of the underlying high-order scheme on par with standard anti-aliasing methods such as over-integration, albeit with marginally worse performance at higher approximation orders. [ABSTRACT FROM AUTHOR]

The transition from regular to apparently chaotic motions is often observed in nonlinear flows. The purpose of this article is to describe a deterministic mechanism by which several smaller scales (or higher frequencies) and new phases can arise suddenly under the impact of a forcing term. This phenomenon is derived from a multiscale and multiphase analysis of nonlinear differential equations involving stiff oscillating source terms. Under integrability conditions, we show that the blow-up procedure (a type of normal form method) and the Wentzel-Kramers-Brillouin approximation (of supercritical type) introduced in [7, 8] still apply. This allows to obtain the existence of solutions during long times, as well as asymptotic descriptions and reduced models. Then, by exploiting transparency conditions (coming from the integrability conditions), by implementing the Hadamard’s global inverse function theorem and by involving some specific WKB analysis, we can justify in the context of Hamilton-Jacobi equations the onset of smaller scales and new phases. [ABSTRACT FROM AUTHOR]

The equation for the fluid velocity gradient along a Lagrangian trajectory immediately follows from the Navier–Stokes equation. However, such an equation involves two terms that cannot be determined from the velocity gradient along the chosen Lagrangian path: the pressure Hessian and the viscous Laplacian. A recent model handles these unclosed terms using a multi-level version of the recent deformation of Gaussian fields (RDGF) closure (Johnson & Meneveau, Phys. Rev. Fluids , vol. 2 (7), 2017, 072601). This model is in remarkable agreement with direct numerical simulations (DNS) data and works for arbitrary Taylor Reynolds numbers $\textit {Re}_\lambda$. Inspired by this, we develop a Lagrangian model for passive scalar gradients in isotropic turbulence. The equation for passive scalar gradients also involves an unclosed term in the Lagrangian frame, namely the scalar gradient diffusion term, which we model using the RDGF approach. However, comparisons of the statistics obtained from this model with DNS data reveal substantial errors due to erroneously large fluctuations generated by the model. We address this defect by incorporating into the closure approximation information regarding the scalar gradient production along the local trajectory history of the particle. This modified model makes predictions for the scalar gradients, their production rates, and alignments with the strain-rate eigenvectors that are in very good agreement with DNS data. However, while the model yields valid predictions up to $\textit {Re}_\lambda \approx 500$ , beyond this, the model breaks down. [ABSTRACT FROM AUTHOR]

This paper attempts to develop a Dynamic Mode Decomposition (DMD)-based Reduced Order Model (ROMs) that can quickly but accurately predict the forces and moments experienced by a road vehicle such that they be used by an on-board controller to determine the vehicle's trajectory. DMD can linearize a large dataset of high-dimensional measurements by decomposing them into low-dimensional coherent structures and associated time dynamics. This ROM can then also be applied to predict the future state of the fluid flow. Existing literature on DMD is limited to low Reynolds number applications. This paper presents DMD analyses of the flow around an idealized road vehicle, called the Ahmed body, at a Reynolds number of 2.7 × 10 6 . The high-dimensional dataset used in this paper was collected from a computational fluid dynamics (CFD) simulation performed using the Menter's Shear Stress Transport (SST) turbulence model within the context of Improved Delayed Detached Eddy Simulations (IDDES). The DMD algorithm, as available in the literature, was found to suffer nonphysical dampening of the medium-to-high frequency modes. Enhancements to the existing algorithm were explored, and a modified DMD approach is presented in this paper, which includes: (a) a requirement of higher sampling rate to obtain a higher resolution of data, and (b) a custom filtration process to remove spurious modes. The modified DMD algorithm thus developed was applied to the high-Reynolds-number, separation-dominated flow past the idealized ground vehicle. The effectiveness of the modified algorithm was tested by comparing future predictions of force and moment coefficients as predicted by the DMD-based ROM to the reference CFD simulation data, and they were found to offer significant improvement. [ABSTRACT FROM AUTHOR]

A novel high-order projection method is presented in this study for the simulation of incompressible turbulent flows, utilising weighted essentially non-oscillatory (WENO) schemes. Spatial discretization involves using WENO schemes for nonlinear convection and standard central differences (CD) for the viscous term. The approach combines various orders of WENO and central difference schemes (WENO3/CD2, WENO5/CD4, and WENO7/CD6) to achieve third, fifth, and seventh-order spatial accuracy for velocity fields, respectively. Validating this method involved a thorough examination using the 2D Taylor Green vortex problem in both space and time, along with simulations of 3D Taylor-Green vortex problems at increasing Reynolds numbers (Re) of 1, 600, 16, 000, and 160, 000. The outcomes encompassed analyses of kinetic energy, enstrophy, energy spectra, and vortex fields, indicating the method's effectiveness in accurately simulating high Reynolds number turbulent flows without additional sub-grid scale models. [ABSTRACT FROM AUTHOR]