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The generation mechanism of wall heat flux is one of the fundamental problems in supersonic/hypersonic turbulent boundary layers. A novel heat decomposition formula under the curvilinear coordinate was proposed in this paper. The new formula has wider application scope and can be applied in the configurations with grid deformed. The new formula analyzes the wall heat flux of an interaction between a shock wave and a turbulent boundary layer over a compression corner. The results indicated good performance of the formula in the complex interaction region. The contributions of different energy transport processes were obtained. While the processes by the mean profiles such as molecular stresses and heat conduction, can be ignored, the contributions by the turbulent fluctuations, such as Reynolds stresses and turbulent transfer of heat flux, were greatly increased. Additionally, the pressure work is another factor that affects the wall heat flux. The pressure work in the wall-normal direction is concentrated close to the reattachment point, while the pressure work in the streamwise direction acts primarily in the shear layer and the reattachment point. [ABSTRACT FROM AUTHOR]

The internal flow scale of a Pelton turbine is variable, and the interaction between the jet and the bucket has strong transient characteristics, resulting in an incomplete understanding of its internal vortex structure evolution and energy dissipation mechanisms. In order to reveal the influence of vortices on the flow regime and energy dissipation mechanisms in the turbine, this paper establishes the correlation between the unsteady flow characteristics and energy dissipation inside the Pelton turbine based on the energy balance equation and quantifies the energy losses inside the turbine. The results indicate that vortex structures present a non-uniform distribution inside the jet, which disrupts the uniformity of the jet velocity distribution, resulting in an uneven distribution of high-vorticity zones on the bucket surfaces and intensifying flow interference among the buckets. The strong shear flow caused by the downstream flow detachment of the needle guide, the turbulent boundary layer on the jet surface, and the wake effect downstream of the needle tip are the main reasons for the dissipation of fluid kinetic energy. The distribution range of energy loss on the rotating bucket closely corresponds to the position of the high-vorticity zone. Energy dissipation inside the turbine is primarily in the form of turbulent kinetic energy, and the injectors and runner are the primary energy dissipation components. Moreover, inside the injectors, each form of energy loss remains relatively constant, whereas inside the runner, its rotation induces fluctuating energy losses attributed to Reynolds stress work. The results contribute to an enhanced understanding of the energy dissipation characteristics and complex flow mechanisms within the turbine, providing reference for the optimisation and efficient operation of the multi-nozzle Pelton turbine. [ABSTRACT FROM AUTHOR]

In this study, wind flow around a telecommunication tower (Milad Tower) model was numerically simulated. Several computational fluid dynamics (CFD) simulations were performed using the Reynolds Averaged Navier–Stokes (RANS) and the Delayed Detached Eddy Simulation (DDES) models. Two cases resembling the tower head with different inlet turbulence intensity profiles were considered, and computational results for wind flow and pressure distributions on various sections around the tower head and structure were presented and compared with the available wind tunnel data. Good agreement of the simulated pressure coefficient distributions around the head section in the windward and leeward sides of the tower with the wind tunnel data was observed. The regions with positive and negative pressures on the tower surface were identified based on the pressure coefficient distributions. In addition, the airflow characteristics around a model of the entire Milad Tower were studied. The simulation results for the airflow and the drag and lift coefficients acting on the tower for the high and low-intensity winds were evaluated and discussed. It was demonstrated that the DDES model was preferred for the simulation of mean pressure coefficients over the tower; however, the RNG k–ε model results, which were obtained at lower computational costs, were also satisfactory. In addition, the DDES showed better performance in simulating the maximum and minimum pressure coefficients on different tower levels and predicting the mean aerodynamics drag and lift coefficients. The results of the DDES model illustrated the higher power spectral density of velocity fluctuations on the leeward side of the tower compared with the windward side. [ABSTRACT FROM AUTHOR]

When planning a new wind farm, optimising the turbine layout to maximise power output is crucial. Traditional approaches based on analytical wake models often fail in complex terrain. In such areas, wake‐to‐wake interactions and terrain‐induced effects need to be taken into account via high‐fidelity optimisation. Building on our prior research on how best to maximise offshore wind farm power production by both micro‐siting (layout optimisation) and wake steering (yaw angle optimisation), this study focuses on the optimisation of two onshore wind turbines located around a 2D hill. The proposed approach is based on high‐fidelity simulations (LES) to account for the complexity of the flow and on Bayesian optimisation (BO) to systematically and efficiently find the optimal configuration for the two turbines given a set of parameters. The LES are performed with the high‐order finite‐difference wind farm simulator WInc3D, which has been modified for this study to deal with complex terrains and turbines with yaw and tilt. Following the validation of the immersed boundary method (IBM) and wall modelling used to model the complex terrain and of a modified actuator disc model (ADM) to account for tilted turbines, two BO campaigns are presented in this study, with different sets of parameters to optimise. The main flow features and some statistics for each optimum case are discussed using the flow fields with and without the optimised configuration of the upstream turbine to analyse its impact on the downstream turbine and on the overall power of the system. For the first optimisation, the design variables include the wind turbines' streamwise locations and their hub heights (4 parameters). This optimisation study benefits from an elevated upstream turbine hub, leading to a significant power increase in the downstream turbine, thanks to an acceleration of the flow in front of the hill. For the second optimisation, the design space is modified based on the data from the first optimisation, by replacing the turbine's hub height with the possibility of changing the tilt angle of the upstream turbine (4 parameters). It is found that considerable power gains can be obtained while maintaining a modest upstream turbine hub height, by introducing a positive tilt angle in the upstream turbine. Overall, it is found that by setting up the turbines in ways that exploit the speed‐up of the flow around the hill, both optimised layouts achieve considerable enhancements in power density over a reference configuration. [ABSTRACT FROM AUTHOR]

We present Atacama Large Millimeter/submillimeter Array observations of SiO, SiS, H2O, NaCl, and SO line emission at ∼30–50 mas resolution. These images map the molecular outflow and disk of Orion Source I (SrcI) on ∼12–20 au scales. Our observations show that the flow of material around SrcI creates a turbulent boundary layer in the outflow from SrcI, which may dissipate angular momentum in the rotating molecular outflow into the surrounding medium. Additionally, the data suggest that the proper motion of SrcI may have a significant effect on the structure and evolution of SrcI and its molecular outflow. As the motion of SrcI funnels material between the disk and the outflow, some material may be entrained into the outflow and accrete onto the disk, creating shocks that excite the NaCl close to the disk surface. [ABSTRACT FROM AUTHOR]

The turbulent boundary layer in a viscous incompressible fluid developing longitudinally past the surface of a thin cone or cylinder at a finite distance from the laminar–turbulent transition zone is studied. The characteristic Reynolds number, determined from the external flow velocity and the length of the body, is assumed to be large, and the thickness of the boundary layer is small and comparable to the radius of the body. The asymptotic method of multiple scales is used to find solutions to the Navier–Stokes equations. Instead of the traditional decomposition of the solution into time-averaged values and their fluctuations, the velocities and pressure are expressed as an asymptotic series consisting of steady and perturbed terms. As a result, the viscous steady flow (‘secondary’) that arises in the boundary layer as a mandatory component of fast turbulent fluctuations was described. Analytical and numerical solutions for the radial steady velocity are presented, describing the self-induced suction of fluid from the external flow into the boundary layer. Further analytical solutions are obtained for the longitudinal and circumferential velocities, which differ markedly from the laminar regime. The solutions found are somewhat similar to the degenerate (one-dimensional) case of self-sustaining longitudinal thin structures in turbulent shear flows. A qualitative comparison with direct numerical simulations is presented. [ABSTRACT FROM AUTHOR]

The article delves into the scaling of vertical coherence and energy profiles for wall-attached eddies during sand and dust storms, using high-frequency observation data. It highlights self-similar features for velocity, temperature, and particulate matter at lower heights during these storms. The research aims to improve near-wall models by enhancing understanding of wall-attached eddies in sand and dust storms, focusing on turbulence characteristics and energy profiles. The study also examines the logarithmic behavior of turbulence intensity for different components, considering factors like buoyancy and dust field effects. [Extracted from the article]

The article in the Journal of Fluid Mechanics delves into the dynamics of thermal and momentum boundary regions in three-dimensional direct numerical simulations of Rayleigh–Bénard convection. It uncovers the absence of a global coherent mean flow, with velocity fluctuations being the dominant force. The study highlights the presence of local shear-dominated flow patches and extensive shear-free incoherent regions near the wall, which have implications for convection layers with larger aspect ratios. The research challenges traditional stability concepts and emphasizes the significance of fluctuations in turbulent convection, comparing findings with previous experiments and simulations to provide a comprehensive understanding of turbulent thermal convection dynamics. [Extracted from the article]

The article delves into the analysis of large-scale coherent structures in turbulent pipe flows at different Reynolds numbers. By utilizing the Karhunen-Loève decomposition and a unique Voronoi diagram method, the research identifies two categories of structures - wall-attached and detached eddies - based on their characteristics and energy contributions. The study demonstrates that as the Reynolds number increases, the energy distribution of these structures changes, with attached eddies scaling linearly with the wall-normal distance while detached eddies maintain a consistent size. This research sheds light on the spatial distribution of these structures, potentially aiding in understanding the variations in mean velocity between pipe and channel flows and enhancing the modeling of large-scale motions. [Extracted from the article]

The article "Dynamics of turbulent energy and dissipation in channel flow" delves into the intricate relationship between turbulent kinetic energy (TKE), turbulence dissipation rate (TDR), and turbulence production rate (TPR) in fully developed turbulent channel flow. The research uncovers time lags between these variables, with correlations that vary based on Reynolds number. The findings emphasize the complex nature of energy transfer and dissipation within different regions of the channel, showcasing correlations that involve both time and wall-distance lags. This study offers valuable insights into the self-sustaining process of turbulent flows, with implications for understanding turbulence dynamics across various contexts. [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]

The article presents a new wall-pressure spectral model for non-equilibrium boundary layers, addressing the limitations of existing semi-empirical models in strong pressure gradient conditions. By analyzing high-fidelity simulations and experimental data, the study identifies optimal scaling variables for accurate wall-pressure spectrum predictions. The proposed model, incorporating local mean velocity profiles, shows improved performance in capturing the effects of pressure gradients on wall-pressure statistics in turbulent boundary layer flows. Overall, the research provides valuable insights into the impact of Reynolds number and pressure gradients on wall-pressure spectra, enhancing our understanding of non-equilibrium boundary layer dynamics. [Extracted from the article]

Considerable efforts have been devoted to the understanding of the small-scale characteristics in turbulent flows. While the universality of small-scale quantities has been established for incompressible flows, their extension to high-pressure transcritical flows remains an open area of research. To address this question, we investigate the real-fluid thermodynamic effects on small-scale velocity statistics of high-pressure transcritical wall-bounded turbulence. We show that in the locally isotropic region for transcritical flows, low-order moments of small-scale statistics collapse for all cases and Kolmogorov's (1941) theory holds. However, real-fluid thermodynamic effects introduce deviations in the tails of the probability density function for the velocity derivative and, consequently, high-order moments of velocity gradients and dissipation rate in transcritical flows cannot collapse in the locally isotropic region. Analysis of the intermittency shows that the low-order structure functions in transcritical flows follow extended self-similarity, while the dependence of the intermittency factor on real-fluid effects is observed for high-order structure functions. The real-fluid effects on intermittency are explained by turbulent structures related to rare events. Additionally, the dissipation rate moments for transcritical flows follow a universal scaling with Reynolds number, and the scaling exponents are different from those of incompressible flows. These results extend the small-scale universality in incompressible flows (Schumacher et al. , Proc. Natl Acad. Sci. USA , vol. 111, 2014, pp. 10961–10965) to realistic flows with significant changes in thermodynamic properties, and provide a physical underpinning of the scaling laws of small-scale statistics at transcritical conditions. [ABSTRACT FROM AUTHOR]

This paper presents the development of fluorescent tracer particles for use in gas flows as a countermeasure for undesired strong light reflections on surfaces of channel walls or obstacles and as a label for the discrimination of multi-constituent flows. The employment of fluorescent dye-doped tracer particles with a wavelength-specific optical filter enables the separation of the Stokes-shifted particle light emission from reflections on surfaces and Mie scattering from non-fluorescing particles. The fluorescent particles were made of Pyrromethene 567 (P567) and Di-Ethyl-Hexyl-Sebacate (DEHS), and the addition of P567 was not found to alter the characteristics of the particles generated. Investigations in a low-speed wind tunnel revealed that the intensity of fluorescent emission is proportional to the dye concentration at least up to 2.0 g l - 1 . The efficacy of reflection removal was investigated in a setup with a metal turbine blade placed in the flow and a laser sheet oriented to impinge the blade surface. With the installation of an appropriate optical filter, undesired light reflections were successfully removed, and reasonable vector calculations were enabled in proximity to the reflective blade surfaces. Finally, the performance of the modified DEHS was compared to conventional DEHS with the measurement of a canonical turbulent boundary layer (TBL). The flow was globally seeded with conventional DEHS and the TBL was locally seeded with fluorescing DEHS; simultaneous imaging with a notch filter confirmed that the flow is accurately tracked by the modified DEHS without additional bias. Furthermore, this indicated the possibility of using the newly developed particles to segregate portions of a flow with multiple constituents. [ABSTRACT FROM AUTHOR]

This paper reports an algorithm for measuring the time-averaged skin friction vector field τ ¯ (X) starting from time-resolved temperature maps, acquired by a functional coating of temperature-sensitive paint. The algorithm is applied to a large area around a wall-mounted cube, immersed in the turbulent boundary layer over a flat plate. The method adopts a relaxed version of the Taylor Hypothesis operating on time-resolved maps of temperature fluctuations T ′ measured on the slightly warmer bounding surface. The procedure extracts U ¯ T (X) , the celerity of displacement of T ′ , as the best approximation of the forecasting provided by the frozen turbulence assumption near the wall, where its rigorous application is inappropriate. The τ ¯ (X) estimation is based on the hypothesis of a linear relationship between U ¯ T (X) and U ¯ U (X) , chained to the one between U ¯ U (X) and U ¯ τ (X) . We assess the outcomes of the proposed algorithm against those derived by the 2D and 3D Lagrangian particle tracking (LPT) methodology 'Shake-The-Box', whose advent has made available high-quality near-wall flow field information. Furthermore, data from high-density 2D time-resolved LPT allows exploring the suitability of the linear relationships chain between U ¯ T (X) and U ¯ τ (X) in the proposed context. [ABSTRACT FROM AUTHOR]

A linear theory of the trapped mountain waves that develop in a turbulent boundary layer is presented. The theory uses a mixing‐length turbulence model based on Monin–Obukhov similarity theory. First, the backward reflection of a stationary gravity wave propagating toward the ground is examined. Three parameters are investigated systematically: the Monin–Obukhov length Lmo$$ {L}_{\mathrm{mo}} $$, the roughness length z0$$ {z}_0 $$, and the limit value of the mixing length λ$$ \lambda $$ aloft the "inner" layer. The reflection coefficient appears to depend strongly on the Richardson number aloft the inner layer (J=λ/κLmo$$ J=\lambda /\kappa {L}_{\mathrm{mo}} $$, with κ$$ \kappa $$ the von Kármán constant), with the reflection decreasing when the stability J$$ J $$ increases. The influence of the roughness and mixing lengths on the reflection is explained in terms of the depth of a "pseudo"‐critical level located below the surface, with the reflection decreasing when the depth of the "pseudo"‐critical level decreases. The preferential modes of oscillations occurring in the presence of mountain forcing are then analysed, with the decay rate of the trapped waves downstream increasing when the reflection decreases. At a certain point nevertheless, when the absorption is large but the boundary‐layer depth deep enough, trapped modes appear that interact little with the surface. [ABSTRACT FROM AUTHOR]

The wind characteristics of the atmospheric boundary layer (ABL) over complex forest terrains are inherently intricate, influenced by the interplay of rugged topography, seasonal climatic fluctuations, and periodic vegetation dynamics. These effects are especially evident in the near-ground wind field, which exhibits substantial seasonal variability. Conventional wind characterization methods, as outlined in current standards, often fail to accurately capture these seasonal variations, thereby complicating the reconstruction of the near-ground ABL turbulent wind field in complex forest terrains. Accordingly, we employ the narrow band synthetic random flow generation (NSRFG) technique within large eddy simulation (LES) to generate inflow turbulence representing the growing and baldness seasons in a complex forest terrain by adjusting parameter equations for seasonal adaptation and introducing new empirical equations for the turbulent spectrum. Subsequently, we verified the seasonal turbulent flow's statistical characteristics and flow structure to assess its feasibility and validity, ultimately establishing a 'seasonal numerical wind field' model. Finally, the seasonally modified LES-NSRFG method was applied to the numerical simulation of turbulent flow around the Commonwealth Advisory Aeronautical Research Council (CAARC) standard high-rise building model. A comprehensive comparison of wind effects was conducted for the CAARC model under varying incoming flow conditions. The results indicate that seasonal winds in a complex forest terrain significantly affect the building's vortex wake, increasing the irregularity and complexity of the structural wind pressure and base moment coefficients. Thus, the seasonal wind effect must be considered when designing wind-resistant engineering structures in forest regions moving forward. [ABSTRACT FROM AUTHOR]

Riblets can be potentially employed to passively reduce the turbulent friction drag. However, the drag reduction performance of riblets does not currently meet expectations, which could assist in emission reduction and energy conservation in green transportation. This study proposes and validates a topological form of hierarchical nested riblets (HNR) that significantly enhances the drag reduction performance. To explore the drag reduction enhancement mechanism, direct numerical simulations are performed for flow simulation on the riblet surface under different Reynolds numbers. The results show that under the riblet dimensionless spacing of the riblet s + ≈ 21 , the drag reduction performance of the HNR surface improves by about 70% compared to that of the uniform riblet surface, which is inspired by shark skin. From the perspective of turbulence statistics, the HNR surface reduces the turbulent mixing near the wall, weakening the momentum transfer. Furthermore, the transient flow field shows that the secondary riblet in HNR prevents some turbulent flow and streamwise vortices from entering the groove, considerably weakening the dispersive stress induced by the secondary flow. Moreover, owing to the influence of the secondary riblet, small-scale turbulence develops and strengthens into large-scale turbulent motion, which is advantageous to the boundary layer flow and results in drag reduction improvement. [ABSTRACT FROM AUTHOR]

Vortex-induced vibration (VIV) of a low mass ratio circular cylinder ( m * = 10), that is free to vibrate in crossflow and in-line directions, at supercritical Reynolds number (3 × 10 5 ) has been studied using large-eddy simulation for a range of reduced speed (2 ≤ U * ≤ 11). In the supercritical regime, the boundary layer transitions to a turbulent state via the formation of a laminar separation bubble (LSB). The regime is associated with weakened vortex shedding, resulting in subdued VIV response. Lock-in is observed for U * ≥ 3. The cylinder vibration frequency is identical in the crossflow and in-line directions, leading to an elliptical trajectory. The rms of the force coefficients is similar to that for a stationary cylinder in the desynchronization regime while it decreases with increase in U * during lock-in. The spatiotemporal dynamics of LSB is explored. The LSB is sedentary in the desynchronization regime, while it undergoes significant circumferential movement, in each cycle of cylinder oscillation, in the lock-in regime. The mode of vortex shedding, determined from the arrangement of vortices in the span-averaged instantaneous flow as well as the phase difference between the lift and cylinder response, is C(2S) in the desynchronization regime. It is 2 P 0 in the lock- in regime. [ABSTRACT FROM AUTHOR]

Using riblet structures to modulate the turbulent boundary layer to reduce frictional drag is important for vehicle stability and range. The flow on the airfoil surface at different angles of attack is different, and the effect of the riblets structure on the turbulent boundary layer flow will be changed, which in turn affects the drag reduction effect. In this paper, the drag reduction characteristics of V-shaped riblets are investigated experimentally by the particle image velocimetry technique. The distributions of mean velocity, friction coefficient, drag reduction rate, and coherent structure in the boundary layer on the airfoil surface at angles of attack from 0° to 12° are analyzed, and the effects of smooth and riblets surfaces on the turbulent flow characteristics near the wall are compared. Based on these results, the airfoil surface was zonally covered with riblets to investigate further the effects of the riblets' covering positions on the drag reduction of the airfoil at different angles of attack. The results show that the riblets reduce the intensity, distribution, and probability of the eject and sweep events in the turbulence bursts by suppressing the velocity fluctuations at the near-wall surface, thus realizing the reduction of turbulence drag. The zoned arrangement of riblets can effectively regulate the airfoil surface flow and reduce the unfavorable effects of the riblets on the laminar flow region and flow separation region on the surface of the airfoil with different angles of attack, and the results of the study can provide a reference for the influence of the coverage range of the riblets on the drag reduction effect. [ABSTRACT FROM AUTHOR]

Time-marching of turbulent flow fields is computationally expensive using traditional Computational Fluid Dynamics (CFD) solvers. Machine Learning (ML) techniques can be used as an acceleration strategy to offload a few time-marching steps of a CFD solver. In this study, the Transformer (TR) architecture, which has been widely used in the Natural Language Processing (NLP) community for prediction and generative tasks, is utilized to predict future velocity flow fields in an actuated Turbulent Boundary Layer (TBL) flow. A unique data pre-processing step is proposed to reduce the dimensionality of the velocity fields, allowing the processing of full velocity fields of the actuated TBL flow while taking advantage of distributed training in a High Performance Computing (HPC) environment. The trained model is tested at various prediction times using the Dynamic Mode Decomposition (DMD) method. It is found that under five future prediction time steps with the TR, the model is able to achieve a relative Frobenius norm error of less than 5 % , compared to fields predicted with a Large Eddy Simulation (LES). Finally, a computational study shows that the TR achieves a significant speed-up, offering computational savings approximately 53 times greater than those of the baseline LES solver. This study demonstrates one of the first applications of TRs on actuated TBL flow intended towards reducing the computational effort of time-marching. The application of this model is envisioned in a coupled manner with the LES solver to provide few time-marching steps, which will accelerate the overall computational process. [ABSTRACT FROM AUTHOR]

In this work, a modal matching approach is developed to investigate the impact of the spatial correlation of wall pressure fluctuations on structural vibration and its role in suppressing flow-induced vibration in aircraft. The modal matching method defines the wavelength ratio between the convection wave and the flexural wave to capture the spatial characteristics of both the flow mode and the structural mode. In comparison to conventional methods such as the finite element method and analytical method, the proposed modal matching approach allows for obtaining the vibration response in the spatial-wavenumber domain. To validate the spatial correlation effect on structural vibrations obtained with the modal matching approach, a wind tunnel test was conducted to measure flow-induced vibrations. The calculated spatial-wavenumber data for flow-induced vibrations reveal that the crests and troughs of the vibration velocity alternate periodically as the wavelength of the wall pressure fluctuations changes. At the resonance frequency, a vibration velocity trough can be observed when the spatial correlation of wall pressure fluctuations aligns with the structural mode. Additionally, the concept of resonance independence of flow-induced vibrations is introduced to describe the phenomenon where the most severe structural vibration, determined by the matching value between the aerodynamic mode and the structural mode, does not always occur at the resonance frequency. The modal matching approach effectively suppresses structural vibration by utilizing the spatial correlation of wall pressure fluctuations, offering a new perspective on controlling flow-induced vibrations. [ABSTRACT FROM AUTHOR]

Wall-modeled large-eddy simulations of turbulent boundary-layer flows over a flat plate at Mach 3.5, 7.87, and 13.64 were carried out, and the aero-optical distortions resulting from density fluctuations were investigated. The conditions for the Mach 3.5 case match those of a direct numerical simulation in the literature. The Mach 7.87 conditions are representative of the Hypersonic Wind Tunnel at Sandia National Laboratories, and the Mach 13.64 conditions match those of the Arnold Engineering Development Complex Hypervelocity Tunnel 9. The wall-modeled large-eddy simulations were validated using available experimental and DNS reference data. The normalized root-mean-square optical path difference for all three cases is in good agreement with respective data obtained from reference direct numerical simulations and experiments (within 1.53% for M∞=3.5, 1.25% for M∞=7.87, and 12% for M∞=13.64, compared to numerical data). Above M∞=5.0, the normalized path difference obtained from the simulations is above the value predicted by a semi-analytical relationship by Notre Dame University. With increasing Mach number, the bulk contribution to the total optical distortion shifts toward the boundary-layer edge. In addition, a proper orthogonal decomposition reveals that the nature of the dominant density fluctuations, which are located near the boundary-layer edge, changes from three-dimensional to two-dimensional. Understanding how the coherent structures contribute to the optical path difference may pave a way toward devising active flow control strategies geared toward a reduction of the optical path difference. [ABSTRACT FROM AUTHOR]

In this work we assess the impact of the limited availability of wall-embedded sensors on the full 3D estimation of the flow field in a turbulent channel with R e τ = 200 . The estimation technique is based on a 3D generative adversarial network (3D-GAN). We recently demonstrated that 3D-GANs are capable of estimating fields with good accuracy by employing fully-resolved wall quantities (pressure and streamwise/spanwise wall shear stress on a grid with DNS resolution). However, the practical implementation in an experimental setting is challenging due to the large number of sensors required. In this work, we aim to estimate the flow fields with substantially fewer sensors. The impact of the reduction of the number of sensors on the quality of the flow reconstruction is assessed in terms of accuracy degradation and spectral length-scales involved. It is found that the accuracy degradation is mainly due to the spatial undersampling of scales, rather than the reduction of the number of sensors per se. We explore the performance of the estimator in case only one wall quantity is available. When a large number of sensors is available, pressure measurements provide more accurate flow field estimations. Conversely, the elongated patterns of the streamwise wall shear stress make this quantity the most suitable when only few sensors are available. As a further step towards a real application, the effect of sensor noise is also quantified. It is shown that configurations with fewer sensors are less sensitive to measurement noise. [ABSTRACT FROM AUTHOR]

This paper reports on the recent NATO Advanced Vehicle Technology (AVT) effort associated with smooth-wall two-dimensional turbulent boundary layer flows subjected to streamwise pressure gradients. The effort considered experiments, Reynolds Averaged Navier-Stokes (RANS) simulations, modelling, scaling and flow physics relative to the subject flows. Special attention is given to the current predictive capabilities and deficiencies of RANS simulations, and the interplay between experiments and RANS validation and development. In addition, the efficacy of the prediction of velocity field response and wall pressure statistics are respectively demonstrated via Resolvent and Gene Expression Programming based models. The persistence of a logarithmic mean velocity profile is evaluated and measures of non-equilibrium are described and discussed. A number of open issues are described and recommendations for future research are suggested. [ABSTRACT FROM AUTHOR]

The statistical behaviours of the density-weighted displacement speed and its curvature and strain rate dependence during head-on interaction of statistically planar premixed flames have been analysed based on Direct Numerical Simulations. The analysis has been conducted within turbulent boundary layers featuring inert walls, under both isothermal and adiabatic wall boundary conditions. The flame quenches due to heat loss when it reaches close to the wall in the case of isothermal wall boundary condition. By contrast, the flame can propagate all the way to the wall before extinguishing due to the consumption of reactants in the case of adiabatic wall boundary conditions. Thus, the effects of thermal expansion, quantified by dilatation rate, and flame normal acceleration during flame-wall interaction in the case of the isothermal wall are weaker than that in the case of the adiabatic wall case due to flame quenching. This gives rise to significant differences in the curvature and strain rate dependences of the reactive scalar gradient magnitude, which affects the statistical behaviours of the reaction and normal diffusion components of density-weighted displacement speed including their mean values, widths of their probability density functions, and their local strain rate and curvature dependences. It has been found that the interaction of near-wall vortical structures with flame surface increases the range of curvature variation during head-on interaction, which acts to widen the probability density function of the density-weighted displacement speed. This trend is relatively stronger for the isothermal wall boundary condition because of the larger variation of the reaction rate component of the density-weighted displacement speed due to local flame quenching, which also acts to widen the range of the density-weighted displacement speed variation in comparison to that in the case of adiabatic wall boundary conditions. Although the qualitative nature of curvature, strain rate, and stretch rate dependences of the density-weighted displacement speed remain unaffected by the wall boundary condition, the strength of the correlation changes with the progress of head-on interaction. It has been found that the negative correlation between the density-weighted displacement speed and flame curvature weakens with the progress of head-on interaction, which gives rise to a reduction in the strength of the correlation between the density-weighted displacement speed and flame stretch rate. Similarly, the correlation between the tangential strain rate and the density-weighted displacement speed weakens with the progress of head-on interaction. Detailed physical explanations are provided for these behaviours, and their modeling implications are indicated. [ABSTRACT FROM AUTHOR]

Abstract In this study, wind flow around a telecommunication tower (Milad Tower) model was numerically simulated. Several computational fluid dynamics (CFD) simulations were performed using the Reynolds Averaged Navier–Stokes (RANS) and the Delayed Detached Eddy Simulation (DDES) models. Two cases resembling the tower head with different inlet turbulence intensity profiles were considered, and computational results for wind flow and pressure distributions on various sections around the tower head and structure were presented and compared with the available wind tunnel data. Good agreement of the simulated pressure coefficient distributions around the head section in the windward and leeward sides of the tower with the wind tunnel data was observed. The regions with positive and negative pressures on the tower surface were identified based on the pressure coefficient distributions. In addition, the airflow characteristics around a model of the entire Milad Tower were studied. The simulation results for the airflow and the drag and lift coefficients acting on the tower for the high and low-intensity winds were evaluated and discussed. It was demonstrated that the DDES model was preferred for the simulation of mean pressure coefficients over the tower; however, the RNG k–ε model results, which were obtained at lower computational costs, were also satisfactory. In addition, the DDES showed better performance in simulating the maximum and minimum pressure coefficients on different tower levels and predicting the mean aerodynamics drag and lift coefficients. The results of the DDES model illustrated the higher power spectral density of velocity fluctuations on the leeward side of the tower compared with the windward side.

ABSTRACT When planning a new wind farm, optimising the turbine layout to maximise power output is crucial. Traditional approaches based on analytical wake models often fail in complex terrain. In such areas, wake‐to‐wake interactions and terrain‐induced effects need to be taken into account via high‐fidelity optimisation. Building on our prior research on how best to maximise offshore wind farm power production by both micro‐siting (layout optimisation) and wake steering (yaw angle optimisation), this study focuses on the optimisation of two onshore wind turbines located around a 2D hill. The proposed approach is based on high‐fidelity simulations (LES) to account for the complexity of the flow and on Bayesian optimisation (BO) to systematically and efficiently find the optimal configuration for the two turbines given a set of parameters. The LES are performed with the high‐order finite‐difference wind farm simulator WInc3D, which has been modified for this study to deal with complex terrains and turbines with yaw and tilt. Following the validation of the immersed boundary method (IBM) and wall modelling used to model the complex terrain and of a modified actuator disc model (ADM) to account for tilted turbines, two BO campaigns are presented in this study, with different sets of parameters to optimise. The main flow features and some statistics for each optimum case are discussed using the flow fields with and without the optimised configuration of the upstream turbine to analyse its impact on the downstream turbine and on the overall power of the system. For the first optimisation, the design variables include the wind turbines' streamwise locations and their hub heights (4 parameters). This optimisation study benefits from an elevated upstream turbine hub, leading to a significant power increase in the downstream turbine, thanks to an acceleration of the flow in front of the hill. For the second optimisation, the design space is modified based on the data from the first optimisation, by replacing the turbine's hub height with the possibility of changing the tilt angle of the upstream turbine (4 parameters). It is found that considerable power gains can be obtained while maintaining a modest upstream turbine hub height, by introducing a positive tilt angle in the upstream turbine. Overall, it is found that by setting up the turbines in ways that exploit the speed‐up of the flow around the hill, both optimised layouts achieve considerable enhancements in power density over a reference configuration.

This paper analysed the relationships between various scales by considering the condition of the upstream flow fields. The feasibility of flow control for drag reduction through intermittent energy input from active drag reduction systems was discussed. In the experiment, a piezoelectric oscillator was used to impose local disturbances on a turbulent boundary layer. Flow field information was collected using fixed probes upstream of the piezoelectric oscillator and moving probes downstream (moving along the wall-normal direction). Correlation analyses were performed on velocity signals of different scales both upstream and downstream of the piezoelectric oscillator to determine their spatiotemporal relationships. Distur-bance signals and their higher-order harmonics were identified through the pre-multiplied energy spectrum, and different signal scales were categorized. The interactions between large scales and disturbance scales, as well as between disturbance scales and small scales, were particularly discussed under large-scale high-speed/low-speed inflow backgrounds. It is observed that the large-scale high-speed background modulates the disturbance signals. Under both large-scale high-speed and low-speed inflow backgrounds, there exists a fixed phase correspondence between the disturbance signals and small-scale signals, which remains unaffected by the inflow background. It is explicitly stated that when actively controlling the flow field intermittently using a piezoelectric oscillator, it is advisable to operate under a large-scale high-speed inflow background.

We present the first analysis of frame‐indifferent (objective) fluxes and material vortices in Large Eddy Simulations of atmospheric boundary layer turbulence. We extract rotating fluid features that maintain structural coherence over time for near‐neutral, transitional, and convective boundary layers. In contrast to traditional analysis of coherent structures in turbulent boundary layers, we provide the first identification of vortex boundaries that are mathematically defined to behave as tracer transport barriers. Furthermore, these vortices are indifferent to the choice of observer reference frame and can be identified without user‐dependent velocity field decompositions. We find a strong agreement between the geometric qualities of the coherent structures we extract using our new method and classical descriptions of horizontal rolls and convective cells arising from decades of observational studies. We also quantify trends in individual vortex contributions to turbulent and advective fluxes of heat under varying atmospheric stability. Using recently developed tools from the theory of transport barrier fields, we compare diffusive momentum and heat barrier fields with the presence of rolls and cells, and determine a strong connection between heat and momentum orthogonality with the physical drivers of roll‐cell transformation. This newly employed frame‐indifferent characterization of coherent turbulent structures can be directly applied to numerical model output, and thus provides a new Lagrangian approach to understand complex scale‐dependent processes and their associated dynamics. Plain Language Summary: As uniform temperature wind flows over flat terrain that is the same temperature as the air, special eddies, called horizontal rolls, begin to appear in the wind and organize the flow into quasi‐recurrent patterns. When air flows over terrain that is much hotter than the air, the air warms and begins to rise and subsequently cool. This gives rise to a different kind of organizing structure, the convective cell, much like you see in fluids on a warm stove. The physics of the transition from roll structures to cell structures in the atmosphere is poorly understood, but important for quantifying exchanges of energy and moisture between the land, ocean, and atmosphere. In the present research, we utilize recent advances from nonlinear dynamical systems to explicitly identify the boundaries of individual coherent fluid structures, and quantify their role in the transport of momentum and heat. This gives us a new way to objectively quantify atmospheric fluxes and determine how much a certain eddy is behaving like a roll or a cell. Key Points: Lagrangian‐Averaged Vorticity Deviation (LAVD) identifies eddy boundaries in large eddy simulations of near‐neutral and convective atmospheresLAVD contour eddies exhibit specific frame‐indifferent momentum and heat transport characteristics for different atmospheric stabilitiesDifferentiating roll and convective cell type coherent structures is possible via the orthogonality of momentum and heat barrier fields [ABSTRACT FROM AUTHOR]

The article delves into heat-transfer effects in compressible turbulent boundary layers, highlighting the significance of the Eckert number (Ec) and the diabatic parameter Θ in comparing heat-transfer effects. It categorizes data into different cooling cases based on the wall-normal position of the temperature peak, offering insights into how heat transfer effects vary with Mach number and wall-to-recovery temperature ratio. The regime diagram presented in the text illustrates the impact of Reynolds number, Mach number, and high-enthalpy effects on the classification of heat transfer effects, with a focus on temperature ratios and the influence of cooling intensity. The study also discusses the influence of high stagnation enthalpy conditions on heat transfer in vehicles, emphasizing the importance of maintaining wall temperatures below material limits. [Extracted from the article]

In rivers, the addition of finer sediment to a coarser riverbed is known to increase the mobility of the coarser fraction. Two mechanisms have been suggested for this: a geometric mechanism whereby smaller sizes smooth the bed, increasing near-bed velocity and thus mobility of the larger sizes, and a viscous mechanism whereby a transitionally smooth turbulent boundary layer forms, rendering the coarser grains more mobile. Here, we report on experiments using two sediment mixtures to better understand these proposed mechanisms. In Mixture 1, we used 0.5 and 5 mm grains, and in Mixture 2, we used 2 and 20 mm grains. If the entrainment of coarse gravel by finer sediment is a purely geometric effect, then the addition of finer material should produce the same effect on the mobility of the coarser material for both mixtures because they have the same size ratio. We show that addition of finer material has a different effect on the two mixtures. We observed an increase in the mobility of the coarse fraction for both mixtures, but the increase in coarse fraction mobility for Mixture 1 was almost twice that for Mixture 2. Our experiments show that in addition to the geometric effect, enhancement of coarse gravel transport by finer sediment is also driven by a viscous effect. [ABSTRACT FROM AUTHOR]

A neutrally buoyant float deployed in an atmospherically driven turbulent ocean boundary layer on the dense side of a submesoscale front was repeatedly carried across the boundary layer by the turbulence and then trapped beneath the slumping front. Lagrangian particles in a large‐eddy simulation of a similar baroclinically unstable front forced by surface cooling move along convergent surface filaments toward filament junctions. They are also caught by convective plumes that downwell them at speeds similar to those of the float. Subsequently, some are trapped in the pycnocline by frontal slumping due to ageostrophic secondary frontal circulations. In both observations and simulations, boundary layer turbulence and frontal circulations work together to trap and subduct particles from the mixed layer. The small‐scale boundary layer motions move them vertically within the boundary layer and larger, submesoscale frontal circulations move them laterally out of the boundary layer and under the slumping fronts. Plain Language Summary: The present study provides evidence for enhanced tracer transport at ocean fronts in both observation and numerical simulation. The finding suggests a pathway for pollutants (e.g., microplastics) to accumulate in the ocean interior. The transport pathway is driven by both large‐scale eddy dynamics and finescale turbulence, of which the latter plays a larger role in downwelling speed while the spatial sites of subduction are determined by the former. Key Points: Lagrangian float observations reveal downwelling across and trapping beneath the turbulent mixed layer at a submesoscale frontSimilar particle trajectories in convectively forced LES result from joint effects of boundary layer turbulence and submesoscale dynamicsDownwelling is due to finescale convective plumes while trapping and restratificationare due to the submesoscale circulation of the front [ABSTRACT FROM AUTHOR]

The influence of symmetry-breaking effects of ridge-type roughness on secondary currents in turbulent channel flow is investigated using direct numerical simulations. The ridges have triangular cross-section, which is systematically varied from isosceles to right-angled triangle, introducing an imbalance to the slopes of the ridges' lateral surfaces while the streamwise homogeneity of the surfaces is maintained. In all cases, secondary current vortices are produced, but asymmetric ridge cross-sections break the symmetry of these vortices. As a result of the asymmetry-induced misalignment and imbalance in the secondary current vortices, net spanwise flow emerges. The magnitude of the spanwise flow increases with the slope ratio of the ridge lateral surfaces and significantly modifies the mean flow topology, leading to the merging of critical points in the case of the right-angled triangular ridge shape. Within the cavities, the net spanwise flow is accompanied by a non-zero mean spanwise pressure gradient, while from the perspective of the outer flow, the scalene ridge surfaces have a similar effect as a wall that is slowly moving in the spanwise direction. Overall, the present results suggest the existence of a special type of Prandtl's secondary currents of the second kind, namely those that result in net spanwise flow. [ABSTRACT FROM AUTHOR]

In the present study, the response of a hypersonic turbulent boundary layer at an inflow of Ma∞ = 6 and Re∞ = 16·106 1/m to a smooth and rough surface along a sharp cone is examined. The model consisted of three segments with exchangeable parts to consider smooth and rough surfaces with a roughness topology of square bar elements with a nominal wavelength of four times the height of the elements. In selected regions of interest, the flow field was measured by particle image velocimetry (PIV) which enabled analysis of mean velocity fields and Reynolds stresses. Van Driest transformed smooth wall mean velocity profiles showed the expected incompressible behavior and compared well to previous investigations. A combination of an integral and fitting approach is discussed to enable inner scaling of the rough wall profiles, which showed the expected shift below the smooth wall profile. The smooth wall turbulence profiles from PIV agreed to artificially filtered DNS in case of the streamwise component. Turbulence profiles above the smooth and rough wall agreed to within measurement accuracies. Additionally, two−point correlations were used to investigate turbulent structures above the smooth and rough wall. Both, length scales and orientations of the correlations, showed high level of agreement between smooth and rough walls, with only differences close to the wall. Furthermore, uniform momentum zones could be identified with similar behavior along both smooth and rough walls. Information from turbulence data support outer layer similarity, whereas mean velocity profiles show an increase in Coles wake parameter for the rough wall data. This might be influenced by transitional roughness effects. [ABSTRACT FROM AUTHOR]

This study utilized the WRF model to investigate the track evolution and rapid intensification (RI) of Typhoon Doksuri (2023) as it moved across the Luzon Strait and through the South China Sea (SCS). The simulation results indicate that Doksuri has a smaller track sensitivity to the use of different physics schemes, while having a greater intensity sensitivity. Sensitivity numerical experiments with different physics schemes can well capture its northwestward movement in the first two days, but they predict less westward track deflection as the typhoon moves across the Luzon Strait and through the SCS. Moreover, all the experiments successfully simulated Doksuri's RI, albeit with quite different rates and a time lag of 12 h. Among different combinations of physics schemes, there exists an optimal set of cumulus parameterization and cloud microphysics schemes for track and intensity predictions. Doksuri's track changes as the typhoon moved across the Luzon Strait and through the SCS were influenced by the topographic effects of the terrain of the Philippines and Taiwan, to different extents. The track changes of Doksuri are explained by the wavenumber-one potential vorticity (PV) tendency budget from different physical processes, highlighting that the horizontal PV advection dominates the PV tendency throughout most of the simulation time due to the offset of vertical PV advection and differential diabatic heating. In addition, this study applies the extended Sawyer–Eliassen (SE) equation to compare the transverse circulations of the typhoon induced by various forcing sources. The SE solution indicates that radial inflow was largely driven in the lower-tropospheric vortex by strong diabatic heating, while being significantly enhanced in the lower boundary layer due to turbulent friction. All other physical forcing terms were relatively insignificant for the induced transverse circulation. The coordinated radial inflow at low levels may have led to the eyewall development in unbalanced dynamics. Intense diabatic heating thus was vital to the severe RI of Doksuri under a weak vertical wind shear. [ABSTRACT FROM AUTHOR]

Shark skin is composed of denticles, or complex scale-like features, which have been shown to reduce drag in turbulent and laminar flow. The denticle crown features undulating structures called riblets that interact with the turbulent boundary layer to reduce drag. Two mechanisms thought to contribute to the drag-reducing properties of riblets include the lifting of streamwise vortices and the hampering of spanwise vortex interactions to reduce crossflow, which could translate to similar flow mechanisms for denticles. Because of the varied morphologies of dermal denticles on different shark species, which also depend on body location, the impact of these denticle geometries on flow is of interest to the biology community, including related fields such as fluid mechanics and oceanography. This review highlights the past 15 years of manufacturing techniques and experimental measurements of drag over denticle-inspired surface structures, including real shark skin samples and engineered denticles and riblets. State-of-the-art additive manufacturing and other techniques are primarily limited to mm-length denticle scales, which have demonstrated drag reduction in lower flow speeds, under 1 m s−1. New manufacturing approaches can create sub-mm length denticles and nanotextured surface structures, which have achieved reported drag reductions of up to 31%. We synthesize results from the literature to illustrate the drag reduction properties of bioinspired denticles and riblets according to their geometry and flow conditions. Using these trends, we suggest design features and focus areas for future research, such as increasing studies of different denticle morphologies, hydrophobicity, antifouling properties, and acoustic noise reduction. Continued work on bioinspired denticles for drag reduction has wider implications in comparative biology and applications to design more energy-efficient, persistent vehicles for environmental monitoring. [ABSTRACT FROM AUTHOR]

The accurate prediction of the trailing-edge noise and the determination of its sources are essential to reduce fans and propellers noise. This noise component is due to the scattering of the turbulent boundary layer into acoustic waves by the trailing edge. In this paper, the noise emanating from a CD (Controlled-Diffusion) airfoil is simulated and computed via the hybrid methods of aeroacoustics. In these methods, the aerodynamic and acoustic fields are computed separately. The flow data are obtained using the in-house LES solver SFELES. ACTRAN acoustic solver has been used to solve the acoustics and to provide the near and far fields propagation via Lighthill's analogy. Curle's analogy is applied as well in its integral compact formulation which takes the presence of walls into account. Curle's formulation is applied proposing an approach where the volume and surface integrals have been implemented in SFELES to be calculated simultaneously with the flow in order to avoid the storage of noise sources which requires a huge space. In Lighthill's analogy, sources and near field maps show that the turbulent boundary layer and wake are the more efficient sources and the center of radiation is the trailing edge. The comparison of the numerical results with the experimental measurements, performed by Moreau and Roger and Moreau et al., shows an overall excellent agreement confirming the capability of SFELES (LES sources) combined with ACTRAN (Lighthill's analogy) to predict correctly the noise generated by turbulent flows around airfoils. The acoustic spectrum presents an overprediction up to 5 dB at the frequencies 300 Hz and 550 Hz and an underprediction about 5 dB at the frequencies 1100 Hz and 1750 Hz. The sound pressure level (SPL) obtained using the proposed approach of Curle's analogy matches very well the experimental results. Thus, Curle's analogy can be used to obtain a fast, approximated and acceptable results about the noise radiation of airfoils avoiding the storage of noise sources which requires a huge space and time. [ABSTRACT FROM AUTHOR]

The characteristics of uniform momentum zones (UMZs) and UMZ interfaces in turbulent channel flow (TCF) at friction Reynolds number ranging from 600 to 2000 are investigated. The focus is on the Reynolds number effect and the difference among the turbulent channel flow, turbulent pipe flow (TPF), and turbulent boundary layer (TBL). The UMZ is identified using the approach of probability density histograms, a discrete approximation of the probability density function. We find that the average number of UMZs increases with the Reynolds number, similar to the findings in the TBL. The first group of UMZ in the TCF, namely the quiescent core, has a thicker thickness at a higher Reynolds number. Therefore, the UMZ closer to the channel centre significantly influences high Reynolds number wall turbulence by occupying a large portion of the channel. With increasing Reynolds number, the intensity of spanwise vorticity at the UMZ interface increases, while the thickness of the local maximum decreases. The dominance of prograde vorticity at the interface also increases with the Reynolds number. Moreover, the contortion of the UMZ interface is significantly enhanced at high Reynolds numbers than at low Reynolds numbers in both the streamwise and spanwise directions. Furthermore, the number of UMZ for a given snapshot is related to the overall velocity fluctuations in the subregion, increasing with large-scale Q2 events and decreasing with large-scale Q4 events. Comparing the three flow configurations, UMZs show similarities in their trends, along with two notable differences. First, the TPF has more UMZs compared to the TCF and TBL. Second, the innermost group in the TPF is considerably thicker than that of the higher ranks, a pattern absent in the TCF and TBL. The present findings advance the understanding of UMZs in TCF and demonstrate the similarities and differences between the cases of TCF and TBL. [ABSTRACT FROM AUTHOR]

"Double-loop" theory was determined by deriving a correlation between turbulent fluctuating kinetic energy and water vapor volume fraction from the momentum equation, which further logically revealed the mystery of cavitation breaking around a three-dimensional symmetry hydrofoil based on the numerical results of large eddy simulation and Zwart-Gerber-Belamri cavitation model. When the second-order fluctuation moment V'xV'x and the streamwise velocity Vx are depleted, a vortex is generated, leading to alternating cavitation interface fluctuations. In one state, cavitation naturally breaks outward from the inner zone, triggering an up-and-down fluctuation in the normal velocity in the gap vortex and transferring external energy to the inner zone. In another state, cavitation collapse caused by a reentrant jet stagnates the reverse Vx so that V'xV'x tends to zero. It triggers a rise in an upward normal velocity in the attached vortex, creating an exchange of energy through the wake. The pressure implosion resulting from the Shrinkage of the "Like-Rayleigh-Plesset" cavity at the cavitation onset is stronger than the pressure implosion created by the vortex field during cavitation breaking. [ABSTRACT FROM AUTHOR]

Transient conjugate heat transfer measurements under varying temperature and velocity inlet boundary conditions at incompressible flow conditions were performed for flat plate and ribbed channel geometries. Therefrom, local adiabatic wall temperatures and heat transfer coefficients were determined. The data were analyzed using typical heat transfer correlations, e.g., Nu = C Re m Pr n , determining the local distributions of C and m. It is shown that they are closely linked. A relationship ln C = A − m B is observed, with A and B as modeling parameters. They could be related to parameters in log-law or power-law representations for turbulent boundary layer flows. The parameter m is shown to have a close link to local pressure gradients and, therewith, near wall streamlines as well as friction factor distributions. A normalization of the C parameter allows one to derive a Reynolds analogy factor and, therefrom, local wall shear stresses. [ABSTRACT FROM AUTHOR]

The efficient and stable application of periodic forcing for drag-reduction can help underwater vehicles operate at high speed for long durations and improve their energy-utilization efficiency. This study considers flow control around a body-of-revolution model subjected to periodic blowing or suction through annular slots. The focus is on the boundary-layer structure, properties, and drag of the control fluid under a wide range of body variables (size, free-flow velocity, slot area, and blowing/suction velocity) and control parameters (normalized periodic-forcing amplitude and relative slot sizes). Body variables differ in their effects on the drag-reduction rate, with the surface pressure pushing the model vehicle when S and v are higher than S0 and v0. In particular, the lowest pressure drag was −26.4 N with v increasing, and the maximum drag-reduction rate of total drag exceeded 135%. At a fixed Reynolds number, increasing the values of the control parameters leads to larger-scale unstable vortex rings downstream from the slots; the surface-velocity gradient is reduced, effectively lowering the drag. A simple model relating the periodic fluctuation of pressure drag to the body variables is developed through quantitative analysis and used to determine navigational stability. [ABSTRACT FROM AUTHOR]

A common denominator between conical-symmetry and conical shock interaction is the spanwise pressure gradient, which perform more non-uniformity and its interaction flow is more complicated than the spanwise-homogeneous planar shock wave. Recent advances in conical-symmetry and conical shock interactions with turbulent boundary layer are reviewed in specific areas: (i) quasi-conical swept interactions due to compression ramps and sharp fins, (ii) impinging conical shock wave with interactions of plate wall, (iii) laminar double cone interactions with consideration of real-gas effects. Substantial success has been achieved in describing the phenomena of the time averaged and instantaneous flow features and the low-frequency unsteadiness, including correlations and coherent structures in the separation bubble, through complementary experimental and numerical studies of swept shock interactions. All available observations are here scrutinized to infer underlying mechanisms of interactions in conical flow, and provide theoretical foundation and hints for fluidic control techniques. Comparison with high-fidelity direct numerical simulations is used to quantified the uncertainty of RANS turbulence models in complex interactions. Regarding heat transfer, extensive studies of hypersonic flow over double cone geometries have shown that those can be predicted with reasonable accuracy, even in the presence of high-temperature effects. [ABSTRACT FROM AUTHOR]

We recorded aerodynamic roughness and shear velocity along transects on and around mature crescent‐shaped barchan dunes of 4.5m $4.5\ \mathrm{m}$ and 27m $27\ \mathrm{m}$ height above the horizontal rock‐covered Qatar desert by fitting to the log‐law time‐averaged vertical velocity profiles acquired from triads of ultrasonic anemometers penetrating the inner turbulent boundary layer. Shear velocity first decreased, then recovered as air climbed on the dune, with a local maximum ahead of the crest as predicted by the Jackson and Hunt (1975, https://doi.org/10.1002/qj.49710143015) theory. Unlike flows over gentler bedforms without a slope discontinuity, an anomalous peak of shear velocity also arose on the dune centerline at the brink, which the theory attributed to skewness in the dune transect profile. The onset of aeolian transport produced a log‐law passing through the Bagnold (1941, https://doi.org/10.1007/978‐94‐009‐5682‐7) focal point. It was bracketed by noticeable hysteretic peaks in the correlation between wind speed and entrained sand flux. The dunes' rocky surroundings and topography produced an aerodynamic roughness at odds with the Nikuradse (1933, https://ntrs.nasa.gov/citations/19930093938) data for fully developed turbulent boundary layers. Large‐eddy numerical simulations illustrated the sensitivity of shear velocity to wide changes in aerodynamic roughness from desert floor to dune surface. Plain Language Summary: Wind friction is the engine that erodes sand dunes, relentlessly pushing them over roads, houses, and infrastructure. Our records of wind speed on crescent‐shaped mobile dunes challenge the conventional understanding of this process. Comparing field measurements and models, we show that the highest friction occurs where the gentle upward dune surface abruptly gives way to a steeper avalanching downward slope. Our data also reveals that the "aerodynamic roughness," a measure of wind friction on sand, is at odds with historical data meant for turbulent pipe flow. Because numerical simulations are used to predict flow over landforms that are inaccessible to detailed measurements, we validate them against data on a large dune. Our observations imply that, to achieve greater fidelity, simulations should subdivide the fluid neighborhood of the dune more finely, and revisit how they treat aerodynamic friction on its surface. Although our work involved large desert dunes, we expect these suggestions to apply more broadly to atmospheric, fluvial or submarine landforms that are surrounded by rougher terrain or that feature sudden changes in slope. Key Points: At the brink of a mature barchan, theory, simulations and field data reveal a peak of shear stress rising with dune skewnessAerodynamic roughness on dunes with rocky surroundings disagree with Nikuradse's (1933, https://ntrs.nasa.gov/citations/19930093938) data for fully developed turbulent boundary layersWe discuss challenges that large‐eddy simulations face to reproduce turbulent flow on field‐size dunes [ABSTRACT FROM AUTHOR]

Large‐eddy simulations (LES) of the Marine Atmospheric Boundary Layer (MABL) commonly utilize roughness length to parameterize wave effects on the sea surface. However, this method might not adequately capture the intricacies of wind‐wave interactions, especially in swell‐dominated conditions. In this study, we integrated a wave‐induced stress parameterization method into the open‐source LES code, PArallel Large‐eddy simulation Model (PALM), and assessed its performance under low wind conditions with varying wave ages and directions. Our method treats windsea effects as surface friction while employing an exponentially decaying profile to represent swell‐induced stress. We examined the model's sensitivity to wind and wave parameters and calibrated the wave damping rate values across various scenarios, leveraging data from wave‐phase‐resolved simulations and field measurements. The enhanced LES model with the proposed wave parameterization effectively reproduces wave‐influenced wind characteristics such as upward momentum flux, low‐level wind maximum, and negative wind gradients, showing good agreement with previous numerical and observational data. Moreover, our model accounts for wind‐wave misalignment scenarios by calculating each directional and spectral wave stress component individually and integrating them over the wave spectrum. We found that the swell‐induced stress is the dominant force in the wave boundary layer (WBL) with low wind speed, with its magnitude significantly varying with the wave direction relative to the wind. This variation influences the dynamic equilibrium within the WBL, modifying both wind shear and wind veer, and these effects persist up to the top of the boundary layer flow. Plain Language Summary: Ocean waves significantly impact the atmospheric boundary layer flow by modulating the momentum and energy exchange at the wind‐wave interface. Understanding these effects is crucial for an accurate modeling of the atmosphere and ocean systems. We incorporated the wave‐induced stress into the conventional logarithmic wind profile near the sea surface to improve the presentation of the wind‐wave interaction in the numerical simulation. The enhanced model can reproduce complex processes such as the wind‐wave misalignment effects, energy transfer from wave to wind field, and wave‐induced jet flows. Our findings show that the wave stress is the dominant force in the wave boundary layer and its influence on wind varies with their direction relative to the wind. Swells that follow the wind reduce friction and accelerate wind speed, while opposing swells increase drag, potentially causing slower or even reversed near‐surface winds. These wave impacts can extend to the top of the atmospheric boundary layer through turbulent mixing. Incorporating wave‐induced stress into large‐eddy simulations significantly enhances the representation of offshore wind fields and contributes to research on atmosphere‐ocean interactions and offshore meteorology. Key Points: Parameterization of wave‐induced stress is integrated into a large‐eddy simulation model and verified for the first timeThe wave growth/damping rate for the proposed parameterization is calibrated by comparison with numerical and observational dataWind‐wave misalignment impacts the wind and turbulence by modifying the dynamic equilibrium in the wave boundary layer [ABSTRACT FROM AUTHOR]

Roughness of the surface underlying the atmospheric boundary layer causes departures of the near-surface scalar and momentum transport in comparison with aerodynamically smooth surfaces. Here, we investigate the effect of $56\times 56$ homogeneously distributed roughness elements on bulk properties of a turbulent Ekman flow. Direct numerical simulation in combination with an immersed boundary method is performed for fully resolved, three-dimensional roughness elements. The packing density is approximately $10\,\%$ and the roughness elements have a mean height in wall units of $10 \lesssim H^+ \lesssim 40$. According to their roughness Reynolds numbers, the cases are transitionally rough, although the roughest case is on the verge of being fully rough. We derive the friction of velocity and of the passive scalar through vertical integration of the respective balances. Thereby, we quantify the enhancement of turbulent activity with increasing roughness height and find a scaling for the friction Reynolds number that is verified up to $Re_\tau \approx 2700$. The higher level of turbulent activity results in a deeper logarithmic layer for the rough cases and an increase of the near-surface wind veer in spite of higher $Re_\tau$. We estimate the von Kármán constant for the horizontal velocity $\kappa _{m}=0.42$ (offset $A=5.44$) and for the passive scalar $\kappa _{h}=0.35$ (offset $\mathbb {A}=4.2$). We find an accurate collapse of the data under the rough-wall scaling in the logarithmic layer, which also yields a scaling for the roughness parameters $z$ -nought for momentum ($z_{0{m}}$) and the passive scalar ($z_{0{h}}$). [ABSTRACT FROM AUTHOR]