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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]

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]

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]

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]

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]

The article discusses the existence of weak solutions of the Kelvin–Voigt model in the context of the motion of water with added polymers. The model considers the delay in establishing equilibrium states due to relaxation time and internal reorganization processes. The study aims to prove the weak solvability of the initial–boundary value problem by introducing regular Lagrangian flows and defining weak solutions based on a priori estimates and the theory of the topological degree of contracting vector fields. The research was funded by the Russian Science Foundation and the author declares no conflicts of interest. [Extracted from the article]

This paper presents the results of an analysis of the influence of submesoscale internal gravity waves (IGWs), registered with the help of a sodar in the atmospheric boundary layer, on turbulence characteristics measured in the surface layer. Data obtained in rural areas in the Moscow region are used. Two types of IGWs are identified visually on sodar echograms: internal gravity-shear waves (IGSW) of the Kelvin–Helmholtz billow type and buoyancy waves (BWs). For 28 episodes of IGSWs and 10 episodes of BWs, turbulent kinetic energy, as well as heat and momentum fluxes, is calculated based on the data of eddy covariance measurements carried out with an ultrasonic thermometer–anemometer located on a mast 56 m above ground level. Variations of these characteristics accompanying the passage of wave trains are investigated, quantitative estimates are made, and the degree of influence of IGSWs and BWs on the turbulence is compared. [ABSTRACT FROM AUTHOR]

The effects of inlet Mach number on the unsteadiness of shock-boundary layer interactions (SBLIs) over curved surfaces are investigated for a supersonic turbine cascade using wall-resolved large eddy simulations. Three inlet Mach numbers, 1.85, 2.00, and 2.15 are considered at a chord-based Reynolds number 395,000. The curved walls of the airfoils impact the SBLIs due to the state of the incoming boundary layers and local pressure gradients. On the suction side, due to the convex wall, the boundary layer entering the SBLI evolves under a favorable pressure gradient and bulk dilatation. On the other hand, the concave wall on the pressure side imposes an adverse pressure gradient and bulk compression. Variations in the inlet Mach number induce different shock impingement locations, enhancing these effects. A detailed characterization of the suction side boundary layers indicates that a higher Mach number leads to larger shape factors, favoring separation and larger bubbles, while the reverse holds for the pressure side. A time-frequency analysis reveals the presence of intermittent events in the separated flow occurring predominantly at low-frequencies on the suction side and at mid-frequencies on the pressure side. Increasing the inlet Mach number leads to an increase in the time scales of the intermittent events on the suction side, which are associated with instants when high-speed streaks penetrate the bubble, causing local flow reattachment and bubble contractions. Instantaneous flow visualizations show the presence of streamwise vortices developing on the turbulent boundary layers on both airfoil sides and along the bubbles. These vortices influence the formation of the large-scale longitudinal structures in the boundary layers, affecting the mass imbalance inside the separation bubbles. [ABSTRACT FROM AUTHOR]

The author has presented results of numerical modeling of the influence of the angle of attack of a thin width-limited plate with a short chord on the velocity field of the internal region of a turbulent boundary layer (TBL) and skin friction. The computations were performed by the large eddy simulation (LES) method at the Reynolds number Reθ = 540 calculated from the velocity on the channel axis and the momentum thickness. Changes in the average and pulsation characteristics of the three-dimensional velocity field under the impact of the wake of the plate and of tip vortices as functions of the angle of attack of the plate in the range ±1° have been analyzed. It has been shown that a determining influence on the formation of the velocity field is exerted by the tip vortices whose vorticity is opposite with the positive and negative angles of attack. The mean value of the skin-friction coefficient decreases by 6% at the positive angle of attack and by 14% at the negative angle with respect to the value of this coefficient in an unperturbed TBL. [ABSTRACT FROM AUTHOR]

The rapid decarbonisation of the power generation and aviation sectors will require a move away from incremental development, exposing designers and researchers to the risk of unexpected results from uncertainty in boundary layer state. This problem already exists for parts developed with fully turbulent assumptions, but in novel design spaces the risk increases for both real components, where previous knowledge of similar designs may be inapplicable, and particularly in experimental testing of scaled models, where reducing Reynolds number can result in a drastic change in flow topology that skews the conclusions of a test. Computational methods struggle to reliably predict boundary layer state so experimental techniques for diagnosing boundary layer state are needed. Infrared thermography (IR) is a non-invasive technique that offers simple, fast visualisation of boundary layer state with no additional instrumentation. IR is relatively uncommon in the literature and there is minimal information available on the best practices for its use. This paper aims to encourage the adoption of IR as a diagnostic tool by demonstrating routes for optimisation and pointing out pitfalls to avoid. A low-order model is developed and used to predict how the signal-to-noise ratio (SNR) of an IR visualisation changes depending on the thermal design of the test piece. It is shown that in low-speed flows with active heating from the surface the SNR is maximised through a suitable choice of surface insulation, while in high-speed flows, where passive temperature differences are used, there is a crossover between heat transfer and recovery temperature effects that results in an SNR of zero, an effect that can arise in both steady-state and transient experiments. Experimental validation of the 1D model in both flow regimes is shown alongside two case studies on the use of IR in sub-scale testing where uncertainty in boundary layer state results in critical differences from the full-scale flow. [ABSTRACT FROM AUTHOR]

The streamwise effective slope ( ES x ), which is the mean absolute streamwise gradient of the roughness, is considered to be a key parameter in predicting the drag penalty of rough-wall turbulent boundary-layers. However, many real-world rough surfaces are multi-scaled. For such surfaces, ES x can be unbounded and its value can be dominated by scales of the topography that are invisible to the flow. To illustrate this, a campaign of drag balance measurements was conducted with a set of machined surfaces. A baseline surface is prepared with fine machining parameters. By coarsening the machining precision, artefacts called 'scallops' are introduced which increases the 'measured' ES x without changing other geometrical statistics. The drag of the 'scalloped' surfaces is higher than the baseline surface, with the relative drag increase scaling with the viscous scaled scallop height, but only when their height exceeds 2 ∼ 3 times the viscous length scale. Further, the drag of one of the 'scalloped' cases, even when the scallop height is O (10) viscous units, is seen to be much lower ( ∼ 28 %) than a case with matched ES x , but where the ES x results from larger scale features. These findings confirm that for multi-scaled surfaces, ES x may be a misleading topographical metric for drag (or k s ) prediction and one must consider which scales contribute to the average slope of a surface. [ABSTRACT FROM AUTHOR]

The disturbances generated by external turbulence in the shear layer on a flat plate suddenly set in motion are found. As the initial conditions, turbulent flow found using direct numerical simulation of the development of isotropic homogeneous turbulence is used. The solution obtained models laminar-turbulent transition in the boundary layer on a flat plate under relatively low free-stream turbulence when the transition is caused by Tollmien–Schlichting waves. The solution makes it possible to describe the process of generating various disturbances, namely, low-frequency streaky structures and instability waves and also their development in the initial stage of laminar-turbulent transition. Based on the processing of the obtained results, a simple model is proposed that relates the spectra of instability waves in the boundary layer and turbulent pulsations in free-stream flow. The dependences of the initial amplitude of instability waves and their critical amplification factors (N-factors) on the degree of flow turbulence are also obtained. [ABSTRACT FROM AUTHOR]

Among the different passive and active techniques for skin friction drag reduction for turbulent boundary layers, near wall forcing through moving walls is one of the most promising techniques at low Re τ . Fewer studies have looked at the mechanism at high Re τ , closer to flight conditions, largely because, in this regime, numerical simulations become harder and experiments more challenging. To that end, there is the need of a systematic study for different surface waves and flow conditions. This work introduces a new model using a kagome lattice and an experimental setup which combines simultaneous measurements of surface displacement and velocity in the boundary layer. Here the results from a shortened version of the model at Re τ ≈ 1000 are presented to demonstrate the capability of the experimental setup which is developed in view of further investigation at higher Reynolds number. [ABSTRACT FROM AUTHOR]

The present study considers uniform blowing in turbulent boundary layers as active flow control scheme for drag reduction on airfoils. The focus lies on the important question of how to quantify the drag reduction potential of this control scheme correctly. It is demonstrated that mass injection causes the body drag (the drag resulting from the stresses on the body) to differ from the wake survey drag (the momentum deficit in the wake of an airfoil), which is classically used in experiments as a surrogate for the former. This difference is related to the boundary layer control (BLC) penalty, an unavoidable drag portion which reflects the effort of a mass-injecting boundary layer control scheme. This is independent of how the control is implemented. With an integral momentum budget, we show that for the present control scheme, the wake survey drag contains the BLC penalty and is thus a measure for the inclusive drag of the airfoil, i.e. the one required to determine net drag reduction. The concept of the inclusive drag is extended also to boundary layers using the von Kàrmàn equation. This means that with mass injection the friction drag only is not sufficient to assess drag reduction also in canonical flows. Large Eddy Simulations and Reynolds-averaged Navier–Stokes simulations of the flow around airfoils are utilized to demonstrate the significance of this distinction for the scheme of uniform blowing. When the inclusive drag is properly accounted for, control scenarios previously considered to yield drag reduction actually show drag increase. [ABSTRACT FROM AUTHOR]

Chevron-shaped protrusions have been proposed in the literature for turbulent skin friction reduction. However, there is no consensus on the performance of this passive flow control technique; both an increase and a decrease in drag have been observed in previous studies. There is also no experimental evidence to support the working mechanism behind the drag reduction effect that has been postulated in the literature. In this study, direct force measurements were used to replicate experiments from the literature and, in addition, were used to test new array configurations to characterise the effect of individual design parameters on drag performance. A total of 23 different protrusion configurations were investigated in a turbulent boundary layer flow. In addition to the integral force measurements, particle image velocimetry was used to measure wall-parallel velocity fields in order to extract the statistical sizing and energy of the near-wall cycle turbulence. All configurations increased the drag between 2% and 10% for a friction Reynolds number of 1700. The drag reduction reported in the literature could not be replicated; however, these findings agreed with an experimental and numerical study that reported drag increase. The trend observed in the low-speed streak spacing from the PIV experiments was consistent with that observed in the balance data. Nevertheless, no evidence was found to support the working mechanism proposed in the literature. These results cast doubt on the proposed drag reduction potential of chevron-shaped protrusions. In the authors' view, the results of this study strengthen previous conclusions regarding their minor increase in drag. Future studies to further approach a consensus are proposed. [ABSTRACT FROM AUTHOR]

Miniature vortex generators (MVGs) have the potential to control turbulent boundary layers (TBLs). Analyzing the scaled turbulent structure of MVG TBLs in experiments typically requires indirect methods for accurate friction velocity U τ determination. The established methods for U τ determination need to be verified for non-generic TBLs, such as MVG TBLs, especially at near-wake stations. This study compares the performance of the five most common U τ -determination methods when applied to MVG TBLs. The methods are tuned in terms of their free parameters and fitting ranges using direct measurements of U τ from large eddy simulation data, and applied to a corresponding experiment to assess their accuracy. Based on the findings, an "inner" method, which is the Musker function with a drifting buffer layer incorporating a bump function, is recommended for MVG TBLs of the form investigated, as it provides a good estimation of U τ with uncertainty < 3 % for all streamwise stations. The method is applied to three further experimental tests with different flow conditions to study trends in U τ . The error of the drag variation is < 6 % , which indicates the method is reliable for MVG TBLs at high Reynolds numbers. [ABSTRACT FROM AUTHOR]

We present an experimental realisation of spatial spanwise forcing in a turbulent boundary layer flow, aimed at reducing the frictional drag. The forcing is achieved by a series of spanwise running belts, running in alternating spanwise direction, thereby generating a steady spatial square-wave forcing. Stereoscopic particle image velocimetry in the streamwise–wall-normal plane is used to investigate the impact of actuation on the flow in terms of turbulence statistics, drag performance characteristics, and spanwise velocity profiles, for a non-dimensional wavelength of λ x + = 397 . In line with reported numerical studies, we confirm that a significant flow control effect can be realised with this type of forcing. The scalar fields of the higher-order turbulence statistics show a strong attenuation of stresses and production of turbulence kinetic energy over the first belt already, followed by a more gradual decrease to a steady-state energy response over the second belt. The streamwise velocity in the near-wall region is reduced, indicative of a drag-reduced flow state. The profiles of the higher-order turbulence statistics are attenuated up to a wall-normal height of y + ≈ 100 , with a maximum streamwise stress reduction of 45% and a reduction of integral turbulence kinetic energy production of 39%, for a non-dimensional actuation amplitude of A + = 12.7 . An extension of the classical laminar Stokes layer theory is introduced, based on the linear superposition of Fourier modes, to describe the non-sinusoidal boundary condition that corresponds to the current case. The experimentally obtained spanwise velocity profiles show good agreement with this extended theoretical model. The drag reduction was estimated from a linear fit in the viscous sublayer in the range 2 ≤ y + ≤ 5 . The results are found to be in good qualitative agreement with the numerical implementations of Viotti et al. (Phys Fluids 21, 2009), matching the drag reduction trend with A + , and reaching a maximum of 20%. [ABSTRACT FROM AUTHOR]

It has long been known that under the right circumstances, inertial particles (such as sand, dust, pollen, or water droplets) settling through the atmospheric boundary layer can experience a net enhancement in their average settling velocity due to their inertia. Since this enhancement arises due to their interactions with the surrounding turbulence it must be modelled at coarse scales. Models for the enhanced settling velocity (or deposition) of the dispersed phase that find practical use in mesoscale weather models are often ad hoc or are built on phenomenological closure assumptions, meaning that the general deposition rate of particles is a key uncertainty in these models. Instead of taking a phenomenological approach, exact phase-space methods can be used to model the physical mechanisms responsible for the enhanced settling, and these individual mechanisms can be estimated or modelled to build a more general parameterization of the enhanced settling of inertial particles. In this work, we use direct numerical simulations (DNS) and phase-space methods as tools to evaluate the efficacy of phenomenological modeling approaches for the enhanced settling velocity of inertial particles for particles with varying friction Stokes numbers and settling velocity parameters. We use the DNS data to estimate profiles of a drift–diffusion based parameterization of the fluid velocity sampled by the particles, which is key for determining the settling velocity behaviour of particles with low to moderate Stokes number. We find that by increasing the settling velocity parameter at moderate friction Stokes number, the magnitude of preferential sweeping is modified, and this behaviour is explained by the drift component of the aforementioned parameterization. These profiles indicate that that when eddy-diffusivity-like closures are used to represent turbulent transport, empirical corrections used in phenomenological models may be potentially compensating for their incompleteness. Finally, we discuss opportunities for reinterpreting phenomenological approaches for use in coarse-scale weather models in terms of the exact phase-space approach. [ABSTRACT FROM AUTHOR]

The study analyzes the basic properties of the scalar wavenumber-frequency spectrum of turbulent pressures, representing the total energy of the wave components of the turbulent pressure field with a given wave vector modulus. Consideration of the scalar spectrum, which has independent applied significance, makes it possible to visualize the energy distribution of turbulent pressures in a wide range of frequencies and wave numbers. Based on well-known vector wave field models, relations are proposed for estimating the relative scalar spectrum. The degree and nature of the parametric influence of the Mach and Reynolds numbers are determined. [ABSTRACT FROM AUTHOR]

As a typical nonlinear wave, forward-leaning waves can be frequently encountered in the near-shore areas, which can impact coastal sediment transport significantly. Hence, it is of significance to describe the characteristics of the boundary layer beneath forward-leaning waves accurately, especially for the turbulent boundary layer. In this work, the linearized turbulent boundary layer model with a linear turbulent viscosity coefficient is applied, and the novel expression of the near-bed orbital velocity that has been worked out by the authors for forward-leaning waves of arbitrary forward-leaning degrees is further used to specify the free stream boundary condition of the bottom boundary layer. Then, a variable transformation is found so as to make the equation of the turbulent boundary layer model be solved analytically through a modified Bessel function. Consequently, an explicit analytical solution of the turbulent boundary layer beneath forward-leaning waves is derived by means of variable separation and variable transformation. The analytical solutions of the velocity profile and bottom shear stress of the turbulent boundary layer beneath forward-leaning waves are verified by comparing the present analytical results with typical experimental data available in the previous literature. [ABSTRACT FROM AUTHOR]

The validity of the usual laws of the wall for Favre mean values of the streamwise velocity component and temperature for non-reacting flows has been assessed for turbulent premixed flame-wall interaction using Direct Numerical Simulation (DNS) data. Two different DNS databases corresponding to friction velocity-based Reynolds number of 110 and 180 representing unsteady head-on quenching of statistically planar flames within turbulent boundary layers have been considered. The usual log-law based expressions for the Favre mean values of the streamwise velocity and temperature for the inertial layer have been found to be inadequate at capturing the corresponding variations obtained from DNS data. The underlying assumptions of constant shear stress and the equilibrium of production and dissipation of turbulent kinetic energy underpinning the derivation of the usual log-law for the mean streamwise velocity have been found to be rendered invalid within the usual inertial layer during flame-wall interaction for both cases considered here. The heat flux does not remain constant within the usual inertial layer, and the turbulent flux of temperature exhibits counter-gradient transport within the so-called inertial layer for the cases considered in this work. These render the assumptions behind the derivation of the usual log-law for temperature to be invalid for application to turbulent flame-wall interaction. It has been found that previously proposed empirical modifications to the existing laws of the wall, which account for density and kinematic viscosity variations with temperature, do not significantly improve the agreement with the corresponding DNS data in the inertial layer and the inaccurate approximations for the kinematic viscosity compensated wall normal distance and the density compensated streamwise velocity component contribute to this disagreement. The DNS data has been utilised here to propose new expressions for the kinematic viscosity compensated wall normal distance and the density compensated streamwise velocity component, which upon using in the empirically modified law of wall expressions have been demonstrated to provide reasonable agreement with DNS data. [ABSTRACT FROM AUTHOR]

A novel modeling paradigm, named as SED-SL-RBF, enhances the structural ensemble dynamics-stress length (SED-SL) model of wall-bounded turbulence using limited aerodynamic data from NACA airfoils. It constructs a multi-layer structure (MLS) of the turbulent boundary layer (BL) over the airfoils and uses machine learning to reconstruct model parameters from experimental data. This approach has been applied to turbulent flows over nine distinct NACA airfoil types, with a broad spectrum of Reynolds numbers and angles of attack. The model parameters, l0∞ and ybuf∞, are reconstructed using a radial basis function (RBF) neural network and applied to an SED-SL computational fluid dynamics (CFD) solver. This results in improved predictions of lift and drag coefficients for geometries and flow conditions previously calculated using the Menter shear stress transport (SST) turbulence model. The accuracy of the predictive lift coefficient CL exceeded 95%, while the error in the predictive drag coefficient CD was less than 6 counts. The neural network-augmented SED-SL model also demonstrated exceptional predictive accuracy for the pressure field. The MLS parameters for NACA 2421 exhibit similarities with angle of attack (AOA), which can be treated as functions of the Reynolds number. These findings suggested that the MLS parameters for NACA 2421 are independent of the AOA prior to stall. This similarity behavior provides a promising approach to model airfoil flows under various physical conditions. The broader vision is to integrate data to reveal innate model discrepancies in terms of model parameters, thereby extending the applicability of the SED-SL-RBF model to a wider range of flow scenarios. [ABSTRACT FROM AUTHOR]

Time series of large samples of instantaneous velocity fields with a 7.26δ0.99×1.18δ0.99 size in a turbulent boundary layer have been obtained by experimental measurement using time-resolved particle image velocimetry (TRPIV) with a four-camera array. The resultant experimental Reynolds number (Reτ) is 1046. The continuous spatial wavelet transform has been employed to convert the streamwise velocity fluctuations into multi-scale components in the streamwise direction at all wall-normal positions. The distribution of turbulence kinetic energy across all spatial scales and all wall-normal positions are determined from the multi-scale wavelet coefficients. The continuous temporal wavelet transform has also been utilized to resolve the temporal series of longitudinal velocity fluctuations into multi-scale temporal components at all wall-normal positions. The use of the multi-scale wavelet coefficients delivered the turbulent kinetic energy distribution across all temporal scales and all wall-normal positions. Moreover, the distribution of multi-scale flatness factors with spatial scales and normal position, before and after removal of multi-scale coherent structures, has been calculated. The turbulent structures are detected by zero-crossing the wavelet coefficients. The conditional sampling scheme and spatial phase-locked method have been employed to establish typical multi-scale structures at different wall-normal positions. The universality of the small-scale structures has been confirmed where the associated vorticity is characterized by an alternating positive and negative quadrupole along the longitudinal direction and wall-normal direction, while the streamlines can be considered a dynamic system composed of a saddle point and focal points. The scaling exponent Ϛ(p), before and after the removal of the coherent structures, has been calculated, confirming that the multi-scale coherent structures are responsible for the anomalous scaling law. [ABSTRACT FROM AUTHOR]

The statistical behaviour of the transport of reaction progress variable variance, c ″ 2 ~ , and non-dimensional temperature variance, T ″ 2 ~ , have been analysed using three-dimensional Direct Numerical Simulation (DNS) data of turbulent premixed flame-wall interaction with isothermal inert walls within turbulent boundary layers for (i) an unsteady head-on quenching of a statistically planar flame propagating across the boundary layer, and (ii) a statistically stationary oblique wall quenching of a V-flame. It has been found that the reaction rate contribution acts as a leading order source term to the transport of both reaction progress variable variance, c ″ 2 ~ , and non-dimensional temperature variance, T ″ 2 ~ , whereas the molecular dissipation term remains the leading order sink term for both configurations analysed here. With the progress of flame wall interaction, the magnitude of all the source terms for the transport equations of both reaction progress variable and non-dimensional temperature variances vanish in the near-wall region with the onset of flame quenching. However, the molecular dissipation term continues to act as a sink term. The performances of the existing models for turbulent scalar flux, reaction rate and scalar dissipation rate contributions have been assessed for both flame-wall interaction configurations based on a priori DNS analysis. The existing available models for scalar dissipation rate for temperature and the reaction rate contribution in the variance transport equations even with the previously proposed wall corrections do not adequately predict the behaviour in the near-wall region. Modifications have been suggested to the existing closure models for the scalar dissipation rate and the reaction rate contribution to the scalar variance transport equations to improve the predictions in the near-wall region. Furthermore, the recommended closures for the unclosed terms of both reaction progress variable variance, c ″ 2 ~ , and non-dimensional temperature variance, T ″ 2 ~ , are shown to accurately capture the corresponding variations obtained from DNS data for both near to and away from the wall. [ABSTRACT FROM AUTHOR]

In this paper, we present an overview of numerical simulation methods for the flow around typical underwater vehicles at high Reynolds numbers, which highlights the dominant flow structures in different regions of interest. This overview covers the forebody, midbody, stern, wake region, and appendages and summarizes flow phenomena, including laminar-to-turbulent transition, turbulent boundary layers, flow under the influence of curvatures, wake interactions, and all associated complex vortex structures. Furthermore, the current issues and challenges of capturing these flow structures are addressed. This overview provides a deep insight into the use of numerical simulation methods, including the Reynolds-averaged Navier–Stokes (RANS) method, large eddy simulation (LES) method, and the hybrid RANS/LES method, and evaluates their applicability in capturing detailed flow features. [ABSTRACT FROM AUTHOR]

The effects of vibrational nonequilibrium processes on turbulence-generated acoustic noise were investigated in a Mach-2.8 shock-tunnel facility. Gas mixtures with relevant absorption characteristics were first identified from measurements of attenuation coefficients using a heated acoustic chamber. In the shock-tunnel facility, CO 2 , N 2 , He, and He/CO 2 mixtures were injected into the lower boundary layer of the flow through a porous plate. A four-point Focused Laser Differential Interferometer (FLDI) positioned above the turbulent boundary layer was used to obtain simultaneous freestream measurements of entropic fluctuations propagating along streamlines and acoustic disturbances along Mach lines. Correlated fluctuations of Mach-line and streamline FLDI signal pairs were analyzed with a cross-power spectral density (CPSD). Compared to a boundary layer of pure air, the injection of 30%, 35%, and 40% He/CO 2 mixtures resulted in reduced fluctuation powers correlated along a Mach line in the frequency range of 200–800 kHz. Minimal reductions in fluctuation power were found along a streamline, indicating that the vibrationally active gas is affecting acoustic disturbances and not entropic disturbances. A mathematical disturbance model was created to examine the sensitivity of the measured attenuation to acoustic disturbances propagating from the lower boundary layer only. Disturbances were modeled as Gaussian wave packets of finite width, propagating in the streamwise direction and along Mach lines from the four walls of the test section. Modeling the acoustic disturbances from the lower boundary layer with a 15–30% amplitude reduction resulted in amplitude spectral densities and CPSDs that agreed well with the FLDI measurements. [ABSTRACT FROM AUTHOR]

An experimental investigation was carried out to study the fluid–structure interactions on a compliant panel subjected to an impinging shock wave and an incoming turbulent boundary layer. These experiments were aimed at understanding the time-averaged and unsteady characteristics of fluid–structure interaction at Mach 2. Two shock impingement locations on the panel (aspect ratio of 2.82), namely the central and three-fourths of the panel length, were tested. The shock boundary layer interactions on a rigid flat plate served as a baseline case. Measurements include shadowgraph and surface oil flow visualizations, panel deflections using a capacitance probe, cavity acoustics using a pressure sensor, surface pressures using discrete pressure sensors, and pressure-sensitive paints. Results show that the interaction on the compliant panel is relatively three-dimensional as compared to a rigid plate with a nominally two-dimensional interaction. Pressure fluctuations on the compliant panel are significantly higher than on the rigid plate, and the fluctuation spectra are multi-modal. Strong coupling at some frequencies was observed between the shock and the panel for both shock impingement locations. The present study suggests that for a compliant panel, the shape of pressure spectra is sensitive to the measurement location on the panel, the panel modifies the pressure distribution around the interaction, and the energy in dominant modes depends on the shock impingement location. [ABSTRACT FROM AUTHOR]

The results of shock-induced turbulent boundary layer separation simulations using scale-resolving methods with wall modelling (WMLES) are presented. A test case is chosen for which experimental data and reference large eddy simulation results are available. The IDDES method based on the SST turbulence model in the WMLES mode is compared with a similar method in which the explicit subgrid scale model is turned off. In the second approach, two schemes of the WENO family are considered. In each case, two grids of different densities are employed. The separation size, pressure, and friction coefficient distributions along the wall, and one-point pressure probability density functions at two reference points are compared. An unexpected result is demonstrated: in all the eddy-resolving simulations, the separation region's length turns out to be underestimated by 15–28%, and no significant reduction in the error with grid refinement is observed. Possible reasons for this phenomenon are discussed. [ABSTRACT FROM AUTHOR]

Within the framework of a three-dimensional boundary layer, the region of developed turbulent flow of an incompressible fluid under the action of a longitudinal unfavorable (positive) pressure gradient (hereinafter, referred to as UPG) in the diffuser is simulated. For a turbulent flow regime, the Reynolds-averaged Navier–Stokes equations in the boundary layer approximation are closed using a differential turbulence model based on the introduction of turbulent viscosity and the Kolmogorov–Prandtl hypotheses. Using the nonlinear least squares method (NLSM), correlation dependencies are found for the initial experimental data, which are further used in the numerical simulation. The calculated and experimental velocity profiles are compared in the region close to separation. Based on the numerical solutions of the turbulent boundary layer equations, the mechanisms of the interaction of the flows in the presence of transverse pressure gradients in an incompressible fluid are studied. [ABSTRACT FROM AUTHOR]

An original numerical framework is developed in the present research work in order to estimate the free field sound radiation from baffled structural panels subjected to nonhomogeneous turbulent boundary layer flow-induced excitation. A sequence of semi-analytical methods is used to estimate the nonhomogeneous turbulent boundary layer wall pressure spectrum, which is decomposed using the Cholesky technique to obtain the random wall pressure in the frequency domain. Structural panels are modeled using the finite element technique, and a coupled finite element-boundary element modeling technique is developed to estimate the sound power level radiated into the free field. Results are obtained for laminated composite structural panels with various fiber orientations. [ABSTRACT FROM AUTHOR]

Past work has shown that coupling can exist between atmospheric air flows at street scale (O(0.1 km)) and city scale (O(10 km)). It is generally impractical at present to develop high-fidelity urban simulations capable of capturing such effects. This limitation imposes a need to develop better parameterisations for meso-scale models but an information gap exists in that past work has generally focused on simplified urban geometries and assumed the buildings to be on flat ground. This study aimed to begin to address this gap in a systematic way by using the large eddy simulation method with synthetic turbulence inflow boundary conditions to simulate atmospheric air flows over the University of Southampton campus. Both flat and realistic terrains were simulated, including significant local terrain features, such as two valleys with a width about 50 m and a depth about average building height, and a step change of urban roughness height. The numerical data were processed to obtain averaged vertical profiles of time-averaged velocities and second order turbulence statistics. The flat terrain simulation was validated against high resolution particle image velocimetry data, and the impact of uncertainty in defining the turbulence intensity in the synthetic inflow method was assessed. The ratio between realistic and flat terrains of time-mean streamwise velocity at the same ground level height over a terrain crest location can be >2, while over a valley trough it can be <0.5. Further data analysis conclusively showed that the realistic terrain can have a considerable effect on global quantities, such as the depth of the spanwise-averaged internal boundary layer and spatially-averaged turbulent kinetic energy. These highlight the potential impact that local terrain features (O(0.1 km)) may have on near-field dispersion and the urban micro-climate. [ABSTRACT FROM AUTHOR]

The penalized wall function method for simulation of compressible near-wall turbulent flow regions in the numerical modeling of viscous compressible flows is developed. The method is formulated as a differential condition to match the outer and the wall function solutions and is based on a generalized characteristic-based volume penalization method to transfer shear stress from the outer region of the boundary layer to the wall. The method is modified to extend its applicability to turbulent flows with adverse pressure gradient, when separation and reattachment zones are formed, as well as to use computational meshes with coarser near-wall resolution. These advantages are demonstrated for two test problems, namely, the flow over a flat plate with zero and adverse pressure gradients. [ABSTRACT FROM AUTHOR]

Variations in the turbulent Schmidt and Prandtl numbers in the boundary layer on a wall with film cooling when helium is injected into xenon flow through a porous insert is investigated numerically using the three-parameter RANS turbulence model supplemented with transport equations for turbulent heat and mass transfer. The results obtained are compared with calculated data for constant turbulent Schmidt and Prandtl numbers. [ABSTRACT FROM AUTHOR]

The wall pressure fluctuation fields on fairing surfaces and in the surrounding turbulent boundary layer are experimentally investigated. The fairings were in the shape of semi-ellipsoids and were mounted on the wall of a subsonic low-noise wind tunnel. Their heights amounted to 25% of the oncoming boundary layer thickness. The main physical features of the flows under consideration are determined by means of the surface oil-flow visualization. The fluctuating wall pressure field is compared with the flow pattern. It is shown that the greatest pressure fluctuation levels are recorded in the nose region of the fairing surface. It is established that lengthening the model leads to a considerable reduction of the pressure fluctuation strength in the region of flow separation from the fairing surface. [ABSTRACT FROM AUTHOR]

Wall-modeled large eddy simulation (WMLES) is used to investigate turbulent fluctuations around an axisymmetric body of revolution. This study focuses on evaluating the ability of WMLES to predict the fluctuating flow over the axisymmetric hull and analyzing the evolution of turbulent fluctuations around the body. The geometry is the DARPA SUBOFF bare model and the Reynolds number is 1.2×107, based on the free-stream velocity and the length of the body. Near-wall flow structures and complex turbulent fluctuation fields are successfully captured. Time-averaged flow quantities, such as time-averaged pressure and skin-friction coefficients, and time-averaged velocity profiles on the stern, achieved great agreements between WMLES results and experimental data. Self-similarity of time-averaged velocity defects within a self-similar coordinate up to twelve diameters from the tail. A comprehensive analysis of second-order statistics in the mid-body, stern, and wake regions is condutced. Numerical results agree well with experimental data and previous wall-resolved large eddy simulation (WRLES) results about root mean square (rms) of radial and axial fluctuating velocities at the stern. Turbulent fluctuations including turbulent kinetic energy (TKE) and second-order velocity statistics are identified as dual peak behavior and non-self-similar over the wake length, consistent with previous findings in the literature. This assessment enhances the understanding of WMLES capabilities in capturing complex fluctuating flow around axisymmetric geometries. [ABSTRACT FROM AUTHOR]

A variational problem for a conditional extremum of the Mayer type on the construction of the distribution law of the normal component of the velocity of injection of a cooled gas into a turbulent boundary layer under a supersonic flow regime that provides the minimum value of the convective heat flux transmitted from the hot gas to the streamlined surface is considered. The isoperimetric condition is the power of the injection control system, calculated taking into account Darcy's filtration law. To solve the optimal problem, we use the first integral for the conjugate system with respect to Lagrange multipliers, obtained earlier by the authors using the classical theorem of E. Noether on invariant variational problems and the Lie-Ovsyannikov infinitesimal apparatus. A.A. Dorodnitsyn's method of generalized integral relations, which has proven itself well in calculating the characteristics of boundary layers under various flow regimes, is used for the calculations. A computational experiment conducted for the case of a flow around a sphere shows the effectiveness of the optimal control law found in comparison with a uniform injection: the gain in the value of the minimized functional is 16.8%. The novelty of the study lies in the development of a method for solving the variational problem using the first integral for the conjugate system, as well as the Dorodnitsyn's method of generalized integral relations. The scientific significance of the study lies in the development of the theory of an optimally controlled boundary layer under a turbulent flow regime in supersonic gas flows. The results obtained may be of interest in the design of systems for the active thermal protection of surfaces in high-velocity gas flows. [ABSTRACT FROM AUTHOR]

Measurements of surface heat flux via IR thermography and boundary layer density fluctuations using a variation of focused laser differential interferometry (FLDI) were performed concurrently on two models in a Mach 6 flow with freestream Reynolds number sweeps. For a flared cone geometry, the local surface heating and mean amplitude of turbulent wave packets are shown to increase in tandem. Furthermore, the combined data show that a significant increase in surface heat transfer coincides with the appearance of nonlinear interactions associated with the second-mode instability. For a tangent ogive model with roughness-dominated transition, a similar correlation between FLDI data and surface heat flux is observed. In both cases, the correlation is observed when using band-passed content near the characteristic frequency of the boundary layer, which suggests structures on the order of the boundary layer in scale dominate the surface heating process. In total, the present work suggests that analysis of wave packet amplitudes in FLDI can be directly tied to information about surface heating and identifying the eventual transition to fully turbulent flow. [ABSTRACT FROM AUTHOR]

Full-scale drag penalty predictions of flows over rough walls require surface roughness characterisation from laboratory experiments or numerical simulations. In either approach, it is necessary to determine the so-called equivalent sand-grain roughness height ( k s ). There are several steps involved in determining this aerodynamic roughness lengthscale, but its procedure typically includes a combination of measurement of wall-shear stress ( τ w ) using direct or indirect methods as well as analysis of velocity profiles. Indirect methods usually rely on assumptions made about flow and its scaling including the validity of universal outer-layer similarity. However, the implications of the underlying assumptions involved in full-scale drag prediction are unclear. In this work, we carry out wind tunnel measurements over a realistic rough surface (from a fouled ship-hull) to evaluate the impact of different methods with an emphasis on using the outer-layer similarity hypothesis for full-scale drag predictions. Wall-shear stress is measured using an in-house floating-element drag balance (DB), and velocity profiles are obtained using particle image velocimetry (PIV), allowing the evaluation of k s , and the associated wake parameters through several methods. The aerodynamic roughness parameters hence obtained are used for full-scale drag penalty calculations. It is observed that the predicted drag penalty can vary by over 15 % among the different methods highlighting the care that should be taken when employing such methods. [ABSTRACT FROM AUTHOR]

Systematic experimental investigations on the influence of deep gaps on the location of laminar–turbulent transition are reported. The tests were conducted in the Cryogenic Ludwieg–Tube Göttingen, a blow-down wind tunnel with good flow quality, at eight different unit Reynolds numbers ranging from R e 1 = 17.5 × 10 6 m - 1 to 80 × 10 6 m - 1 , three Mach numbers, M = 0.35 , 0.50 and 0.65, and various pressure gradients. A flat-plate configuration, the extended two-dimensional wind tunnel model PaLASTra was modified in order to allow the installation of gaps with nominal widths of 30 μ m, 100 μ m and 200 μ m and a depth of d = 9 mm . A maximum Reynolds number based on the gap width R e w = R e 1 · w ≈ 16 , 000 was reached. Transition Reynolds numbers ranging from R e tr ≈ 1 × 106 to 11 × 106 were measured, as a function of gap width, pressure gradient and Mach and Reynolds number. This systematic investigation facilitates a linear approximation of R e tr dependent on the boundary layer shape factor H 12 for various flow conditions and gap widths. It was therefore possible to conduct an investigation of R e tr depending on R e 1 and the relative change of the transition location depending on the gap width w. Incompressible linear stability analysis was used to calculate amplification rates of Tollmien–Schlichting waves and determine critical N-factors by correlation with measured transition locations. The change in the critical N-factor Δ N by installation of the gap is investigated as a function of w and R e w . It was found that a gap width of 30 μ m reduces the critical N-factors in the range of Δ N ≈ 0.5 ± 0.25 , while gap widths of 100 μ m and 200 μ m reduce the critical N-factor in the range of Δ N ≈ 1.5 ± 1 . Interestingly, an increase in gap width from 100 to 200 μ m was not found to induce smaller transition Reynolds numbers or reduced N-factors, which might be due to resonance effects. [ABSTRACT FROM AUTHOR]

A new formula is derived for the random error of sample central moments from correlated data which does not assume an underlying distribution and is accurate to leading order in the number of sample elements. Central moments, being important quantities in turbulence research, require accurate error estimation. Many approaches have been followed in the past for estimating the random errors of central moments from correlated data. These include: simple extensions of the formula for independent data, using the formula for the random error of generic averages, assuming an underlying normal distribution, and using block bootstraps. All of these approaches are compared with the present formula using datasets from a turbulent boundary layer, freestream grid turbulence, and a turbulent round jet. For even-order sample central moments, many of the existing approaches perform well with differences of <15%. However, for odd-order sample central moments, only the block bootstrap methodology performs similarly well. For the same sample central moments, the other methods differ by as much as 200–1000%. [ABSTRACT FROM AUTHOR]

The ablation of thermal protection systems in hypersonic vehicles would generate randomly distributed surface roughness, which prominently influences laminar-turbulent boundary-layer transition. While discrete and regular-shaped roughness elements have been extensively studied, the effect of distributed roughness on hypersonic boundary-layer stability and transition is poorly understood. In the present study, experiments were performed on a 7-degree half-angle sharp cone with sandpaper-type roughness in a Mach 6 Ludwieg tube. The transition onset locations were obtained by infrared thermography. Single/two-point focused laser differential interferometer was utilized to characterize the instability waves over distributed roughness along both the streamwise and wall-normal directions within the laminar, transitional and turbulent regions. On distributed roughness, the transition onset location moves forward, and the second mode was still the dominant instability. It exhibits lower frequency, faster growth, and earlier decay compared with the smooth surface. Linear stability calculations were carried out to assist with experiments. Furthermore, bispectral analysis indicates complicated nonlinear phase-locked energy exchanges within the boundary layer. It is shown that the earlier saturation and faster decay of the second mode on distributed roughness is associated with intensified nonlinear interactions. [ABSTRACT FROM AUTHOR]

The three-dimensional computer model based on the Reynolds-averaged Navier–Stokes equations together with the Baldwin–Lomax and Prandtl algebraic models of turbulent mixing is used to calculate radiative–convective heat transfer on the surface of the MSL descent space vehicle. The intensification of convective heat transfer on the leeward side of the frontal aerodynamic shield and the superiority of the radiative heat flux density over the convective one on the rear surface are demonstrated. The calculations are performed using the physically and chemically nonequilibrium gas model. The results are compared with the calculations based on other computational models and the flight data on the heat load to the descent vehicle obtained during MSL descent in the dense layers of the Martian atmosphere. [ABSTRACT FROM AUTHOR]

Using direct numerical simulation, a problem of the interaction of a Mach wave with a boundary layer on a flat plate, streamlined by a supersonic perfect gas flow at the Mach number M = 2.5 is considered. The influence of the intensity of the incident Mach wave on the laminar-turbulent transition (LTT) is studied. It is shown that the incidence of a Mach wave with an amplitude of 5%, simulating the relative thickness of the roughness on the side wall of the wind tunnel, on the boundary layer leads to the formation of a turbulent wedge in the boundary layer on a flat plate. [ABSTRACT FROM AUTHOR]

Eddies affect the motion of particles within the turbulence boundary layer on different length and velocity scales. Attached eddies, which extend to the wall, are primarily responsible for most of the turbulent kinetic energy and Reynolds shear stresses in the turbulence boundary layer. The attached eddies can be devided into two types: type-A eddies are directly attached to the wall, while type-B eddies are not. The physical characteristics of type-A and type-B eddies are quite distinct, as such their individual effect on sediment particle movement should be more precisely considered in sediment transport modeling. This study combines the attached eddies hypothesis with the stochastic diffusion particle tracking model (SD-PTM) to simulate suspended sediment particle transport in a turbulence boundary layer under the influence of attached eddies. Sediment particles in turbulent flow not only move with the mean drift but also diffuse randomly due to turbulence. Hence, the trajectory of such particles is treated as governed by a stochastic process in this study. The proposed model, incorporating the detailed properties of type-A and type-B eddies, presents an eddy-constrained random walk (ECRW) that describes the circular motion of flow in turbulence. The proposed model is verified using the concentration profile obtained from available data. The ensemble means and variances of particle trajectories can be obtained. Anomalous diffusion of sediment particles under the effect of attached eddies and the contributions of type-A and type-B eddies to sediment particle movement are discussed. [ABSTRACT FROM AUTHOR]

Turbulent flow over urban-like roughness has been numerically studied for various purposes, such as the clarification of turbulent characteristics over rough walls, visualization of turbulent structures around block arrays, and evaluation of urban ventilation and pedestrian winds. In such simulations, a portion of the developing boundary layer is extracted as a numerical domain, assuming periodic boundary conditions in the horizontal direction to reproduce laterally homogeneous rough surfaces. However, the conditions required to drive the airflow by an artificial momentum source uniquely determine the turbulent statistics, which are different from those in developing boundary layers. Therefore, this study presents a new approach for driving the airflow over urban-like block arrays. The new method is based on spatially averaged Navier–Stokes equations to prove the necessity of height-dependent momentum provision. The turbulent flow over a cubical-block array is determined using large-eddy simulations driven by four different momentum sources. Regardless of the driving force, the vertical profiles of the streamwise velocity components are identical. In contrast, slight differences in the vertical Reynolds stress, variances in the velocity components, and turbulence kinetic energy are appropriately reproduced in the new approach. In addition, the budget equations of the turbulent statistics reveal that a change in the vertical Reynolds stress affects the energy production and its redistribution into variance components. The proposed method can contribute to the reproduction of a realistic turbulent flow and provide a theoretical understanding of the momentum provision. [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]

Rotation in hurricane flows can significantly impact the dynamics and structure of the turbulent boundary layer. Despite this unique feature of hurricane boundary layers, the current planetary boundary layer (PBL) schemes in weather models are neither specifically designed nor comprehensively tested for intensifying hurricane flows. The objective of this paper is to bridge this knowledge gap by characterizing the role of vertical diffusion depth and magnitude in simulated hurricane intensity, size, and track. To this end, five major hurricane cases undergoing an intensification period are simulated using two widely used local and non-local PBL schemes in Weather Research and Forecasting (WRF) model. In total, eighty WRF simulations are conducted by varying the grid resolution, PBL scheme, eddy diffusivity depth and magnitude, and PBL height. By decreasing the existing vertical diffusion depth and magnitude, on average, ~ 38 and ~ 24% improvements in hurricane intensity forecasts were obtained compared to the default models. Hence, the results indicate that the current PBL schemes in WRF are overly diffusive for simulating major hurricanes since they do not account for turbulence suppression effects in rotating hurricane flows. The paper yields new insights into the role of vertical diffusion in simulated hurricane dynamics and provides some guidance to enhance the PBL schemes of NWPs for improved hurricane forecasts. [ABSTRACT FROM AUTHOR]

Vertical turbulent and convective fluxes and corresponding velocities of the dust aerosol particle uplift under conditions of quasiperiodic emission have been determined by the measurements of the dust aerosol particle concentrations and the turbulent fluctuations of the wind speed components in a desertified area in the Astrakhan region. It is shown that convective fluxes are caused by the combined action of both regular upward motions (updrafts) in the convective coherent structure of the atmospheric boundary layer and turbulent diffusion. In the surface layer of the atmosphere at heights from 0.3 to 6.0 m, the vertical profiles of the dust aerosol concentration are approximated by power functions. It is demonstrated that the vertical turbulent and convective fluxes of dust aerosol can be estimated by the gradient method using the scales of turbulent and convective vertical motions in the atmospheric boundary layer. [ABSTRACT FROM AUTHOR]

The work is devoted to the development of a new experimental method for entering controlled disturbances with a given frequency-wave structure into a supersonic boundary layer. Experimental data on the formation of disturbances from two pulsed sources (pulsed glow discharge) in the laminar flat-plate boundary layer at the Mach number equal to 2 are given. The experiments were carried out in the T-325 wind tunnel of ITAM SB RAS. The localized sources were spaced out at the same distance from the leading edge of the plate at 6 mm from each other spanwise. Flow pulsations were measured using a hot-wire probe of constant temperature anemometer, the signal was recorded synchronously with ignition of the discharges. This made it possible to distinguish the pulsations from the discharges against the background of random uncontrolled "natural" pulsations of the boundary layer. The spatial-temporal and frequency-wave structure of the generated disturbances from a single or two discharges, operating synchronously or with a time delay, are analyzed. It is found that the maximum difference in the structure of disturbances from one and two sources is observed in the central region, while at the side boundaries of the disturbances, the pulsations are close in all considered cases. In the wave spectra of disturbances from two discharges, nodes and antinodes are formed. Their position is determined by the distance between the sources and the time delay in their operation. [ABSTRACT FROM AUTHOR]