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Our ability to reliably and efficiently predict complex high-Reynolds-number ( R e ) turbulent flows is essential for dealing with a large variety of problems of practical relevance. However, experiments as well as computational methods such as direct numerical simulation (DNS) and large eddy simulation (LES) face serious questions regarding their applicability to high R e turbulent flows. The most promising option to create reliable guidelines for experimental and computational studies is the use of analytical conclusions. An essential criterion for the reliability of such analytical conclusions is the inclusion of a physically plausible explanation of the asymptotic turbulence regime at infinite R e in consistency with observed physical requirements. Corresponding analytical results are reported here for three canonical wall-bounded turbulent flows: channel flow, pipe flow, and the zero-pressure gradient turbulent boundary layer. The asymptotic structure of the mean velocity and characteristic turbulence velocity, length, and time scales is analytically determined. In outer scaling, a stable asymptotic mean velocity distribution is found corresponding to a linear probability density function of mean velocities along the wall-normal direction, which is modified through wake effects. Turbulence tends to decay in this regime. In inner scaling, the mean velocity is governed by a universal log-law. Turbulence does survive in an infinitesimally thin layer very close to the wall. [ABSTRACT FROM AUTHOR]

The surface drag coefficient plays a crucial role in determining the momentum exchange between the Earth's surface and the atmosphere. Previous studies have observed a tendency for the drag coefficient to decrease at high wind speeds over land, primarily due to the inverse energy cascade. This study aims to examine the impacts of the reduced drag coefficient on the typhoon forecasting skills in terms of the track, the central pressure, and the 10‐m maximum wind speed. Drag reduction is approached through the logarithmic wind profile and a new formula that considers the reduction in roughness length. The results show that the track and the 10‐m maximum wind speed forecasts are improved, but these improvements are primarily evident over plain areas, as the typhoon structure is significantly disrupted by the topography over mountainous areas. Additionally, the reduced drag coefficient results in less dissipation of the typhoon's wind fields over land. As the wind speeds at coastal areas are enhanced compared with those in operational model, the changes in the Coriolis force are responsible for the track prediction improvement. These findings highlight the importance of parameterizing coherent processes in current numerical models. Plain Language Summary: Momentum transport efficiency directly determines the energy dissipation of weather systems. Recently the reduced momentum transport efficiency in typhoon high wind conditions over land surface has been documented. Our objective is to comprehend the impact of this reduced efficiency on numerical simulations of typhoons over land. The results show that considering the reduced momentum transport efficiency yields improved typhoon prediction skills in terms of the track and the 10‐m maximum wind speed, over plain areas. Due to the reduced drag coefficient causing the less dissipation of wind fields at the coastal areas, it is found that the changes in the Coriolis force contribute to the track prediction improvement. Key Points: The consideration of drag reduction over land improves the typhoon forecasting skills in terms of the track and the 10‐m maximum wind speedThe improvement of typhoon forecasts is mainly evident over plains, as the topography disrupts the typhoon structure over mountainous areasThe track prediction improvement can be attributed to the changes in Coriolis force [ABSTRACT FROM AUTHOR]

The present paper aims to evaluate the effect of emergent rigid vegetation density on the flow's turbulence structure and hydraulic parameters at the non-prismatic floodplains. The experiments were performed using the physical model of the asymmetric non-prismatic compound channel. The results show that the velocity distribution in the vegetation flow is more influenced by the drag force caused by the vegetation than by the bed shear stress and does not follow the law of logarithmic velocity distribution throughout the non-prismatic section. The intense velocity gradient at the interface of the main channel and the floodplain leads to the development of strong secondary currents, increased Reynolds shear stresses, apparent shear stresses and momentum exchange in this region. Vegetation also decreases mean kinetic energy in the floodplain and increases it in the main channel. The mean turbulence exchange coefficient for the non-prismatic compound channels without vegetation was 0.23 and for the divergent and convergent compound channels was 0.035 and 0.020, respectively. The comparison of the local drag coefficient results shows that the fluctuations of this parameter are greater in the divergent section than in the convergent section due to the strong secondary currents in the interface. HIGHLIGHTS This paper is the result of original research in the field of river engineering.; Its innovation is in the field of compound open channel and flow turbulence structure.; The Acoustic Doppler Velocimeter (ADV) profiler has been used with high accuracy to understand the flow structure.; Vegetation on flood plains and its effects on flow hydraulic is novel.; Comparison has been made between the effects of divergence and convergence on flow hydraulics.;

This work compares the threshold applied to the swirling strength as well as the vortex orientation statistics in the total and fluctuating velocity fields using direct numerical simulations of compressible and incompressible turbulent channel flows. It is concluded that the difference in the swirling strength for vortex identification is minimal in the logarithmic region such that these two situations share the same threshold. Regarding the vortex orientation, the inclination angle remains similar. However, as the wall-normal distance increases, a more and more obvious distinction is noticed for its orientation with respect to the spanwise (z) direction. It is mainly due to their intrinsic differences and attendant contrasting preference for the vortex identification, i.e., vortices rotating in the −z direction for the total velocity field and in the z direction for the fluctuating one. These observations function as a reasonable explanation for various remarks in previous studies.

Our ability to reliably and efficiently predict complex high-Reynolds-number (Re) turbulent flows is essential for dealing with a large variety of problems of practical relevance. However, experiments as well as computational methods such as direct numerical simulation (DNS) and large eddy simulation (LES) face serious questions regarding their applicability to high Re turbulent flows. The most promising option to create reliable guidelines for experimental and computational studies is the use of analytical conclusions. An essential criterion for the reliability of such analytical conclusions is the inclusion of a physically plausible explanation of the asymptotic turbulence regime at infinite Re in consistency with observed physical requirements. Corresponding analytical results are reported here for three canonical wall-bounded turbulent flows: channel flow, pipe flow, and the zero-pressure gradient turbulent boundary layer. The asymptotic structure of the mean velocity and characteristic turbulence velocity, length, and time scales is analytically determined. In outer scaling, a stable asymptotic mean velocity distribution is found corresponding to a linear probability density function of mean velocities along the wall-normal direction, which is modified through wake effects. Turbulence tends to decay in this regime. In inner scaling, the mean velocity is governed by a universal log-law. Turbulence does survive in an infinitesimally thin layer very close to the wall.

The vertical distribution of longitudinal velocity and the turbulence structure of open channel flows with natural‐like reedy emergent vegetation were investigated in this study. The natural‐like reedy vegetation consisted of a basal stem and flexible blades with shape and reconfiguration behaviour comparable with those found in actual riparian areas. The 3D velocity field was measured using an acoustic Doppler velocimeter. The vertically non‐uniform distribution of the frontal area could dramatically affect the exchange of mass and momentum between emergent vegetation and their surroundings. The average time velocity profile varied inversely with the frontal area. The high velocity gradient appearing at the middle section of the zone with frontal area varied considerably. The structures of vortices and vertical transport were analysed using spectral analysis and conditional sampling to identify the prominent vortical structures and their contribution to momentum transport. Spectral analysis suggested that the stem‐scale vortices set by stem spacing where the vertical frontal area had no clear change were dominant, and shear‐layer vortices where vertical frontal area dramatically changed cannot be disregarded. Quadrant analysis showed that inward and outward interactions were the dominant contributors to Reynolds stress, excluding the zones approaching the channel boundary and the vertical frontal area that dramatically changed. A four‐layer model was developed to predict the vertical distribution of longitudinal velocity with a newly established profile of local drag coefficient. The predicting results agreed well with the experimental data. [ABSTRACT FROM AUTHOR]

Scaling of turbulent wall-bounded flows is revealed in the gradient structures, for each of the Reynolds stress components.Within the “dissipation” structure, an asymmetrical order exists, whichwe can deploy to unify the scaling and transport dynamics within and across these flows. There are subtle differences in the outer boundary conditions between channel and flat-plate boundarylayer flows, which modify the turbulence structure far from the wall. The self-similarity exhibited in the gradient space and corresponding transport dynamics establish capabilities and encompassing knowledge of wall-bounded turbulent flows. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of turbulent boundary layers with roughness elements on a wall were performed to investigate the spatial characteristics of rough-wall turbulence and establish a corresponding prediction method. When the roughness height was larger than the buffer layer, the rough-wall turbulence exhibited different spatial characteristics of the turbulence structures from those pertaining to a smooth wall. A novel spatial scaling method was established to examine the universal spatial characteristics of turbulence structures in the presence and absence of wall roughness. Specifically, the viscous length was determined by modifying the definition of the friction velocity in the region in which the roughness influenced the flow. The rough-wall turbulence could be accurately predicted by performing large eddy simulations using the subgrid scale model with the filter width, which was modified using the proposed spatial scaling method. The proposed model can be used to design more efficient fluid machinery in engineering applications. [ABSTRACT FROM AUTHOR]

Ice cover formed on a river surface is a common natural phenomenon during the winter season in cold high latitude northern regions. For the ice‐covered river with compound cross‐section, the interaction of the turbulence caused by the ice cover and the channel bed bottom affects the transverse mass and momentum exchange between the main channel and floodplains. In this study, laboratory experiments are performed to investigate the turbulent flow of a compound channel with shore ice covered on the floodplains. Results show that the shore ice resistance restricts the development of the water flow and creates a relatively strong shear layer near the edge of the ice‐covered floodplain. The mean streamwise velocity in the main channel and on the ice‐covered floodplains shows an opposite variation pattern along with the longitudinal distance and finally reaches the longitudinal uniformity. The mixing layer bounded by the velocity inflection point consists of two layers that evolve downstream to their respective fully developed states. The velocity inflection point and strong transverse shear near the interface in the fully developed profile generate the Kelvin‐Helmholtz instability and horizontal coherent vortices. These coherent vortices induce quasi‐periodic velocity oscillations, while the dominant frequency of the vortical energy is determined through the power spectral analysis. Subsequently, quadrant analysis is used in ascertaining the mechanism for the lateral momentum exchange, which exhibits the governing contributions of sweeps and ejections within the vortex center. Finally, an eddy viscosity model is presented to investigate the transverse momentum exchange. The presented model is well validated through comparison with measurements, whereas the constants α and β appeared in the model need to be further investigated. Key Points: The longitudinal evolution of the streamwise velocity in a compound channel with shore ice is presented using laboratory experimentsThe shore ice induced resistance restricts the flow development and affects the width and momentum thickness of the mixing layerWhether the floodplain is covered by the shore ice, the transverse momentum exchange mechanism is similar [ABSTRACT FROM AUTHOR]

We performed direct numerical simulations of a turbulent channel flow in viscoelastic fluids to assess various spatial scaling methods and to find a scaling method that can visualise the spatial features regardless of fluid properties. The Giesekus and Oldroyd-B models were used as constitutive equations to model the viscoelastic fluids alongside a Newtonian fluid. The initial scaling method that can consider local viscosity variations demonstrated an increase in the size of the velocity streaks and a decrease in the number of quasi-streamwise vortices, with no change in the observed spatial features of the turbulence structures. After scaling using the ratio of the wall friction coefficients, we observed typical spatial features of the principal turbulence structures that coincided with those of the Newtonian fluid. The results indicate that the characteristics peculiar to the turbulent flow of viscoelastic fluids can be predicted in a manner similar to that for Newtonian fluids. [ABSTRACT FROM AUTHOR]

Scaling of turbulent wall-bounded flows is revealed in the gradient structures, for each of the Reynolds stress components. Within the “dissipation” structure, an asymmetrical order exists, which we can deploy to unify the scaling and transport dynamics within and across these flows. There are subtle differences in the outer boundary conditions between channel and flat-plate boundary-layer flows, which modify the turbulence structure far from the wall. The self-similarity exhibited in the gradient space and corresponding transport dynamics establish capabilities and encompassing knowledge of wall-bounded turbulent flows.

Winds in the atmospheric surface layer (ASL) over distinctive urban morphology are investigated by building-resolved large-eddy simulation (LES). The exponential law is applied to urban canopy layers (UCLs) unprecedentedly to parameterize vertical profiles of mean-wind-speed u¯ z and examine the influence of morphological factors. The skewness of streamwise velocity S u is peaked at the zero-plane displacement d (drag center) where flows decelerate mostly. The dynamics and intermittency in roughness sublayers (RSLs) are further contrasted. It helps determine the critical strength of the organized structures (ejection, Q2 and sweep Q4) in their contributions to the average momentum transport (i.e., 3 to 5). Two key factors of the local-scale dynamics are revealed - building heterogeneity and upstream giant wakes that could amplify turbulence kinetic energy (TKE) and energetic intermittent Q4 by different mechanisms. The former is conductive for large-eddy generation that promotes vertical fluctuating velocity w", stimulating intermittent, energetic Q2 and Q4. The latter, whose footprints are identified by the two-point correlation of streamwise velocity R uu with specific size and inclination, facilitates intermittent, fast streamwise fluctuating velocity u", forming vigorous Q4. Nevertheless, excessive planar density λ p (≈ 0.7) is detrimental to both transport processes. These findings contribute to the theoretical and empirical wall models of large-scale roughness that help urban planners and policymakers to improve air quality., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Chun-Ho LIU reports financial support was provided by Hong Kong (HK) Research Grants Council (RGC)., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Turbulence structures in flow over three types of wall roughness: sand-grain, cube roughness and a realistic, multi-scale turbine-blade roughness, are compared to structures observed in flow over a smooth wall in open channel flow at R e τ = 1000 , using direct numerical simulations. Two-point velocity correlations, length scales, inclination angles, and velocity spectra are analysed, and the applicability of Townsend's outer layer similarity hypothesis [Townsend. The structure of turbulent shear flow. Cambridge: Cambridge University Press; 1976] to these parameters was examined. Results from linear stochastic estimation suggest that, near the wall, the quasi-streamwise vortices observed in smooth-wall flow are present in the large-scale recessed regions of multi-scale roughness, whereas they are replaced by a pair of 'head-up, head-down' horseshoe structures in sandgrain and cube roughness, similar to those observed by Talapatra and Katz [Coherent structures in the inner part of a rough-wall channel flow resolved using holographic PIV. J Fluid Mech. 2012;711:161–170]. The configuration of conditional eddies near the wall suggests that the kinematical process of vortices differ for each kind of rough surface. Eddies over multiscale roughness are conjectured to obey a growth mechanism similar to those over smooth walls, while around the cube roughness the head-down horse-shoe vortices of Talapatra and Katz [Coherent structures in the inner part of a rough-wall channel flow resolved using holographic PIV. J Fluid Mech. 2012;711:161–170] may undergo solid-body rotation on top of a cube roughness on account of the strong shear layer, shortening the longitudinal extent of near-wall structure and promoting turbulence production during this process. These results illustrate the sensitivity of turbulence structure to the roughness texture, particularly within the roughness sublayer. [ABSTRACT FROM AUTHOR]

Turbulence structures in flow over three types of wall roughness: sand-grain, cube roughness and a realistic, multi-scale turbine-blade roughness, are compared to structures observed in flow over a smooth wall in open channel flow at R e τ = 1000 , using direct numerical simulations. Two-point velocity correlations, length scales, inclination angles, and velocity spectra are analysed, and the applicability of Townsend's outer layer similarity hypothesis [Townsend. The structure of turbulent shear flow. Cambridge: Cambridge University Press; 1976] to these parameters was examined. Results from linear stochastic estimation suggest that, near the wall, the quasi-streamwise vortices observed in smooth-wall flow are present in the large-scale recessed regions of multi-scale roughness, whereas they are replaced by a pair of 'head-up, head-down' horseshoe structures in sandgrain and cube roughness, similar to those observed by Talapatra and Katz [Coherent structures in the inner part of a rough-wall channel flow resolved using holographic PIV. J Fluid Mech. 2012;711:161–170]. The configuration of conditional eddies near the wall suggests that the kinematical process of vortices differ for each kind of rough surface. Eddies over multiscale roughness are conjectured to obey a growth mechanism similar to those over smooth walls, while around the cube roughness the head-down horse-shoe vortices of Talapatra and Katz [Coherent structures in the inner part of a rough-wall channel flow resolved using holographic PIV. J Fluid Mech. 2012;711:161–170] may undergo solid-body rotation on top of a cube roughness on account of the strong shear layer, shortening the longitudinal extent of near-wall structure and promoting turbulence production during this process. These results illustrate the sensitivity of turbulence structure to the roughness texture, particularly within the roughness sublayer. [ABSTRACT FROM AUTHOR]

As a milestone research in vortex identification (VI), the physical quantity of Liutex, including its forms of scalar, vector and tensor, was systematically explored and rigorously obtained as the third-generation (3G) of the vortex definition and identification methods distinguished from the first generation (1G) by vorticity and the second generation (2G) by the vortex identification (VI) criteria solely dependent on the velocity gradient tensor eigenvalues. Based on these findings, the vortex-core lines were abstracted from the well-defined Liutex, and for the first time, were automatically generated and massively visualized using computer. The distinctive characteristics of these vortex cores with the intriguing threshold-independency make them be the uniquely appropriate entity to represent and to depict the vortex structures in turbulence. The letter made use of the DNS data for the natural transition in a zero-pressure gradient flat-plate (Type-A turbulent boundary layer (TBL)) and the fully-developed turbulence in a square annular duct (Type-B TBL) to demonstrate the vortex structure represented by the vortex-core lines. The 3G VI approach based on the vortex-core lines is capable of profoundly uncovering the vortex natures. Moreover, the capability of automatically identifying the vortex cores and massively visualizing the large number of vortex-core behaviors in a transient way will enable the fluid-mechanics and other related-science communities to step into a new era to explore the intrinsic natures of the centennial puzzle of turbulence and other vortex-related phenomena in future. [ABSTRACT FROM AUTHOR]

When using point measurement for environmental or sediment laden flows, there is well-recognised risk for not having aligned measurements that causes misinterpretation of the measured velocity data. In reality, these kinds of mismeasurement mainly happen due to the misinterpretation of bed orientation caused by the complexity of its determination in natural flows, especially in bedload laden or rough bed flows. This study proposes a novel bed realignment method to improve the measured data benchmarking by three-dimensional (3D) bed profile orientation and implemented it into different sets of experimental data. More specifically, the effects of realignment on velocity profile and streamwise turbulence structure measurements were investigated. The proposed technique was tested against experimental data collected over a water-worked and an experimentally arranged well-packed beds. Different from the well-packed rough bed, the water-worked bed has been generated after long sediment transport and settling and hence can be used to verify the proposed bed-alignment technique thoroughly. During the flow analysis, the corrected velocity, turbulence intensity and Reynolds stress profiles were compared to the theoretical logarithmic law, exponential law and linear gravity (universal Reynolds stress distribution) profiles, respectively. It has been observed that the proposed method has improved the agreement of the measured velocity and turbulence structure data with their actual theoretical profiles, particularly in the near-bed region (where the ratio of the flow measurement vertical distance to the total water depth, z/h, is limited to ≤0.4).

A new particle subgrid scale model is proposed to consider the effect of gas flow on particle motions. Multiphase gas-particle turbulent flow is modeled by a second-order moment two-phase turbulence model involving a four-way coupling strategy to describe the interactions among gas-particle, particle-gas and particle-particle collisions. A large eddy simulation algorithm is developed to solve the hydrodynamic parameters of confined swirling and non-swirling particle-laden flow in coaxial chamber. Results show that predictions are well agreed with experimental data. Vortex structures and vortices distributions of gas and particle flow are quietly different. Coherent structures of swirling particle flows are not observed, and length of recirculation region is almost one quarter of non-swirling flow. Compared to developed flow region of non-swirling flow, standard deviation values of granular temperature at near entrance decreased by 5.5 times. Dominant frequencies of particle number density of non-swirling and swirling flows are 18 Hz and 10 Hz. Particle dispersions exhibit anisotropic characteristics, and their distributions are not in accordance with the normal distribution. The 2D turbulence model needs to be further improved due to the failure of describing vortex stretch in this job. • A new particle subgrid scale model is proposed to consider the effect of gas flow on particles. • Coherent structure and vortices of confined particle-laden flow are revealed by large eddy simulation. • Recirculation length of non-swirling flow is almost 4.0 times larger than that of swirling flow. • Standard deviations of granular temperature of non-swirling flow have decreased by 6.5 times. • Dominant frequencies of particle number density are 18 Hz and 10 Hz in non-swirling and swirling flow. [ABSTRACT FROM AUTHOR]

Abstract Separated and reattaching flows over sharp-leading-edge bluff bodies are important to investigate in order to improve our understanding of practical flows such as the case of low-rise buildings in the atmospheric boundary layer. In this study, Particle Image Velocimetry measurements of the separated-reattaching flows over the roof surface of a low-rise building model were taken for six different turbulent boundary layer conditions. The results were analyzed to understand how the incident turbulence affects the flow field of the separation bubbles above the low-rise building roof. The mean flow field above the roof-surface was found to be approximately similar across the six terrain conditions using the mean reattachment length in the streamwise direction and the maximum mean thickness of the separated shear layer in the vertical direction. However, the turbulence stresses are not similar which is attributed to high levels of initial turbulence kinetic energy in the separated shear layer. This leads to fundamental differences in the initial development of the separated flow when compared to flows with lower turbulence in the incident stream. The results indicate that the Kelvin-Helmholtz instability may be altered, or perhaps even suppressed, in the initial flow development region. This leads to substantially different turbulence statistics and characteristics within the separated shear layers. Highlights • PIV measurements were made in the separated flow above roof of a low-rise building. • Six upstream terrain conditions, from open water to suburban, were examined. • Streamwise Reynolds normal stresses have strong dependence on incident turbulence. • Vertical and shear stresses are similar over most of separated flow for all terrains. • There is indication that Kelvin-Helmholtz instability is suppressed at leading edge. [ABSTRACT FROM AUTHOR]

Clustering of coherent fine-scale eddies in a turbulent mixing layer has been analyzed by using direct numerical simulation (DNS) data at Re λ  ≃ 250. The coherent fine-scale eddies are defined based on the second invariant of the velocity gradient tensor and the vorticity vector. The clustering is evaluated by the number density of coherent fine-scale eddies, and the large-scale structures are extracted by low-pass filtered velocity fields. Conditional averaging shows that the large-scale enstrophy increases with the number density, whereas the large-scale strain rate stays around the average in the high number density region. The alignments of the vorticity vector and the eigenvectors of the large-scale strain rate tensor are conditioned by the number density or the strain rate magnitude. The eigenvectors and the vorticity vector indicate strong preferential alignments under the intense large-scale strain rate condition. On the other hand, those alignments become weak in the high number density regions. The inter-scale energy transfer between grid and subgrid scales is significantly correlated with the magnitude of the large-scale strain rate while there is no apparent correlation with the number density. [ABSTRACT FROM AUTHOR]

The results are presented from an experimental study to investigate three-dimensional turbulence structure profiles, including turbulence intensity and Reynolds stress, of different non-uniform open channel flows over smooth bed in subcritical flow regime. In the analysis, the uniform flow profiles have been used to compare with those of the non-uniform flows to investigate their time-averaged spatial flow turbulence structure characteristics. The measured non-uniform velocity profiles are used to verify the von Karman constant κ and to estimate sets of log-law integration constant Br and wake parameter П, where their findings are also compared with values from previous studies. From κ, Br and П findings, it has been found that the log-wake law can sufficiently represent the non-uniform flow in its non-modified form, and all κ, Br and П follow universal rules for different bed roughness conditions. The non-uniform flow experiments also show that both the turbulence intensity and Reynolds stress are governed well by exponential pressure gradient parameter β equations. Their exponential constants are described by quadratic functions in the investigated β range. Through this experimental study, it has been observed that the decelerating flow shows higher empirical constants, in both the turbulence intensity and Reynolds stress compared to the accelerating flow. The decelerating flow also has stronger dominance to determine the flow non-uniformity, because it presents higher Reynolds stress profile than uniform flow, whereas the accelerating flow does not. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of shock wave and supersonic turbulent boundary layer interaction in a 24° compression ramp with adiabatic and cold-wall temperatures are conducted. The wall temperature effects on turbulence structures and shock motions are investigated. The results are validated against previous experimental and numerical data. The effects of wall cooling on boundary layer characteristics are analysed. Statistical data show that wall cooling has a significant effect on the logarithmic region of mean velocity profile downstream the interaction region. Moreover, the influence of wall temperature on Reynolds stress anisotropy is mainly limited in the near-wall region and has little change on the outer layer. As the wall temperature decreases, the streamwise coherency of streaks increases. Based on the analysis of instantaneous Lamb vector divergence, the momentum transport between small-scale vortices on cold-wall condition is significantly enhanced. In addition, spectral analysis of wall pressure signals indicates that the location of peak of low-frequency energy shifts toward higher frequencies in cold case. Furthermore, the dynamic mode decomposition results reveal two characteristic modes, namely a low-frequency mode exhibiting the breathing motion of separation bubble and a high-frequency mode associated with the propagation of instability waves above separation bubble. The shape of dynamic modes is not sensitive to wall temperature. [ABSTRACT FROM PUBLISHER]

Turbulent flow in the axial direction along a cylinder representing a monofilament yarn was reproduced for a relatively wide range of radius Reynolds numbers using direct numerical simulations. In the simulation of the thinnest cylinder, the friction coefficient agreed with a previously published formula. A pair of high- and low-speed streaks was detected around even the thinnest cylinder, whereby it was confirmed that turbulence was sustained according to the analysed turbulence statistics. Even when only a single pair of streaks around the cylinder was detected, the characteristics of the turbulence structures, such as the mean streak spacing based on the viscous length scale, were the same as those in flows over a flat plate. It was found that the friction coefficient changes in a way that maintains the structural characteristics of the flow, consistent with the view that universal characteristics of turbulence structures exist. [ABSTRACT FROM PUBLISHER]

The turbulence structure in a shock wave/turbulent boundary layer interaction at incipient separation was investigated in order to get insight into turbulence generation and amplification mechanisms in such flow fields. The flow along a two-dimensional $$11.5^{\circ }$$ compression corner was studied experimentally at a Mach number of $$M=2.53$$ and with a momentum-thickness Reynolds number of $$\mathrm{Re}_{\theta }=5370$$ . From hot-wire boundary layer traverses and surface heat-flux density fluctuation measurements with the fast-response atomic layer thermopile, the turbulence structure and amplification was described. Space-time correlations of the mass-flux fluctuations across the boundary layer and the surface heat-flux density fluctuations were measured to further characterize the development of the turbulence structure across the interaction. The large-scale boundary layer structures are concealed by shock-related effects in the strongly disturbed shock-foot region. Shortly downstream, however, large-scale structures dominate the signal again, just as in the incoming flow. A mechanism explaining this behavior is suggested. [ABSTRACT FROM AUTHOR]

• Lateral distribution of velocity may have two dramatically increasing segments. • Lateral distribution of the Reynolds stress shows a bimodal characteristic. • Large-scale quasi-2-D coherent structures exist in the shear layer and mixing layer. • Ejections and sweeps events are dominant in the shear layer and mixing layer. Aquatic plants can alter flow structure, affect sediment transport and contaminants diffusion, and is noteworthy for channel restoration and navigation. In the present work, experiments are conducted to investigate the velocity and turbulence structures in a compound channel with varying coverage of riparian vegetation. Lateral profile of velocity may change from a traditional single sharply increasing segment to two sharply increasing segments, the shear layer (SL) and the mixing layer (ML), which are influenced by bed morphology and vegetation. Lateral profile of Reynolds stress is bimodal, with a crest value that decreases with increasing ratio of the width of nonvegetated floodplain to the bankfull height (B NV / h). Quasi-two-dimensional (Quasi-2-D) coherent structures can be found in both SL and ML when B NV / h is small according to power spectral density and temporal autocorrelation function, which have a significant influence on Reynolds stress. Brief and intense ejections and sweeps events dominate the contribution to Reynolds stress, and according to quadrant analysis, an ejections event of the SL may occur simultaneously with each event of the ML. [ABSTRACT FROM AUTHOR]

Turbulent plane jets are prototypical free shear flows on which fundamental research can expand the overall understanding of turbulent flows. In this study, flow characteristics of the turbulent plane jets are studied by means of direct numerical simulation (DNS) based on the finite difference method. The effects of the initial conditions at the jet exit (i.e. Reynolds number and velocity profile) on the spatial development of velocity and scalar fields are mainly investigated. Meanwhile the instantaneous coherent structures, based on the Q-criterion and the local minimum of pressure, are presented. The results suggest that the plane jet flow with the higher Re (=Ubd/ν : Ub is the bulk mean velocity at the jet exit, d is the slot width, and ν is the kinematic viscosity) and the parabola velocity profile at the jet exit will correspond to the shorter potential core, furthermore, the lower Re and the parabola velocity profile will strengthen the decay of the mean field in the process of flow transition to full turbulence. The flow self-similarity will be achieved at shorter streamwise distance from the jet exit for the higher Re and the parabola velocity profile at the jet exit. Moreover, the dependency of coherent structures on the initial Reynolds number is remarkable, especially in the flow development region, but the change of initial velocity profile at the jet exit hardly acts on the scale of coherent structures. It is also observed that the scalar field is more sensitive to changes of initial conditions than the velocity field.

To elucidate the unsteady characteristics of a nonlinear pressure loss generated in a restricted flow, pressure drops across and flow rates through an orifice were precisely measured in sinusoidally oscillating oil flows. It has turned out that the unsteady relationship between the nonlinear pressure loss and the flow rate describes a closed loop turning around along the characteristic curve of the steady-state one in the counter-clockwise direction, which indicates that the change of the nonlinear pressure loss is delayed behind that of the flow rate in an unsteady orifice flow. The phenomenon occurs probably because the structure of turbulence in an orifice jet flow, where a nonlinear pressure loss is generated by energy dissipation, has inertia against a change of the flow rate and cannot follow it without a time lag. A mathematical model incorporating a constant time lag into the steady-state nonlinear pressure loss was proposed to simulate the unsteady characteristics of the nonlinear pressure loss, successfully explaining the long-term question in acoustics why the reactive part of an acoustic orifice impedance decreases as the amplitude increases.