Reynolds Stress Distributions Research Articles

Influence of a Meandering Channel on the Threshold of Sediment

River meanders and channel curvatures play a significant role in sediment motion, making it crucial to predict incipient sediment motion for effective river restoration projects. This study utilized an artificial intelligence method, multiple linear regression (MLR), to investigate the impact of channel curvature on sediment incipient motion at a 180-degree bend. We analyzed 42 velocity profiles for flow depths of 13, 15, and 17 cm in a laboratory flume. The results indicate that the velocity distribution was influenced by the sediment movement threshold conditions due to channel curvature, creating a distinct convex shape based on the bend’s position and flow characteristics. Reynolds stress distribution was concave in the upstream bend and convex in the downstream bend, underscoring the bend’s impact on incipient motion. Bed Reynolds stress was highest in the first half of the bend (0 to 90 degrees) and lowest in the second half (90 to 180 degrees). The critical Shields parameter at the bend was approximately 8–61% lower than the values suggested by the Shields diagram, decreasing from 0.042 at the beginning to 0.016 at the end of the bend. Furthermore, our findings suggest that the MLR method does not significantly enhance the understanding of sediment movement, highlighting the need for a more comprehensive physical rationale and an expanded dataset for studying sediment dynamics in curved channels.

Hydrodynamic characterization of the surfacing attitude of the manned deep-sea submersible

Hydrodynamic characteristics of manned deep-sea submersibles are important for optimizing design, increasing safety, and improving handling and control. To study the hydrodynamic characteristics of manned deep-sea submersibles in different surfacing attitudes, this paper carries out hydrodynamic tests on the scaled-down simplified model of “Jiaolong” submersible of 1:36. The drag, lift, and pitching moment of the manned deep-sea submersible in different surfacing attitudes are obtained by six-component sensors, and the time-averaged streamlines, turbulence kinetic energy, normal velocity and Reynolds stress distribution are obtained by photographing the wake flow field of the submersible using PIV particle image velocimetry. The results show that compared with the vertical surfacing attitude, the drag decreased by 38.2%, the lift magnitude rose by 98.9%, the pitching moment decreased by 20.8%, the vortex disappeared in the stern wake area, and the maximum turbulent kinetic energy decreased by 33.6% when surfacing at a 30° rake angle.

Effect of the yaw angle on the riblet performance

Direct numerical simulations are performed to investigate the effect of the yaw angle (α) on the drag-reduction performance by riblets in a turbulent channel flow. The yaw angles of α=0° and 90° correspond to the riblets parallel and perpendicular to the free-stream direction, respectively. It is well known that maximum drag reduction occurs at α=0°, and the drag-reduction performance by riblets is degraded with increasing α. We obtain the critical yaw angle below which drag reduction can be achieved and investigate the corresponding mechanism. From a parametric study, drag reductions are achieved for α<20°. As α increases, the form drag increases more rapidly than the skin-friction drag decreases, resulting in the increase in the total drag. At drag reducing and increasing yaw angles, the mean velocity profiles in wall units shift upward and downward, respectively, and the latter profile suggests that the drag-increasing riblet is similar to a rough surface. With increasing α, the wall-normal and spanwise velocity fluctuations and Reynolds shear stress increase, and stronger near-wall vortices are generated. By applying a scaling analysis to the momentum equation in the riblet-perpendicular direction and the FIK (Fukagata–Iwamoto–Kasagi) identity [Fukagata et al., “Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows,” Phys. Fluids 14, L73–L76 (2002); Peet and Sagaut, “Theoretical prediction of turbulent skin friction on geometrically complex surfaces,” Phys. Fluids 21, 105105 (2009); Bannier et al., “Riblet flow model based on an extended FIK identity,” Flow Turbul. Combust. 95, 351–376 (2015)] to the riblet-parallel direction, the total drag is expressed as a linear function of sin2α.

Ship Flow of the Ryuko-maru Calculated by the Reynolds Stress Model Using the Roughness Function at the Full Scale

The k-omega SST turbulence model is extensively employed in Reynolds-averaged Navier–Stokes (RANS)-based Computational Fluid Dynamics (CFD) calculations. However, the accuracy of the estimation of viscous resistance and companion flow distribution for full-sized vessels is not sufficient. This study conducted a computational analysis of the flow around the Ryuko-maru at model-scale and full-scale Reynolds numbers utilizing the Reynolds stress turbulence model (RSM). The obtained Reynolds stress distribution from the model-scale computation was compared against experimental measurements to assess the capability of the RSM. Furthermore, full-scale computations were performed, incorporating the influence of hull surface roughness, with the resulting wake distributions juxtaposed with the actual ship measurements. The full-scale calculation employed the sand-grain roughness function, and an optimal roughness length scale was determined by aligning the computed wake distribution with Ryuko-maru’s measured data. The results of this study will allow for the direct performance estimation of full-scale ships and contribute to the design technology of performance.

Experimental study of the Reynolds stress distribution and its self-similarity in a semi-confined impinging jet

This paper investigates the Reynolds stress distribution and its self-similarity of a semi-confined impinging jet using the two-dimensional particle image velocimetry (2D-PIV) technology. The confinement is achieved by placing a plate parallel to the impinging plate at the nozzle exit. Nine sets of measurements are conducted at three impingement distances (4, 5, and 6 times the nozzle diameter) and three Reynolds numbers (5084, 8179, and 13,926). The spatial distribution and the self-similarity of Reynolds stresses are obtained. The results indicate that high turbulence occurs mainly in the free jet shear layer and the wall jet shear layer. The variation of Reynolds stresses with impingement distances and Reynolds numbers are determined. The Reynolds stresses in the radial wall jet region do not exhibit self-similarity in the outer-scaled coordinates. Scaling parameters based on Reynolds stresses are proposed, and the Reynolds stress velocity scales decay by a power-law function in the self-similar region, while the Reynolds stress length scales increase linearly. The Reynolds stresses in the radial wall jet region show extremely strong self-similarity using the Reynolds stress scaling parameters normalization. Finally, the corresponding scaling laws for Reynolds stresses are obtained.

Analysis of improved digital filter inflow generation methods for compressible turbulent boundary layers

We propose several enhancements to improve the accuracy and performance of the digital filter turbulent inflow generation technique and assess their efficacy in the context of wall-resolved large-eddy simulations of a compressible turbulent boundary layer. Improvements of accuracy include a more realistic correlation function for the transversal directions, target length scales that vary with wall-distance, and a counter-intuitive approach that involves the suppression of streamwise velocity fluctuations at the inflow. For improving the computational performance, we propose to generate the inflow data in parallel in single precision and at a prescribed time interval based on the turbulence time scale, and not at every time-step of the simulation. Based on the results of 7 wall-resolved large-eddy simulations, we find that the new correlation functions and the considered performance improvements are beneficial and therefore desired. Suppressing streamwise velocity fluctuations at the inflow leads to the fastest relaxation of the pressure fluctuations; however, this approach increases the adaptation length defined in terms of compliance with the von Kármán integral equation. The adaptation length can be shortened by artificially increasing the wall-normal Reynolds stresses, thereby preserving the desired turbulence kinetic energy level. A detailed inspection of the Reynolds stress transport budgets reveals that the observed spurious spatial transients are largely driven by pressure-related terms. For instance, increased values of u′p′¯ are found throughout the computational domain when a physical Reynolds stress distribution is prescribed at the inflow. Therefore, efforts to enhance digital filter techniques should aim at modeling pressure fluctuations as well as their correlation with the velocity components.

Comparison of non-reactive flow fields in conventional swirl-air combustion and swirl distributed combustion

Flow field distribution in traditional swirl flow and simulated distributed reaction swirl flow configuration was examined at 3 kHz frequency using high-speed Particle Image Velocimetry (PIV). Non-reactive flow field conditions in distributed reaction zones were fostered from the conventional swirl flow case using the designed inlet mixture preparation with CO2 dilution of the inlet airstream that corresponded to the combustion case at a heat release intensity of 5.72 MW/m3-atm. High-speed imaging of flow fields was performed at a framing rate of 3000 frames/s using a Neodymium-doped Yttrium Lithium Fluoride (Nd:YLF) laser beam at 527 nm wavelength to capture the Mie scattering images of the flow field. Mean flow velocity obtained from the post-processing of raw PIV images showed higher momentum flux injection in the swirl distributed flow zone case as compared to the normal swirl flow case. Enhanced velocity fluctuations were also observed in both the axial and radial direction for the distributed swirl flow case resulting in higher turbulence for the examined condition. The distributed swirl flow case showed a prominent corner recirculation zone along with higher flow entrainment within the recirculation zone. Enhanced interaction of average streamlines with vortices was found in the swirl distributed flow case. Increased turbulent fluctuation in the two different spatial directions was responsible for higher Reynolds stress distribution (u′v′) in the distributed flow case, indicating improved mixing behavior at lower global oxygen concentrations from the carbon dioxide (CO2) dilution, and showed different length scales of turbulent eddies. Furthermore, the spatial local Damkӧhler number values for distributed flow case (for its implication in reacting case) were found to be less than unity. The results showed a shorter flow timescale than the corresponding chemical timescales in distributed reaction zone. These results help support substantiating the difference of reaction regimes on the Borghi diagram (plot of velocity fluctuation/ laminar flame speed versus the integral length scale/ flame thickness) between the distributed reaction zone (that possesses thickened reaction zone) and conventional swirl reaction flow regimes having corrugated flamelet zone.

Budgets of Reynolds stresses in film cooling with fan-shaped and cylindrical holes

The compressible budget terms in the transport equations of Reynolds stresses are examined from the large eddy simulation (LES) result of the film cooling. The capability of LES and the statistical post-processing procedure were first validated. The compressible Reynolds stress budget terms are then analyzed for both fan-shaped and cylindrical cooling films. The balance of all budget terms is shown. The effect of the blowing ratio on each budget term is examined. The mechanisms by which energy is extracted from the mean flow and distributed among the normal Reynolds stresses are highlighted. The sources of anisotropy in the Reynolds stress distributions are examined in detail, and their relation to the flow patterns of the mean and instantaneous flow is explored. The downstream development of the Reynolds stress budgets is studied, and it is shown that the jets of both fan-shaped and cylindrical films can be split into a near field and a far field with different properties. Far downstream of the cooling films, the Reynolds stress budgets near the wall present similarities with the Reynolds stress budgets in a boundary layer, while the Reynolds stress budgets further away from the wall resemble budgets in a free-shear flow. It is shown that the budgets of the Reynolds stress in the three-dimensional wall jets object of this study obey approximate similarity laws. These laws are based on easily obtained integral scales but need to be modified by suitable powers of the distance from the orifice producing the jet.

Comparison of Velocity and Reynolds Stress Distributions in a Straight Rectangular Channel with Submerged and Emergent Vegetation

Vegetation in rivers and streams plays an important role in preventing erosion and improving bank stability. Comparison between emergent vegetation (bank vegetation) and submerged vegetation, in terms of velocity and Reynolds stress distributions, for the same aspect ratio and flow discharge, has received limited attention in the literature. This study investigates the velocity and Reynolds stress, as well as the log law for submerged and emergent vegetation in a laboratory flume and compares the results for a different set up with different sediment size and aspect ratio but the same discharge. The results indicate that the influence of submerged vegetation on the secondary currents generation is less than emergent vegetation. In addition, the log law application is valid for both submerged and emergent vegetation cases, however, it is valid up to y/h = 0.75 for emergent vegetation (in which the vegetation cover in banks is partly out of the water) but up to y/h = 0.25 for vegetation bank. For both submerged and emergent vegetation, Reynolds stress distribution presents a convex form but with a different turning point. Comparison of the results with those in an artificial pool over submerged vegetation and low aspect ratio (<5) keeps almost the same form for velocity and Reynolds stress distributions but decreases the turning point in Reynolds stress distribution. For the submerged vegetation cover, the location of zero shear stress superposes that of maximum velocity, but for the emergent vegetation approaching the bank vegetation and shifting the maximum velocity towards the bed, the location of zero shear stress approaches the bed.

Wake flow control of a square cylinder via distributed jets over the rear porous surface

An active control method was used to attenuate the unsteady vortex shedding in the wake of a square cylinder. Distributed jets were applied to the rear porous surface at a Reynolds number of 1.07 × 104. The strength of the jets was measured using the dimensionless jet momentum coefficient CQ. The dynamic mode decomposition, instantaneous vorticity evolution, and time-averaged characteristics, e.g., turbulent kinetic energy, streamlines, velocity profiles and Reynolds stress distributions of the wake flow field were used to analyze the control effects. The results suggest that with increasing CQ, two jet vortices gradually form near the rear cylinder wall and compete with the vortex shedding. Consequently, the vortex shedding pattern became nearly symmetrical, whereas the jet vortices remained asymmetrical. The vortex shedding process was pushed downstream. The fluctuations in the wake, particularly near cylindrical walls, were significantly suppressed. Thus, this scheme was successful, indicating potential for drag reduction and energy efficiency. Better control effects were associated with higher CQ in the present study.

Flow field characteristics of a 127-pin rod bundle with hexagonal spacer grids

A determination of nominal flow phenomena in liquid metal fast reactor (LMFR) fuel assemblies is critical toward generation-IV reactor development. Axially positioned spacer grids are used to maintain the geometry of hexagonal rod bundles and simultaneously introduce perturbations in the flow. Three-dimensional (3D) printed asymmetric honeycomb spacer grids were installed in a prototypical 127-pin LMFR fuel assembly model to study complex fluid dynamics interactions induced by the spacer grid and rods. To characterize flow dynamics in this intricate geometry, time-resolved particle image velocimetry (TR-PIV) using the matched-index-of-refraction method was employed to obtain non-intrusive velocity measurements for three axial planes (one near-wall and two interior planes) at a Reynolds number of 6000. The statistical TR-PIV results compared sub-channel-dependent normalized time-averaged velocity, velocity fluctuations, Reynolds stress, vorticity, and turbulence kinetic energy distributions. TR-PIV line profiles characterized downstream spacer grid flow dynamics. Two-point spatial and spatial–temporal cross-correlation fields revealed local coherent structures and quantified convection velocities of traveling vortices. Spatial–temporal decomposition using dynamic mode decomposition (DMD) applied to the near-wall vorticity fields extracted turbulent structures and flow instabilities in the wake region of the spacer grid, along with their decay and frequency rates. Reduced-order velocity fields from DMD reconstructions identified the most energy-containing coherent structures persistent in the near-wall region. This research provides experimental data sets and analyses of flow behavior in rod bundles with hexagonal spacer grids. The results are critical toward LMFR design and geometry optimization, crucial for the validation of computational fluid dynamics and reduced-order flow models.

Decomposition of the wall-heat flux of compressible boundary layers

We use the method developed by Elnahhas and Johnson [“On the enhancement of boundary layer skin friction by turbulence: An angular momentum approach,” J. Fluid Mech. 940, A36 (2022)] and Xu et al. [“Decomposition of the skin-friction coefficient of compressible boundary layers,” Phys. Fluids 35, 035107 (2023)] for the decomposition of the skin-friction coefficient to integrate the mean temperature equation for high-Reynolds-number compressible boundary layers and arrive at an identity for the decomposition of the wall-heat flux. The physical interpretation of the identity and the limitations of this approach are discussed. We perform an integration on the mean temperature equation to obtain an identity that is the heat-transfer analog to the compressible von Kármán momentum integral equation for the skin-friction coefficient. This identity is applied to numerical data for laminar and turbulent compressible boundary layers, revealing that the mean-flow dissipation and production of turbulent kinetic energy given by the Favre–Reynolds stresses dominate the thermal-energy balance. The term related to the growth of the turbulent boundary layer opposes the wall cooling. Other identities for the wall-heat flux, inspired by the method of Fukagata et al. [“Contribution of Reynolds stress distribution to the skin friction in wall-bounded flows,” Phys. Fluids 14(11), L73–L76 (2002)], are studied numerically and by asymptotic methods. The terms of these identities depend spuriously on the upper integration bound because this bound is a mathematical quantity used in the derivation. When the bound is asymptotically large, the integral identities simplify to the heat-transfer analog to the von Kármán momentum equation. We also prove that an existing multiple-integration identity reduces to the definition of the wall-heat flux when the number of integrations is asymptotically large. No information about the wall-heat transfer is extracted because the impact of the integration number is nonphysical.

Effects of Channel Width Variations on Turbulent Flow Structures in the Presence of Three-Dimensional Pool-Riffle

Changes in the width of channels or rivers that have three-dimensional pool-riffle can affect the key parameters of river engineering and flow resistance. Understanding the effect of width changes on flow structures helps to control erosion and sedimentation in coarse-bed rivers and better design ecological restoration projects. The present study investigates the effect of the sequential pool-riffle and its interaction with the bank narrowness on the turbulent flow characteristics. For this purpose, an experimental study was conducted in variable and fixed width flume with an aspect ratio greater than five. The results showed that when the flow decelerates (entrance of the pool), the negative and low longitudinal velocities expand as the flow depth increases. From both sides of the central axis, longitudinal velocities decreased when entering the middle part of the pool and reduced the flow width. The changes in the maximum turbulence intensity values, from the central axis towards the channel bank, in the variable and fixed width modes had an increasing trend. In all three longitudinal directions along the flume, the maximum turbulence intensity and the maximum Reynolds shear stress in the variable width mode were larger than those in fixed one. Knowledge of the flow pattern along a variable width stream and better understanding of velocity and Reynolds stress distribution will help engineers to better estimate the controlling parameters in river restoration and improve hydraulic models’ performance.

Coupling of Volumetric Flowfield and Surface Effectiveness Measurements for Flat-Plate Film Cooling With Cylindrical Holes Using Tomographic Particle Image Velocimetry and Pressure-Sensitive Paint

AbstractIn this study, a series of time-averaged tomographic particle image velocimetry (TPIV) measurements are completed for simple angle, cylindrical film cooling holes on a flat plate with an inclination angle of 30 deg and a diameter of 4 mm. The flat plate is installed in a low-speed wind tunnel with a mainstream turbulence intensity of Tu=8% and an average velocity of 21.6 m/s. The blowing ratios of the jet range from M = 0.3 to M = 1.5, and the density ratio of the jet is fixed at DR = 1. The Reynolds number of the cooling jet varies from 1700 to 8400. The repeatability and accuracy of the tomographic particle imaging velocimetry (TPIV) measurements are compared against the results obtained by other flowfield measurement techniques. The results of the TPIV measurements are presented in velocity and vorticity iso-surface distributions in 3D, as well as 2D planar slices of velocity, vorticity, turbulence intensity, and Reynolds Stress distributions within the measurement volume. The characteristics of the flowfield are coupled with the detailed film cooling effectiveness distribution obtained using the pressure-sensitive paint (PSP) technique. An inverse relation among volumetric turbulence accumulation (TA) and surface film effectiveness, η, can be correlated.

Decomposition of the skin-friction coefficient of compressible boundary layers

We derive an integral formula for the skin-friction coefficient of compressible boundary layers by extending the formula of Elnahhas and Johnson [“On the enhancement of boundary layer skin friction by turbulence: An angular momentum approach,” J. Fluid Mech. 940, A36 (2022)] for incompressible boundary layers. The skin-friction coefficient is decomposed into the sum of the contributions of the laminar coefficient, the change of the dynamic viscosity with the temperature, the Favre–Reynolds stresses, and the mean flow. This decomposition is applied to numerical data for laminar and turbulent boundary layers, and the role of each term on the wall-shear stress is quantified. We also show that the threefold integration identity of Gomez et al. [“Contribution of Reynolds stress distribution to the skin friction in compressible turbulent channel flows,” Phys. Rev. E 79(3), 035301 (2009)] and the twofold integration identities of Wenzel et al. [“About the influences of compressibility, heat transfer and pressure gradients in compressible turbulent boundary layers,” J. Fluid Mech. 930, A1 (2022)] and Xu et al. [“Skin-friction and heat-transfer decompositions in hypersonic transitional and turbulent boundary layers,” J. Fluid Mech. 941, A4 (2022)] for turbulent boundary layers all simplify to the compressible von Kármán momentum integral equation when the upper limit of integration is asymptotically large. The dependence of these identities on the upper integration bound is studied. By using asymptotic methods, we prove that the multiple-integration identity of Wenzel et al. [“About the influences of compressibility, heat transfer and pressure gradients in compressible turbulent boundary layers,” J. Fluid Mech. 930, A1 (2022)] degenerates to the definition of the skin-friction coefficient when the number of integrations is asymptotically large.

Dynamics of multi-scale vortical structures behind a barchan dune

In this study, multi-scale vortical structures and vortex dynamics around a fixed-bed barchan dune have been experimentally investigated based on the particle image velocimetry technique, wavelet transform, and the finite-time Lyapunov exponent method. It was found that the dynamic characteristics of a dune wake are predominated by large- and intermediate-scale coherent structures. Quadrant analysis of the Reynolds-stress distribution for the corresponding wavelet components revealed that ejection and sweep events are the main contributors to the whole field, while outward and inward interaction events just dominate the region near the dune crest. In addition, the process of ejection and sweeping motions associated with the turbulent bursting sequence can also be captured by applying proper orthogonal decomposition analysis of the decomposed velocity field for the different wavelet components. Finally, a continuous development process of the different wavelet scale structures in the shear layer was visualized in the Lagrangian framework. The small-scale waves grow exponentially and gradually develop into larger-scale vortices when convected downstream until the reattachment point, and larger-scale vortices break into the smaller ones.

Turbulence Characteristics in Mild and Steep Entrance Slopes of Pool-Riffle Sequences

This study investigates the distributions of velocity and Reynolds stress in an artificial pool-riffle sequence for two entrance slopes of 5° and 20° in a laboratory and compares the results with a similar pool-riffle sequence in a gravel-bed river in central Iran. Quadrant analysis is applied to find out the contributions of different events on turbulent flow structures. At an entrance slope of 5°, the velocity distribution follows an exponential trend without flow separation, but for the entrance slope of 20°, the velocity distribution indicates a S-shaped pattern with flow separation. Variation of entrance slope does not influence the convex form of Reynolds stress (RS) distribution, but it influences the location of the maximum value of RS and the flow separation zone length. The results reveal that outward and ejection are the dominant events for both field and laboratory settings. The sweep displays a decreasing trend from near the bed toward the water surface, however, the outward contribution indicates an increasing trend from the bed toward the water surface. The agreement between laboratory and field results in velocity and Reynolds stress will help the river engineers to better manage complex fluvial processes. Bursting process events depend on the aspect ratio for the same entrance pool slope. The agreement in the results of velocity and Reynolds stress distributions and the dominant events of bursting process in the laboratory and field are encouraging for better restoration of rivers and decreasing the cost of projects.

Wave Boundary Layer at the Ice–Water Interface

On re-examining the problem of linear gravity waves in two layers of fluids with a viscous ice layer overlaying water of deep depth, we give a detailed analysis of the fluid velocities, velocity shear, and Reynolds stress associated with wave fluctuations in both the ice layer and the wave boundary layer just beneath it. For the turbulent wave boundary layer, water eddy viscosity is used. Comprehensive discussions on various aspects of the velocity fields are made in terms of a Reynolds number based on the ice-layer thickness and viscosity, and the ice-to-water viscosity ratio. Speculation of the wave-induced steady streaming is made based on the Reynolds stress distribution, offering a preliminary insight into the mean flows in both the ice layer and wave boundary layer in the water. For wave attenuation, the results using a typical ice viscosity and a reasonable water eddy viscosity show good agreement with data over the range of frequencies for field and lab waves, significantly outperforming those assuming an inviscid water.

A Mixing Process Influenced by Wall Jet-Induced Shock Waves in Supersonic Flow

With the development of hypersonic air-breathing propulsion systems, such as the supersonic combustion ramjet (Scramjet) and rocket-based combined cycle (RBCC) engines, the mixing process of supersonic airstream with fuel in the engine combustor has been drawing more and more attention. Due to the compressibility effects, the mixing process in a supersonic condition is significantly inhibited. In the present paper, the novel strategy of wall-jet induced shock waves (WJISW) is put forward to realize mixing enhancement. The interaction process between WJISW and the supersonic mixing layer is researched and the enhanced-mixing mechanism is revealed, employing large eddy simulation (LES) methods. The fine vortex structures of the flow field are well captured and presented, utilizing the numerical schlieren technique. Detailed visualization results indicate that WJISW in a low frequency condition can result in the ‘region action mode’ (RAM) never reported before. The drastic dynamic behaviors including growth, deformation, and distortion in the interaction region can undoubtedly promote the mixing of upper and lower streams. The Reynolds stress distributions along the streamwise x-direction suggest that more intense fluctuations can be achieved with a low frequency WJISW. Moreover, a sharp increase in mixing layer thickness can be realized in the interaction region. The dynamic mode decomposition (DMD) analysis results show that the mixing layer evolution process is dominated by the mode induced by WJISW, which leads to the coexistence of both large- and small-scale structures in the flow field. The entrainment process corresponding to large-scale vortices and the nibbling process corresponding to small-scale vortices can obviously promote mixing enhancement. It is suggested that the present proposed strategy is a good candidate for enhanced-mixing with application to Scramjet and RBCC combustors.

Interaction of Irregular Distribution of Submerged Rigid Vegetation and Flow within a Straight Pool

The interaction of bedform and vegetation cover significantly affects the turbulent flow parameters. To investigate this interaction, experiments were carried out in both a gravel-bed river and a laboratory flume. The purpose of field investigations was to find the slopes for both the entrance section and exit section of pools, the grain size of the bed material, and the flow condition. Based on field data, without considering any scaling analysis, a straight pool was constructed in a laboratory flume that was 0.9 m wide, 0.6 m deep, and 14 m long. The entry and exit slopes of the straight pool were 7.4° and 4°, respectively. The straight pool had vertical side walls and a gravel bed with a median grain size of d50 = 23.3 mm. Plastic cylinders planted in an irregular pattern in the channel beds were used to model rigid submerged vegetation. The velocity components were recorded by using an ADV at 200 Hz. In this study, the distributions of velocity, Reynolds stress, and TKE were investigated for flows in the presence of submerged rigid vegetation in channel beds with various area densities of vegetation. Results show that the shape of Reynolds stress distribution depends on the entrance and exit slopes of the pool, as well as the irregular distribution pattern of vegetated elements. Inside the pool with the presence of submerged vegetation in the channel bed, the maximum TKE appears above the bed surface with a larger distance depending on the area density of vegetation in the channel bed. However, the momentum exchange and turbulent energy are likely influenced by the secondary circulation of the flows associated with the irregular distribution of vegetated elements in the channel bed. Results of the quadrant analysis show that the momentum between the flow, bedform, and vegetated elements is mostly transferred by sweep and ejection events. The outward event tends to grow toward the water surface, reaching the highest amount near the water surface. At the pool entrance section where the flow is decelerating, the ejection event is dominant near the bed while the sweep event is strong near the water surface. With the decrease in the vegetation density in the pool bed, both the ejection and outward events become dominant.