Sweep Events Research Articles

Characteristics of Shallow Flows in a Vegetated Pool—An Experimental Study

Pools are often observed in gravel-bed rivers, together with the presence of vegetation patches. In the present study, a conceptual model of a gradual varied flow with both convective deceleration and acceleration flow sections has been constructed in a flume to study turbulent flow structures. Vegetation patches with extended canopies were planted in the pool sections in order to increase the thickness of the boundary layer inside the inner zone. The effects of different flows (namely decelerating, uniform and accelerating flows) along an artificial pool on flow velocity, shear stress and bursting events have been investigated. In addition, due to the occurrence of secondary currents in shallow streams, the characteristics of turbulent shallow flow have been investigated along two axes that are parallel to the sidewall of the flume. The results showed that the application of the log law should be used with care to estimate shear velocity along a pool with a vegetated bed. The presence of a vegetation patch causes an increase in Reynolds shear stress, especially along the entrance section of the pool where the flow decelerates. The results of the quadrant analysis reveal that the sweep and ejection events have the most dominant influence over the vegetation patch in the pool; however, the contributions of outward and inward events increase near the bed, especially in the entrance section of the pool where the flow is decelerating. The distribution of stream-wise RMS of turbulence intensity along the pool generally presents a convex shape.

Coherent structures, turbulence intermittency, and anisotropy of gravity currents propagating on a rough and porous bed

This study experimentally investigated the impacts of rough and porous (RP) bed and sedimentation processes on the coherent structures, turbulence intermittency, and anisotropy of saline and turbidity currents. The results reveal that the local current concentration responds immediately (saline current) or languidly (turbidity current) to turbulence bursting events. Inside the dense current, the turbulent momentum fluxes in the streamwise (u′u′¯) and vertical (w′w′¯) directions transfer downstream and downward, which favor the sweep events. Inside the ambient water, u′u′¯ and w′w′¯ transfer upstream and upward, contributing to the formation of ejection events. At the current–ambient water interface, u′u′¯ and w′w′¯ do not tend to transfer in particular directions resulting in almost equal quantities of sweep and ejection events. The Gram–Charlier series expansion is strictly applicable to probability density functions (PDFs) of the sweep and ejection events but not suitable ideally for PDFs of the outward and inward interaction events. The primary anisotropy invariant map (AIM) of gravity currents starts from the two-component plain strain limit (near the bed). It is followed by the three-dimensional isotropy (inside the dense current and ambient water) and the axisymmetric contraction limit (current–ambient water interface). Finally, it ends in two-dimensional isotropy (near the free surface). This AIM is sensitive to the RP boundary and the sedimentation processes. Along the streamwise direction, the RP boundary causes alternations between the anisotropic and isotropic turbulence, but the arranged pattern of the rough units determines the period of this alternation.

The dynamics of cylinder-wake/boundary-layer interaction revealed by turbulent transports

The flow past a cylinder near a plane wall for small gap ratios (G/D=0.1, 0.3, and 0.9) and fixed ReD = 1000 is numerically studied. The fundamental flow features are characterized by the instantaneous and mean fields. Then, the dynamics of cylinder-wake/boundary-layer interaction are revealed by the turbulent momentum transport and kinetic energy production. The turbulent fluctuations caused by the secondary vortex (SV) (at G/D=0.3, 0.9) and the novel tertiary vortex (TV) (at G/D=0.9) can be observed in the distributions of Reynolds stresses. For G/D=0.1 and G/D=0.3, the wake/boundary-layer interaction is dominated by ejection and sweep events, which are related to the generation of the hairpin vortex. These two bursting events lead to the momentum transport between the high- and low-speed sides. For G/D=0.9, the ejection event is not found in the interaction region because the head of the hairpin vortex is entrained into the wake. The upper roller (RU) helps to transport high-momentum fluid toward the wall in this case, although it does not take part in the interaction directly. The shedding of RU, the lower roller (RL), SV (at G/D=0.3 and 0.9), and KH (Kelvin–Helmholtz) vortex (at G/D=0.1) and the generation of the hairpin vortex are crucial to turbulent kinetic energy (TKE) production. The RU, KH vortex, and SV transfer ⟨u′u′⟩ out to ⟨v′v′⟩ and ⟨w′w′⟩ resulting redistribution of the TKE. While RL, surviving for a shorter time, transfers ⟨v′v′⟩ out to ⟨u′u′⟩ and ⟨w′w′⟩, helping explain why it disappears quickly, TV only transfers out ⟨v′v′⟩ out to ⟨u′u′⟩, and its TKE comes from other terms rather than the production term. The redistribution of TKE due to the generation of the hairpin vortex can result in the slower growth rate of the secondary disturbance growth stage, promoting the wall boundary layer transition.

Fine‐Sediment Erosion and Sediment‐Ribbon Morphodynamics in Coarse‐Grained Immobile Beds

AbstractIn rivers, fine sediments are often transported over immobile coarse grains. With low sediment supply, they tend to aggregate in longitudinal ribbons. Yet, the long‐term evolution of such ribbons and the influence of immobile grains on the erosion of fine sediments are still not well understood. Flume experiments without sediment supply were therefore performed to investigate the erosion of an initially uniform fine‐sediment bed covering an immobile bed of staggered spheres through topographic and flow measurements. The topographic measurements yielded the spheres' protrusion above the fine sediment (P) and revealed long‐lived ribbons with ridges and troughs. The ridges are the main long‐term sediment source as the troughs are quickly eroded to a stable bed level resulting from the spheres' sheltering. The ridges stabilize with a spacing of 1.3 effective water depths, their number resulting from the integer number of wavelengths fitting into the effective channel width which excludes side‐wall accumulations. The ridges' erosion is damped by the local upflow of secondary current cells, which displaces the strongest sweep events above the bed. The upflow intensity is controlled by the ridges' height for low P, while for high P by the lateral roughness heterogeneity. The trends in erosion rates over ridges and troughs are similar and characterized by the following sequence of four regimes with increasing P: a drag sheltering, a turbulence‐enhancement, a wake‐interference sheltering, and a skimming‐flow sheltering regime. The critical P levels at the transitions are independent of the flow above the canopy, depending only on the geometrical configuration of the immobile bed.

Flow leakage and Kelvin–Helmholtz instability of turbulent flow over porous media

In the present paper, turbulent flow in a composite porous–fluid system, including a permeable surface-mounted bluff body immersed in a turbulent channel flow, is investigated using pore-scale large eddy simulation. The effect of Reynolds number (Re) on the flow leakage from porous to non-porous regions, Kelvin–Helmholtz (K–H) instabilities, as well as coherent structures over the porous–fluid interface are elaborated by comparing cases with three Reynolds numbers (Re = 3600, 7200, and 14 400). Results show that more than 52% of the fluid entering the porous blocks leaks from the first half of the porous region to the non-porous region through the porous–fluid interface. As the Re number increases from 3600 to 14 400, the flow leakage decreases by 24%. Flow visualization shows that the Re number affects the size of counter-rotating vortex pairs (CRVPs) and coherent hairpin structures above the porous block. At the Re = 3600, the CRVPs are larger and their centers locate farther away from the porous–fluid interface at Y/D ∼ 0.3 (D is two times of the distance between the centers of two consecutive pores), while at the highest Re number (Re = 14 400), they are 200% smaller and their centers become closer to the interface at Y/D ∼ 0.1. Moreover, turbulence statistics show that by reducing the Re number, turbulence production is delayed downstream; at the Re = 14 400, it begins from the leading edge of the porous block (X/D = 0), while at the Re = 3600, turbulence production is postponed and starts nearly at the middle of the porous block (X/D = 4.6). Also, quadrant-hole analysis indicates that increasing the Re number from 3600 to 14 400 strengthens sweep events and submerses the legs of coherent hairpin structures into the interface. Finally, the distribution of the pressure gradient for the three Re numbers confirms the occurrence of the K–H instability vortices over the porous–fluid interface. For Re = 3600, the K–H instability vortices show a linear growth rate in the vertical and horizontal directions with the slope of 0.136 and 0.05, respectively. However, by increasing the Re from 3600 to 14 400, the growth rate slope in the horizontal direction decreases by nearly 33.8%, while in the vertical direction, it increases by 200%.

Evidence of hard-selective sweeps suggests independent adaptation to insecticides in Colorado potato beetle (Coleoptera: Chrysomelidae) populations.

Pesticide resistance provides one of the best examples of rapid evolution to environmental change. The Colorado potato beetle (CPB) has a long and noteworthy history as a super-pest due to its ability to repeatedly develop resistance to novel insecticides and rapidly expand its geographic and host plant range. Here, we investigate regional differences in demography, recombination, and selection using whole-genome resequencing data from two highly resistant CPB populations in the United States (Hancock, Wisconsin and Long Island, New York). Demographic reconstruction corroborates historical records for a single pest origin during the colonization of the Midwestern and Eastern United States in the mid- to late-19th century and suggests that the effective population size might be higher in Long Island, NY than Hancock, WI despite contemporary potato acreage of Wisconsin being far greater. Population-based recombination maps show similar background recombination rates between these populations, as well as overlapping regions of low recombination that intersect with important metabolic detoxification genes. In both populations, we find compelling evidence for hard selective sweeps linked to insecticide resistance with multiple sweeps involving genes associated with xenobiotic metabolism, stress response, and defensive chemistry. Notably, only two candidate insecticide resistance genes are shared among both populations, but both appear to be independent hard selective sweep events. This suggests that repeated, rapid, and independent evolution of genes may underlie CPB's pest status among geographically distinct populations.

Spatial and spectral characteristics of information flux between turbulent boundary layers and porous media

The interaction between boundary layer turbulence and a porous layer is the cornerstone of interface engineering. In this study, the spatial and spectral-resolved transfer entropy is used to assess the asymmetry of the causal interaction next to the permeable wall. The analysis is based on pore-resolved direct numerical simulation of turbulent channel flow over cylinder arrays. The spatial map of transfer entropy reveals the information flux between the porous medium and arbitrary nearby positions, and paths connecting locations with maximum information transfer are identified. The paths in the ‘top-down’ and ‘bottom-up’ directions, respectively, lean upstream and downstream, demonstrating that the coupling process is directionally dependent. The scale dependence of transfer entropy is inspected with a surrogate data strategy. As wall permeability increases, the active scale range in causal interaction shifts from near-wall vortices to Kelvin–Helmholtz type eddies. In addition, linear stochastic estimation is used to determine the statistical velocity field for a local informative event. In an average sense, the interaction between a convecting sweep or ejection event and the up/down-welling motions at the pore unit is the core mechanism that contributes to the causal coupling. The statistical findings derived from the transfer entropy are then validated using a neural network-based remote sensing model.

Reynolds number and submergence ratio effects on turbulence structures in the shallow wake of a horizontal pipe located on a rough bed

The effects of Reynolds number and submergence ratio on mean velocity, separation length, Reynolds shear stresses (RSS), correlation coefficient, anisotropy, and turbulent kinetic energy (TKE) are evaluated by modifying the free-stream velocity and pipe diameter to understand the fluid–structure interaction in the presence of a rough channel bed. Acoustic Doppler velocimetry is used to acquire three-dimensional velocity data in the experiments. The experimental data demonstrate that the non-dimensional separation length is linearly varying with the Reynolds number up to a critical Reynolds number, and after that, it becomes independent of the Reynolds number. The vertical profiles of the mean velocity, RSS, correlation coefficient, anisotropy, and TKE are not affected by the Reynolds number; however, their profiles are affected by the submergence ratio. The RSS and TKE are found to be larger for the higher submergence ratio below the top level of the pipe throughout the wake region. The reattachment point is found to be the most turbulent as the most pronounced peaks of RSS and TKE are occurring at this point. The quadrant analysis reveals that the RSS contribution of ejection events is higher for a lower submergence ratio, whereas the RSS contribution of the sweep events is higher for a higher submergence ratio. In conclusion, this study provides an in-depth understanding of the turbulent flow characteristics in the wake region of a bed-mounted horizontal circular pipe located on a fixed sand bed, especially the modification of the turbulence properties due to changes in the Reynolds number and submergence ratio.

Experimental observations on turbulent boundary layers subjected to a step change in surface roughness

Based on experimental data acquired with particle image velocimetry, we examine turbulent boundary layers that are subjected to an abrupt change in wall roughness in the streamwise direction. Three different sandpapers (P24, P36 and P60) together with a smooth wall are used to form a number of different surface transition cases, including both R $\rightarrow$ S (where upstream surface is rough and second surface is either smooth or smoother compared with the upstream surface) and S $\rightarrow$ R (where upstream surface is smoother compared with the downstream surface; both surfaces are rough). This enables us to investigate the effect of the surface transition strength ( $M = \ln [y_{02}/y_{01}]$ , where $y_{01}$ and $y_{02}$ are the roughness lengths of the upstream and downstream surfaces, respectively) on the growth of the internal boundary layer (IBL) and the corresponding flow structure. The results show that for each surface transition group (i.e. R $\rightarrow$ S and S $\rightarrow$ R), the thickness of the IBLs is proportional to the strength of the surface transition, and that the IBLs are thicker for the S $\rightarrow$ R cases compared with their R $\rightarrow$ S counterparts for similar $|M|$ , when normalised by the initial boundary layer thickness ( $\delta _0$ ). The results also show that the growth rates of the IBLs could be represented by a power law, consistent with the previous studies. However, despite a wide range of scatter in the literature for the power-law exponent, an average value of $0.75$ (varies between $0.71$ and $0.8$ with no clear trend) is obtained in the present study considering all the surface transition cases. The pre-factor for the power-law fit, on the other hand, is found to be related to the strength of the surface transition. In addition to the variations in the velocity defect and diagnostic plots with the growth of the IBLs, sweep and ejection events appear to differ significantly (depending on the type of the step change). Two-point spatial correlations, moreover, show that the structure is more elongated in the wall-normal and streamwise directions, as the flow accelerates over the downstream surface (i.e. R $\rightarrow$ S cases). For the reverse transition cases (i.e. S $\rightarrow$ R, where the flow decelerates over the downstream rougher surface), however, the correlation coefficients shrink in size in both directions.

Drag reduction using velocity control in Taylor–Couette flows

Direct numerical simulation of Taylor–Couette flow subject to opposition control is investigated at Reynolds number (Re) of 3000. The idea is to impose exact opposite velocities of the detection plane at the walls to counteract near-wall stream-wise vortices. In this study, various velocity control strategies, namely wall-normal, axial, combined and blowing only, have been investigated from the viewpoint of skin-friction drag reduction. Further, the effects of skipping spatial points in azimuthal and axial directions and in time have been investigated from a drag reduction point of view. Based on the emergence of a virtual wall that hinders the vertical transport of momentum (i.e. on reduction of Reynolds shear stress production as well as sweep ejection events), flow physics has been explained via statistical analysis of fluctuations, Reynolds shear stresses, and near-wall coherent structures. The spatial density of near-wall vortical structures shows a marked reduction, followed by quadrant contribution analysis of Reynolds shear stresses reveals a decrease in ejection and sweep events, leading to reduced production of Reynolds shear stresses and skin-friction drag.

Physical mechanism of vertical gradient of pressure flux and its impact on turbulent flux estimation

• Flux-variance method is an effective way to indirectly obtain pressure flux. • Pressure flux convergence/divergence is partially related to larger-scale eddies. • Ejection and sweep events lead to opposite vertical gradient of pressure flux. • There's a quadratic relation between pressure flux gradient and friction velocity. • Taking pressure transport into account refines estimation of turbulent flux in CBL. Using multi-layer pressure fluctuations and other conventional turbulence observational data over a nearly flat and homogeneous dune underlying surface, the temporal and spatial characteristics of pressure flux and its profile were analyzed to illustrate the physical mechanism of pressure flux convergence/divergence and the impact of pressure transport terms on the estimation of turbulent fluxes in this study. The flux-variance (FV) method is an effective approach to indirectly obtain pressure flux through momentum flux when no fast-response observations of pressure fluctuations are available. The fitted functions are w ′ p ′ ¯ = 0.16 · ( − F Z ( z ) · u ′ w ′ ¯ σ p σ u ) and u ′ p ′ ¯ = 0.55 · ( F X ( z ) · u ′ w ′ ¯ σ p σ w ) , where F Z ( z ) and F X ( z ) are the height normalized functions for vertical and horizontal directions. Under the circumstance of pressure flux divergence (FD), the transport efficiency of pressure and contribution of larger-scale eddies to pressure flux at higher levels are greater than those at lower levels, and flows in the atmospheric surface layer (ASL) are mainly dominated by the sweep events; while the pressure flux convergence (FC) cases are exactly the opposite situations. A good quadratic function relationship was found between the vertical gradient of pressure flux and friction velocity under different atmospheric stabilities: ∂ w ′ p ′ ¯ ∂ z = 0.01 u * ( 16 u * − 1 ) . Turbulence kinetic energy (TKE) and its vertical transport are greater in the FD cases than those in the FC cases. The momentum and sensible heat fluxes in the convective boundary layer (CBL) were overestimated by their common second-order diagnostic equations and Rotta model (pressure redistribution term parameterization) which neglect the diffusion terms. After adding the contribution of pressure transport terms into the diagnostic equations and Rotta model and modifying the original Rotta constants, the overestimations were well corrected and the estimated turbulent fluxes were more consistent with the observations. This work provides a novel direction and idea for the future improvement of the planetary boundary layer (PBL) parameterization schemes in many numerical weather and climate models.

Large Eddy Simulation of a sediment particle under entrainment conditions

A Large Eddy Simulation of a fully developed turbulent open-channel flow with a Reynolds number of was performed to study dynamics of coherent structures in the vicinity of a stationary sediment particle. The sediment particle was positioned at the bottom on a smooth channel bed. The impulse concept was employed by computing three-dimensional hydrodynamic forces acting on the sediment particle in order to predict the onset of particle motion. Results of quadrant analyses at high impulse events yielded mostly outward and sweep events. Auto- and cross-correlation of the hydrodynamic forces and the flow field were studied and discussed. An identification of coherent structures at high impulse events showed that large-scale, long-lived hairpin-like vortices whose bed-normal lengths are larger than viscous units were resulting in an increase of lift and drag forces simultaneously, facilitating initiation of particle motion.

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.

Investigation of Vortex Structure Modulation by Spume Droplets in the Marine Atmospheric Boundary Layer by Numerical Simulation

Direct numerical simulation (DNS) of a droplet-laden, turbulent Couette airflow over a waved water surface is performed modeling the marine atmospheric boundary (MABL) layer carrying idealized spume droplets. Both the instantaneous and mean flow properties, the characteristics of the vortex structures and the momentum exchange between air turbulence and waved water surface and droplet-mediated momentum transfer are investigated. A Eulerian–Lagrangian approach is employed in DNS where full, 3D Navier–Stokes equations for the carrier air are solved in a Eulerian frame, and the trajectories of individual droplets are simultaneously tracked in a Lagrangian frame. The impact of the droplets on the carrier air flow is modeled via a point force approximation. The droplets size is considered in the range of spume droplet sizes observed in MABL. Various water surface roughness and droplet injection scenarios are considered, and both instantaneous and phase-averaged flow fields, the Reynolds stresses and the eigenvalues of the local air velocity gradient tensor are evaluated in DNS. Numerical results show a strong dependence of the droplet-mediated airflow modification on-the-droplet injection mechanism. Droplets injected with the surrounding air velocity effectively mitigate the vortex structures by reducing their swirling strength and suppress the momentum flux from air turbulence to water surface by weakening both ejections and sweeping events, and thus accelerating the mean flow as compared to the droplet-free flow. On the other hand, droplets injected with the velocities of the Lagrangian particles of the water surface enhance both the swirling strength of the vortex structures and air-flow turbulent stresses and decelerate the mean wind. The results also show that these effects of droplet-mediated flow modification become less pronounced as the water surface wave steepness increases.

Influence of surface modification on droplet mobility during dropwise condensation

AbstractA two‐step surface fabrication technique is used to modify the wetting characteristics of copper sheets for heat transfer application. It consists of oxygen plasma treatment that results in the formation of surface nanostructures, and a post‐functionalization process using self‐assembled monolayers. The copper samples are characterized by contact angle measurements, scanning electron microscopy, and atomic force microscopy. Results show that surface roughness, described using hybrid and functional roughness parameters, varies with irradiation power. The plasma‐treated copper samples become superhydrophobic with water contact angles ranging from 150° to 160° after the post‐functionalization. Condensation experiments are conducted to explore the influence of surface wettability on condensate formation. Results show that droplets formed on the superhydrophobic copper sheets are smaller and more mobile with sweep events that are 9x more frequent compared to those on untreated copper sheets. Sweep events are shown to be influenced by surface porosity, which is defined using standard roughness parameters. It is demonstrated that higher surface porosity results to higher sweep interval.

Turbulence Structure and Burst Events Observed in a Tidally Induced Bottom Boundary Layer

AbstractIn estuarine environments tidal variation affects turbulence and associated sediment transport processes near the bed. In situ turbulence and sediment data from the benthic boundary layer of the subaqueous Yangtze delta were collected over several tidal cycles and are used to gain new insights into turbulence structure and sediment mobility in the benthic boundary layer. Our analysis reveals that turbulent kinetic energy production was balanced by dissipation during flood, while during ebb an imbalance was found (dissipation > production) attributed to turbulence advection. Quadrant analysis showed a tidal variability of the four quadrants with ejections (E) and sweeps (S) being the dominant and equal contributors to Reynolds stress and sediment resuspension. Increased dissipation due to turbulent kinetic energy advection during ebb coincides with an increase in the timeshare of inward‐ and outward‐interaction events, resulting in a higher Reynolds stress contribution from ejections and sweeps. The contribution of outward (O) and inward (I) ‐interactions to sediment resuspension was found to be negligible. Turbulence intermittency is characterized using the frequency, duration, strength, and intensity of the quadrant events. The frequency and duration of outward (O) and inward (I) ‐interactions controlled bursts were 1/3∼1/2 that of ejections (E) and sweeps (S), which lead to them having limited effects on Reynolds stress. A comparison with settling velocity suggests that vertical sediment transport is mainly controlled by ejection and sweep events. Inward‐ and outward‐interactions appear to be more important for the resuspension of fine material with settling velocities of the order of 1 mm/s.

Probabilistic Characterization of Sweep and Ejection Events in Turbulent Flows and Its Implications on Sediment Transport

AbstractTurbulent boundary layers are populated by a hierarchy of recurrent structures normally referred to as “coherent structures.” Among others, ejection and sweep events are critical coherent structures responsible for sediment transport. This study focuses on gaining a better understanding of the spatial‐–temporal probabilistic characteristics of sweep and ejection events. The existence of uniform momentum zones (UMZs) is demonstrated to affect the spatial distribution of large‐scale motions (LSMs), and the ejection and sweep events tend to present near UMZ edges where interfacial layers of high shears exist. In this study, UMZ detection is employed to identify coherent structures. The conditional velocity decomposition is then applied to the available direct numerical simulation (DNS) flow data after UMZ edges were determined. Based on the integrated criterion for distinguishing ejection and sweep events, one can determine the probabilistic characteristics of coherent structures such as the center point, wall‐normal length, and streamwise length. Physical insights such as joint probability density functions of wall‐normal length and streamwise length can be established. The attached and detached features of the sweeps and ejections can then be classified and characterized, respectively. Durations of sweep and ejection events are demonstrated to follow a lognormal distribution in this study. The occurrence ratio of sweep events in the (LSMs) is quantified from the DNS data. The implications of the results on sediment transport are discussed. The contributions of coherent structures with respect to wall‐normal height and the intermittency and geometrical properties of the coherent structures are presented.

Thermochemical non-equilibrium effects in turbulent hypersonic boundary layers

A hypersonic, spatially evolving turbulent boundary layer at Mach 12.48 with a cooled wall is analysed by means of direct numerical simulations. At the selected conditions, massive kinetic-to-internal energy conversion triggers thermal and chemical non-equilibrium phenomena. Air is assumed to behave as a five-species reacting mixture, and a two-temperature model is adopted to account for vibrational non-equilibrium. Wall cooling partly counteracts the effects of friction heating, and the temperature rise in the boundary layer excites vibrational energy modes while inducing mild chemical dissociation of oxygen. Vibrational non-equilibrium is mostly driven by molecular nitrogen, characterized by slower relaxation rates than the other molecules in the mixture. The results reveal that thermal non-equilibrium is sustained by turbulent mixing: sweep and ejection events efficiently redistribute the gas, contributing to the generation of a vibrationally under-excited state close to the wall, and an over-excited state in the outer region of the boundary layer. The tight coupling between turbulence and thermal effects is quantified by defining an interaction indicator. A modelling strategy for the vibrational energy turbulent flux is proposed, based on the definition of a vibrational turbulent Prandtl number. The validity of the strong Reynolds analogy under thermal non-equilibrium is also evaluated. Strong compressibility effects promote the translational–vibrational energy exchange, but no preferential correlation was detected between expansions/compressions and vibrational over-/under-excitation, as opposed to what has been observed for unconfined turbulent configurations.

Evaluating the concrete grade-control structures built by modified fish-nest bricks in the river restoration: A lab-based case study

Concrete grade-control structures (CGCSs) have broad application prospects in the restoration of large rivers. But there is a lack of indicators to evaluate CGCSs at laboratory study. In this study, we proposed two evaluation indicators from the perspective of the impact of CGCSs on geomorphology change and fish habitat, namely the spatial-averaged occurrence probability of sweep events near the bed and flow diversity. To verify the reasonableness of these indicators, flume experiments were conducted with CGCSs built by modified fish-nest bricks in different Reynolds number and layout condition. Data of the flow field around structures in streamwise, transverse and vertical direction was obtained and analyzed. Results of mean flow field show that large recirculation zones are found in the cavity and behind the element. The mechanism of suspended sediment deposition around CGCSs in the flow can be further clarified by combining sweep and ejection according to quadrant analysis. In the vertical direction, the ratio of sweep to total events near bed after spatial-averaged processing is found to be higher for the staggered array. According to the Shannon's entropy, water flow diversity was calculated to quantify the fish habitat. The water flow diversity index around the CGCSs is higher for the staggered. It can be concluded that the elements of CGCSs in staggered manners have a better protection for riverbed and can provide a more stable fish habitat suitability. The results anticipated by the spatial-averaged occurrence probability of sweep events near bed and flow diversity in the experiment are consistent with the result of previous research on landform change and fish habitat. The research could provide a theoretical basis for the application of CGCSs for river restoration.

Large-Eddy Simulation on the Aerodynamic and Thermal Characteristics in a Micropipe of the Hypersonic Engine Precooler.

The precooling air-breathing technique has become a study focus in the aerospace field. Research on the internal flow and heat-transfer mechanism of the precooler is important for design and optimization. A large-eddy simulation was used to study the aerodynamic and thermal characteristics in a micropipe of the hypersonic engine precooler with supercritical methane as coolant and fuel. Under the effect of buoyancy, the high-temperature and low-density fluid near the wall in the circumferential direction gradually accumulate to the top wall. The accumulation of low-density fluid enhances the thermal acceleration effect at the top wall, which intensifies the local turbulent relaminarization and forms an M-shaped velocity distribution, resulting in the weakening of the heat transfer. On the other hand, the high-density fluid gathers to the bottom wall under the influence of gravity, the local thermal acceleration effect is weakened, and the flow heat transfer is enhanced. The influence of the relationship between the turbulent burst and the turbulent heat transfer under the effect of buoyancy is analyzed. It is found that the low-speed ejection events and high-speed sweep events are strengthened at the bottom wall, especially the low-speed ejection. However, the occurrence of these events at the top wall is restrained to a certain extent.