Streamwise Shear Research Articles

3D numerical modelling and laboratory study of flow field induced by a group of submerged vegetations

The three-dimensional (3D) numerical modelling in an open channel flow field of a group of submerged vegetations using computational fluid dynamics (CFD) platform of FLOW-3D HYDRO was performed in this study. A set of acoustic Doppler velocimetry (ADV) measurements have been conducted as benchmark to validate the numerical model. A quantitative comparison was performed on several hydrodynamic variables that impacted the vegetated open channel flow, such as flow depth, streamwise water velocity, turbulent intensity, and Reynolds shear stress. In the numerical analysis, the flow turbulence was treated using the RANS approach (within RNG k-ε); while the Volume Of Fluid (VOF) method was used to track the air-water interface. Structured meshes with hexahedral elements were used to discretize the channel geometry. In the findings, the numerical model reasonably reproduced the flow field and presented corresponding agreement with the experimental turbulent structures. This study showed that the differences in results between various analyses were all less than 10% and concludes that the presented numerical approach can be utilised as an efficient tool for simulations of the flow field within a vegetation patch (i.e. by using the simplified RANS approach).

The riverbank vegetation for mitigating the adverse effects of sediment dredging

AbstractSediment dredging has been an ever‐growing issue, especially in developing nations with high demand for concrete filler material. River systems are adversely affected by sediment mining, resulting in decreased stability of the riverbed and riverbanks. Nature‐based solutions for riverbank erosion have been a research topic that has led to the proposal of vegetation on the riverbanks. However, little is known about the extent of riverbank vegetation required to negate the devastating effects of sediment mining because dense vegetation severely affects the flow structure and becomes a waste trap. This experimental study uses sparsely dense, flexible, and bladed vegetation to study the annulment effects of vegetation against the existing mining pit. Near‐bed turbulence and sediment transport have increased in the test section in the presence of a mining pit. The increase in near‐bed streamwise and transverse Reynolds shear stresses helped us understand the increased sediment movement in streamwise and lateral directions. The morphology of the test section showed increased riverbed erosion at the beginning of the test segment. The entire cross‐section was levelled at the end of the test section, and aggradation was downstream of the test segment. In contrast, in the vegetated riverbank case, the initial profile of the bank was almost unchanged for the same discharge of flow and upstream sand pit. The sparse vegetation overperformed the intended negation effects. This study establishes that sparse vegetation would perform better in maintaining the channel morphology, which otherwise in dense vegetation would have faced a high erosion rate in the main channel while giving the same protection to the riverbanks.

An analysis of drag reduction using spanwise forcing on rough walls

Spanwise opposed wall-jet forcing has been shown to reduce the skin-friction drag of wall-bounded turbulent flows by suppressing the near-wall turbulent motion (Yao et al., 2018). In the present work, the response of this drag reduction mechanism to the presence of surface roughness is studied. To this end, direct numerical simulations of flow in smooth and rough plane channels at a matched friction Reynolds number (Reτ=180) are carried out. The roughness is generated by distributing discrete roughness elements at two different values of frontal solidity, namely 0.021 and 0.045. The roughness elements height is k+=18 (＋indicates viscous scaling). By varying the forcing amplitude A+, it is shown that, when other forcing parameters are fixed, maximum drag reduction in a rough channel is achieved at a considerably larger A+ than in a smooth channel. Notably, the strength of the wall-jet at which the maximum drag reduction is achieved, is comparable for the smooth and both rough cases. Additionally, it is observed that the maximum drag reduction values attained in rough channels are smaller compared to those observed in smooth channels, for both lower and higher frontal solidity cases — 2.4% and 2%, respectively. The reduced drag reduction potential originates from two observations; firstly, drag on roughness elements, which is dominated by pressure drag, is not reduced by the method in question. Secondly, the drag on channel floor is reduced to a lesser extent when roughness elements are present. The latter can be attributed to the observation that, while in all cases the forcing suppresses streamwise random turbulent fluctuations and shear stress, this suppression is less pronounced when roughness elements are present.

NUMERICAL INVESTIGATION OF TURBULENCE CHARACTERISTICS AND SELF-SIMILARITY IN A HIGHLY AERATED STABLE HYDRAULIC JUMP USING LARGE EDDY SIMULATION

This work presents a numerical study of a stable hydraulic jump at Froude number 4.25 and Reynolds number 1.15×105 inside a horizontal and rectangular channel with a length of 3.2 m, a width of 0.5 m and a height of 0.4 m using large eddy simulation (LES). Classical hydraulic jump characteristics are obtained, such as conjugate depths, jump length, void fraction and velocity profiles. The hydraulic jump maximum streamwise velocity decay and shear layer spreading rate are simulated and compared with experimental data. For these parameters, numerical results demonstrate that is possible to stablish an analogy with other shear flows, such as the horizontal plane wall jet. Profiles of streamwise and vertical components of mean velocity are simulated, and self-similarity is observed for cross-sections located at the recirculation region of the jump. Self-similarity is also observed in terms of turbulent fluctuations, insofar as LES simulations indicate a high level of turbulence in the recirculation region. The simulated root mean square of streamwise velocity fluctuations, , ranges from 0.5 to 0.7 of the maximum cross-sections velocity, whereas the root mean square of vertical component of velocity fluctuations, , stays around 0.5 of the maximum cross-sections velocitiy. All validation comparisons show good agreement with the selected experimental data of Kramer and Valero (2020) and Wang (2014), presenting average deviations always lesser than 5%.

Drag reduction by means of an array of staggered circular cavities at moderate Reynolds numbers

Further insight into the physics of boundary layers developing over an array of circular cavities at moderate Reynolds numbers (Reθ up to 3400) is gained from hot wire boundary layer surveys on a range of parameters. The findings are consistent with literature as to the possibility of reducing skin friction drag with circular perforations. The results evidence a modification of the mean velocity profile, an increase of the wake parameter and the shape factor associated to a decrease of the skin friction. The modifications of the boundary layer as well as the skin friction reduction are more pronounced when the open area ratio is increased, which is achieved by increasing the cavity diameter or equivalently by decreasing the inter-cavity spacing. The turbulent activity is shifted in the upward direction, the burst intensity decreases in the buffer layer (10<Y+<35) and increases in the upper part of the logarithmic region/beginning of outer layer (80<Y+<200) with the formation of an outer peak in the turbulence intensity profile. The premultiplied spectra and the streamwise shear stress profiles evidence an energy surplus in the same part of the boundary layer and for the highest open area ratios a “bi-hump” behaviour. The formation of outer quasi-streamwise rollers is believed to be the cause of this energy hump in the spectrum. The turbulence statistics show a striking similarity with the turbulent boundary layer in presence of wall-normal blowing. It is believed that the cavities promote a vertical velocity component that displaces the turbulent activity towards the outer region. Different hypotheses to explain the modification of the flow statistics are detailed in the paper.

Mathematical Analysis of Transverse Wall-Shearing Motion via Cross Flow of Nanofluid

The investigation of nanofluid’s cross flow, which is caused by a nonlinear stretching sheet within the boundary layer, is presented. The proper mathematical detail is provided for three distinct cross flow instances with the streamwise flow. A uniform transverse stream located far above the stretched plate, in one instance, creates the cross flow. Two further situations deal with cross flows caused by surface transverse shearing motions. Weidman’s work was used to find a similarity solution by making the necessary changes. It has been found that two parameters, namely nanoparticle volume frictions ϕ and a nonlinear stretching parameter β, have a significant impact on the flow of fluids in cross flow scenarios. Graphical representations of transverse and streamwise shear stresses and velocity profiles are provided. From this study, we found that nanoparticle volume fraction ϕ reduces the momentum boundary layer in both streamwise and cross flow scenarios while increasing the temperature of the fluid and, hence, increasing thermal boundary layer thickness. The same is observed for the nonlinear stretching parameter β.

DES AND RANS MODELING OF PRIMARY ATOMIZATION IN A COAXIAL SWIRLING LIQUID-GAS JET

This study assesses different turbulence modeling approaches for simulation of two-phase coaxial annular swirling jet flows. The problem selected from literature for comparison involves an analytical inlet profile for an annular liquid sheet sandwiched between two coaxial annular gaseous jets. The liquid-gas interface is resolved using the volume-of-fluid model with continuum surface force approximation. Reynolds-averaged Navier-Stokes simulations and detached eddy simulations (DES) are conducted to obtain transient multiphase numerical solutions. Different turbulence models explored include the &lt;i&gt;k-&amp;epsilon;&lt;/i&gt; renormalization group (RNG) with swirl modification, the Reynolds stress model (RSM), RSM with scale-adaptive simulation (RSM-SAS), and DES. Comparisons with the direct numerical results from literature suggest that the &lt;i&gt;k-&amp;epsilon;&lt;/i&gt; RNG and RSM approaches simulate only the streamwise shear of the liquid jet and are inadequate in capturing the swirling aspect of the jet flow and expected instabilities. DES can predict several expected features such as radial asymmetry, surrounding gas vortices causing jet instabilities, and eventual jet breakup with reasonable accuracy. While RSM-SAS predicts radial asymmetry, some jet instability, and is much more accurate than &lt;i&gt;k-&amp;epsilon;&lt;/i&gt; RNG and RSM, it fails to predict instabilities as good as DES and does not predict a complete jet breakup. RSM-based methods are found to be computationally very expensive compared to the &lt;i&gt;k-&amp;epsilon;&lt;/i&gt; RNG model, suggesting DES as the better alternative than RSM methods for such applications if resources are available.

Linear instability and nonlinear flow states in a horizontal pipe flow under bottom heating and transverse magnetic field

Linear stabilities of the liquid metal mixed convection in a horizontal pipe under bottom heating and transverse magnetic field are studied through linear global stability analyses. Three branches of the linear stability boundary curves are determined by the eigenvalue computation of the most unstable modes. One branch is located in the region of large Hartmann number and determined by the linear unstable mode which was first revealed by the numerical simulations of Zikanov, Listratov &amp; Sviridov (J. Fluid Mech., vol. 720, 2013, pp. 486–516). This branch curve shows that the global unstable mode exists above a threshold of Hartmann number, which agrees with the experiment of Genin et al. (Temperature fluctuations in a heated horizontal tube affected by transverse magnetic field. In Proc. 8th PAMIR Conf. Fund. Appl. MHD, Borgo, Corsica, France, pp. 37–41, 2011). The other two branch curves determined by two different long-wave unstable modes intersect with each other in the region of small Hartmann number. The critical Grashof number on these two curves increases exponentially with the increase of the Hartmann number. Through energy budget analyses at the critical thresholds of these unstable modes, it is found that, for the unstable mode at large Hartmann numbers, buoyancy is the dominant destabilizing term which demonstrates the hypothetical explanation of Zikanov et al. (2013) who regard natural convection as a destabilization mechanism. It is further revealed that, with respect to the unstable modes on the critical stability curves of small Hartmann numbers, the dominant destabilization comes from the streamwise shear of the basic flow. Finally, within the linear unstable region, fully developed nonlinear flow states of the mixed convection are investigated by direct numerical simulations (DNS) with several sets of selected dimensionless parameters. The spatio-temporal structures of these nonlinear flow states are discussed in detail with comparison with the linear unstable global modes.

Large-eddy simulation and analytical modeling study of the wake of a wind turbine behind an abrupt rough-to-smooth surface roughness transition

The evolution of a wind turbine wake situated downstream of an abrupt change in surface roughness is investigated using large-eddy simulations (LES). The results are compared with the evolution of the wake of a turbine sited on a homogeneously rough surface, and with the flow over a surface undergoing an abrupt roughness transition without a turbine. The changed surface roughness affects the turbulent statistics such as streamwise velocity, turbulence intensity, and shear stress. Different velocity deficits can be constructed based on different definitions of “background” velocity. The usual definition, that is, the difference between the velocity upstream and downstream of a turbine, attains negative values over a significant portion of the turbine wake, rendering it difficult to model using the usual Gaussian radial shape-based framework. An alternative definition, that is, the difference between the velocity over a heterogeneous surface in the absence and in the presence of a turbine, has mostly positive values and is amenable to modeling. A new model accounting for streamwise and vertical variations of the background velocity profile is developed. The new model yields more accurate predictions of the LES results than the existing Gaussian wake-shape model, which is applicable only for turbines sited on homogeneously rough surfaces.

Numerical study of wall shear stress fluctuations and near-wall structures in a square duct at low Reynolds numbers

Direct numerical simulations (DNS) of turbulent flow through a square duct at two different low Reynolds numbers is performed using OpenFOAM to study the instantaneous wall shear stress fluctuations and near-wall structures. We first benchmarked our simulation results against other DNS studies which used higher-order spectral methods, thereby confirming the accuracy of our results. The mean profile of streamwise wall shear stress is well studied in the literature, however its higher order statistics has not been yet explored. There is also a lack of information on the spanwise component of wall shear stress and its higher-order statistics. Here, we provide an extensive study on streamwise and spanwise wall shear stresses, and their higher-order statistics. The intensities of both components of wall shear stress fluctuations are close to zero near the corner, and monotonically increase while moving towards the duct center. This is in contrast with their mean profiles which do not exhibit similar monotonic trends. However, the third-order moment, i.e. skewness of the streamwise wall shear stress again shows a non-monotonic behavior from the corner to the center. The profiles of the mean as well as the higher-order statistics of wall shear stress follow a better scaling between the two Reynolds number studied, when the spanwise location is normalized using inner scaling. Streamwise velocity correlations near the wall show that the flow structures in a square duct are similar to that of a channel flow near the center, whereas there is a cross-talk of the flow structures between the neighboring walls near the corner. An investigation into the effect of corner on the probability of back flow events is made at a Reynolds number lower than previously reported. The probability follows a non-monotonic behavior as we move from the corner towards the center, where it reaches a minimum at around 50 wall units from the corner. Similar to the wall shear stress profiles, the behavior of the near-wall flow structures as well as the spanwise variation of the probability of backflow events also show a qualitatively similar trend for the two Reynolds numbers, when the spanwise location is normalized using inner scaling.

Stochastic Model for Quasi-One-Dimensional Transitional Turbulence with Streamwise Shear Interactions.

The transition to turbulence in wall-bounded shear flows is typically subcritical, with a poorly understood interplay between spatial fluctuations, pattern formation, and stochasticity near the critical Reynolds number. Here, we present a spatially extended stochastic minimal model for the energy budget in transitional pipe flow, which successfully recapitulates the way localized patches of turbulence (puffs) decay, split, and grow, respectively, as the Reynolds number increases through the laminar-turbulent transition. Our approach takes into account the flow geometry, as we demonstrate by extending the model to quasi-one-dimensional Taylor-Couette flow, reproducing the observed directed percolation pattern of turbulent patches in space and time.

A Novel Approach for Computational Fluid Dynamics Analysis of Mean Wind Loads on Heliostats

Abstract A computational fluid dynamics study about the aerodynamic loads over a heliostat due to an atmospheric boundary layer flow using a modified k–ɛ turbulence model is presented. A new formulation is used, in which the model quantities vary with the velocity field. Modified wall functions for roughness were used at the bottom of the computational domain to achieve horizontal homogeneity of the airflow. Good horizontal homogeneity for the streamwise and spanwise velocity and turbulence intensity profiles were found. The incident profiles were compared with the inlet ones. The average percentage differences were 0.22% for velocity and 0.43% for turbulent intensity. Good agreement was found between the numerical data and theoretical values of the streamwise and spanwise shear stress at the bottom of the domain. The aerodynamic coefficients of the heliostat at different elevation angles were obtained, and a good agreement was found between the numerical data concerning the wind tunnel experimental values. An average percentage difference of 3.1% was found for drag, 6.5% for lift, and 6.0% for overturning. A significant improvement was obtained by using this new formulation with respect to a non-modified k–ɛ turbulence model. The average differences of the aerodynamic coefficients were 6.6% for drag, 12.4% for lift, and 10.1% for overturning. The velocity, turbulent kinetic energy, and pressure fields at different elevation angles were analyzed. It was found that at an elevation of 60 deg, the stagnation point of the flow occurs at the superior edge of the heliostat, causing the maximum lift force over the structure.

Bottom wall shear stress fluctuations in shallow-water Langmuir turbulence

In neutrally stratified shallow water, full-depth Langmuir cells (LCs) can interact with the turbulent benthic boundary layer and, thus, influence bottom wall shear stresses. In this paper the impacts of full-depth LCs on the streamwise and spanwise wall shear stresses are systematically studied using the database obtained from wall-resolved large-eddy simulation of shallow-water Langmuir turbulence. Analyses focus on the instantaneous wall shear stress fluctuations and the joint probability density functions between the stress fluctuations and the LCs parts of the velocity fluctuations, which show that the linear superimposition effect and nonlinear modulation effect of LCs are responsible for the spanwise organized distribution of wall shear stress fluctuations. Compared with the statistics in pure shear-driven turbulence without LCs, the mean square values of wall shear stress fluctuations in shallow-water Langmuir turbulence are enhanced by the strong linear superimposition effect of LCs, while the skewness and kurtosis are reduced by the combination of the linear superimposition effect and nonlinear modulation effect of LCs. Based on the scalings of these effects, a new predictive model of wall shear stress fluctuations is proposed for shallow-water Langmuir turbulence. The proposed model can predict the spatial distribution and statistics of wall shear stress fluctuations using the LCs parts of velocity fluctuations measured above the water bottom. Owing to the persistence of the spanwise inhomogeneity of wall shear stresses induced by full-depth LCs, the new predictive model will be useful for improving the wall-layer modelling for shallow-water Langmuir turbulent flows.

Correlation analysis of flow and sound in non-isothermal subsonic jets based on large eddy simulations

The sound radiation mechanism is investigated in non-isothermal subsonic jets at acoustic Mach number 0.9 and at temperature ratios of 0.86, 1.0, and 2.7, using causality methods on the results predicted by large eddy simulations. Turbulence signals in the jet flows are rearranged according to the acoustic analogies, such as the streamwise and radial shear signals, ux′ and ur′, the self-signal, ui′uj′, the entropy fluctuation, s′, and the vortex norm, |ω|. The ray-tracing method is applied to locate the acoustically correlated regions, and the two-point correlation is employed to analyze the acoustic correlation. The results show that the cancelation between the s′ and ux′, and between the s′ and ur′, vary with temperature ratio, and, thus, may affect the radiation directionality. The density fluctuation, ρ′, suppresses the acoustic correlations of the shear- and self-signals in the cold jet, respectively, but strengthens them in the hot jet; thus, the ρ′ enhances the cancelation both in cold and in hot jets. The intense negative fluctuation of the ux′ is found to be associated with the cancelation mechanism. It induces the peaks of ρ′ and s′ by disturbing the average fields. The jet temperature changes the averaged density and entropy so as to change the cancelation mechanism. Two large-scale acoustically correlated parts of the coherent structures are visualized by the zonal correlation of the ρ′. Their acoustic correlations cancel each other in the hot jets, but enhance each other in the cold jets. The acoustically correlated regions represented by the zonal correlations of the s′ and |ω| are in small scale and gather around the end of the potential core region.

Analytical model for the suspended sediment concentration in the ice-covered alluvial channels

Ice cover formed on an alluvial channel can significantly alter the flow characteristics, such as the vertical distributions of streamwise velocity and shear stress, and hence the water and sediment transport process. The vertical profile of the suspended sediment concentration in the ice-covered alluvial channels with steady uniform flows is investigated in this study. To calculate the suspended sediment concentration, we are based on the Schmidt O’Brien equation and deduce an analytical model that employs an existing eddy viscosity model and a modified formula of the sediment fall velocity considering the common effects of the upper and lower boundaries. The proposed analytical model is then validated by using available experimental data reported in the literature. The predicted accuracy of the proposed model is evaluated through error statistics by comparing to previous modeled results. The relative concentration profiles of the suspended sediment are subsequently simulated by applying the validated analytical model with different characteristic parameters. Results show that the relative concentration decreases with the increase of both the ice cover roughness and the sediment fall velocity. The uniformity of the relative concentration distribution is closely related to the value of the proportionality parameter σ, revealing the physical mechanism that the more prominent the turbulent diffusion effect is, the more uniform the relative concentration profile is.

Analytical solution for depth-averaged velocity and boundary shear in a compound channel

Estimation of streamwise depth-averaged velocity and boundary shear stress are important requisites for modelling of river flows associated with flood events. Recently, many methods have emerged to predict these flow variables with great accuracy, but they provide unsatisfactory results in shear layer regions. This paper presents an improved methodology to predict depth-averaged velocity and bed shear stress for a straight compound channel flow. An analytical solution to the depth-integrated turbulent form of the Navier–Stokes equation is obtained. The transverse shear stress in the mixing region is modelled using an effective eddy viscosity concept that contains horizontal coherent structures and three-dimensional bottom turbulence. The secondary flow term is modelled by considering the log-law profile for streamwise velocity and half cosine curve for the transverse velocity component. The analytical solution is successfully applied to a wide range of experimental compound channels and field cases. The efficacy of the present solution has been successfully tested by comparing with observed values.

Blowing-only opposition control: Characteristics of turbulent drag reduction and implementation by deep learning

Opposition control is an effective active control strategy for drag reduction, which has been extensively investigated. In the current work, the essential characteristics of drag reduction by the blowing-only opposition control scheme (i.e., opposition blowing) in turbulent channel flow are investigated. It is demonstrated that, under the condition of constant wall-normal mass flux, the drag reduction achieved by the opposition blowing scheme is almost independent of the allocation of the blowing velocity among all the effective blowing points. This feature simplifies the complexity of the control scheme and provides great convenience for the application of the convolutional neural network (CNN) to implement the opposition blowing scheme, i.e., only the direction of the wall-normal velocity at the detection plane needs to be predicted. In this paper, both the streamwise and spanwise wall shear stresses are taken as the input of the CNN model, and the reasonability of the CNN model is verified from a statistical perspective. It is found that as long as the directions of the large wall-normal velocity fluctuations are accurately predicted, the opposition blowing scheme can be successfully implemented, in which the CNN model is able to ensure a high prediction accuracy. Furthermore, applying the trained network model to a flow at a higher Reynolds number than the training set can still accurately predict the directions of the large wall-normal velocity fluctuations, which generalizes the applicability of the CNN model.

Wall-Shear-Stress-Based Conditional Sampling Analysis of Coherent Structures in a Turbulent Boundary Layer

Abstract Coherent structures are critical for controlling turbulent boundary layers due to their roles in momentum and heat transfer in the flow. Turbulent coherent structures can be detected by measuring wall shear stresses that are footprints of coherent structures. In this study, wall shear stress fluctuations were measured simultaneously in a zero pressure gradient turbulent boundary layer using two house-made wall shear stress probes aligned in the spanwise direction. The wall shear stress probe consisted of two hot-wires on the wall aligned in a V-shaped configuration for measuring streamwise and spanwise shear stresses, and their performance was validated in comparison with a direct numerical simulation result. Relationships between measured wall shear stress fluctuations and streamwise velocity fluctuations were analyzed using conditional sampling techniques. The peak detection method and the variable-interval time-averaging (VITA) method showed that quasi-streamwise vortices were inclined toward the streamwise direction. When events were simultaneously detected by the two probes, stronger fluctuations in streamwise velocity were detected, which suggests that stronger coherent structures were detected. In contrast to the former two methods, the hibernating event detection method detects events with lower wall shear stress fluctuations. The ensemble-averaged mean velocity profile of hibernating events was shifted upward compared to the law of the wall, which suggests low drag status of the coherent structures related with hibernating events. These methods suggest significant correlations between wall shear stress fluctuations and coherent structures, which could motivate flow control strategies to fully exploit these correlations.

Direct numerical simulation of turbulent channel flow over random rough surfaces

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Machine-learning-based feedback control for drag reduction in a turbulent channel flow

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