Effects of finite-size heavy particles on the turbulent flows in a square duct

A parallel direct-forcing fictitious domain method is applied in fully-resolved numerical simulations of particle-laden turbulent flows in a square duct. The effects of finite-size heavy particles on the mean secondary flow, the mean streamwise velocity, the root-mean-square velocity fluctuation, and the particle concentration distribution are investigated at the friction Reynolds number of 150, the particle volume fraction of 2.36%, the particle diameter of 0.1 duct width, and the Shields number ranging from 1.0 to 0.2. Our results show that the particle sedimentation breaks the up-down symmetry of the mean secondary vortices, and results in a stronger secondary-flow circulation which transports the fluids downward in the bulk center region and upward along the side walls at a low Shields number. This circulation has a significant impact on the distribution of the mean streamwise velocity, whose maximum value occurs in the lower half duct, unlike in the plane channel case. The flow resistance is increased and the turbulence intensity is reduced, as the Shields number is decreased. The particles accumulate preferentially at the face center of the bottom wall, due to the effect of the mean secondary flow. It is observed that the collision model has an important effect on the results, but does not change the results qualitatively.     

A hydrodynamic investigation of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redd selection at the riffle scale

High‐resolution velocity profile measurements were taken over a series of riffles on a gravel‐bed stream using a Pulse Coherent Acoustic Doppler Profiler (PCADP) to quantify the fluid structure of riffles and nests (redds) where brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) spawned. Velocity profiles were obtained on a highly discretized planometric scale ranging between 20 and 40 cm grid spacings, with vertical observations occurring every 1.6 cm. From the velocity profiles, between 15 000 and 38 000 velocity measurements were obtained over each riffle on any given day of measurement. Velocity profiles were converted to streamwise velocity magnitude, flow depth, Reynolds number, Froude number, shear stress, vertical velocity components and turbulent kinetic energy per unit area to evaluate the spatial structure of the riffles and the spatial structure of redds (pits and tailspills) relative to the surrounding riffle structure. Semi‐variograms were employed to evaluate the persistence of the fluid structure based upon the metrics evaluated. Results showed that discrete velocity observations poorly described the spatial structure of the flow system and poorly correlated with redd locations. Reynolds number analysis identified a relatively consistent fluid property for distances typically 2–3 times the longitudinal length of redds. Turbulent kinetic energy per unit area consistently identified common regions on all riffles studied that corresponded with the location selections for redds where flow was identified as essentially uni‐directional. Froude number was found to be insensitive in predicting the fluid spatial structure in wadeable flow depths and relating it to the fluid structure of redds. Results indicated that a series of metrics at varying spatial scales of turbulence may be necessary to understand the spatial complexity of redd selection. Copyright © 2011 John Wiley & Sons, Ltd.     

ON THE ORIGIN OF TURBULENT SECONDARY FLOW IN A STRAIGHT DUCT OF NON-CIRCULAR CROSS-SECTION

The origin of turbulent secondary flow in a straight duct of non-circular cross-section is discussed from a theoretical standpoint and examined from a numerical simulation ofthe turbulent flow in a square duct,It is reached that the driving force of such secondary flowcomes from the second-order terms of the difference in gradients of normal and transverseReynolds stresses in the axial vorticity equation.The present results obtained compare well withavailable experimental results for both primary and secondary flow quantities.     

Numerical study of high-order Lagrangian structure functions in a turbulent channel flow with low Reynolds number

The scaling exponents of Lagrangian velocity structure functions from orders 1 to 10 in a low Reynolds number turbulent channel flow are investigated by using direct numerical simulation. The Reynolds number Re_τ is 80 (based on friction velocity on the wall). The Lagrangian velocity structure functions are shown to obey the scaling relations <Δv^q(τ)>∼τ^ζL(q). The scaling exponents are normalized by ζ_L(2) (so-called ESS procedure). The coincidence between the theoretical predictions and numerical calculations is very good for the longitudinal scaling exponent in the channel center. It is also found that the high-order longitudinal scaling exponents agree with theoretical values better than those for the transverse direction.

     

A high‐resolution hydrodynamic investigation of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redds

High‐resolution velocity measurements were taken over a series of redds on a gravel‐bed stream using a Pulse Coherent Acoustic Doppler Profiler (PCADP) to quantify the hydrodynamics of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) redds. On redds studied, over 4500 velocity measurements per redd were acquired per day to quantify the flow velocity, flow depth and related fluid mechanics metrics of Reynolds numbers, Froude numbers and turbulent kinetic energy per unit area. Results showed that velocity and Froude numbers varied widely at the redd scale, but consistently showed higher velocities and Froude numbers over the tailspill regions relative to the surrounding study limits. Results of Reynolds numbers calculations showed no apparent correlations to spawning location preference and redd structure. Turbulent kinetic energy per unit area consistently demonstrated a strong correlation with redd locations. The metric maintained low values (i.e. unidirectional flow with little turbulence) where all redds and attempted redds were observed. The study also demonstrates that a number of hydraulic metrics and several spatial scales will likely be necessary to understand any inherent relationship between river hydraulics and redd placement. Copyright © 2010 John Wiley & Sons, Ltd.     

添加网格对增强表面活性剂减阻流体传热性能的试验研究

本文应用热电偶测温系统和PIV（Particle Image Velocimetry）,激光测量系统对二维流道内添加网格后CTAC（Cetyltrimethyl Ammonium Chloride）表面活性剂减阻流体的传热性能和流动结构进行试验研究。研究表明：在完全减阻区内,减阻流体的传热性能大幅度降低;当雷诺数超过临界雷诺数后,传热性能呈指数级快速增长;网格的添加破坏了表面活性剂的胶束网状结构,在一定程度上恢复了流场的湍流强度,从而使传热效果显著增强。     

壁面微结构流动控制技术的减阻机理研究

为了研究壁面微结构流动控制技术的减阻效应及其产生的原因,利用循环管路系统的方形管道进行了压降测定试验,并利用粒子成像测速仪测量了边界层内部结构和对应的参数。试验采用了沟槽和肋条两种不同类型的微结构壁面,每种形状的微结构各有3种不同的结构尺寸。试验研究结果表明:在一定的无量纲宽度s+范围内,6种不同的微结构壁面都具有减阻效果;减阻率随着s+的增大,呈现先增大后减小的趋势,其中沟槽壁面2的减阻效果最好,最大减阻率为9.90%;壁面微结构通过影响流场内部的涡结构、湍流脉动、雷诺切应力和平均流速等使得不同壁面微结构具有减阻效果。     

Simulation of fine sediment transport in oscillatory boundary layer

Sediment transport and resulting morphodynamics take place at vastly different spatial and temporal scales. Bottom boundary layer dynamics is one of the most critical small-scale processes controlling sediment transport. However, it is very challenging as it involves highly nonlinear turbulent flow phenomenon that occurs at a scale as small as Kolmogorov scales. The unsteady nature of the flow forcing in the coastal environment, such as waves, is another important factor that further complicates the process. This paper aims to take a step to better understand wave-driven turbulent boundary layer and corresponding transport process. The problem of wave-driven sediment transport is simplified into fully developed oscillatory channel flow. Particles are assumed to be fine and dilute and hence they can be approximately considered as passive except gravitational settling with its settling velocity estimated from Stokes' law. Highly accurate pseudo-spectral flow solver has been employed and Direct Numerical Simulation (DNS) is carried out to resolve all scales of flow turbulence without sub-grid closure. Sediment concentration is then updated via mass conservation. Clear fluid simulation for intermittently turbulent condition (i.e., Stokes Reynolds number Re_Δ = 1000) is validated with earlier DNS results reported by Spalart and Baldwin [1989. Direct simulation of a turbulent oscillating boundary layer. Turbulent Shear Flows 6. Springer]. Fine sediment with dilute concentration is then added in the numerical simulation to study sediment suspension events under oscillatory flow. Suspension events are evaluated both qualitatively, by observing vortex structures, and quantitatively by the statistics of vertical fluxes. For fine particles and dilute concentration considered here, it is observed that particle phase is well-mixed in the boundary layer. Suspension is highly dependent on turbulent vortical structures at different wave phases.     

LARGE-EDDY SIMULATION OF FLOW AROUND CYLINDER ARRAYS AT A SUBCRITICAL REYNOLDS NUMBER

The complex three-dimensional turbulent flows around a cylinder array with four cylinders in an in-line square configuration at a subcritical Reynolds number of 1.5 × 10^4 with the spacing ratio at L/D = 1.5 and 3.5 were investigated using the Large Eddy Simulation （LES）. The full field vorticity and velocity distributions as well as turbulent quantities were calculated in detail and the near wake structures were presented. The results show that the bi-stable flow nature was observed at L/D = 1.5 and distinct vortex shedding of the upstream cylinders occurred at L/D = 3.5 at Re = 1.5 × 10^4. The techniques of Laser Doppler Anemometry （LDA） and Digital Particle Image Velocimetry （DPIV） are also employed to validate the present LES method. The results show that the numerical predictions are in excellent agreement with the experimental measurements. Therefore, the full field instantaneous and mean quantities of the flow field, velocity field and vorticity field can be extracted from the LES results for further study of the complex flow characteristics.     

明渠柱体上游马蹄涡的运动学特征研究

马蹄涡是水下柱体结构周围床面冲刷的主要动力。为掌握马蹄涡的运动学特征,引入了基于旋转强度的马蹄涡识别方法,提出了基于椭圆涡拟合的马蹄涡特征提取方法,其中,椭圆涡由Oseen涡和倾斜单向剪切流叠加而成。基于上述方法,通过开展明渠圆柱绕流PIV实验,对柱体上游对称面时均流场中马蹄涡的运动学特征进行了研究。结果表明,在柱体雷诺数大于104的紊流柱体绕流中,马蹄涡上游的流动分离点随柱体雷诺数的增大而缓慢向下游移动,但马蹄涡的位置保持不变,涡中心距柱体中轴约0.67D(D为柱体直径或宽度),距床面约0.06D;随着柱体雷诺数的增大,马蹄涡的半径不变而强度增加,使得马蹄涡作用于床面的剪切应力增大。研究结果建立了客观提取马蹄涡运动学特征的数学方法,得到了流动分离点、马蹄涡位置等运动学特征随水流条件的变化规律,揭示了明渠柱体绕流与其它流动中柱体绕流的差异,初步构建了马蹄涡的动力作用概念模型。     

Simulation of buoyancy-induced turbulent flow from a hot horizontal jet简

Experimental visualizations and numerical simulations of a horizontal hot water jet entering cold water into a rectangular storage tank are described. Three different temperature differences and their corresponding Reynolds numbers are considered. Both experimental visualization and numerical computations are carried out for the same flow and thermal conditions. The realizable k-e model is used for modeling the turbulent flow while the buoyancy is modeled using the Boussinesq approximation. Polynomial approximations of the water properties are used to compare with the Boussinesq approximation. Numerical solutions are obtained for unsteady flow while pressure, velocity, temperature and turbulence distributions inside the water tank as well as the Froude number are analyzed. The experimental visualizations are performed at intervals of five seconds for all different cases. The simulated results are compared with the visualized results, and both of them show the stratification phenomena and buoyancy force effects due to temperature difference and density variation. After certain times, depending on the case condition, the flow tends to reach a steady state.     

泵站钟形进水流道吸水室后壁形状的研究

基于三维不可压缩流体的雷诺平均N-S方程和RNGk-ε紊流模型,对矩形、半圆形和"ω"吸水室后壁形状的钟形进水流道泵装置进行了三维紊流数值模拟,并分析了钟形进水流道后壁形状对泵装置水力特性的影响。相同流量下,"ω"形吸水室进水流道流线分布最规则,漩涡比其他形状的进水流道小,"ω"形吸水室进水流道水力损失比矩形吸水室进水流道小1cm;出口断面的流速均匀度达到93%,比矩形和半圆形吸水室进水流道高约2个百分点;出口断面速度加权平均角度达到83.5°,比矩形吸水室进水流道高0.6°,比半圆形吸水室进水流道高0.2°;泵装置运行高效区流量范围比半圆形的拓宽了7.3%,比矩形的拓宽了30%。该研究对于完善泵站钟型进水流道吸水室优化设计具有一定意义。     

The direct numerical simulation of pipe flow

The conservative difference scheme and the third-order Runge-Kutta scheme in combination with the the Crank-Nicholson scheme are used to directly simulate the flow field in a pipe with the Reynolds number of 2 600. The flow field, including the velocity distribution and the turbulence intensity, is obtained by the direct numerical simulation. From the calculated results, the ratio of the linear average velocity along the ultrasonic propagation path to the profile average velocity on the pipe cross-section is also obtained in an ultrasonic flow meter. It is concluded that the direct numerical simulation method can be used to study the ratio of the profile-linear average velocity at low Reynolds number conditions in the transition region and to improve the measurement accuracy of the ultrasonic flow meter.     

部分冰盖下水流水力特性数值模拟

运用Fluent软件建立雷诺应力模型,模拟不同冰盖覆盖度下恒定均匀流流场,从纵向时均流速、二次流、雷诺应力、紊动能等方面揭示部分冰盖下水流的水力特性。结果表明：岸冰的形成会导致冰盖下水流在断面上重新分配,明流区流速增大,冰盖下流速减小;横向动量交换产生复杂的二次流结构,涡体形状、数量、大小和位置随着冰盖覆盖度的增加而变化;雷诺应力断面分布较复杂,冰盖附近区域雷诺应力为负值,负值区范围与冰盖覆盖度密切相关,并凸向明流区;冰盖、边壁和床面的粗糙度不同导致紊动能在断面上的分布呈现出明显的差异,冰盖区紊动能大于明流区的值,反映出岸冰对冰盖下水流结构影响的复杂性。     

Analysis of shear rate effects on drag reduction in turbulent channel flow with superhydrophobic wall

We examined the shear rate effect on drag reduction of superhydrophobic surfaces with different slip lengths. For this purpose, turbulent channel flow was considered at the friction Reynolds numbers of Reτ = 180, 395, 500. By using Navier＇s slip condition it is shown that increasing shear rate leads to the greater reduction in drag force and also more reduction occurs in larger slip length. Based on the results, more than 25% drag reduction happens at a friction Reynolds number of Reτ= 500 for slip length of 1 ×10 5 m. The simulation results suggest that reduction in drag force occurs because slip condition reduces the Reynolds stresses, also weakens vorticity filed and the near-wall coherent structures, and therefore turbulence production is decreased.     

Large eddy simulation of water flow over series of dunes

Large eddy simulation was used to investigate the spatial development of open channel flow over a series of dunes. The three-dimensional filtered Navier-Stokes (N-S) equations were numerically solved with the fractional-step method in sigma coordinates. The subgrid-scale turbulent stress was modeled with a dynamic coherent eddy viscosity model proposed by the authors. The computed velocity profiles are in good agreement with the available experimental results. The mean velocity and the turbulent Reynolds st...     

上方来水来沙对细沟水流水力学参数的影响

利用供沙土槽和细沟土槽双土槽径流小区系统 ,定量分析了粘黄土陡坡地在 5 0、90和 1 30mm h降雨强度下上方来水来沙对坡下方细沟水流水力学参数变化的影响。结果表明 ,上方来水的汇入和降雨强度的增大 ,可使细沟水流流态由层流转变为紊流。上方来水来沙使细沟水流的流速、雷诺数、弗罗得数明显增大 ,而阻力系数明显减少 ,从而导致细沟侵蚀产沙量增大。定量分析了细沟水流流速、雷诺数和阻力系数与上坡来水引起坡下方净细沟侵蚀产沙量的关系 ,建立了一定条件下净细沟侵蚀产沙量与流速相对增量、雷诺数相对增量和阻力系数相对减量的数学统计模型     

分岔管道三维湍流水力特性数值模拟

建立了分岔管道水动力学的计算模型,求解三维贴体坐标系下水流控制方程。计算中采用修正的k-ε湍流模型,应用有限体积法离散方程,物理变量采用同位网格布置,控制体表面上各变量的值用界面动量插值法计算,速度-压力耦合计算采用SIMPLEC算法。本文应用该数值模型模拟了三维弯道水流的水力特性,并与模型试验实测值进行了比较,结果吻合较好。最后应用该数值模型预测了35°分岔管道连接区域三种流动情况的水力特性。     

开孔促淤板周围水流紊动特性试验研究

为深入了解开孔促淤板周围的水流特性,通过室内概化水槽试验分析了开孔促淤板前后的水流紊动特性,采用声学多普勒流速仪(ADV)测量了开孔促淤板前后典型断面上的三维瞬时流速,分析了典型断面上的时均流速、紊动强度、雷诺切应力和紊动能的分布规律。试验结果表明:板后近板区出现回流区,且回流区与开孔促淤板格栅位置相对应;板后水流相对紊动强度、雷诺切应力及相对紊动能明显大于板前,开孔促淤板对板后近板区水流影响较大;随着与开孔促淤板距离的增大,回流现象消失,流速变化趋于稳定,水流相对紊动强度减小且最终趋于稳定,相对紊动能出现折减且折减率逐渐降低;靠近近板区xOy面雷诺切应力大于yOz面雷诺切应力,靠近下游xOy面雷诺切应力与yOz面雷诺切应力相差较小。     

PIV MEASUREMENTS OF THE NEAR-WAKE FLOW OF AN AIRFOIL ABOVE A FREE SURFACE

The near-wake flow of a NACA0012 airfoils mounted above a water surface were experimentally studied in a wind/wave tunnel. The main objective of this study is to investigate the influence of the free surface on the structure of the airfoil trailing wake. The flow structure was measured with different ride heights between the airfoil and free surface using a Particle Image Velocimetry (PIV) system. The Reynolds number based on the chord length of the airfoil was about 3.5×103. For each experimental condition, large amount of instantaneous velocity fields were captured and ensemble-averaged to get the spatial distributions of mean velocity and mean vorticity, as well as turbulence statistics. The results show that the flow structures of the airfoil wake varies remarkably with the change in the ride height.