Heat exchange at laminar flow in rectangular channels

Numerical modeling of heat exchange at a laminar stationary and pulsatile flow in rectangular channels with different aspect ratios of side lengths γ has been carried out by a finite difference method for two boundary conditions: a constant wall temperature and a constant heat flux density on the wall. For the boundary condition of the first kind, the similarity of distributions of the heat flux density and shear stress on the walls over the channel perimeter has been established. The reasons for a nonmonotonous dependence of the initial thermal interval length on γ are discussed. For the boundary condition of the second kind, the difference of the Nusselt number averaged over the perimeter at γ → 0 from its value for a flow in a flat channel has been explained. An increase in the Nusselt number averaged over the perimeter and the period of oscillations has been revealed for a pulsatile flow in the quasi-stationary regime at large amplitudes of the oscillations of the velocity averaged over the cross section.     

Lattice Boltzmann method simulating hemodynamics in the three-dimensional stenosed and recanalized human carotid bifurcations

By using the lattice Boltzmann method(LBM)pulsatile blood flows were simulated in three-dimensional moderate stenosed and recanalized carotid bifurcations to understand local hemodynamics and its relevance in arterial atherosclerosis formation and progression.The helical flow patterns,secondary flow and wall dynamical pressure spatiotemporal distributions were investigated,which leads to the disturbed shear forces in the carotid artery bifurcations.The wall shear stress distributions indicated by time-averaged wall shear stress(TAWSS),oscillatory shear index(OSI),and the relative residence time(RRT)in a cardiac cycle revealed the regions where atherosclerotic plaques are prone to form,extend or rupture.This study also illustrates the point that locally disturbed flow may be considered as an indicator for early atherosclerosis diagnosis.Additionally the present work demonstrates the robust and highly efficient advantages of the LBM for the hemodynamics study of the human blood vessel system.     

Local characteristics of the upward bubbly flow in an annular channel

Results of experimental investigation of the bubbly gas-liquid flow in a vertical annular channel are presented. The average and pulsation shear stresses and distributions of local void fraction were measured by the electrochemical method on both channel walls. It is shown that with a rise of gas flow rate ratio the value of wall shear stress increases significantly, and this effect becomes higher at a decrease in superficial liquid velocity. A presence of the gas phase effects significantly shear stress on the inner wall. Relative intensity of shear stress pulsations increases similarly on both channel walls.     

Influence of bubbly clusters on the characteristics of the two-phase gas-liquid flow in a flat channel

Results of experimental investigation of a bubbly gas-liquid flow in horizontal and weakly inclined (from −20° to +20°) flat channel are presented. These measurements were carried out within the 0.2–1 m/s range of superficial velocities and volumetric gas flow rate ratio of up to 0.2. The hydrodynamic structure was measured by the electrochemical method with application of wall shear stress and conductivity microprobes. During the experiments signals of shear stress on the upper channel wall and local gas flow rate ratio were recorded completely. After numerical treatment of recorded signals the profiles of local gas flow rate ratio were obtained, average shear stress and its relative mean square pulsations on the upper channel wall were determined. It is shown that under the studied regimes the bubbles are grouped into clusters, and the bubbly flow is presented by alternation of bubbly clusters and single-phase liquid with separate bubbles and without them. Average wall shear stress and absolute shear stress pulsations in the range of bubbly clusters and beyond them were determined. Histograms of probability density distribution were obtained for the wall shear stress on the upper wall. It is shown that average shear stress and absolute pulsations in clusters are significantly higher than those in the flow zone free from bubbles. The work was financially supported by the Russian Foundation for Basic Research (No. 07-08-00405a).     

纳米通道内气体剪切流动的分子动力学模拟

采用分子动力学模拟方法研究了表面力场对纳米通道内气体剪切流动的影响规律.结果显示通道内的气体流动分为两个区域:受壁面力场影响的近壁区域和不受壁面力场影响的主流区域.近壁区域内,气体流动特性和气体动力学理论预测差别很大,密度和速度急剧增大并出现峰值,正应力变化剧烈且各向异性,剪切应力在距壁面一个分子直径处出现突变.主流区域的气体流动特性与气体动力学理论预测相符合,该区域内的密度、正应力与剪切应力均为恒定值,速度分布亦符合应力-应变的线性响应关系.不同通道高度及密度下,近壁区域的归一化密度、速度及应力分布一致,表明近壁区域的气体流动特性仅由壁面力场所决定.随着壁面对气体分子势能作用的增强,气体分子在近壁区域的密度和速度随之增大,直至形成吸附层,导致速度滑移消失.通过剪切应力与切向动量适应系数(TMAC)的关系,得到不同壁面势能作用下的TMAC值,结果表明壁面对气体分子的势能作用越强,气体分子越容易在壁面发生漫反射.     

Large Eddy Simulation of Turbulent Channel Flow with 3D Roughness Using a Roughness Element Model

Large eddy simulation of turbulent channel flow with dense and small 3D roughness elements is carried out using a roughness element model Profiles of mean Reynolds stress, mean velocity and rms velocity as well as turbulent structures near the wall are obtained. The shear stress in the rough wall is larger than that in the smooth wall side and the rough wall has a larger influence on the channel flow. Profiles of mean streamwise velocity near the wall have logarithmic velocity distributions for both smooth and roughness walls, while there is a velocity decrease for the rough wall due to larger fractional drag. All the three components of rms velocities in the rough wall region are larger than that in the smooth wall region, and the roughness elements on the wall increase turbulent intensity in all directions. The s~reak spacing and average diameter of near wall quasi-s~reamwise vortices increase with the presence of roughness elements on the wall and it is shown that the rough wall induces complex and strong streamwise vortices. Results of dense and small 3D roughness elements in both turbulent statistics and structure, obtained with a relatively simple method, are found to be comparable to related experiments.     

Surface roughness effects on the turbulence statistics in a low Reynolds number channel flow

A high-resolution particle image velocimetry was used to characterize a low Reynolds number turbulent flow in a channel. Experiments were conducted over a sand grain-coated surface of large relative roughness, and the results were compared with measurements over a smooth surface. The roughness perturbation significantly modified the outer layer. Even though the streamwise Reynolds stress shows less sensitivity in the outer layer to the boundary condition, significant enhancements were observed in the wall-normal Reynolds stress and the Reynolds shear stress. These modifications were considered as footprints of the larger-scale eddies transporting intense wall-normal motions away from the rough wall. A quadrant decomposition shows that strong and more frequent ejections are responsible for the larger values of the mean Reynolds shear stress over the rough wall. The results also indicate that spanwise vortex cores with mean vorticity of the same sign as the mean shear are the dominant smaller-scale vortical structures over the smooth and rough walls. A linear stochastic estimation-based analysis shows that the average larger-scale structure associated with these vortices is a shear layer that strongly connects the outer layer flow to the near-wall flow. A proper orthogonal decomposition of the flow suggests that the large-scale eddy is more energetic for the rough wall, and contributes more significantly to the resolved turbulent kinetic energy and the Reynolds shear stress than the smooth wall.     

横向粗糙元壁面槽道湍流的大涡模拟研究

采用大涡模拟和浸没边界法相结合对不同高度和不同间距横向粗糙元壁面槽道湍流进行了模拟,得到了光滑壁面和粗糙壁面湍流的流向平均速度分布,雷诺剪切应力,脉动速度均方根和近壁区拟序结构。结果发现横向粗糙元降低了流向平均速度,增大了流动阻力,粗糙壁面湍流的雷诺剪切应力大于光滑壁面。粗糙元降低了流向脉动速度,增强了展向和法向脉动速度。粗糙元高度越高,对湍流流动影响越大,而粗糙元间距对湍流统计特性的影响不大。粗糙壁面仍然存在着和光滑壁面类似的条带结构。     

Numerical study of the pulsatile flow depending on non-Newtonian viscosity in a stenosed microchannel

Considering the shear-thinning feature of blood viscosity, the characteristics of non-Newtonian fluids are important in pulsatile blood flows. Stenosis, with an abnormal narrowing of the vessel, blocks blood flow to downstream tissues and leads to plaque rupture. In smaller arteries of diameters up to a few hundred micrometers, such stenosis can result in severe consequences. Therefore, a systematic analysis of the blood flow around the stenosed microchannel is important. In this study, non-Newtonian behaviors of the blood flow around a microchannel of diameter 500 μm, with 60% severe stenosis, were examined using CFX under pulsatile flow condition, with a period of 1 s and Reynolds number of 14.025 at the systolic phase. The viscosity information of the two non-Newtonian samples and the used pulsatile profile were based on our previous study. For comparison, water at room temperature was used as the Newtonian fluid. During the pulsatile phase, wall shear stress (WSS) is highly oscillated. In the case of the water flow, the recirculation occurred downstream the stenosis. This recirculation made the vortex structures travel the longest and induced a low WSS distribution and rapid normalized pressure drop at downstream of the stenosis. Conversely, the non-Newtonian feature of viscosity made flow structures almost symmetric, with respect to the stenosis. However, the highly oscillating WSS enhances the tendency of plaque instability and damage to the endothelium. Our findings on the influence of blood viscosity on stenotic lesions may help clinicians understand relevant mechanisms.     

带外力项的iDdQ(q-1)多松弛格子Boltzmann模型

以3维14速（iD3Q14）多松弛格子Boltzmann（MRT LB）模型为例，在iDdQ（q-1）MRT LB模型的基础上，采用多尺度展开和反向设计法构造出带外力项的iDdQ（q-1）MRT LB模型，该模型在低Mach数的假设下可恢复到带外力项的不可压Navier-Stokes方程.通过对三维方形管道内泊肃叶流、脉动流以及二维Taylor涡衰减流的模拟发现，数值结果与解析解吻合很好，并且空间精度达到二阶，从而验证了新模型模拟外力驱动的稳态和非稳态不可压流动的有效性.     

Asymmetry of the wall shear stress in a stimulated ascendant flow of a liquid with monodisperse bubbles

Results of an experimental investigation of the two-phase wall shear stress averaged over the tube perimeter and the pulsation of wall shear stress in a stimulated ascendant flow with monodisperse bubbles with an average diameter of 1.2 and 2.2 mm are presented. Regimes with various hydrodynamic parameters such as high shear stress on the wall, low and negative wall shear stress, a high level of shear stress pulsation on the wall, and possible decrease in this level of pulsation are found. An increase in the void gas fraction results in a monotonic increase of perturbation of the single-phase flow. The dependences of the ratio of two-phase and single-phase wall shear stresses for two average bubble diameters seem to be qualitatively similar. The analysis of data revealed a complex dependence of the shear stress pulsation on the bubble diameter. The averaged flow characteristics quantitatively change upon the decrease in the bubble diameter. A further decrease in the average bubble diameter at the same void gas fraction will probably increase the heat-and mass-transfer characteristics of the flow. This is an issue for the futures study.     

Wall Shear Stress and Heat Transfer of Downward Bubbly Flow at Low Flow Rates of Liquid and Gas

The effect of suppression of turbulence in a downward bubbly flow and its impact on the wall shear stress and heat transfer are discussed. Measurements were carried out for Reynolds numbers Re = 5000–10000, which were calculated from the velocity of the liquid phase and with the gas volumetric flow rate ratio β = 0–0.05. Data on the size of bubbles detaching from the edges of an array of capillaries in a liquid flow are given. The influence of the disperse phase dimensions on the wall shear stress and heat transfer is discussed. It is shown that change in the size of the dispersed phase can lead to both intensification and deterioration of heat transfer as compared with a single-phase flow at constant flow rates of liquid and gas at the channel inlet. The cause of the heat transfer deterioration is “laminarization” of the flow in the near-wall region. An analysis of the spectral power of signals is given.     

Suboptimal control of wall turbulence with arrayed dimple actuators for drag reduction

Direct numerical simulation of a turbulent channel flow with moving dimples at the bottom wall is carried out using the pseudo-spectral method and the curvilinear coordinate system. Suboptimal control based on the spanwise wall shear stress is applied for skin-friction drag reduction, and is implemented by the finite-size sensor-actuator system. The control law is realised in physical space by using a cross-shaped truncation of the wall shear stress information, which can be measured by the sensor. Only the information of wall shear stress inside the sensor area is utilised and that outside the sensor area is obtained by a linear reconstruction from the averaged value over the sensor. To effectively intervene the near-wall coherent structures, low-pass filtering of the spanwise wall shear stress is additionally implemented to eliminate the extra disturbances induced by the presence of dimple actuators, and the filtered stress is used as the control input. Numerical tests on the present control strategy show that the pressure form drag caused by the presence of dimples is reduced significantly as compared with the original suboptimal control, and the reduction of total drag is comparable with that of the opposition control. The underlying mechanism is further analysed by looking into the interaction between the moving dimples and the near-wall coherent structures.     

Heat transfer and shear stress in a gas-liquid flow in an inclined flat channel

Results of experimental investigation of a bubbly gas-liquid flow in an inclined flat channel are presented. Themeasurements were carried out in the range of superficial liquid velocities of 0.3–1.1 m/s and with different values of the volumetric gas flow rate ratio. The hydrodynamic structure wasmeasured bymeans of an electrochemical method using miniature shear stress probes. Values of average shear stress and heat transfer coefficient for different orientation of the channel were found. It is shown that in a bubbly gas-liquid flow the shear stress and heat transfer depend substantially on the channel inclination angle.     

Visualisation of dynamic flow birefringence of cardiovascular models

In this paper, the method of dynamic flow birefringence (DFB) have been studied extensively under the consideration of an application to cardiovascular models. The method utilises the optical interference patterns observed in the birefringent flow for determination of the fluid shear stress and velocity distribution. In order to measure a flow in a cardiovascular model, an assumption of a simplified stress–optical relation in a pulsatile flow is suggested and special experimental techniques such as birefringent fluid for simulating blood and new experimental system have been developed. Application studies focus on pulsatile flows in typical models, namely arterial bifurcation and mechanical heart valves. Experimental results are discussed and compared with those of other researchers.     

Experimental study of the Taylor bubbles shear stress in an upward flow in a vertical tube

The modification of electrodiffusional method of the wall shear stress measurements is applied for registration of the Taylor bubble shear stress in an upward liquid flow. Time realization of shear is considered as a structure frozen into the flow, which moves together with a bubble. Experiments were carried out in laminar and transitional liquid flows. The wall shear stress in the liquid film around bubble averaged over the tube perimeter is presented for different flow Reynolds numbers and different lengths of the bubble.     

倾斜上升管内气液两相弹状流壁面传质特性研究

采用VOF模型对倾角为45°、80°、85°三种情况下倾斜上升管内弹状流的壁面传质特性进行了研究.传质特性通过其与壁面切应力的类比关系来体现.数值模拟结果表明,低混合物流速时,上管壁面切应力在液膜区有明显波动,而下管壁面切应力分布则比较光滑.随着混合流速的增大,液膜区上下壁面切应力分布趋于一致.管子下壁面切应力平均值大于管子上壁面,在Taylor气泡运动速度较低时较为突出.随着Taylor气泡速度的增大,管子上下壁面的切应力平均值趋于相同.相同的混合流速下倾斜角度越大,上下管壁的切应力分布越趋于近似.下降液膜区的壁面切应力平均值大于Taylor气泡尾迹区域.根据Chilton-Colburn的类比关系,壁面切应力的规律完全适用于壁面传质系数.     

Nonlinear dynamics of an interface between shear bands

We study numerically the nonlinear dynamics of a shear banding interface in two-dimensional planar shear flow, within the nonlocal Johnson-Segalman model. Consistent with a recent linear stability analysis, we find that an initially flat interface is unstable with respect to small undulations for a sufficiently small ratio of the interfacial width l to cell length L(x). The instability saturates in finite amplitude interfacial fluctuations. For decreasing l/L(x) these undergo a nonequilibrium transition from simple traveling interfacial waves with constant average wall stress, to periodically rippling waves with a periodic stress response. When multiple shear bands are present we find erratic interfacial dynamics and a stress response suggesting low dimensional chaos.     

Strengthened opposition control for skin-friction reduction in wall-bounded turbulent flows

An opposition control scheme with strengthened control input is proposed and tested in turbulent channel flows at friction Reynolds number Re_τ = 180 by direct numerical simulations. When the detection plane is located at less than 20 wall units, the drag reduction rate can be greatly enhanced by increasing the control amplitude parameter. The maximum drag reduction rate achieved in the present study is around 33%, which is much higher than the best value of 25% reported in literature. The strengthened control can be more efficient to attain a given drag reduction rate. Based on the total shear stress at the virtual wall established between the real wall and the detection plane by the control, a new friction velocity is proposed and the corresponding coordinate transform is made. Scaled by the proposed friction velocity, the wall-normal velocity fluctuation and the Reynolds shear stress of the controlled flows are collapsed well with those of the uncontrolled flow in the new coordinate. Based on the similarity, a relation between drag reduction rate and the effectiveness of the virtual wall is deduced, which disclosed that the elevation and residual Reynolds shear stress at the virtual wall are the key parameters to determine the drag reduction rate. The conclusion are also validated at Re_τ = 395 and 590. The decrease of the drag reduction rate with the increase of the Reynolds number is attributed to the enhanced residual Reynolds shear stress at the virtual wall.     

狭窄程度对锥缩血管中脉动流的影响

建立了一个轴对称的狭窄锥缩刚性动脉模型.利用不可压缩的N-S方程作为计算的理论基础,采用有限差分方案进行数值分析.提供了3种狭窄程度下的管壁切应力分布曲线并进行了分析和比较,同时给出了模型M3(h/R₀=1/3)在一个流动周期内的流线图.结果显示狭窄附近的流场受到了干扰,特别是狭窄的喉部及其下游区域的流场,并且狭窄程度越严重,狭窄对流场的影响就越大.