Viscous near-wall flow in a wake of circular cylinder at moderate Reynolds numbers

Here we present the results of experimental investigation of a cross flow around a circular cylinder mounted near the wall of a channel with rectangular cross section. The experiments were carried out in the range of Reynolds numbers corresponding to the transition to turbulence in a wake of the cylinder. Flow visualization and SIV-measurements of instantaneous velocity fields were carried out. Evolution of the flow pattern behind the cylinder and formation of the regular vortex structures were analyzed. It is shown that in the case of flow around the cylinder, there is no spiral motion of fluid from the side walls of the channel towards its symmetry plane, typical of the flow around a spanwise rib located on the channel wall. The laminar-turbulent transition in the wake of the cylinder is caused by the shear layer instability.     

Stability of time-periodic flow with laminar boundary-layer separation

The influence of the low-frequency modulation of flow behind a rectangular backward-facing step on the amplitude characteristics of disturbances in the separated laminar boundary layer has been studied. The experimental data were obtained by the method of hot-wire anemometry in a wind tunnel at a low subsonic velocity. Response of the separated flow to the long-wave oscillations generated by a local source of disturbances on the surface of the experimental model was clarified. The low-frequency nonstationarity of the separation region leads to a growth of velocity fluctuations in the separated boundary layer, which dominate the laminar-turbulent transition and the state of the flow in the near-wall region.     

Structure of the channel flow behind a surface-mounted rib under conditions of laminar-turbulent transition

The PIV technique was used to measure the instantaneous vector fields of flow velocity and vorticity behind a thin cross-flow rib installed in a channel with laminar, transient, or turbulent flow. The data were treated statistically to determine the fields of mean longitudinal flow velocity and the correlations of pulsating velocity components 〈u′ν′〉 and 〈u′u′〉. Some features of the flow structure developing under conditions of laminar-turbulent transition behind the rib have been revealed.     

三维边界层内诱导横流失稳模态的感受性机理

边界层感受性问题是层流向湍流转捩的初始阶段,在转捩过程中起关键性作用,尤其是三维边界层流动.因此,研究三维边界层感受性问题对进一步理解层流向湍流转捩机理以及湍流成因具有重要的理论意义.采用数值方法研究自由来流湍流与三维壁面局部粗糙相互作用下三维边界层的感受性问题,确定是否能在三维边界层内寻找一种新的横流失稳模态;确定在何种条件下三维边界层内能诱导出定常、非定常的横流失稳模态;探索自由来流湍流的强度、展向波数和法向波数以及三维壁面局部粗糙的大小和结构类型等因素在自由来流湍流与三维壁面局部粗糙作用下三维边界层内被激发出的感受性过程中有何影响,并确定何种横流失稳模态在三维边界层感受性过程中占据何种地位.对自由来流湍流与三维壁面局部粗糙作用激发三维边界层内感受性问题的深入研究,将有助于完善流动稳定性与湍流理论,为层流向湍流转捩过程的预测与控制提供合理的理论依据.     

Acoustic testing of the vortex structure produced by an air flow about an array of cylinders

Results of experiments on the scattering of a plane ultrasonic wave from a vortex wake formed in an air flow behind a lattice of vertical cylinders are presented. The lattice is periodic in the direction perpendicular to the oncoming flow. The experiments are performed in a wind tunnel for two values of the Reynolds number, namely, Re = 75 and 500, and for lattices with different numbers of cylinders and with different lattice periods g = (2.5–15)d (where d is the diameter of the cylinders). The measured parameters of the scattered waves are used to estimate the degree of transverse correlation between the vortex wakes formed behind the cylinders for flows with different Reynolds numbers. The results obtained from an analysis of the characteristics of the scattered sound are compared with the results of direct hot-wire anemometer measurements and with the data obtained by other researchers.     

三维圆柱体绕流的发展和演化

结合三阶精度格式求解可压缩NS方程，本文研究了绕过三维圆柱体的流动结构，阐明了流动的演化机理。大攻角下，在三维圆柱体背风区形成了一个脱落涡序列，其截面流态非常类似于二维圆柱绕流，主涡、二次涡以及tertiary涡形成了一个层次结构。前一个主涡脱体后，tertiary涡将演化为其后续的新生主涡，并且合并圆柱体对称面另一侧的二次涡。     

湍流边界层中下扫流与“反发卡涡”

用氢气泡法观测湍流边界层的下扫流和有关的流动结构.实验中发现一种新型涡结构，它的特征与典型的发卡涡正好相反.发卡涡的头部指向下游，而它的头部指向上游； 发卡涡的两腿之间，由于涡的诱导产生上升流，而它则在其两腿之间，由于涡的诱导产生下扫流. 关键词： 湍流边界层 流动显示 流动结构 发卡涡     

磁通密度对第Ⅱ类超导体磁通动力学的影响

计算了二维无序钉扎系统中磁通运动的平均速度、微分电阻、纵向电压噪声和静态结构因子.通过在不同磁通密度下的磁通运动形式,给出了磁通运动的动力学相图.研究表明,磁通晶格存在钉扎相、塑性流相、近晶流相,和运动玻璃相.在运动玻璃相中,随着驱动力的进一步增加,横向玻璃态和运动Bragg玻璃态相继出现.磁通密度增大有利于有序相的出现.当磁通密度增大到一定程度时,近晶流动相会消失.磁通运动随着外加驱动电流增大发生从塑性流动相到运动玻璃相的转变 关键词： Ⅱ类超导体 磁通动力学 运动玻璃     

Stability of a subsonic gas microjet

The stability of and the laminar-turbulent transition in a plane subsonic helium microjet flowing out into the atmosphere is studied experimentally. The microjet experiences both natural perturbations and controlled periodic acoustic effects. The averaged and instantaneous flow fields are visualized using the Schlieren method and particle tracking method. The pulsation parameters of the mass flow rate are measured, and data for nonlinear interaction between the perturbations at the laminar-turbulent transition in the microstructure are obtained by bispectral analysis.     

前缘曲率对三维边界层内被激发出非定常横流模态的影响研究

三维边界层感受性问题是三维边界层层流向湍流转捩的初始阶段,是实现三维边界层转捩预测与控制的关键环节.在高湍流度的环境下,非定常横流模态的失稳是导致三维边界层流动转捩的主要原因;但是,前缘曲率对三维边界层感受性机制作用的研究也是十分重要的课题之一.因此,本文采用直接数值模拟方法研究在自由来流湍流作用下具有不同椭圆形前缘三维（后掠翼平板）边界层内被激发出非定常横流模态的感受性机制;揭示不同椭圆形前缘曲率对三维边界层内被激发出非定常横流模态的扰动波波包传播速度、传播方向、分布规律、感受性系数以及分别提取获得一组扰动波的幅值、色散关系和增长率等关键因素的影响;建立在不同椭圆形前缘曲率情况下,三维边界层内被激发出非定常横流模态的感受性问题与自由来流湍流的强度和运动方向变化之间的内在联系;详细分析了不同强度各向异性的自由来流湍流在激发三维边界层感受性机制的物理过程中起着何种作用等.通过上述研究将有益于拓展和完善流动稳定性理论,为三维边界层内层流向湍流转捩的预测与控制提供依据.     

Dynamic evolution of transverse energy flow in focused asymmetric optical vector-vortex beams

We present here controlled generation of asymmetric optical vector-vortex beams using a two-mode optical fiber and study the dynamic evolution of the transverse energy flow (TEF) when focused through a spherical lens. The dependence of the TEF on various factors such as the vortex charge, vortex anisotropy and polarization structure around the vortex core is explored. It is found that the TEF is directly proportional to the phase gradient and its direction is governed by the vortex charge. The presence of C-point polarization singularity in the beam and the polarization structure around it results in vibrational phase gradient which is the major factor deciding the TEF in vector-vortex beams.     

Visualization of turbulent flow around a sphere at subcritical reynolds numbers

The shear layer evolution and turbulent structure of near-wake behind a sphere atRe= 11,000 and 5,300 were investigated using a smoke-wire visualization method. A laminar flow separation was found to occur near the equator. The smooth laminar shear layers appeared to be axisymmetrically stable to the downstream location of aboutx/d=1.0 atRe=11,000 andx/d= 1.7∼1.8 atRe=5,300, respectively. At Re=11,000, the vortex ring-shaped protrusions were observed with the onset of shear layer instability. Moreover, the transition from laminar to turbulence in the separated flow region occurred earlier at the hiher Reynolds number ofRe=11,000 than atRe=5,300. The PIV measurements in the streamwise and cross-sectional planes atRe=11,000 clearly revealed the turbulent structures of the sphere wake such as recirculating flow, shear layer instability, vortex roll-up, and small-scale turbulent eddies.     

Numerical analyses of flow around airfoils subjected to flow induced vibration

This paper describes extensive computer-based analytical studies on the details of unsteady flow behavior around airfoils subjected to flow induced vibration in turbo-machinery. To consider the time-dependent motions of airfoils, a complete Navier-Stokes solver incorporating a moving mesh based on an analytic solution of motion equation for airfoil translation and rotation was applied. The drag and lift coefficients for the cases of stationary airfoils and airfoils subjected to flow induced vibration were examined. From the numerical results in non-coupling case as out of consideration of the airfoil motion, it was found that the separation vortex consisted of large-scale rolls with axes in the span direction, and rib substructures with axes in the stream direction. In the coupling simulation including the airfoil motion, both the translation and the rotation displacement were gradually increased when the airfoil translation and rotation natural frequencies synchronize exactly with the oscillation frequency of the fluid force. In addition, the transformation from complex structure with rolls and ribs to two-dimensional aspect of only rolls could be visualized in three-dimensional simulation.     

Numerical simulation of the evolution of unstable disturbances of various modes and initial stages of the laminar-turbulent transition in the boundary layer at the freestream Mach number М = 6

Direct numerical simulations of linear and nonlinear stages of the evolution of unstable disturbances of various modes and initial stages of the laminar-turbulent transition in the boundary layer on a flat plate at the freestream Mach number M = 6 are performed on the basis of full unsteady Navier–Stokes equations for a compressible gas. A considerable effect of three-dimensional unstable disturbances on initiation of the laminar-turbulent transition is demonstrated.     

Pattern of breakdown of laminar flow into turbulent spots

The breakdown into turbulent spots is the least understood stage of the laminar-turbulent transition process. With cellular-automaton stochastic simulations and stability analysis, we show that the pattern of breakdown in boundary-layer flow bears a connection to laminar instability and may be reconstructed using macroscopic properties of the transition zone, such as persistence times and transitional intermittency. We propose experimental tests of our ideas.     

Hysteresis of laminar-turbulent transition in a pipe with a divergent inlet section

Gas flow behavior near the laminar-turbulent transition condition in a pipe with a divergent inlet section has been studied experimentally. Hysteresis in the laminar-turbulent transition regime is observed. Also, we have observed unstable, regular vacillation of the flow rate when the time-averaged flow is controlled at the laminar-turbulent critical point with stationary background conditions. The text was submitted by the authors in English.     

The mechanism of breakdown in laminar-turbulent transition of a supersonic boundary layer on a flat plate-temporal mode

Recently, more and more attentions have been paid to the transition and turbulence of compressible flows, especially supersonic flows, but progress is very slow. In general, attentions were paid to the evolution of disturbances in laminar flow region, and the mechanism of breakdown process in laminar-turbulent transition remains unclear. The conventional idea of what leads to laminar-turbulent transition is that the transi-tion starts from the amplification of disturbances, and when the distur…     

Heat transfer peculiarities in separated flow past an oblique rib under different external turbulence

Results of an experimental study of turbulent flow past a flat rib installed at an angle to the free-stream direction are reported. In the experiments, external flows with two different turbulence numbers were used, and the angle of rib inclination to the free stream was varied from 50 to 90°. The experiments were performed for ribs of various heights under conditions with natural and high (13.4 %) free-stream turbulence levels. Visualization tests were performed to elucidate the vortex formation pattern and the direction of flow streamlines. Deformations of the recirculation region and secondary-vortex zone as well as enhanced effects due to 3D flow structure observed on decreasing the angle ϕ, and also notable restructuring of the flow at a high free-stream turbulence intensity, were identified. A comparison between pressure coefficients in different longitudinal sections of the channel is reported for ribs of various heights installed at various angles ϕ. The influence of rib inclination angle, rib height, and free-stream turbulence number on local heat-transfer coefficients and heat-transfer intensification is analysed. This work was financially supported by the Russian Foundation for Basic Research (Grant No. 06-08-00300).     

High-order shock-fitting methods for direct numerical simulation of hypersonic flow with chemical and thermal nonequilibrium

In recent years, much progress has been made in the direct numerical simulation of laminar-turbulent transition of hypersonic boundary layer flow. However, most of the efforts at the direct numerical simulation of transition previously have been focused on the idealized perfect gas flow or “cold” hypersonic flows. For practical problems in hypersonic flows, high-temperature effects of thermal and chemical nonequilibrium are important and cannot be modeled by a perfect gas model. Therefore, it is necessary to include the real gas models in the numerical simulation of hypersonic boundary layer transition in order to accurately predict flow field parameters. Currently most numerical methods for hypersonic flow with thermo-chemical nonequilibrium are based on shock-capturing approach at relatively low order of accuracy. Shock capturing schemes reduce to first-order accuracy near the shock and have been shown to produce spurious oscillations behind curved strong shocks. There is a need to develop new methods capable of simulating nonequilibrium hypersonic flow fields with uniformly high-order accuracy and avoid spurious oscillations near the shock. This paper presents a fifth-order shock-fitting method for numerical simulation of thermal and chemical nonequilibrium in hypersonic flows. The method is developed based on the state-of-the-art real gas models for thermo-chemical nonequilibrium and transport phenomena. Shock-fitting approach is used because it has the advantage of capturing the entire flow field with high-order accuracy and without any oscillations near the shock. The new method has been tested and validated for a number of test cases over a wide span of free stream conditions. The developed method is applied for the study of receptivity of free stream acoustic waves over a blunt cone for hypervelocity flow. Some preliminary results of the computations of the high order shock fitting method for the above mentioned study have also been presented.