基于UGKS的过渡区方柱绕流数值研究

本文基于统一气体动理学格式(Unified Gas Kinetic Scheme,UGKS),对微尺度过渡区气体绕流方柱开展数值模拟研究,计算分析了Kn数对气体流动传热过程的影响规律。研究发现,随着Kn数增加,方柱壁面气体速度滑移和温度跳跃增大,壁面上压力、剪切力和热流也相应增大,方柱壁面-气体之间的换热得到强化;方柱对气体流动的阻碍作用减小,方柱前滞止区影响范围相对增大,方柱温度对柱后区域气体温度影响相对减小。     

串列双圆柱时均流场特性分析

对Re=12000,间距比L/D=1.167,2.333,3.500和4.667的串列双圆柱后方速度场进行了实验测量,分析串列双圆柱后方不同剖面处的速度分布规律和湍流度分布规律.并通过流函数理论模型对小间距比串列双圆柱后方流场进行了分析,得出如下结论:与单圆柱相比,当串列双圆柱间距比较小时,后方圆柱的自由剪切层明显向内...     

电磁力控制湍流边界层分离圆柱绕流场特性数值分析

在亚临界区高雷诺数Re=1.4×105下,采用脱体涡模拟结合湍流分离的方法对弱电解质中电磁力作用下湍流边界层分离圆柱绕流场及其升(阻)力特性进行了数值模拟和分析.结果表明,电磁力可以提高圆柱体湍流边界层内的流体动能,延缓圆柱体湍流边界层的流动分离,减弱圆柱体湍流绕流场中在流向和展向上大尺度漩涡的强度,减小圆柱体阻力时均值及其升力脉动幅值.当电磁力作用参数大于某个临界值后,湍流边界层流动分离消失,在圆柱体尾部产生射流现象,从而电磁力对圆柱体产生净推力作用,出现负阻力现象,而且升力脉动幅值接近于零,出现圆柱体升力消失现象.     

Effects of roughness elements on bypass transition induced by a circular cylinder wake

Abstract  

The bypass transition of flat-plate boundary layer induced by a circular cylinder wake under the influence of roughness elements is experimentally investigated. The hydrogen-bubble visualization results show that the boundary layer separation occurs upstream of the roughness, and the separated shear layer is incised by roughness to different extent, resulting in different kinds of secondary vortices formed immediately downstream of the roughness. During the evolution of the secondary vortex, two types of instabilities are observed, which are denoted as large- and small-scale instabilities, respectively, according to different spatial scale of the hairpin vortices formed afterward. A merging process of hairpin vortices is also observed when the secondary vortices undergo the small-scale instability, and a potential new transition control strategy based on the present observation is proposed.     

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.     

Structure conditioned velocity statistics in a high pressure swirl flame

A piloted, partially premixed, liquid-fueled swirl burner is operated at high pressure (1 MPa). High-speed (6 KHz) stereoscopic PIV is used to investigate the characteristics of the stagnation line separating the pilot jet and the central recirculation zone (CRZ) with varying pilot-main ratio and global equivalence ratio. The mean curvature of the stagnation line displayed a large spatial scale pattern that was present for all operating conditions. All three components of velocity, in-plane shear, and swirling strength are conditioned upon the instantaneous stagnation line. Mean distributions of the velocity normal to the stagnation line show that velocity is oriented towards the CRZ when the stagnation line is found nearer the centerline of the combustor. The conditioned out-of-plane velocity (w) shows a distinct concentration of large mean and fluctuation RMS values towards the center of the measurement domain. Varying fuel flow does not significantly change this spatial structure, only the magnitudes of the w statistics. The in-plane shear stress was the largest for the pilot biased condition as a stronger shear layer develops. For the leanest flame, large fluctuation RMS values of shear stress were confined to a region where the pilot jet begins to interact more heavily with the main jet. Operating with less pilot fuel flow enhanced the mean conditional swirling strength indicating that the pilot shear layer was shedding more intense eddies. Disregarding spatial relations, a scatter plot of w, shear stress, and swirling strength displayed trends between the variables. The largest swirling strength values coincide with highest magnitude shear stresses and the widest range of w. These conditioned statistics highlight how certain aspects of the combustor flow field are invariant with fuel distribution. This is desirable for aeropropulsive combustors that must maintain stable ignition from a range of conditions from landing/take-off to cruise.     

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.     

Handling of temperature dependence of viscosity in problems of incompressible medium flow around a cylinder

The viscous incompressible medium (water, air) flow past a circular cylinder is considered with regard for the temperature T dependent viscosity v. The influence of different boundary conditions for temperature on flow structure, the drag coefficient and its components due to the pressure and viscosity is investigated in the problem of the flow past a cylinder at rest for the (diameter-based) Reynolds number Re_D = 40. A relation between the viscosity gradient along a normal to the body surface and the integral vorticity flux from the body surface into the boundary layer is discussed. Unlike the constant viscosity case the vorticity flux may be different from zero, which must lead because of the integral conservation law for the vorticity to an alteration of the far-field boundary conditions for the velocity. In the same connection, the problem is analysed on the heat spot entry into the computational region under consideration for the flow past a circular cylinder. The examples of the symmetrization of separated flow past a cylinder performing rotation oscillations in a uniform free stream (the Taneda problem) are considered. A comparison with flow computations for low Mach numbers M « 1 for the flow of a medium past a cylinder at rest is carried out. At the computation of the equation for heat transfer under the assumption of incompressibility of such media as air, it is proposed to retain the pressure derivative, which is typical of gases. In this case, a better agreement with the computations of compressible flows (for M « 1) is achieved, for example, at the determination of the sizes of a symmetric zone of flow separation past a circular cylinder. An unsteady flow in the neighborhood of the point of joining the zero streamline bounding a closed region of separated flow (the cavity) in a wake of the cylinder at rest is obtained by a numerical simulation at the Reynolds number equal to 40.     

Effect of elasticity of a polymer solution on the Hele-Shaw flow

In this study, the Hele-Shaw cell is used to examine the effect offluid elasticity on the flow patterns of two-dimensional potential flow. Flows around a circular cylinder, a square cylinder and flows through abruptly converging-diverging channels (slits) with different throat lengths are tested for water and 0.2 wt % polyacrylamide aqueous solution (PAA-solution). The viscosity of the latter is well modeled by the power law, and the first normal stress difference in the steady shear flow is around ten times higher than the shear stress. Although the PAA-solution is highly shear-thinning, the flows of PAA-solution well reproduce the two-dimensional potential flow patterns that correspond to the respective flow configurations when the flow rate is very low. The potential flow patterns ofPAA solution are disturbed in the opposite way of inertia effect observed for water. The streamlines near the upstream stagnation point of cylinders are shifted upstream separating from the cylinder surface when the flow rate is higher, while streamlines in the wake approach closer to the downstream stagnation point. Streamlines offlow through the slit at flow rates higher than the potential flow region show that a pair ofvortices is formed upstream the slit entrance, while the streamlines remain attached to the downstream wall after passing the slit.     

Flow visualization behind a streamwise oscillating cylinder and a stationary cylinder in tandem arrangement

Interference is investigated between a stationary cylinder wake and that of a downstream streamwise oscillating cylinder. Experiments were carried out in a water tunnel. A laser-induced fluorescence technique was used to visualize the flow structure behind two inline circular cylinders of identical diameterd. The downstream cylinder was forced to oscillate harmonically at the amplitude of 0.5d and the frequency ratiof _e f _s=1.8, wheref _e is the oscillation frequency of the downstream cylinder andf _s is the vortex shedding frequency from an isolated stationary cylinder. The investigation was conducted for the cylinder center-to-center spacingL/d=2.5 ∼ 4.5. Two flow regimes have been identified, i.e. the ‘single-cylinder shedding regime’ atL/d<-3.5 and the ‘two-cylinder shedding regime’ atL/d>3.5. At smallL/d, the upstream cylinder does not appear to shed vortices; vortices are symmetrically formed behind the downstream cylinder as a result of interactions between the shear layers separated from the upstream cylinder and the oscillation of the downstream cylinder. This is drastically different from that behind two stationary cylinders atL/d<-3.5, where vortices are shed alternately from the downstream cylinder only. AtL/d=4.5, both upstream and downstream cylinders shed vortices. This is true with or without the oscillation of the downstream cylinder. The flow structure is now totally different from that atL/d=3.5. The vortices are shed alternately from the upstream cylinder; a staggered spatial arrangement of vortices occurs behind the downstream cylinder.     

Energy transport enhancements in a transitional thermal plasmaplume

The evolution of heat and mass transfer in the initial region of a transitional plasma plume is investigated and discussed. The results show that these transport processes are controlled and limited by the plume shear layer instability. This process of laminar-turbulent transition is consecutively controlled by the plume core shear layer instability, by the “jet mode” induced by unstable shear layer, and by resonance in the jet and arc chamber. These three processes are manifested in three events. Between the first and second phase, there is a maximum of average enthalpy. The other two thresholds occur at maximum and minimum stagnation heat flux from the plume core. It seems that these processes also influence the thermal energy production in the arc chamber cavity     

Influence of metallic nanoparticles in water driven along a wavy circular cylinder

In the present study, simultaneous effects of metallic nanoparticles and magnetohydrodynamic due to stagnation point flow of nanofluid along a wave circular cylinder is presented. The effect of induced magnetic field is incorporated to deal the boundary and thermal boundary layer domain. Mathematical modelling for momentum and energy equation is constructed that is based upon three different kinds of nanoparticles namely: copper (Cu), Titanium di oxide (TiO₂), and alumina (Al₂O₃) within the working fluid water. Each mixture is analysed at the individual level and made comparison amongst all the mixture to examine the resistance and thermal conductivity of nanofluid within the boundary layer region. The solutions are exposed via boundary value problem using shooting method along with the Runge-Kutta-Fehlberg method. The characteristics of emerging parameters for the fluid flow and heat transfer are discussed through graphs and tables. The effects of ϕ (nanoparticle volume fraction) on heat transfer and shear stress at the wall are analysed in detail. It is finally concluded that by increasing the ratio of nanoparticles there is a significant increase in the temperature but slight decrease in the velocity profile.     

Visualization of flow structure around a hypersonic re-entry capsule using the electrical discharge method

The spatial flowfield around a model of the re-entry capsule of the Mars Environmental Survey (MESUR) Pathfinder probe afterbody configuration traveling at a speed of Mach 10 was investigated utilizing the electrical discharge method. The shock shape ahead of the capsule was observed using a technique for visualizing 3-D shock shapes, then the streamline following the shock wave was observed utilizing a technique for visualizing streamlines crossing a shock wave. Subsequently, the flowfield behind the capsule was observed by applying a technique for visualizing flow patterns. From these observations, the spatial flow construction including the wake region such as a separation, free shear layer, and rear stagnation location behind the capsule was made clear. These experiments utilizing the electrical discharge method qualitatively demonstrated the spatial flow structure before and behind the hypersonic re-entry capsule, which had been very difficult to visualize. These experiments were carried out by using a pulsed facility of 18 ms duration.     

Models base study of inclined MHD of hybrid nanofluid flow over nonlinear stretching cylinder

Focus of the present analysis is on the stagnation point flow of hybrid nanofluid with inclined magnetic field over a moving cylinder. The extended version of two models (e.g. Xue model and Yamada-Ota model for hybrid nanofluids) are considered in this study). A mathematical model of hybrid nanofluid flow is developed under certain flow assumptions. Boundary layer approximations are also utilized to model a system of partial differential equations. The systems of partial differential equations are further converted to dimensionless systems of ordinary differential equations by means of suitable similarity transformations. A numerical solution is obtained by applying bv4c technique. Effects of variation in physical parameters involved are depicted through graphs. Skin friction coefficient and Nusselt number are highlighted through tables. Our main objective is to investigate the heat transfer rate on the surface of the nonlinear stretching cylinder. The results of Xue model and Yamada-Ota model for the hybrid nanofluid due to nonlinear stretching cylinder are computed for comparison. In both cases, velocity and temperature profiles are best compared to the decay results.     

Numerical simulation and experiments of a control method to suppress the Bénard von Kármán instability

We report both two-dimensional numerical simulations and experimental results that confirm the robustness of a new method for inhibiting vortex shedding associated to the Bénard-von Kármán (BvK) instability in the wake of a cylinder. Using the SIMPLER algorithm on a 2D channel, we solve the Navier-Stokes equations and we show that pressure suction at the front stagnation point of a circular cylinder, modelled here through a point sink located at the front stagnation point, can completely suppress the Bénard-von Kármán instability for super-critical Reynolds numbers. Comparison with recent experimental results are in close agreement. Received 7 March 2002 / Received in final form 12 September 2002 Published online 29 November 2002     

圆柱面上再附边界层流动的级数分析

按普朗特边界展理论,将圆柱面上再附边界层流动表示成级数的形式,并根据实验所测圆柱表面上的压强分布导出再附边界层外边界上的流速分布,取有限项级数解的形式解出了圆柱表面上再附边界层内的流速分布及流动再分离点。     

Stagnation Region Heat Transfer for Circular Jets Impinging on a Flat Plate

The heat transfer characteristics in a stagnation region were investigated experimentally for five circular free jets impinging into a heated flat plate. The local temperature distributions are estimated from the thermal images obtained from an infrared camera. To get a precise heat transfer data over the plate, fully developed straight pipe jets were used in this study. Mean jet Reynolds number varied from 1,000 to 45,000, jet-to-plate vertical non-dimensional distance H/D varied from 2 to 6, and the spacing distance jet-to-jet S/D varied from 2 to 8. A geometrical arrangement of one jet surrounded by four jets an in-line array was tested. The results show that the stagnation point Nusselt number is correlated to a jet Reynolds number as Nu_st∝Re^0.61. The average Nusselt number is higher at a separation distance of 2D for three cases of spacing distances, S/D = 2, 4, and 6.     

Turbulent mixing of small-obstacle-induced perturbations with the separated shear layer behind a backward-facing step

In the present paper, we consider one of the most efficient and simple methods to additionally intensify the exchange processes and heat transfer in the separated flow behind a backward-facing step. The method uses small obstacles installed upstream the step; such obstacle act as turbulators smaller in size than the main obstacle. As the turbulators, solid mini ribs, comb ribbings, and wall-detached mini ribs were used. Intensification of the turbulent mixing process behind the main obstacle occurs due to the introduction of small-obstacle-induced 2D and 3D perturbations into the separated shear layer behind the step. Results of a detailed experimental study of the distributions of pressure and heat transfer for different heights of the small intensifier and its positions with respect to the step are reported. The influence of intensifier shape on the thermal and dynamic characteristics of the flow has been analyzed. The distributions of pressure and heat-transfer coefficients were used to evaluate the effectiveness of the various mini obstacles and the limits of their action on the drag and heat transfer.     

非定常瞬态流动过程中的Lagrangian拟序结构与物质输运作用

从Lagrangian角度数值分析了圆柱瞬时起动过程中的非定常瞬态流动现象,如分离泡产生、破裂和涡脱落等及其产生的非定常效应,揭示了所列现象诱导的物质输运和迁移效应,首先采用双时间步长的特征线算子分裂算法数值模拟了圆柱起动过程中的瞬时流场,然后采用数值方法从流场中提取出Lagrangian拟序结构(LCSs),并根据非线性动力学理论研究了流动分离和旋涡演化过程中的物质输运作用,结果表明,圆柱瞬时起动后所产生的非定常阻力与相应瞬态现象中的物质输运有密切的关系:对称分离泡产生及其在流向方向的生长,能够使分离泡内压力升高且分布均匀,从而减小阻力;对称分离泡的失稳增强了分离泡与主流之间的物质输运作用,最终导致涡的脱落,并有利于推迟流动分离和减小分离区域,非定常流动中LCSs所描述的物质输运和迁移作用对流动控制的机理研究具有一定指导意义。     

Phase averaged velocity field in the near wake of a square cylinder obtained by a PIV method

Phase averaged velocity fields in the near wake region behind a square cylinder have been successfully obtained using randomly sampled PIV data sets. The Reynolds number based on the flow velocity and the model height was 3,900. To identify the phase information, we examined the magnitude of circulation and the center of peak vorticity. The center of vorticity was estimated from lowpass filtered vorticity contours adopting a sub-pixel searching algorithm. Due to the sinusoidal nature of circulation which is closely related to the instantaneous vorticity, the location of peak vorticity fits well with a sine curve of the circulation magnitude. Conditionally averaged velocity fields represent the Karman Vortex shedding phenomenon quite successfully within ±5° phase uncertainty. The oscillating nature of the separated shear layers and the separation bubble at the upper and lower surfaces are clearly observed. With the hot-wire measurement of Strouhal frequency, we found that the convection velocity changes its magnitude very rapidly from 25 to 75 percent of the free stream velocity along the streamwise direction when the flow passes the recirculation region.