近壁面静止圆球的尾迹特性

圆球在大空间均匀来流下的绕流特性已得到了广泛的研究,但是壁面对圆球绕流特性的影响还未清楚。通过实验方法研究了与壁面接触的静止圆球在明渠流中的绕流特性,重点关注了圆球后尾迹的特性。研究发现,圆球尾迹存在三种状态:稳定状态、非稳定对称状态和非稳定非对称状态。当Re150时,尾迹呈稳定状态,尾迹关于垂直于壁面的平面对称;当150Re400时,尾迹呈对称状态,Reynolds数较高时,圆球后存在规则的、周期性的涡脱落,Strouhal数为0.26～0.33,脱落涡关于垂直于壁面的平面对称,此时尾迹功率谱的频带分布较宽;而当Re400时,圆球后的涡脱落不再对称,尾迹随着Reynolds数的增大呈现混沌特征。     

凹壁面切向射流近壁面单颗粒尾流特性的大涡模拟

采用大涡模拟研究了凹壁面切向射流作用下近壁面圆球型颗粒对流体流动特性的影响,模拟获得的尾涡结果与实验示踪剂图像吻合较好。研究了颗粒尾流的涡旋结构及其演变过程,考察了雷诺数Re=700～10 000时颗粒周围速度、涡量及流线变化。结果表明,随着雷诺数增加,颗粒的影响区域涡量增强,涡量的峰值始终出现在颗粒迎流面,颗粒后侧的回流区显著收缩。Re≥2000时在射流展向颗粒后侧存在两个尾涡,流体的切向速率和涡量均发生周期性波动。对颗粒的升力和阻力进行了监控,Re=2000时旋涡脱落频率对应的斯特劳哈尔数St=0.000 854,升力功率谱中峰值对应的St=0.001 52;Re=10 000时阻力功率谱没有发现峰值,升力功率谱中峰值对应的St=0.008 74。     

波纹壁面上流动液膜涡的生成特性

针对液膜在非平整壁面上流动过程中生成涡的现象,基于VOF方法,采用FLUENT软件模拟了三维波纹壁面上的液膜流动。研究了波纹结构内涡结构的演化过程,分析入口Reynolds数、波纹结构、壁面倾角、流体黏度和表面张力对波纹结构内涡结构的影响。结果表明：随着时间的演化,涡的大小和形状不断变化,最终达到稳定。且涡结构变化对自由液面的波动影响显著。较低Re和波形度时,波纹结构内不易形成涡,随着Re和波形度增大,产生涡且涡呈增大趋势,涡的形态也随之改变,自由液面位置升高,其相位滞后于波纹壁面。当壁面倾角改变时,波纹结构内的涡特性变化较大,液膜厚度略有增加,而自由液面相位不明显。表面张力对涡结构有显著影响,液膜流动过程中不容忽视。流体黏性改变时,波纹结构内涡的大小和形状无明显的变化。黏度变小和忽略表面张力时,液膜厚度均变薄。以上结果为工业设备生产、运行和设计提供了一定参考依据。     

The influence of fluid elasticity on wall effects for creeping sphere motion in cylindrical tubes

Experimental results are presented of wall effect for the slow motion of spheres in elastic, constant-viscosity liquids. The results are correlated in terms of diameter ratio for d/D < 0.3, and Weissenberg number We < 5. Weissenberg number is defined as We = 2θV_m/d, with θ the Maxwellian relaxation time (θ = N₁/2τγ). The wall effect is found to be adequately described by Newtonian expressions for small Weissenberg number, We < 0.01. For larger values of the Weissenberg number, We > 0.2, virtually no wall effect is discernible; the small effect observed is correlated by the wall factor expression The wall effect observed is ascribed to the influence of fluid elasticity alone, since all the fluids used were elastic to a greater or lesser extent, but showed no shear thinning.      

Sedimentation of a sphere along the axis of a long square duet filled with non-newtonian liquids

Based on extensive experimental results, it is shown that the retardation effect caused by the confining walls on the free settling velocity of a sphere is smaller with square walls than that with cylindrical boundaries. This is true for both Newtonian and power law fluids, provided the particle Reynolds number is small (< about 5). The values of the wall factor for Newtonian liquids are in excellent agreement with theory (up to R / L ≤ 0.1) while those for power law fluids have been correlated empirically via a linear relationship. The results reported here encompass the following ranges of conditions: 1 ≥ n ≥ 0.7; Re < 15 and 0.024 < R/L < 0.238.     

On near‐wall jets in a disc‐like gas vortex unit

To clarify the three‐dimensional (3D) structure of near‐wall jets observed in disc‐like gas vortex units (GVUs), experimental and numerical studies are performed. The experimental results are obtained using stereoscopic particle image velocimetry (PIV), laser doppler anemometry, pressure probes and surface oil flow visualization techniques. The first three techniques have been used to investigate the bulk flow hydrodynamics of the vortex unit. Surface oil flow visualization is adopted to visualize streamlines near the end‐walls of the vortex unit. The surface streamlines help to determine the azimuthal and radial velocity components of the radial near‐wall jets. Simulations of the vortex unit using FLUENT^® v.14a are simultaneously performed, computationally resolving the near‐wall jet regions in the axial direction. The simulation results together with the surface oil flow visualization establish the 3D structure of the near‐wall jets in GVUs for the first time in literature. It is also conjectured that the near‐wall jets develop due to the combined effect of bulk flow acceleration and swirl. The centrifugal force diminishes in the vicinity of the end‐walls. The radially inward pressure gradient in these regions, no longer balanced by the centrifugal force, pushes gas radially inward thus developing the near‐wall jets. © 2016 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 63: 1740–1756, 2017     

A numerical study of the accelerating motion of a dense rigid sphere in non-newtonian power law fluids

The equations of motion of an accelerating sphere falling through non-Newtonian fluids with power law index n in the range 0.2 ≤ n ≤ 1.8 were integrated numerically using the assumption that the drag on the sphere was a function of both power law index and terminal Reynolds number, Re_t For 10^?2 ≤ Re_t ≤ 10³ both dimensionless time and distance travelled by the sphere under transient conditions showed a much stronger dependence on the flow behaviour index, n, for shear-thinning than for shear-thickening fluids. The form of this dependence is investigated here. Furthermore, results in four typical shear-thinning fluids suggested a strong correlation between the distance and time travelled by the sphere under transient conditions and the value of the fluid consistency index. The analysis reported herein is, however, restricted to dense spheres falling in less dense fluids, when additional effects arising from the Basset forces can be neelected.     

Drag of a cylinder moving near a wall in a yield stress fluid

The drag of a cylindrical obstacle moving at a constant velocity in a yield stress fluid close to a wall is studied experimentally and numerically. The wall influence has been explored for gap values between the cylinder of diameter D and the wall ranging from 0.01D to 100D, which corresponds, respectively, to hydrodynamic lubrication and to unconfined domain conditions. A model yield stress fluid (Carbopol gel) is used in the experiments. The viscous and plastic drag coefficients have been calculated and measured as depending on the Oldroyd number, in conditions where the yield stress effects are more important than those of viscosity and the inertia negligible. We have performed experimental and numerical validations in the Newtonian case and provided more specifically comparisons of our measured data on yield stress materials with those resulting from viscoplastic flow simulations. © 2018 American Institute of Chemical Engineers AIChE J, 64: 4118–4130, 2018     

Numerical study of separated cross‐flow near a two‐dimensional rough wall with narrow apertures and suction

The turbulent flow (Re = 1.5 × 10⁵) near a rough wall with narrow apertures has been numerically analysed to study the effect of the aperture geometry and wall suction on the flow characteristics. The aperture entry geometry is characterized by roughness height and roughness width. The roughness height is varied from 0.3 to 1.2 mm and roughness width is varied from 2.6 to 4.0 mm. The wall suction is characterized by slot velocity which is varied from 0.25 to 5 m/s. The flow characteristics in terms of fluid streamlines, flow resistance, wall pressure, and wall shear have been presented for several cases. The results show that the flow through the apertures is dominated by a separation vortex that covers the aperture. As roughness height increased (or slope of the roughness), the vortex size increased. With increasing wall suction, the vortex size decreased and moved towards the aperture opening. The flow resistance characterized by pressure drop across the aperture is significantly high for very low wall suction and it is increased with increasing roughness slope. At higher wall suction the slot velocity and roughness geometry has less influence on flow resistance. Wall pressure and skin friction coefficients are dependent on the ratio of roughness height to width.