Suggestive correctional methods for PIV image biasing caused by a rotating mirror system

A rotating mirror is widely used to generate the velocity shift that can resolve the directional ambiguities of PIV (particle image velocimetry) measurements. The rotating mirror system inevitably creates the normal displacement of the resulting PIV images and causes systematic image errors. Two corrective methods are proposed to eliminate or reduce the image biasing in PIV system. The use of two linearly traversing mirrors, instead of a single rotating mirror, shows that image biasing can be eliminated and the velocity shift well generated. As a second option, two co-rotating mirrors, instead of one, can reduce the image biasing with a maximum velocity shift available. Detailed imaging kinematics of the two suggestive methods are presented to lead to designing of practical devices that improve the PIV capabilities by reducing the systematic image errors. The authors like to acknowledge the partial financial support provided from the Turbo and Power Machinery Research Center (TPMRC) of Seoul National University, Seoul, Korea.     

旋转圆柱绕流流场特性分析

对雷诺数Re = 20000 ~ 90000、相对转速ɑ = 0 ~ 0.72的旋转圆柱后方流场进行了实验测量, 分析了旋转圆柱后方不同剖面处的速度分布规律和湍流度分布规律. 采用LES方法对旋转圆柱绕流问题进行了数值模拟, 分析旋转圆柱周围流场特性和自由剪切层变化规律, 最后通过理论模型对流场变化进行分析, 得出如下结论: 当圆柱逆时针旋转时, 同一雷诺数下随着相对转速的增加, 旋转圆柱尾迹区域下方速度突变处的位置随着相对转速的增加而上移, 而上方速度突变处的位置不变, 雷诺数的增加使旋转圆柱尾迹区域下方速度突变处位置有小幅度的下移. 通过数值模拟发现, 圆柱旋转之后, 圆柱后方下侧涡的位置明显上移, 且幅度较大. 下方的自由剪切层有明显的上移, 上方的自由剪切层位置变化较小. 最后通过理论分析发现, 圆柱后侧下方涡位置的上移对圆柱升力影响十分显著, 在高雷诺数、低相对转速的条件下, 旋转圆柱后侧下方涡位置的改变对旋转圆柱的升力、尾流区自由剪切层的变化起到了重要的影响.      

高速转动圆盘流动数值模拟研究

重点研究高速离心压气机叶轮与机匣间的间隙流动及其温度分布。研究将离心压气机简化为高速转动圆盘,搭建了相关实验平台,并开展了相应的数值模拟研究。通过改变转动圆盘的转速和轴向进入的冷却流的流量,研究了转速和流量对于间隙内温度和速度分布的影响。结果显示,转速是影响温度变化的最主要因素,转速越大,温度越高;同等幅度的流量变化对温度的影响则较小。研究发现,在实验和模拟对应的大雷诺数条件下,无量纲的速度分布基本不受到圆盘转速、冷却流量和温度场的影响。     

Unsteady mixed convection flow from a rotating vertical cone with a magnetic field

An analysis is developed to study the unsteady mixed convection flow over a vertical cone rotating in an ambient fluid with a time-dependent angular velocity in the presence of a magnetic field. The coupled nonlinear partial differential equations governing the flow have been solved numerically using an implicit finite-difference scheme. The local skin friction coefficients in the tangential and azimuthal directions and the local Nusselt number increase with the time when the angular velocity of the cone increases, but the reverse trend is observed for decreasing angular velocity. However, these are not mirror reflection of each other. The magnetic field reduces the skin friction coefficient in the tangential direction and also the Nusselt number, but it increases the skin friction coefficient in the azimuthal direction. The skin friction coefficients and the Nusselt number increase with the buoyancy force.     

MODELLING AND CALCULATION OF THE TURBULENT BOUNDARY LAYER ON A ROTATING CYLINDER SURFACE WITH STRONG SUCTION 1)

提出了湍流边界层的一种简单、快速计算方法, 用以求解强吸气作用下旋转圆筒表面边界层流动. 首先, 理论分析了同心圆筒间的旋转流体运动, 外筒静止、内筒旋转且为多孔吸气条件. 强吸气情况下旋转流动主要表现为内筒壁面附近的边界层流动, 基于这一事实得到了周向速度分布的解析表达式. 其次, 通过引入新参数扩展Cebeci-Smith代数湍流模型, 使其能考虑流线曲率、壁面吸气、低Reynolds数效应等因素. 针对这些因素的综合影响, 采用解析修正和经验参数对模型进行调整. 同时, 基于Reynolds应力湍流模型的仿真结果, 校准代数湍流模型中的经验参数. 最后, 给出基于广义Cebeci-Smith湍流模型的旋转壁面边界层流动的迭代算法, 该算法适用于需要特殊迭代过程的轴向及周向流动均匀情况. 计算了不同旋转速度和吸气强度组合工况下的边界层流动, 其周向速度和湍流强度分布与基于Reynolds应力湍流模型的计算结果非常接近. 并且表明, 当Reynolds应力湍流模型数值模拟预测内筒边界层为稳定层流时, 该方法也再现了相同初始条件下的层流边界层.     

强吸气旋转圆筒壁面湍流边界层建模及计算

提出了湍流边界层的一种简单、快速计算方法, 用以求解强吸气作用下旋转圆筒表面边界层流动. 首先, 理论分析了同心圆筒间的旋转流体运动, 外筒静止、内筒旋转且为多孔吸气条件. 强吸气情况下旋转流动主要表现为内筒壁面附近的边界层流动, 基于这一事实得到了周向速度分布的解析表达式. 其次, 通过引入新参数扩展Cebeci-Smith代数湍流模型, 使其能考虑流线曲率、壁面吸气、低Reynolds数效应等因素. 针对这些因素的综合影响, 采用解析修正和经验参数对模型进行调整. 同时, 基于Reynolds应力湍流模型的仿真结果, 校准代数湍流模型中的经验参数. 最后, 给出基于广义Cebeci-Smith湍流模型的旋转壁面边界层流动的迭代算法, 该算法适用于需要特殊迭代过程的轴向及周向流动均匀情况. 计算了不同旋转速度和吸气强度组合工况下的边界层流动, 其周向速度和湍流强度分布与基于Reynolds应力湍流模型的计算结果非常接近. 并且表明, 当Reynolds应力湍流模型数值模拟预测内筒边界层为稳定层流时, 该方法也再现了相同初始条件下的层流边界层.      

Velocity profile measurement by ultrasonic doppler method

The ultrasonic velocity profile measuring method has been developed at PSI for application in fluid mechanics and fluid flow measurement. It uses pulsed ultrasonic echography together with the detection of the instantaneous Doppler shift frequency. This method has the following advantages over the conventional techniques: (1) an efficient flow mapping process, (2) applicability to opaque liquids, and (3) a record of the spatiotemporal velocity field. After a brief introduction of its principle, the characteristics and specifications of the present system are given. Then examples in fluid engineering for oscillating pipe flow, T-branching flow of mercury, and recirculating flow in a square cavity are described that confirm the method's advantages. Several other works under way by other investigators are introduced. A potential for in-depth study of fluid dynamics is demonstrated by several examples from an investigation of modulated wavy flows in a rotating Couette system. The position-averaged power spectrum and the time-averaged energy spectral density were used to study the dynamic characteristics of the flow, and subsequently the velocity field was decomposed into its intrinsic wave structure based on two-dimensional Fourier analysis.     

Heterodyne Doppler global velocimetry

Doppler Global Velocimetry (DGV) is an imaging flow measurement technique which allows the measurement of the velocity distribution in a plane. In DGV the frequency shift of scattered light from moving particles within the flow is used to determine the local flow velocity. Heterodyne Doppler Global Velocimetry (HDGV) is a new approach which combines the imaging and geometrical characteristics of DGV with the measurement principles of reference beam laser Doppler anemometry. The frequency shifted scattered light from the flow tracers is heterodyned with a reference beam from the same light source. Due to interference the result of this superposition is a harmonic intensity modulated signal. This signal is detected using a smart pixel detector array to obtain the velocity distribution. Two different experiments are presented. The first experiment compares the measured velocity distribution of a rotating disk with its actual velocity. The second experiment demonstrates the capability of the technique to measure a real flow.     

Imaging laser Doppler velocimetry

Imaging laser Doppler velocimetry (ILDV) is a novel flow measurement technique, which enables the measurement of the velocity in an imaging plane. It is an evolution of heterodyne Doppler global velocimetry (HDGV) and may be regarded as the planar extension of the classical dual-beam laser Doppler velocimetry (LDV) by crossing light sheets in the flow instead of focused laser beams. Seeding particles within the flow are illuminated from two different directions, and the light scattered from the moving particles exhibits a frequency shift due to the Doppler effect. The frequency shift depends on the direction of the illumination and the velocity of the particle. The superposition of the two different frequency-shifted signals on the detector creates interference and leads to an amplitude modulated signal wherein the modulation frequency depends on the velocity of the particle. This signal is detected using either a high-speed camera or alternatively a smart pixel imaging array. This detector array performs a quadrature detection on each pixel with a maximum demodulation frequency of 250 kHz. To demonstrate the feasibility of the technique, two experiments are presented: The first experiment compares the measured velocity distribution of a free jet using ILDV performed with the smart pixel detector array and a high-speed camera with a reference measurement using PIV. The second experiment shows an advanced setup using two smart pixel detector arrays to measure the velocity distribution on a rotating disk, demonstrating the potential of the technique for high-velocity flow measurements.     

Heat Transfer Influenced by Turbulent Airflow Inside an Axially Rotating Diffuser

The paper presents a study of heat transfer between the turbulent airflow and the inner wall surface of an axial diffuser rotating around its longitudinal axis. Heat transfer was assessed through the measurement of a time-dependent temperature field of the diffuser inner wall surface. Measurements of the instantaneous flow velocity components were performed by a laser–Doppler anemometry system, which delivered information on mean velocity components as well as on the turbulence intensity. A significant increase of all three mean velocity components was observed near the rotating diffuser wall in comparison with a non-rotating diffuser. Temperature field measurements were carried out by means of infrared thermography. The experiment showed a significant dependence of the temperature field on the turbulent flowfield induced by diffuser rotation. A strong influence of the flow separation and reattachment on the temperature distribution was observed, while rotation was found to suppress the occurrence of flow separation from the diffuser wall. Properties of the velocity field such as turbulent kinetic energy were directly coupled with the temperature distribution in order to gain the information on how to enhance or reduce heat transfer by changing the integral parameters of the diffuser (e.g. rotation frequency or amount of flow).     

Experimental investigations of the unsteady rotating flow field in a cylindrical vessel

The rotating flow field in a cylindrical vessel — the so-called whirlpool — is widely used in food engineering as a method for separating particles out of a suspension (Cup-of-tea-method). However many of these whirlpools do not operate adequately or fail entirely. In order to solve this problem, the first step was to investigate the flow field and its time dependency which has not been sufficiently understood until now.The rotating flow in a cylindrical vessel — induced by a fluid jet during the filling period of this vessel — is slowed down by fluid friction after the closing of the inlet valve. The velocity fields to be found mainly near, and pressure distributions at the bottom of the vessel, are measured during this unsteady flow. The results, especially those which describe vortex systems, are used to improve the separation system. This paper is restricted to the hydrodynamic aspect. Therefore success in industrial applications can only be indicated.     

Flow structure measurement by beam scan type LDV

A new type of laser Doppler velocimeter called SLV (Scan-Type Laser-Doppier Velocimeter) which can measure the velocity field almost continuously and simultaneously has been developed and tested. The principle of the apparatus is to traverse the focal point of split laser beams by reflecting them with a rotating polygon mirror or an oscillating mirror which is driven (and controlled) by a stepping motor and to receive the scattered Doppler signals by a photomultiplier by focusing them through a cylindrical lens. The signals from the photomultiplier and the driving pulse of the stepping motor were transmitted to a persona] computer and processed on-line.

Experiments on the oscillatory boundary layer, Kármán vortices, and vortex rings were carried out to check the performance.     

Unsteady MHD Flow on a Rotating Cone in a Rotating Fluid

Unsteady flow over an infinite permeable rotating cone in a rotating fluid in the presence of an applied magnetic field has been investigated. The unsteadiness is induced by the time-dependent angular velocity of the body, as well as that of the fluid. The partial differential equations governing the flow have been solved numerically by using an implicit finite-difference scheme in combination with the quasi-linearization technique. For large values of the magnetic parameter, analytical solutions have also been obtained for the steady-state case. It is observed that the magnetic field, surface velocity, and suction and injection strongly affect the local skin friction coefficients in the tangential and azimuthal directions. The local skin friction coefficients increase when the angular velocity of the fluid or body increases with time, but these decrease with decreasing angular velocity. The skin friction coefficients in the tangential and azimuthal directions vanish when the angular velocities of fluid and the body are equal but this does not imply separation. When the angular velocity of the fluid is greater than that of the body, the velocity profiles reach their asymptotic values at the edge of the boundary layer in an oscillatory manner, but the magnetic field or suction reduces or suppresses these oscillations.     

The influence of Hall current on the rotating oscillating flows of an Oldroyd-B fluid in a porous medium

In this article, analysis is presented to study the effect of Hall current on the rotating flow of a non-Newtonian fluid in a porous medium taking into consideration the modified Darcy's law. The Oldroyd-B fluid model is used to characterize the non-Newtonian fluid behavior. The governing equations for unsteady rotating flow have been modeled in a porous medium. The analysis includes the flows induced by general periodic oscillations and elliptic harmonic oscillations of a plate. The effect of the various emerging parameters is discussed on the velocity distribution. The analytical results are confirmed mathematically by giving comparison with previous studies in the literature. It is observed that the velocity distribution increases with an increase of Hall parameter. The behavior of permeability is similar to that of the Hall parameter.     

INSTABILITY OF COUPLED THERMO-SOLUTE CAPILLARY CONVECTION IN LIQUID BRIDGE AND CONTROL BY ROTATING MAGNETIC FIELD

浮区法因具有无坩埚接触污染的生长优点而成为生长高完整性和高均匀性单晶材料的重要技术.但熔体中存在的毛细对流会给浮区法晶体生长带来极大挑战,这是由于对流的不稳定会导致晶体微观瑕疵的产生和宏观条纹等缺陷的形成.为了提高浮区法生长单晶材料的品质,研究浮区法晶体生长中毛细对流特性及如何控制其不稳定性显得尤为重要.本文采用数值模拟的方法对半浮区液桥内Si_xGe_1-x体系中存在的热质毛细对流展开研究并施加旋转磁场对其进行控制.结果表明：纯溶质毛细对流表现为二维轴对称模式,温度场主要由热扩散作用决定,而浓度场则由对流和溶质扩散共同支配;纯热毛细对流呈现三维稳态非轴对称流动,浓度分布与熔体内热毛细对流的流向密切相关,等温线在对流较大的区域发生弯曲;耦合溶质与热毛细对流则为三维周期性旋转振荡流.施加旋转磁场后,熔体周向速度沿径向向外增大,熔体内浓度场和流场均呈现二维轴对称分布.     

Fluid flow in a rotating channel of square section

Laminar flow in a channel rotating about a transverse axis has been studied numerically [1–3] and analytically [4–7] at small Reynolds numbers. The drag coefficient of rotating channels with straight and curvilinear axes has been measured [4, 8, 9]. The present paper gives the results of an experimental investigation into the kinematics of water flow in a channel rotating with different intensities. The flow was visualized by means of hydrogen bubbles and a dye. A study was made of the process of flow separation in a rapidly rotating channel into a core with homogeneous velocity distribution in the direction parallel to the rotation axis and thin shear layers on the walls normal to this axis. The values of the dimensionless numbers were found that correspond to flow rearrangement accompanied by formation of longitudinally oriented vortex structures in the region of higher pressure, and also the values of the rotation parameter needed for the almost complete suppression of turbulence in the region of lower pressure. A general analysis is made of the forms of instability in the different regions of the flow and of the possible flow regimes in a rotating channel.     

Rapid diagnosis of flow separation around an axial impeller in a mixing vessel

 The relative flows around a flat plate axial impeller in a mixing vessel were visualized directly using a combined image shifting and image de-rotation technique. The image shifting technique used a rotating mirror to produce a velocity bias equal to the blade velocity so that the relative flow field could be studied in a co-axial plane cutting through the 2-dimensional blade section. The technique provides a rapid means of locating local flow separation on the blade. To visualize the relative flow field in the plane of the blade span, an image de-rotation method was used. The method includes using a dove-shaped prism which, when rotated, produce a rotation of an image about the optical axis. It was observed in the relative frame of reference through the prism that a new vortex structure, not reported previously, with vorticity sign opposite to that of the rotation of the shaft, exists near the hub at the high pressure side of the blade. Received: 17 June 1996/Accepted:12 November 1996     

Rotating flow of a third grade fluid in a porous space with Hall current

This study investigates the rotating magnetohydrodynamic (MHD) flow of a third-grade fluid in a porous space. Modified Darcy's law has been utilized for the flow modeling. The Hall effects are taken into consideration. The basic equations governing the flow are reduced to a highly nonlinear ordinary differential equation. This equation has been solved analytically by employing the homotopy analysis method (HAM). The effects of the various interesting parameters on the velocity distribution have been discussed.