Compressible and confined vortex flow

The internal compressible flow of a thin vortex chamber was investigated experimentally by measuring the radial distribution of temperature and pressure, from which the velocity field was calculated. The bulk of the internal vortex was found to be described by u_θr^0.69 = constant. The total resistance of the vortex chamber to the flow was also investigated in the context of fluidic vortex diode behavior under conditions of compressible and choked flow. It was found that the vortex chamber choked at an upstream-to-downstream pressure ratio of about 6 and in doing so passed a mass flow rate of 28% of the equivalent one-dimensional ideal nozzle. The resistance of vortex chambers is known to be strongly influenced by the presence of reversed flow in the exit due to vortex breakdown. Schlieren photography of the swirling exhaust flow was used to show that, while vortex breakdown does occur, it can only do so after the flow has become subsonic downstream of the exit and cannot therefore influence the vortex chamber resistance.     

基于持续同调的边坡土颗粒应力链失稳分析

采用颗粒离散元方法和持续同调理论,研究了内排土场堆叠至不同高度时的边坡稳定性。为便于研究,现采用一水平金属板向下施加压力,代替不同厚度土层的重力荷载,对边坡在竖向荷载作用下的失稳破坏过程进行了颗粒离散元模拟。研究了二维边坡土颗粒速度总矢量、边坡失稳破坏时滑移开裂面的角度以及边坡坡顶y方向的平均速度等宏观响应过程,并构建了自然堆积下边坡堆积体颗粒的法向力链无向网络模型。最后,用持续同调方法对边坡坡顶颗粒接触力链网络的拓扑特征进行分析,获得条码图,建立了岩体结构持续同调特征与失稳演化的关系。本文为研究边坡失稳拓扑识别提供了一种新方法,从而可以有效预测边坡的失稳破坏。     

Experimental Study of the Impact and Penetration of A Cone in Frozen Sand

A reverse experiment technique is used along with the technology of measuring rods to study the impact and penetration of a steel conical body in frozen sandy soil. This paper presents the dependences of maximum values of the force of resistance of cones with base diameters of 10.0, 12.0, and 19.8 mm to penetration into sand on the impact velocity in the range of values 100–400 m/s. The numerical solution of the problem in an axisymmetric formulation with the use of the “Dinamika-2” software package is used to show the effect of waves reflected from the walls of the container on the contact force. A comparative analysis of the forces of resistance to penetration of the shocker into compacted dry, water-saturated, and frozen sandy soils is carried out.     

Experimental study of frontal resistance force in soil cutting

The paper presents an experimental study of the frontal resistance forces in soil cutting, with emphasis on their dependence on tool displacement during the loading and unloading stages under quasistatic and dynamic regimes. Laboratory tests on undisturbed soil, using specially developed equipment, showed that:During the loading stage, at pre-limiting levels of the frontal resistance force, the soil undergoes both reversible and residual deformations.At the onset of the unloading stage, the restoring force undergoes a downward jump.The limiting value of the frontal resistance force increases considerably in the cutting velocity interval of 0.1 to 3–5 mm/s; at higher velocities, up to 25 mm/s, this force slowly. At the initial velocity of 3 m/s, the limiting value of the frontal resistance force exceeds by about 20% its counterpart appearing at the velocity of 0.1 mm/s.The frontal resistance force is linearly related to the tool width and non-linearly to the depth of cutting.     

A divergence‐free interpolation scheme for the immersed boundary method

The immersed boundary approach for the modeling of complex geometries in incompressible flows is examined critically from the perspective of satisfying boundary conditions and mass conservation. It is shown that the system of discretized equations for mass and momentum can be inconsistent, if the velocity is used in defining the force density to satisfy the boundary conditions. As a result, the velocity is generally not divergence free and the pressure at locations in the vicinity of the immersed boundary is not physical. However, the use of the pseudo‐velocities in defining the force density, as frequently done when the governing equations are solved using a fractional step or projection method, combined with the use of the specified velocity on the immersed boundary, is shown to result in a consistent set of equations which allows a divergence‐free velocity but, depending on the time step, is shown to have the undesirable effects of inaccurately satisfying the boundary conditions and allowing a significant permeability of the immersed boundary. If the time step is reduced sufficiently, the boundary conditions on the immersed boundary can be satisfied. However, this entails an unacceptable increase in computational expense. Two new methods that satisfy the boundary conditions and allow a divergence‐free velocity while avoiding the increased computational expense are presented and shown to be second‐order accurate in space. The first new method is based on local time step reduction. This method is suitable for problems where the immersed boundary does not move. For these problems, the first new method is shown to be closely related to the second new method. The second new method uses an optimization scheme to minimize the deviation from the interpolation stencil used to represent the immersed boundary while ensuring a divergence‐free velocity. This method performs well for all problems, including those where the immersed boundary moves relative to the grid. Additional results include showing that the force density that is added to satisfy the boundary conditions at the immersed boundary is unbounded as the time step is reduced and that the pressure in the vicinity of the immersed boundary is unphysical, being strongly a function of the time step. A method of computing the total force on an immersed boundary which takes into account the specifics of the numerical solver used in the iterative process and correctly computes the total force irrespective of the residual level is also presented. Copyright © 2007 John Wiley & Sons, Ltd.     

基于表面设计减阻的水下航行体模型实验与数值分析

针对水下航行体高速航行时提高航速的需要,开展近壁面湍流边界层减阻的研究。进行表面设计使旋涡流动加强导致局部空化从而产生壁面附着的微气泡,以降低流体粘性系数,伴随近壁面湍流结构的改变来实现一定幅度的减阻。设计并进行轴对称体模型在高压循环水洞中的空化现象观测和阻力测量并做了相应条件下的数值计算。。实验和数值结果分析的基本结论比较一致:某种特征表面形貌在湍流中有一定幅度的减阻,局部空化效应在中低流速下牺牲压差阻力而导致航行总阻力升高,在高流速条件下则降低总阻力。     

Effects of variable viscosity on slider bearing

By considering the now in a slider bearing to be a nearly viscometric flow we have examined the effects of variation in viscosity, and normal stress differences on the total resistance and total normal force. We have found that under certain conditions the normal stress differences may be neglected compared with the variation in viscosity. We have found that the effects of variation in viscosity are to reduce the total resistance, and to increase or decrease the total normal force depending on the ratio of the thickness of the liquid at the entry and exit.     

Measurement of radial deformations in thin-walled cylinders

This paper presents a technique for measuring deflections in cylinders. It is based on the Ligtenberg moiré technique which produces fringes that are contours of surface slope. To use the technique in measuring slopes in a cylinder surface, the optical system is modified so that the cylindrical coordinates of the test piece are transformed to a flat polar plot for photographing.     

Determination of yield stress fluid behaviour from inclined plane test

The aim of this paper is to determine precisely under which conditions an inclined plane can be used as a rheometer, which could represent a practical and rapid technique for various types of industrial or natural viscoplastic coarse suspensions. We first examine its efficiency and relevancy for determining fluid yield stress in a straight way by measuring the deepest fluid layer able to stay on the inclined plane. We have made experiments with different materials (clay-water suspensions) whose yield stress ranged from 35 to 90 Pa, using 1 m long open rectangular channels with a slope ranging from 10 to 30° and a width ranging from 5 to 25 cm. Our procedure involved measuring the final fluid depth far from edges a long time after the end of the slow gravity-induced emptying of a dam placed upstream. The fluid yield stress was also estimated independently by fitting a Herschel-Bulkley model to simple shear rheometry data obtained within a relatively wide shear rate range. A good agreement between inclined rectangular channel tests and independent usual rheometrical tests is obtained even for aspect ratios (flow depth to channel width ratio) as large as 1 when one assumes that, when the fluid has stopped, the side and bottom wall shear stresses are equal to the fluid yield stress. These results prove the efficiency of the inclined plane test for determining yield stress when appropriate experimental precautions are taken for both tests. In addition we examine the possibility of determining the simple shear flow curve of a mud suspension from fluid depth, velocity and discharge measurements of different steady flows in a wide open channel (8 m long; 60 cm wide) equipped with a recirculating system. The results obtained from inclined plane tests are in good agreement with independent rheometrical data (with torsional geometries). However it is technically difficult to cover a wide shear rate range from the inclined plane technique since this requires a rather wide channel flow rate range.     

TWO-DIMENSIONAL MOTION OF A PARTICLE ON AN INCLINED PLANE

质点在斜面上受到大小恒定的下滑力和摩擦力而运动,但后者方向随速度而变化。摩擦因子与斜面坡度比值即等效摩擦因子λ> 1 时质点最终在斜面上停止;0:5 <λ≤6 1 时质点不会停止但水平方向运动距离存在限界,而λ≤0:5 时水平方向也可运动至无穷远。相同等效摩擦因子下质点运动轨迹的曲线簇具有包络线。     

The effect of velocity on rigid wheel performance

A simulation model to predict the effect of velocity on rigid-wheel performance for off-road terrain was examined. The soil–wheel simulation model is based on determining the forces acting on a wheel in steady state conditions. The stress distribution at the interface was analyzed from the instantaneous equilibrium between wheel and soil elements. The soil was presented by its reaction to penetration and shear. The simulation model describes the effect of wheel velocity on the soil–wheel interaction performances such as: wheel sinkage, wheel slip, net tractive ratio, gross traction ratio, tractive efficiency and motion resistance ratio. Simulation results from several soil-wheel configurations corroborate that the effect of velocity should be considered. It was found that wheel performance such as net tractive ratio and tractive efficiency, increases with increasing velocity. Both, relative wheel sinkage and relative free rolling wheel force ratio, decrease as velocity increases. The suggested model improves the performance prediction of off-road operating vehicles and can be used for applications such as controlling and improving off-road vehicle performance.     

横摆角下汽车尾部湍流特征量的PIV试验分析

汽车尾部湍流场是汽车压差阻力的主要来源,在HD-2汽车模型风洞中,首先使用测力天平和测压系统,对横摆角工况下汽车模型的气动六分力和纵对称截面48个测点的表面压力进行了测量,然后利用PIV测量技术对模型在横摆角分别为0°、15°的尾部湍流场进行了测量,获得该模型尾流场的速度场、涡量场和雷诺应力流场信息,通过计算得出尾流场区域空间相关系数和湍流积分尺度。结果表明:在横摆角工况下,汽车模型尾部涡流的结构呈现向上发展的趋势;尾流场拖拽涡的范围和强度的增大导致了模型气动力出现较大的增加;湍流积分尺度的变化表明,尾部涡流区的分离噪声与涡流分离位置有关,在汽车尾部造型设计中,要尽量推迟尾部涡流的分离。     

Rigorous theory for transient capillary imbibition in channels of arbitrary cross section

This article addresses a classical fluid mechanics problem where the effect of capillary action on a column of viscous liquid is analyzed by quantifying its time-dependent penetrated length in a narrow channel. Despite several past studies, a rigorous mathematical formulation of this inherently unsteady process is still unavailable, because these existing works resort to a crucial assumption only valid for mildly transient systems. The approximate theories use an integral approach where the penetration is described by equating total force acting on the domain to rate of change of total momentum. However, while doing so, the viscous resistance under temporally varying condition is assumed to be same as the resistance created by a quasi-steady velocity profile. Thus, leading order error appears due to such approximation which can only be true when the variation in time is not strong enough causing negligible transient deviation in the hydrodynamic quantities. The present paper proposes a new way to solve this problem by considering the unsteady field itself as an unknown variable. Accordingly, the analysis applies an eigenfunction expansion of the flow with unknown time-dependent amplitudes which along with the unsteady intrusion length are calculated from a system of ordinary differential equations. A comparative exploration identifies the situation for which the integral approach and the rigorous technique based on eigenfunction expansion deviate from each other. It also reveals that the two methods differ substantially in short-time dynamics at the initial stage. Then, an asymptotic perturbation shows how the two sets of results should coincide in their long-time behavior. In this way, the findings will provide a comprehensive understanding of the physics behind the transport phenomenon.     

Analysis of a rigid body obliquely impacting granular matter

The model of the oblique rigid body impact with a granular matter is studied. The force acting on the body is a linear superposition of a static (velocity-independent) friction force and a dynamic (velocity-dependent) resistance force. The impact of a sphere, a mathematical and a compound pendulum are modeled and simulated using different initial impact velocity conditions and different impact angles. We analyze how rapidly the rigid body impacting a granular media slows upon collision. For most of the analyzed cases the rigid body under high-force impact (higher initial velocity) comes to rest faster in a granular matter than the same body under low-force impacts (lower initial velocity). Researchers were able to explain this interesting phenomena, not shared by solids or liquids, for the vertical impact of spheres. The simulations for some configurations with small initial impact angles show that as the speed at which the rigid body impacts the media increases, the later it will come to rest.

     

一种冲击波速度传感器性能的检测装置

将SDQ307型工程射钉器改制成小型、廉价、可靠的冲击速度检测装置,结合光电技术测速,解决了低压力范围同轴自短路型冲击波速度传感器一致性和可靠性等动态参数先期检验难的问题。实测数据表明,用J5.616(黄色)大威力射钉弹作为冲击力驱动源时,测得(25.50.1) g钢质碰撞器的飞行速度为226.2 m/s;抽样6根同批次同轴冲击波速度传感器结果显示,预测的可靠性不低于66.67%,用标准差表示的一致性不大于1.14%。为准确测量爆轰和冲击压缩实验数据提供了一种有效的方法。     

Slip velocity measurements in an upward bubbly flow by combined LDA and electrodiffusional techniques

 An experimental technique for the measurement of the local slip velocity of spherical bubbles is reported. It is based on the measurement of the local liquid velocity by an electrodiffusional method, and the bubble velocity by a specially adapted LDA (Laser Doppler anemometer) with a short measuring volume. The bubble velocity is measured taking into account the shift between the bubble centre and the centre of the LDA measuring volume. The slip velocity is obtained by subtracting the liquid velocity from the bubble velocity at the point corresponding to the bubble centre. The technique is applicable for flows with high velocity gradients. Results of the slip velocity measurements in an upward bubbly flow at laminar pipe Reynolds numbers are presented. Received: 25 July 1996/Accepted: 13 April 1998     

Pressure pulsations in piping system excited by a centrifugal turbomachinery taking the damping characteristics into consideration

Pressure pulsations excited by a centrifugal turbomachinery such as compressor, fan or pump at the blade passing frequency may cause severe noise and vibrations in piping system. Therefore, the practical evaluation method of pressure pulsations is strongly recommended. In particular, the maximum pressure amplitude under the resonant conditions should be appropriately evaluated. In this study, a one-dimensional excitation source model for a compressor or pump is introduced based on the equation of motion, so as to incorporate the non-linear damping proportional to velocity squared in the total piping system including the compressor or pump. The damping characteristics of the compressor or pump are investigated by using the semi-empirical model. It is shown that the resistance coefficient of the compressor or pump depends on the Reynolds number that is defined using the equivalent velocity of the pulsating flow. The frequency response of the pressure amplitude and the pressure distribution in the piping system can be evaluated by introducing the equivalent resistance of the compressor or pump and that of piping system. In particular, the relation of the maximum pressure amplitude in piping system to the location of the excitation source under resonant conditions can be evaluated. Finally, the reduction of the pressure pulsations by use of an orifice plate is discussed in terms of the pulsation energy loss.