Separating shear flow past a surface-mounted blunt obstacle

The planar flow of incompressible fluid past a blunt obstacle mounted on a flat (horizontal) fixed solid surface of infinite extent is examined in the presence of an incident linear velocity profile, modelling the fluid behaviour close to a small surface roughness for instance. The motion is taken to be steady and laminar. The obstacle is blunt in the sense that its typical surface slopes are not small, a feature which here always induces flow separation both upstream and downstream of the obstacle. Computations and nonlinear theory are applied, together with comparisons. The direct computations of the Navier-Stokes equations, using for example a higher order upwind-difference scheme, deal with a moderate range of Reynolds numbers up to 200, based on the obstacle height and the incident uniform shear. In addition the accuracy is necessarily limited as the Reynolds number increases. The theory is for large Reynolds numbers and is based on viscous-inviscid reasoning, back-pressure effects from the obstacle and slender-layer separation locally, among other influences. The comparisons nevertheless yield encouragingly close agreement, for the present computed cases of a vertical flap or a rectangular block. This is both quantitatively, in terms of the upstream separation and downstream reattachment positions in particular, and generally, in terms of the separating flow structure, even at the notably moderate Reynolds numbers covered accurately by the computations.     

Low-frequency scattering pattern of an arbitrary penetrable obstacle

We consider the scattering of low-frequency sound waves by an arbitrary penetrable obstacle whose density and compressibility are different from those of the surrounding infinite medium. We present solutions both inside and outside the scatterer. The physical properties of the scatterer are implicit in the transition conditions at its surface and in the interior value of the wave length. This leads to a two-parameter boundary value problem which we convert into an integral equation valid both inside and outside the obstacle. This integral equation is then recast into an infinite system of algebraic equations which are judicially truncated to yield the required solutions.     

Decreasing the action of a flow behind a weak shock wave emerging from a channel upon an obstacle

It is experimentally demonstrated that, if the flow behind shock waves outgoing from a channel is subsonic, the action of this flow upon an obstacle can be decreased by changing the shape of the output channel cross section. The results of numerical modeling of the experimental oscillograms show that, by replacing a round channel with a cross-shaped one, the excess action upon the obstacle in a quasi-stationary stage can be reduced by half. This decrease is related to energy dissipation in a three-dimensional flow as a result of the interference of rarefaction and compression waves. The interference arises due to mass redistribution between various symmetry axes in the gaseous medium and due to the flow energy losses for the vortex formation at the channel edges, which are revealed by schlieren photographs.     

On generation of horseshoe vortices by corrugated surfaces, surface roughnesses or pipe bends

This study is motivated by applications to near-wall shear flow (i) as a longitudinal wall shaping starts, (ii) around a surface obstacle, or (iii) through a pipe bend. All are shown to be governed, at relatively high flow rates, by essentially the same theoretical problem. This concerns three-dimensional nonlinear longitudinal vortex-like motion under a prescribed displacement which continues to increase with distance downstream. Symmetry-plane solutions are obtained mainly through forward marching computation followed by analysis of the far-downstream response. The behaviour far downstream is found to involve either a strengthening attachment or an increasing three-dimensional separation (lift-off) with no backflow.     

An Efficient and Stable Spectral-Element Method for Acoustic Scattering by an Obstacle

A spectral-element method is developed to solve the scattering problem for time-harmonic sound waves due to an obstacle in an homogeneous compressible fluid. The method is based on a boundary perturbation technique coupled with an efficient spectral-element solver. Extensive numerical results are presented, in order to show the accuracy and stability of the method.     

Critical free-surface flow over a semi-circular obstruction

Numerical solutions are presented for the problem of two-dimensional critical flow of an ideal fluid over a semi-circular obstacle attached to the bottom of a running stream. The upstream Froude number and downstream flow speed are known in advance, and are therefore computed as part of the solution. The dependence of flow behaviour on obstacle size is discussed.     

Coupled autoregulation models in the cerebro-vasculature

A non-dimensional representation of both myogenic and metabolic autoregulation coupled with an asymmetric binary tree algorithm simulating the cerebro-vasculature has been developed. Results are presented for an autoregulation algorithm of the cerebro-vasculature downstream of the efferent arteries, in this case the middle cerebral artery. These results indicate that, due to the low pressures found in the arteriolar structure, the myogenic mechanism based on the increased open probability due to pressure of stretched activated ion channels does not provide enough variation in the vascular resistance to support constancy of blood flow to the cerebral tissue under variable perfusion pressure. A metabolic model has been developed under the assumption of close proximity between venules and the vascular tree at the arteriolar level. This allows carbon dioxide to diffuse between arterioles and the venous bed causing either a relaxation or contraction of the nearby arteriolar bed. Results show that the metabolic mechanism seems to be the dominant mechanism for cerebral autoregulation.     

Increasing the force with which a shock wave discharged from the channel acts on an obstacle by way of converting a normal pressure shock to a system of oblique shocks

The results are given of experimental and numerical investigations of the effect produced on an obstacle by shock waves discharged from channels of different cross-sectional shapes (circle, square, cross). The pressure distribution on an obstacle mounted normally to the flow axis is measured. The experimental results are compared to the data of numerical calculation for determining the optimal modes as regards the duration of calculation and the cell size that produce the least difference between the experimental and numerical data. Calculations are performed of the gas flow behind a shock wave discharged from a channel of X-shaped cross section, and the distribution of pressure and temperature over the obstacle surface is plotted. It is found that the force with which a flow acts on an obstacle when discharged from a channel of X-shaped cross section is much greater than in the case of being discharged from a channel of round or square cross section. Shadow photographs show that this is due to the reduction of the loss of total pressure in the flow because of the conversion of the normal pressure shock to a system of oblique shocks.Translated from Teplofizika Vysokikh Temperatur, Vol. 42, No. 6, 2004, pp. 900–907.Original Russian Text Copyright © 2004 by T. V. Bazhenova, V. V. Golub, A. L. Kotelnikov, A. S. Chizhikov, M. V. Bragin, and S. B. Shcherbak.     

离心泵流噪声实验研究

搭建了离心泵流噪声测试系统,并对离心泵的流噪声进行实验研究。利用水听器测量了原型叶轮和四种改型叶轮在不同转速下的流噪声,发现水 泵流噪声随着转速的增加而增加,随轮舌间隙的减小而增加。实验结果还表明,水泵下游的流噪声声压级要高于上游。观察水泵两端声压级差随转速以 及叶轮半径的变化关系,并探讨其产生的原因。     

矩形截面隧道交通噪声的计算机仿真

采用计算机仿真的方法计算了三维矩形截面隧道的洞内、洞口和洞外的交通噪声。该方法结合了微观交通仿真技术、噪声排放模型、洞口和洞外传播模型,实现单点声源和动态交通源的噪声分布计算。应用虚声源法计算20阶镜面反射,还考虑直达声或反射声的一阶衍射。结果表明,隧道内点声源在洞内相对于半自由场声压增加量随距离增加而增加,而在洞外呈台阶形下降;隧道外点声源在洞内的声压级增加量随距离呈台阶形增加;动态交通流仿真中,交通流变化对洞内外声压增加量关系不大,吸声系数越大洞内外声压增加量降低越大。     

Determination of the Parameters of a Flow of Particles Colliding with an Obstacle

The paper presents the technique developed and the experimental data (obtained by this technique) on the parameters of a flow of particles, which is formed by the energy of explosion, and the pressure caused by this flow in collision with an obstacle, during which superdeep penetration of the particle material into the obstacle occurs.     

Growth of the total pressure recovery factor in a flow behind a shock wave emerging from a channel with concave corners in cross section

Experimental data on the interaction of an obstacle with a shock wave emerging from a square channel with convex and concave corners in the cross section are considered. The flow structure was imaged by high-speed photography of shadow patterns. The total pressure losses in the flow were determined by measuring pressure at an obstacle oriented perpendicularly to the flow. It is established that the total pressure losses as a result of the flow drag upon efflux from the channel with X-shaped cross section are significantly lower as compared to the case of a channel with a round or square cross section.     

基于COMSOL的超声层析成像仿真及图像重建

为了验证三角形和四边形准则的二值逻辑反投影(Binary Logic Back Projection,BLBP)算法在超声层析成像重建过程中的可行性,以超声层析成像的重建区域中同时含有圆柱形和椭圆柱形障碍物为例,利用COMSOL对成像区域内存在障碍物的声场进行仿真,得到声场声压分布。然后,通过提取和处理所获取的声压分布,获得信号矩阵。最后,将信号矩阵代入二值逻辑反投影算法进行图像重建。仿真结果表明:当入射波的波长λ远小于障碍物的尺寸时,声衍射的现象并不明显。重建结果表明:两种准则在几何近似理论下可行,且四边形准则的效果优于三角形准则。     

The evolution of instabilities in gas microjets under acoustic action

The results of an experimental investigation of the effect of external acoustic perturbations on the stability of millimeter- and submillimeter-scale gas jets flowing out into the atmosphere are presented. Data on flow visualization by the shadow method and the instantaneous velocity fields of the flow by the PIV method are obtained. The sound effect in the jet is shown to lead to the asymmetric mode of instability. The growth of this mode downstream leads to flow bifurcation. Frequency characteristics of the effect for jets of different geometries and jets of different gases are compared.     

驻波场中圆柱形换热管外声流特性的数值研究

基于非线性Navier-Stokes方程推导了一般性的声流控制方程,并利用有限元软件COMSOL对平面驻波声场中单根圆柱形换热管和双圆柱形换热管外包含二阶流场信息的Navier-Stokes方程进行全域求解,得到了一阶和二阶流场信息。研究了雷诺数Re和斯特劳哈尔数Sr对换热管外声流结构的影响规律。研究发现:随Re和Sr的增大,换热管边界层内的涡流区域变小,直至消失,而边界层外的涡流区域逐渐增大;且单换热管外涡流个数由8个减少到4个,双换热管外涡流个数从12个减少到8个。此外,边界层内和边界层外的涡流结构呈反向旋转;边界层外流场沿着振荡方向远离换热管,而在垂直方向流向换热管。     

Sound propagation in isotropically and uni-axially compressed cohesive, frictional granular solids

Using an advanced contact model in DEM simulations, involving elasto-plasticity, adhesion, and friction, pressure-sintered tablets are formed from primary particles and prepared for unconfined tests. Sound propagation in such packings is studied under various friction and adhesion conditions. Small differences can be explained by differences in the structure that are due to the sensitivity of the packing on the contact properties during preparation history. In some cases the signals show unexpected propagation behaviour, but the power-spectra are similar for all values of adhesion and friction tested. Furthermore, one of these tablets is compressed uni-axially and under unconfined conditions and the sound propagation characteristics are examined at different strains: (i) in the elastic regime, (ii) during failure, and (iii) during critical flow: the results vary astonishingly little for packings at different externally applied strains.     

Effect of inlet configuration on the refrigerant distribution in a parallel flow minichannel heat exchanger

The refrigerant R-134a flow distribution was experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet configurations (parallel, normal, vertical) were investigated. Tests were conducted with downward flow for mass flux from 70 to 130 kg m⁻² s⁻¹ and quality from 0.2 to 0.6. Tubes were flush-mounted in the test section, with no protrusion into the header. It is shown that normal and vertical inlet yielded similar flow distribution. As mass flux or quality increased, however, better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results. Correlations were developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.     

基于DSP的啸叫抑制系统的研究与实现

扩声系统中由于声反馈导致啸叫,造成了声音的严重失真。在基于ARC600 DSP核的基础上建立了较为完整的硬件试验系统,首先在计算机上研究并仿真了啸叫抑制算法,之后在此试验系统上对该算法做了实时的实现。实验结果说明,该系统的扩声增益提高了10 d B以上,有效地抑制啸叫并且可以保持较好的音质,实时性良好。     

降膜式蒸发器管束间流量分配均匀性对其性能的影响

通过模拟计算讨论降膜式蒸发器在运行过程中,换热管束内外流体不均匀分布对换热器整体性能的影响。管外性能的模拟计算采用根据管束中换热管位置、沿换热管束长度方向上网格离散的三维网格格式,管内性能的计算采用三维CFD计算方式。计算和分析表明,换热流体在换热器中的分布均匀性对换热器内管外局部干斑出现的比例,以及整体性能有明显的影响。     

室内扩散声场的Monte Carlo模拟方法

本文介绍了ＭｏｎｔｅＣａｒｌｏ方法模拟室内扩散声场的原理，模拟计算了混响室声场中的能量密度衰减曲线、能量脉冲响应、平均自由程及吸声系数等。该方法原理简单，占用内存少，适应于模拟室内声场分布。