矩形截面直管中的振荡流动

通过解析求解纳维尔-斯托克斯方程,本文给出了矩形截面直管中的层流振荡流动解。该解适用于不同的截面高宽比r和不同的振荡流动雷诺数。对典型的r和λ(雷诺数的平方根),给出了振荡流动速度的振幅比和相位差的等值线分布以及振荡流动速度随时间的变化,可以看出流动的速度分布如何从低频高粘性类型向高频低粘性的边界层流型逐渐变化;也可以看出管的边壁和角部对振荡流动的影响。当振荡频率ω为零时,本文结果即成为通过矩形截面定常管流的Stokes解。     

局部狭窄血管中血液振荡流的速度和切应力

本文建立一种分析局部缓慢狭窄血管中血液振荡流的数学模型，给出了血液的轴向流速，径向流速和切应力的包含压力梯度项的解析表达式，并讨论了血管内由局部狭窄引起的压力梯度沿轴向变化的规律。文章以局部余弦狭窄为例进行数值计算，详细讨论上游均匀管段压力梯度的定常部分和不同次谐波对狭窄管段内流速和切应力的影响。数值结果表明，与均匀管情况相比，在狭窄段内，血液振荡流轴向流速无论平均值还是脉动幅值均明显增大，且径向流速不再为零。但径向流速仍远小于轴向流速。同时，切应力也不再仅由轴向流速梯度提供，径向流速梯度也将产生切应力，但是在计算管壁切向上的切应力时，径向流速梯度的贡献仍相当大。与均匀管管壁切应力沿流运方向保持恒定不同。狭窄管管壁切应力（平均值和脉动值）将随着狭窄高度的增大而增大，在狭窄最大高度处达到最大，因而沿流动方向产生了较大的切应力梯度。     

“射流松动泥沙”的机理及工程应用试验研究——射流引起的非均匀流全河段水面比降及垂线流速分布

泥沙清淤技术及其基础研究需要不断发展,通过自制射流发生装置,在水槽中对射流引起的非均匀流水面比降及垂线流速分布全河段变化特点进行了系统试验研究。由于射流的存在,水面比降及垂线流速分布发生较大变化。水面比降射流段上游的比无射流的小,射流段及其下游比无射流的大,而全河段水面比降比无射流时增大。垂线流速分布在射流段及其下游附近发生根本的变化,有射流时,水面附近的流速比底部流速小,垂线平均流速一般小于无射流。上述这些特点与水槽流量和射流量有关,一般而言,水槽流量越大,射流量越大,这些变化越显著。     

圆直管内层流振荡流的摩擦特性

研究了在谐波压力梯度作用下圆管内层流振荡流的流速分布形态、断面平均流速、驱动功率损耗及摩擦特性，并对管内单方向定常流的范宁摩擦系数公式进行扩展，建立了用于管内振荡流的摩擦系数计算式。摩擦系数理论计算结果与实验显示了较好的一致性。     

“射流松动泥沙”的机理及工程应用试验研究：射流引起的非均匀流全河段比降

泥沙清淤技术及其基础研究需要不断发展，通过自制射流发生装置，在水槽中对充引起的非均匀波水面比降及垂线流束分布全河段地系统试验研究。由于射流的存在，水面比降及垂线流速分布发生较在同比降射流段上游的比无射流的小，充段及其下游比无射流的大，而全不面比降比元流时增大。垂线流速分布在射流段及其下游附近发生根本的变化，有射流时，水面附近的流速比底部流速小，垂线平均流速一般小于无射流。上述这些特点与水槽流量和射     

管线振荡绕流对砂床的冲蚀

采用轻质模型砂在带砂槽的Ｕ型振荡水槽中对振荡流中管线下方的砂床冲蚀进行了实验研究,给出了管线下方砂床的平衡冲蚀深度随Ｋｃ数的变化规律以及发生冲蚀的临界ｅ／Ｄ和Ｋｃ的相关,并指出管线的存在使砂床开始形成砂波相应的振荡流临界振幅值下降．     

水中开孔腔流激振荡控制实验研究

水中开孔腔流激振荡是水下航行器的一类突出噪声源. 为探究有效抑制水中开孔腔流激振荡的控制方法和作用特性, 首先以水下航行器的表面开孔结构为对象设计了开孔腔模型, 并提出一种基于来流边界层分流原理的流动控制装置——前缘分流体, 借助循环水洞装置对水中开孔腔的流激振荡特性及其控制进行了实验研究. 通过沿流向和展向安装于腔底的动态压力传感器测量腔内脉动压力, 分别从腔内脉动压力的频谱特性和空间分布特性两方面, 探讨了水中开孔腔在不同流速下的流激振荡特性和前缘分流体对水中开孔腔流激振荡的控制效果, 并对前缘分流体的主要作用机理进行了分析. 研究结果表明: 水中开孔腔流激振荡形式以剪切层自持振荡为主, 在流速较低时, 如2.4 m/s, 就会产生稳定的自持振荡, 且具有随流速升高而急剧增大的趋势; 前缘分流体对水中开孔腔绕流自持振荡具有良好的抑制效果, 且抑制效果随流速增加而显著提升, 对腔内脉动压力频谱峰值和总级的最大抑制量分别达到25.3 dB和15.6 dB; 此外, 前缘分流体对开孔腔流激振荡具有低频频移作用, 有益于避免发生流激空腔共振; 脉动压力空间分布特性表明, 前缘分流体对水中开孔腔流激振荡抑制机理主要在于破坏了腔内流场受到的周期调制作用.      

刚性圆管中血液周期振荡流的切应力分布

本文通过求解圆管内血液振荡流的基本方程，求得圆管内血液流的压力梯度与切应力之间的关系式。在此基础上，详细讲座了圆管中轴向流速和切变率谐波的变化规律，指出流速谐波和切变率谐波的幅值都将随着谐波次数的增大而逐渐减小。为了使所得结果便于应用。文章通过管轴向中心线流速与压力梯度之间的关系式，进一步给出一种利用管轴向中心线流速计算管内切应力分布的简便方法。该方法用于检测活体血管内血液振荡流的切应力分布，具有操作简单，精度较高的优点。最后，以人体颈动脉为例，讨论血液周期振荡流的切应力的分布特性。发现在任意时刻，除了邻近管壁处切应力急剧增大到一定数值之外，沿管截面切应力分布相当均匀且接近于零，呈现出与定常流不同的切应力分布特征。     

均匀来流中旋转圆柱黏性绕流的数值研究

从不可压非定常Ｎ－Ｓ方程出发，首次数值求解了均匀来流中圆柱作周向旋转振荡的黏性绕流问题。探讨了旋转角速度振幅、振荡频率及Ｒｅ数等因素对流场结构及其非定常演化过程的影响，并根据计算结果，给出了在旋转振动频率。速度振幅平面内流场涡结构的分区图。     

小型气驱动式U形振荡水槽

本文介绍气驱动式U形振荡水楷(U形管)的设计特点、流场品质和初步实验结果。在结构设计上采用了理论计算的弯管型线、步进电机带动蝶阀的控制系统和吸气驱动等较先进的技术措施,使得该设备具有流场品质好、结构简单、调节方便等优点,是开展振荡流实验研究的基本设备。     

Numerical simulation of the horizontal Bridgman growth. Part I: Two-dimensional flow

We study the generation of periodic velocity and temperature fields in a plane horizontal crucible of molten metal under the action of a horizontal temperature gradient. The geometry and the boundary conditions are similar to those encountered in the Bridgman growth process of semiconductor crystals, although the present paper is limited to two-dimensional flows. We use transient finite difference and finite element algorithms which lead to identical results. We demonstrate the oscillatory mechanism in two different geometries.     

Heat and mass transfer of a fuel droplet evaporating in oscillatory flow

A numerical study of the heat and mass transfer from an evaporating fuel droplet in oscillatory flow was performed. The flow was assumed to be laminar and axisymmetric, and the droplet was assumed to maintain its spherical shape during its lifetime. Based on these assumptions, the conservation equations in a general curvilinear coordinate were solved numerically. The behaviors of droplet evaporation in the oscillatory flow were investigated by analyzing the effects of flow oscillation on the evaporation process of a n-heptane fuel droplet at high pressure.The response of the time history of the square of droplet diameter and space-averaged Nusselt numbers to the main flow oscillation were investigated in frequency band of 1–75 Hz with various oscillation amplitudes. Results showed that, depending on the frequency and amplitude of the oscillation, there are different modes of response of the evaporation process to the flow oscillation. One response mode is synchronous with the main flow oscillation, and thus the quasi-steady condition is attained. Another mode is asynchronous with the flow oscillation and is highly unsteady. As for the evaporation rate, however, in all conditions is more greatly enhanced in oscillatory flow than in quiescent air.To quantify the conditions of the transition from quasi-steady to unsteady, the response of the boundary layer around the droplet surface to the flow oscillation was investigated. The results led to including the oscillation Strouhal number as a criteria for the transition. The numerical results showed that at a low Strouhal number, a quasi-steady boundary layer is formed in response to the flow oscillation, whereas by increasing the oscillation Strouhal number, the phenomena become unsteady.     

Numerical simulation of the horizontal Bridgman growth. Part II: Three-dimensional flow

We study the steady-state three-dimensional flow which occurs in a horizontal crucible of molten metal under the action of a horizontal temperature gradient. The geometry and the boundary conditions are similar to those encountered in the Bridgman growth process of semiconductor crystals. We find that three-dimensional effects can have a dramatic influence upon the flow, which, before the onset of periodic disturbances, differs appreciably from its two-dimensional counterpart. We also investigate the sensitivity of the flow to non-symmetric disturbances.     

Numerical simulation of the horizontal Bridgman growth. Part III: Calculation of the interface

We study the transient motion of the solidification front during the growth of semiconductor crystals in the horizontal Bridgman geometry. The calculation is based on a two-dimensional flow. We use finite elements which deform with the motion of the interface. The energy equation is coupled with the isothermal constraint of the interface in an implicit transient algorithm. Several examples show the oscillatory motion of the interface caused by the periodic flow of the melt, and they reveal the importance of the growth rate on the shape of the interface.     

Inertial and coriolis effects on oscillatory flow in a horizontal dendrite layer

Flow instability due to oscillatory modes of disturbances in a horizontal dendrite layer during alloy solidification is investigated under an external constraint of rotation. The flow in the dendrite layer, which is modeled as flow in a porous layer and with the inertial effects included, is assumed to rotate about the vertical axis at a constant angular velocity. The investigation is an extension of the work in Riahi (On stationary and oscillatory modes of flow instablity in a rotating porous layer during alloy solidification. J. Porous Media, 6, 177–187, 2003), which was for the case in the absence of the inertial effects. Results of the stability analyses indicate, in particular, that the Coriolis effect can enhance the physical domain for the oscillatory flow, while the inertial effect tends to reduce such domain. Sufficiently strong inertial effect can eliminate presence of the oscillatory mode only for the rotation rate beyond some value. The effect of interaction between the local volume fraction of solid and the flow associated with the Coriolis term was found to be stabilizing.     

Oscillatory flow of second grade fluid in cylindrical tube

The unsteady oscillatory flow of an incompressible second grade fluid in a cylindrical tube with large wall suction is studied analytically. Flow in the tube is due to uniform suction at the permeable walls, and the oscillations in the velocity field are due to small amplitude time harmonic pressure waves. The physical quantities of interest are the velocity field, the amplitude of oscillation, and the penetration depth of the oscillatory wave. The analytical solution of the governing boundary value problem is obtained, and the effects of second grade fluid parameters are analyzed and discussed.     

Hydromagnetic flow in a viscoelastic fluid due to the oscillatory stretching surface

An analysis is carried out to study the unsteady magnetohydrodynamic (MHD) two-dimensional boundary layer flow of a second grade viscoelastic fluid over an oscillatory stretching surface. The flow is induced due to an infinite elastic sheet which is stretched back and forth in its own plane. For the investigated problem, the governing equations are reduced to a non-linear partial differential equation by means of similarity transformations. This equation is solved both by a newly developed analytic technique, namely homotopy analysis method (HAM) and by a numerical method employing the finite difference scheme, in which a coordinate transformation is employed to transform the semi-infinite physical space to a bounded computational domain. The results obtained by means of both methods are then compared and show an excellent agreement. The effects of various parameters like visco-elastic parameter, the Hartman number and the relative frequency amplitude of the oscillatory sheet to the stretching rate on the velocity field are graphically illustrated and analysed. The values of wall shear stress for these parameters are also tabulated and discussed.     

Turbulent Channel Flow with an Artificial Two-Dimensional Wall Layer

Turbulent flow of an incompressible fluid in a plane channel with parallel walls is considered. The three-dimensional time-dependent Navier-Stokes equations are solved numerically using the spectral finite-difference method. An artificial force which completely suppresses lateral oscillations of the velocity is introduced in the near-wall zone (10 % of the channel half-width in the neighborhood of each wall). Thus, the three-dimensional flow zone, in which turbulent oscillations can develop, is separated from the wall by a fluid layer. It is found that the elimination of three-dimensionality in the neighborhood of the walls leads to a significant reduction in the drag. However, complete laminarization does not occur. The flow in the stream core remains turbulent and can be interpreted as a turbulent flow in a channel with walls located on the boundary of the two-dimensional layer and traveling at the local mean-flow velocity. The oscillations developing inside the two-dimensional layer, which have significant amplitude, distort the flow only in the adjacent zone. Beyond this zone the distributions of the mean characteristics and the structure of instantaneous fields completely correspond to ordinary turbulent flow in a channel with rigid walls. The results obtained confirm the hypothesis of the unimportance of the no-slip boundary conditions for the fluctuating velocity component in the mechanism of onset and self-maintenance of turbulence in wall flows.     

Two-dimensional convection in a horizontal fluid layer with spatially periodic boundary temperatures

Two-dimensional thermal convection in a fluid layer confined between two horizontal rigid walls kept at spatially periodic temperatures is investigated by direct numerical simulations. With increasing the Rayleigh number, convection evolves from a steady state to a temporally chaotic flow. It is observed that the transition to the chaos occurs via quasi-periodic states with two or three basic frequencies or via sequences of period-doubling bifurcations, according to the boundary temperature distributions.     

Method of moments for laminar dispersion in an oscillatory flow through curved channels with absorbing walls

The unsteady dispersion of a solute, when the fluid is driven through a curved channel with absorbing walls by an imposed pulsatile pressure gradient, is studied using the method of moments. The study examines the effect of oscillatory Reynolds number, amplitude/frequency of the pressure pulsation and boundary absorption on the longitudinal dispersion. The methodology involves a set of unsteady integral moment equations obtained by applying the Aris-Barton method of moments on the convective-diffusion equation for a curved channel. Central moments are obtained from the moment equations which are solved by a finite-difference implicit scheme. The effect of curvature and boundary absorption on the effective dispersion coefficient from the initial to the stationary stage of the oscillatory flow is studied. Amplitude of the effective dispersion coefficient is found to increase with curvature and decrease with frequency of the pressure pulsation. For large Peclet number and Schmidt number, the amplitude of the dispersion coefficient can be 1.6 times that in a straight channel at large times. Also, for large times, the amplitude of the dispersion coefficient is twice the amplitude of the dispersion coefficient as α, the frequency parameter changes from 0.5 to 1.0. The axial distributions of mean concentration are determined from the first four central moments by using the Hermite polynomial representation. The effect of curvature is to delay the stationary state and also the approach to normality of the concentration distribution. The study has importance in understanding the spreading of pollutants in tidal basins and natural current fields.