随时间变化的非平整壁面对液膜表面波演化特性的影响

本文对非平整壁面上的液膜表面波演化过程进行了研究. 针对非平整壁面随时间变化的特性, 采用小参数摄动法对控制方程和边界条件进行求解, 推导出波动壁面上液膜表面波的扰动方程, 采用导数展开法对其进行求解, 并选取简谐波形状的壁面进行数值研究. 对波动壁面下不同参数的影响规律研究可得, 当壁面频率较小时, 静态波与行进波的波长比较相近, 促进表面波之间的合并, 且壁面频率、壁面振幅及Re数的增加, 均会使表面波的振幅明显增大; 对比波动壁面与非平整壁面可得, 在相同位置处, 随着时间的演化, 非平整壁面上表面波呈周期变化, 而波动壁面上表面波呈波长更大的近周期变化; 壁面振幅和壁面频率的降低, 均会使两种壁面结构下的表面波振幅减小, 但所形成的表面波形有所不同, 即波动壁面引起的表面波可看作波动壁面结构与非平整壁面引起的表面波叠加而成.     

合成冷/热射流控制超声速边界层流动稳定性

为了探究超声速边界层流动稳定性及其转捩控制机理,提出基于合成冷/热射流的边界层速度-温度耦合控制方法,并通过数值模拟研究了Ma=4.5超声速平板边界层不稳定波的传播,采用线性稳定性理论中的时间模式分析了壁面吹吸、射流温度、扰动频率、扰动振幅等对不稳定波控制效果的影响.结果表明:无射流控制时,边界层内同时存在不稳定的第一模态扰动波和第二模态扰动波,且二维波形式的第二模态占主导地位;壁面吹吸作用下,仅出现更加不稳定的第二模态,第一模态被抑制;速度-温度耦合控制下,射流温度对扰动模态的不稳定区域大小及扰动增长率影响显著,射流温度与来流温度不同时,温度的脉动使得流动转捩为湍流的速度加快,边界层速度型更加饱满,抗干扰能力增强,流动稳定性提高;高频的吹吸扰动对流场的控制效果优于低频扰动,扰动频率超过400 Hz时,第二模态扰动波时间增长率降低,扰动分量对边界层速度剖面和温度剖面的修正加快,第二模态更加稳定;扰动振幅减小为主流速度的1%时,仅出现时间增长率较小的第二模态,控制效果较好,进一步减小时,第一模态重新出现,并且波数范围与第二模态先重合后分离,对应的时间增长率先增加后减小.研究结果为边界层转捩控制技术提供了新的思路.     

降液膜流动孤波中的涡流影响因素分析

孤波中的涡流被认为是波动强化传热传质机理之一。本文通过数值模拟方法计算了溴化锂溶液沿平板降膜的流动过程。利用VOF方法捕捉降液膜的自由流动表面,利用CSF模型考虑表面张力对降液膜流动的作用,重点研究了扰动频率,Re和倾斜角对孤波中的涡流的影响。模拟结果表明,随着入口扰动频率的增加,涡流经历了完全开式涡,半开式涡至消失的...     

含活性剂液滴在倾斜粗糙壁面上的铺展稳定性

针对含非溶性活性剂液滴在倾斜粗糙壁面上的铺展过程, 应用润滑理论推导出基态和扰动态下液膜厚度和活性剂浓度的演化方程组, 基于非模态理论研究了液滴铺展的稳定性特征, 探讨了相关参数的影响及其内在机理. 研究表明: 液膜厚度和活性剂浓度扰动量均呈现双驼峰型变化, 且峰值位于液滴底部凹陷处; 随扰动波数k增加, 最易失稳区域由液滴底部右侧凹陷处移至左侧凹陷, 壁面结构的影响逐渐减弱, 液滴铺展历程趋于稳定; 增加壁面倾角θ 将导致液滴铺展不稳定性加剧, 增大壁面高度D和壁面波数k₀均导致液滴铺展稳定性先增强后减弱; 随毛细数C减小, 液滴铺展稳定性下降, 重力的影响逐渐突显, 扰动量最大值呈现先增大后减小的变化趋势.     

前缘形状对钝三角翼边界层稳定性及转捩的影响

选择典型高速流动条件,基于线性稳定性理论研究了不同前缘几何特征对典型大后掠角平板钝三角翼外形高速边界层流动稳定性及转捩的影响.研究表明,椭前缘（截面为椭圆）形状的变化仅影响前缘附近的流场特征和边界层流动稳定性;前缘截面长短轴比（形状因子）变大,前缘形状变尖,则横流速度变大,扰动波增长率变大;对于横流模态和第1模态,不同频率扰动波的中性点流向位置随着前缘形状因子变大而后移;对于第2模态,不同频率扰动波中性点的位置基本不变.转捩由第1模态主导,前缘形状因子变大,转捩流向位置后移.      

壁面结构对非定常薄膜流动表面波特性的影响

采用VOF方法,在入口处引入周期扰动,对沿矩形结构底板下落的二维非定常薄膜流动进行直接数值模拟.计算结果表明,不同频率的外加周期扰动在流场中激发出不同波长的自由表面波.当壁面结构的波长与之相比较小时,由于壁面结构引起的静态波骑行在外加扰动形成的行进波上.若这两个波长相近,非线性效应将促进附近的表面波之间的合并.当薄膜无...     

含活性剂液膜去润湿演化的稳定性特征

针对含非溶性活性剂的液膜在固体基底上的去润湿过程,基于润滑理论建立了基态和扰动态下液膜厚度和表面活性剂浓度的演化模型,应用非模态理论分析了演化过程的稳定性特征,探讨了分子间力对液膜去润湿过程的影响. 研究表明,微扰动波的引入（k=1）有利于液膜去润湿过程的稳定进行,扰动能量逐渐衰减,然而,该效果随着扰动波数的增加而显著改变,k ≥ 2时,液膜演化的稳定性反而恶化,扰动能量被逐步放大,演化呈现出非稳定特征. 增大初始液膜厚度可以有效改善液膜流动的稳定性. 范德华力放大了液膜表面的微扰动,使得液膜演化的稳定性下降;相反,Born斥力和静电斥力具有增强去润湿稳定性的作用. 关键词： 活性剂 去润湿 分离压 稳定性     

倾斜粗糙壁面上含不溶性活性剂溶液的动力学特性

针对倾斜随机粗糙壁面上含不溶性活性剂溶液的流动过程, 采用润滑理论建立了液膜厚度和浓度的时空演化模型, 通过PDECOL程序数值求解得到了液膜流/液滴铺展的动力学特性及壁面结构参数的影响. 研究表明: 在重力分量和Marangoni效应共同作用下, 液膜流/液滴铺展速度加快, 液膜边缘和液滴中心出现毛细隆起, 液膜/液滴底部出现凹陷, 同时受粗糙壁面影响, 液膜表面变形更显著. 增加壁面倾角θ具有使重力分量和Marangoni效应增强, 导致隆起和凹陷程度均有所增加的作用. 增大壁面高度D可使液膜流/液滴铺展速度加快, 表面变形放大. 而壁面波数k₀则使液膜流/液滴铺展过程减缓, 抑制隆起和凹陷产生. 与液膜流相比, D和k₀对液滴铺展速度的影响相对较小. 关键词： 随机粗糙壁面 液膜 Marangoni效应 倾斜流动     

流量分配对逆流微通道内流动沸腾影响的研究

微通道内的沸腾两相流动是解决高热流密度下微电子设备散热最有潜力的手段之一。本文基于逆流式微通道热沉设计,实验研究了不同流量调配下逆流式微通道内的流动沸腾特性。讨论了流量分配对微通道内流动沸腾过程中传热特性、压降分布和壁面温度演化规律的影响。实验结果表明：当逆流式通道两侧的质量流量相同时,壁面呈现较好的温度均匀性,且两侧流动压降基本保持一致。两侧流量相差越大,其对应最大两相压降偏差越大。逆流式微通道的壁面温度分布和局部热点的位置可以通过改变两侧质量流量的大小实现有效控制。同时,微通道内流体的演化周期同样可以根据两侧质量流量的高低实现调控。     

倾斜上升管内气液两相弹状流壁面传质特性研究

采用VOF模型对倾角为45°、80°、85°三种情况下倾斜上升管内弹状流的壁面传质特性进行了研究.传质特性通过其与壁面切应力的类比关系来体现.数值模拟结果表明,低混合物流速时,上管壁面切应力在液膜区有明显波动,而下管壁面切应力分布则比较光滑.随着混合流速的增大,液膜区上下壁面切应力分布趋于一致.管子下壁面切应力平均值大于管子上壁面,在Taylor气泡运动速度较低时较为突出.随着Taylor气泡速度的增大,管子上下壁面的切应力平均值趋于相同.相同的混合流速下倾斜角度越大,上下管壁的切应力分布越趋于近似.下降液膜区的壁面切应力平均值大于Taylor气泡尾迹区域.根据Chilton-Colburn的类比关系,壁面切应力的规律完全适用于壁面传质系数.     

Nonlinear analysis of the time evolution of an unstable flat charged fluid surface

Nonlinear second-and fourth-order corrections to the critical Tonks-Frenkel parameter (which characterizes the stability of the uniformly charged flat surface of an ideal conducting incompressible fluid) are found by asymptotic calculations of the fifth order of smallness in ratio of the wave amplitude to the capillary constant of the fluid. A nonlinear integral equation for the time evolution of the unstable wave amplitude is derived and solved. It turns out that the linear stage of instability development takes a major part of the total time, while the nonlinear stage is very short. It is shown that the characteristic time of instability development on the fluid surface is a rapidly decreasing function of the initial amplitude of a virtual wave and the overcritical surface charge (i.e., the excess of the charge over the critical value).     

Stability of liquid film falling down a vertical non-uniformly heated wall

The main object in this paper is to study the stability of a viscous film flowing down a vertical non-uniformly heated wall under gravity. The wall temperature is assumed linearly distributed along the wall and the free surface is taken to be adiabatic. A long wave perturbation method is used to derive the nonlinear evolution equation for the falling film. Using the method of multiple scale, the nonlinear stability analysis is studied for travelling wave solution of the evolution equation. The complex Ginzburg-Landau equation is determined to discuss the bifurcation analysis of the evolution equation. The results indicate that the supercritical unstable region increases and the subcritical stable region decreases with the increase of Peclet number. It has been also shown that the spatial uniform solution corresponding to the sideband disturbance may be stable in the unstable region.     

非平整基底上受热液膜流动稳定性研究

本文研究了二维黏性流体薄膜沿非平整不均匀加热基底流动时非线性表面波的演化及其流动稳定性.利用长波摄动法推导出非平整线性加热基底上非线性表面波的零阶和一阶演化方程,基于所得演化方程,绘制出正弦波纹基底上液膜的表面波形图,并研究液膜流动的线性稳定性,分析了各无量纲参数对液膜线性稳定性的影响.分析结果表明:在正弦波纹基底上,液膜自由表面随同壁面作相同频率的正弦型波动,且液膜厚度沿流动方向逐渐变小;Marangoni数为稳定影响因素,随Marangoni数的增大,液膜稳定区域增大;Peclet数和倾角θ均为不稳定影响因素,随Peclet数和倾角θ的增大,液膜稳定区域减小;在非平整基底的波峰和波谷处,Peclet数、Marangoni数和倾角θ对稳定性的影响趋势一致,但基底波谷处的液膜稳定区域小于波峰处区域,流动更易失稳.     

Instability and breakup of a thick layer of a viscous magnetic fluid in a tilted magnetic field

A linear partial differential equation describing the evolution of an initial disturbance of a flat free surface of a thin layer of a viscous magnetic fluid covering a horizontal plate in the presence of a uniform magnetic field, is derived within a system of ferrohydrodynamic and magnetostatic equations. The effect of magnetizing the plate on the stability of the flat free surface is investigated. An estimate is obtained for the minimum value of the tangential component of the magnetization vector of the fluid sufficient to radically alter the pattern of the final breakup of the continuous layer. Zh. Tekh. Fiz. 69, 14–22 (October 1999)     

Linear mechanism of surface gravity wave generation in horizontally sheared flow

An analysis is presented of a linear mechanism of surface gravity wave generation in a horizontally sheared flow in a fluid layer with free boundary. A free-surface flow of this type is found to be algebraically unstable. The development of instability leads to the formation of surface gravity waves whose amplitude grows with time according to a power law. Flow stability is analyzed by using a nonmodal approach in which the behavior of a spatial Fourier harmonic of a disturbance is considered in a semi-Lagrangian frame of reference moving with the flow. Shear-flow disturbances are divided into two classes (wave and vortex disturbances) depending on the value of potential vorticity. It is shown that vortex disturbances decay with time while the energy of wave disturbances increases indefinitely. Transformation of vortex disturbances into wave ones under strong shear is described.     

Stability characteristic of hypersonic flow over a blunt wedge under freestream pulse wave

To investigate the stability characteristic of hypersonic flow under the action of a freestream pulse wave, a high-order finite difference method was employed to do direction numerical simulation (DNS) of hypersonic unsteady flow over an 8° half-wedge-angle blunt wedge with freestream slow acoustic wave. The evolution of disturbance wave modes in the boundary layer under a pulse wave and a continuous wave are compared, and the wall temperature effect on the hypersonic boundary layer stability for a pulse wave disturbance is discussed. Results show that, both for a pulse wave and a continuous wave in freestream, the disturbance waves inside the nose boundary layer are mainly a fundamental mode; the Fourier amplitude of pressure disturbance mode in the boundary layer for a pulse wave is far less than that for a continuous wave, and the band frequency of the former is wider than that of the latter. All disturbance modes decay rapidly along the streamwise in the nose boundary layer. In the non-nose boundary layer, the dominant mode is transferred from fundamental mode into second harmonic. The transformation of dominant mode for a pulse wave appears much earlier than that for a continuous wave. Different frequency disturbance modes present different changes along streamline in the boundary layer, and the frequency band narrows around the second harmonic mode along the streamwise. Keen competition and the transformation of energy exist among different modes in the boundary layer. Wall temperature modifies the stability characteristic of the hypersonic boundary layer, which presents little effect on the development of fundamental modes and cooling wall could accelerates the growth of the high frequency mode as well as the dominant mode transformation.     

激光聚焦扰动作用下高超声速边界层稳定性实验研究

在马赫数6、单位雷诺数3.1×10⁶/m的条件下对半锥角7°直圆锥边界层稳定性开展了实验研究.以激光聚焦于流场中局部空间而产生的膨胀冲击波作为人工添加的小扰动,分析了该扰动对高超声速圆锥边界层流动稳定性的影响.实验中利用响应频率达到兆赫兹量级的高频压力传感器对圆锥壁面脉动压力进行测量,通过对压力数据进行短时傅里叶分析和功率谱分析发现,相比于不添加激光聚焦扰动的结果,添加激光聚焦扰动使边界层中第二模态波的出现位置提前,且扰动波的幅值大幅度地增加,在相同的流向范围内,激光聚焦扰动将边界层中的扰动波从线性发展阶段推进到非线性发展阶段,其对边界层中扰动波发展的促进效果明显.同时,激光聚焦位置的不同对边界层中扰动波的发展也具有不同的影响.当激光直接聚焦于圆锥壁面X=100 mm位置时,边界层中频率为90 kHz的扰动波幅值增长最快,在X=500 mm的位置处其幅值放大倍数为3.81,相比而言当激光聚焦位置位于圆锥前方自由来流中时,边界层幅值增长最快的扰动波频率大幅减小为73 kHz,相同范围内,其幅值放大倍数为4.51倍.由此可见,当激光聚焦位置位于圆锥上游的自由来流中时,其对边界层中扰动波的影响更为显著.     

The evolution of synthesised vortices in turbulent boundary layer

Particle image velocimetry technique has been used to investigate the evolution of synthesised vortical structures in the turbulent boundary layer. Synthetic jet actuator is implemented on the flat plate surface to synthesise various vortical structures by operating the actuator at varying operating parameters. The vortices are issued into the boundary layer and their evolution and subsequent interaction with the relatively less energetic near wall fluid is studied. The investigation is based on the quantitative measurements that are made both on the central and parallel lateral planes. Finally, the enhancement of the wall shear stress resulted in by the passing vortices is measured to evaluate the effectiveness of the actuator towards flow separation control.     

Spreading of a viscous liquid film over the corrugated surface and the heat and mass transfer calculations

There are a lot of industrial applications of structured packing. Distillation columns are one of the examples where the liquid flows over the corrugated surface as a thin film to provide a good mass-transfer surface between the liquid and vapor phase. The purpose of the present paper is to study the hydrodynamics and the heat-mass transfer of the liquid film spreading down the corrugated surfaces when the corrugation amplitude is comparable with Nusselt’s film thickness (the amplitude corresponds to a small texture of the structured packing). As a result, a nonlinear type diffusion equation is obtained to describe the evolution of the film thickness profile. The nonlinear diffusion coefficient is obtained for three cases: a smooth inclined plate, a corrugated plate with large ribs, and an inclined corrugated plate with small ribs. The equations are solved numerically. As a result, it has been obtained that the small texture significantly increases the rate of the film thickness evolution in comparison with a smooth plate. To obtain the nonlinear diffusion coefficient in the case of a small texture, the hydrodynamics of the film flow over an inclined corrugated surface are studied. The viscosity, inertia, and surface tension forces are taken into account. The calculations were carried out on the basis of the Navier-Stokes equations. The influence of the microcorrugations on both the heat transfer from the wall and the mass transfer through the free surface was investigated.