平面槽流的二次失稳分析

本文以得自二维直接数值模拟的稳定的二维平衡态解，或将其中的扰动部分乘一小的系数所得到的准定常解取作基本流，考察它们对三维小扰动的不稳定性。导出了只包含二个未知函数的封闭的优动运动方程组。与刘次边条件一起构成了一个常微分方程组的特征值问题。采用Ｃｅｂｙｓｈｅｖ－τ方法将问题转化为一个代数特征值问题，对三维基本波与亚谐波失稳两种情形数值求解获得了三维扰动增长率随二维扰动幅值、雷诺数与展向波数的变化关系     

二维不可压缩平板边界层的紧致有限差分法

针对二维不可压缩平板边界层N-S扰动方程,采用高精度紧致有限差分格式,应用空间模式,直接数值模拟了二维不稳定T-S波传播的过程。该算法的时间离散采用三阶精度混合显隐分裂格式,空间离散采用高精度紧致有限差分逼近。法向采用非等距网格方法,出口边界条件采用嵌边函数法。实例验证,该方法计算结果与流动稳定性分析的结果一致。     

自由剪切流中沿流向变化的二次失稳研究

该文以考虑厚度变化的二维相干结构为背景流动作二次失稳研究,分析流向各位置处三维扰动展向空间尺度选择机制。结果表明:用能量法考虑流向变化影响及采用局部平行假设获得二维相干结构为基础流动,进行二次失稳分析的方法,可用于研究快速增长的二次失稳问题。还说明了三维相干结构的出现是二维相干结构对三维基本扰动的二次失稳的结果,这种失稳过程对展向波数有很强的选择性。     

不同压力梯度平面库埃特流中扰动涡动力学特性

该文以不同压力梯度的平面库埃特流动为基本流,在壁面上构建一个局部微振荡射流扰动,利用直接数值模拟的方法求解三维不可压缩N-S方程,模拟了扰动涡结构的生成与演化过程。模拟结果表明:由于压力梯度的差异,虽然射流扰动期间的扰动涡幅值几乎相同,但幅值在不同的平面库埃特基本流中的演化特性表现出明显的差异;在相同时刻,库埃特基本流压力梯度越大,扰动涡的幅值越大,扰动控制方程中的基本流近壁速度梯度与扰动涡法向速度乘积项对幅值的增长起着主要作用;在不断增长的扰动涡中,呈现高速、低速条纹间隔密集的特性,由于扰动涡的存在,流动区域的近壁面平均切应力系数增加,在增长相对迅速的P??1时的库埃特基本流中形成了相对完整的发卡涡形态的涡量分布。     

沟槽张角对流体减阻机理的数值模拟研究

为了研究边界层涡结构对沟槽壁面摩擦阻力的影响,该文利用FLUENT软件对湍流状态下沟槽壁面进行数值模拟计算,分析了90°、120°和150°三种不同张角下沟槽壁面的减阻效果,获得了不同张角的沟槽结构对减阻效果的影响规律。结果表明:沟槽内形成的旋涡结构可以有效减小沟槽布置区域法向速度梯度,减弱流层间剪切作用,增加条带结构稳定性,湍流猝发频率降低,从而使壁面摩擦阻力降低。当来流速度v=25 m/s,沟槽张角β=90°时,可取得最佳减阻效果。在沟槽结构布置区域,流向涡的涡头上扬角度变大,上扬和下扫强度减弱,有效减小了摩擦阻力;同时,沟槽结构改变近壁区速度分布,使流向涡密度减小,从而减小壁面摩擦阻力。     

二维曲壁扩压流道紊流边界层计算及试验研究

本文利用边界层理论对二维曲壁扩压流道内部的气流流动规律进行了初步的研究。主流流场的计算采用了时间推进有限面积元方法,然后用BOX差分格式对边界层方程进行数值求解,并且在紊流切应力模式中考虑了壁面曲率的影响,由此得到流道两壁上边界层流动沿流动方向上的发展规律,最后通过三个不同来流条件的实验测量,对理论计算和分析进行了验证。结果表明:沿凸壁面的边界层流动与沿凹壁面的边界层流动具有不同的发展规律,凸壁面上的压力分布比四壁面情况下要平缓些,最大压力梯度出现得也更迟,此外,凸壁画上的壁面局部阻力系数也比相应的凹壁面为低,这说明了凸壁面上的边界层流动比较稳定。对于壁面曲率比较大的弯曲流道来说,在边界层的计算中应考虑到曲率的影响。     

基于壁面模化大涡模拟的SUBOFF壁面脉动压力数值研究

该文基于开源计算流体力学平台OpenFOAM,采用壁面模化大涡模拟中的非平衡壁面模型,对雷诺数Re_L=1.2×10⁷(L为艇长)下的SUBOFF艇体绕流问题开展了数值研究。通过Liutex涡识别方法,该文给出了艇体近壁流动的涡结构分布,提取了艇体表面的时均压力系数,与实验数据进行了对比,并在此基础上重点研究了壁面模化大涡模拟对壁面脉动压力的预测,给出了不同区域的壁面脉动压力频率谱、平行中体处流向和周向波数-频率谱。研究结果表明：壁面模化大涡模拟能够准确预测艇体表面时均压力分布;脉动压力预测方面,艇体首部转捩区的压力脉动更为剧烈,脉动压力谱级整体高于充分发展湍流区;从脉动压力流向波数-频率谱中可观察到湍流能量集中的迁移脊,很好地反映了湍流的时空关联特性。     

基于高精度WENO格式的二维溃坝波数值模拟

以浅水波方程组建立二维溃坝洪水模型,对经典WENO格式计算模板重新选取,得到各计算模板上重构多项式,对各个模板的局部光滑因子进行组合得到更高阶的全局光滑因子,建立相应的非线性权,将各模板上重构多项式加权处理得到计算单元界面数值通量的一个新高精度WENO格式,泰勒级数展开证明了该格式的非线性权比经典WENO格式的非线性权更接近理想权。将该格式、Lax-Friedrich通量分裂方法和局部特征分解方法相结合用于二维浅水方程的离散求解,模拟二维局部溃坝流和圆形溃坝流洪水的演进过程,模拟结果与实际流动符合较好,表明采用高精度WENO格式所建立的高分辨率溃坝模型能很好地模拟溃坝波的演进过程。     

大长高比腔体内的混合流体行进波对流

该文报告大长高比腔体（Г=46）中强非线性状态（ψ=-0．47）混合流体行进波对流图案的数值模拟结果。利用MAC法通过二维流体力学扰动方程组进行了数值模拟。获得了发生在对流发生点附近的线性Counter propagatingwave，出现在分叉图非线性分支上的均匀行进波和发生在分叉图saddle-node附近的局部行进波状态等三种行进波对流图案，并探讨了它们的稳定性。最后将数值模拟结果与实验结果进行了比较。     

周期性加热Poiseuille-Rayleigh-Benard流动中局部行波的研究

该文通过二维流体力学基本方程组的数值模拟,研究了水平来流和周期性加热对腔体内流体的运动和动力学特性的影响。当相对瑞利数r(28)9时,随着水平来流雷诺数Re逐渐增加,腔体内的流体发生三种不同的运动,即局部对流运动、局部行波运动和水平流运动。对局部行波运动的周期pT进行研究,局部行波运动的稳定周期随水平来流雷诺数的增大而减小,随相对瑞利数的减小而减小;相对瑞利数越小,稳定周期随水平来流雷诺数变化的增长率越大。局部行波运动的区间(35)Re依赖水平来流雷诺数的强度,当水平来流雷诺数Re大于(35)Re的上限时流体为水平流运动,而当水平来流雷诺数Re小于(35)Re的下限时流体为局部对流运动;对于不同的相对瑞利数r,局部行波运动的区间(35)Re是不同的。随着水平来流雷诺数增大,发现腔体内上游行波区域对流圈的减少速度比下游行波区域更快。     

二维平行壁面剪切流动的转捩与混沌模型

本文利用约化摄动方法，从理论上研究了二维平行壁面剪切流动的转捩与混沌现象，得到了描述二维平行壁面流动中的小振幅Ｔ－Ｓ波的演化方程为Ｋｏｒｔｅｗｅｇ－ＤｃＶｒｉｅｓ－Ｂｕｒｇｃｒｓ方程，反映Ｔ－Ｓ波衰减或增长率的参数为ＫｄＶ－Ｂｕｒｇｅｒｓ方程中的耗散系数Ｖ１与色散系数δ的比值ｖ１√δ，ｖ１和δ由流场结构决定的。当Ｔ－Ｓ波振幅被放大到一定程度后将受到的自由流中的高次或亚谐波的激励。经过分叉而进入混沌     

NUMERICAL ANALYSIS OF THE INFLUENCE OF THE TIP CLEARANCE FLOWS ON THE UNSTEADY CAVITATING FLOWS IN A THREE- D IMENSIONAL INDUCER

To clarify the influences of the tip clearance flows on the unsteady cavitating flow,the three-dimensional unsteady cavitating flows through both the two-dimensional cascades and the three-dimensional inducer with and without tip clearance are performed numerically.The governing equations for the compressible fluid flow with the DES turbulence model are employed with the assumption of the isentropic process of liquid phase.The evolution of cavities is represented as the source/sink of vapor phase.The basic equations in the curve linear coordinate are solved by the finite difference method.As the results of the three-dimensional cavitating flows through the two-dimensional cascades,the tip clearance flows from the pressure side to the suction side of the blade produces the tip vortex cavitation,which affects the sheet cavitation on the leading edge of the next blade and enhances the blockage effect near the casing than the flows without tip clearance.On the other hand,in the case of the three-dimensional inducer,the large backflow cavitation is observed around the inlet of the inducer,where the cavities are developed on the casing by the tip clearance flows.The large pressure gradient between the non-cavitating pressure side and the cavitating suction side enhances the tip clearance flows.The calculation considering the tip clearance reproduces the developed cavitation region similar to that of experimental visualizations.Additionally,the backflow cavitation rotates with the speed slower than the rotation speed of the inducer.Then,the rotation of backflow cavitation causes the periodic fluctuation of the outlet pressure greater than that of the inlet pressure.     

FIVE MODE INTERACTIONS IN A FREE SHEAR LAYER

This study condiders spatially developing free shear layers at moderateReynolds numbers that are formed by the merging of two free streams initially separatedby a splitter plate.To study the nonlinear interactions between the two-and three-dimen-sional fundamental and subharmonic modes in the spatially developing shear layer,the se-lected five modes represent the minimum number that needs to be considered.The objec-tive of this study is to investigate the nonlinear effects arising from the interactions be-tween two-and three-dimensional large-scale coherent structures in a free shear layer.Our attention is focused on the energy exchange mechanisms between the various modes,and the effects of the nonlinear evolution of the phases of the interacting modes.It isshown that depending on the initial modal phase angles the downstream growth of theshear layer and the associated local entrainment can be strongly affected by the presence ofthese nonlinear interactions.This high sensitivity of the shear layer growt     

A STUDY ON NUMERICAL METHOD OF NAVIER-STOKES EQUATION AND NON-LINEAR EVOLUTION OF THE COHERENT STRUCTURES IN A LAMINAR BOUNDARY LAYER

1. INTRODUCTION A high accuracy compact finite difference scheme has been extensively applied to direct simulation of turbulence coherent structure’s complicated and remote physical phenomena [1-5]. The Fourier spectralexpansion was previously commonly …     

ANALYSIS OF HYDRODYNAMICS FOR TWO-DIMENSIONAL FLOW AROUND WAVING PLATES

Hydrodynamic characteristics for two-dimensional flow around a waving plate are investigated. Under large Reynolds number approximation, the flow is assumed to be a combination of the outer potential flow and a thin vortex layer, which consists of a boundary layer and a shed free shear layer. A nonlinear mathematical formulation for describing the outer unsteady potential flow coupled with an unsteady boundary layer equation for the inner viscous flow adjacent to the waving plate is developed. A semi-analytical method with a nonlinear Kutta condition imposed at the trailing edge is used to solve the velocity field of the outer flow and the evolution of wake vortex induced by a large-amplitude waving plate. The unsteady boundary layer equation is solved by extending Pohlhausen’s method to its unsteady counterpart. The thrust and viscous drag coefficients, propulsive efficiency, and the pattern of wake vortex sheet are discussed.     

水平流作用下的混合流体行进波对流

利用高阶紧致有限差分格式,数值模拟研究了物性参数对具有Soret效应的混合流体的Poiseuille-Rayleigh-Bénard流动系统的影响。在正的小分离比下,附加一个微小的温度扰动作为扰动源,流动系统时空结构的发展在经历了初始状态、线性波指数型成长和非线性发展三个阶段之后,最终演变成为稳定的周期性行进波状态。还进一步探讨了水平流强度和Rayleigh数对流场结构的影响。     

ON NONLINEAR STABILITY IN NONPARALLEL BOUNDARY LAYER FLOW

The nonlinear stability problem in nonparallel boundary layer flow for two-dimensional disturbances was studied by using a newly presented method called Parabolic Stability Equations (PSE). A series of new modes generated by the nonlinear interaction of disturbance waves were tabulately analyzed, and the Mean Flow Distortion (MFD) was numerically given. The computational techniques developed,including the higher-order spectral method and the more effective algebraic mapping, increased greatly the numerical accuracy and the rate of convergence. With the predictor-corrector approach in the marching procedure, the normalization condition was satisfied, and the stability of numerical calculation could be ensured. With different initial amplitudes, the non linear stability of disturbance wave was studied. The results of examples show good agreement with the data given by the DNS using the full Navier-Stokes equations.     

Experimental study on modulational instability and evolution of crescent waves

A series of experiments on the instability of steep water wave trains in water with finite water depths and infinite water depths in a wide wave basin were performed. It was found that under the coupled development of modulational instability and class-I instability, the initial two-dimensional steep wave trains evolved into three-dimensional crescent waves, followed by the occurrence of disordered water surfaces, and that the wave energy transferred to sidebands in the amplitude spectrum of the water surface elevation. The results also show that water depth has a significant effect on the growth of modulational instability and the evolution of crescent waves. The larger the water depth, the more quickly the modulational instability suppresses class-I instability.     

ANALYTICAL AND NUMERICAL SOLUTIONS FOR THE FLOW IN MICROTUBE WITH THREE-DIMENSIONAL CORRUGATED SURFACE,PART 1:STEADY FLOW

The influences of three-dimensional corrugated wall on the fully-developed steady no-slip flows in microtube are studied by analytical and numerical methods in this article. Detailed analytical solutions for the space-averaged equations and the numerical method for the solutions of the disturbance equations are given. An iterative arithm of coupled equations with respect to space-averaged velocities and disturbance velocities is suggested. The study shows that a three-dimensional subsidiary stress layer exists in the near-wall region. The relative roughness, wavenumber and Reynolds number are three important parameters influencing the subsidiary stresses and the space-averaged pressure drop. The space-averaged pressure drop subject to an invariable flow rate mainly depends on the position of datum surface. When the datum surface is taken at the balance position of wall function, the value of pressure drop is determined by the dynamic characteristics of the flow.