准三维旋转流体模型及其在部分充液转子稳定性分析中的应用

提了了一种简单的无粘旋转流体准三维模型,并给出了旋转流体对转子作用力的详细表达式,然后用该模型分析了部分充液刚性悬臂转子系统的稳定性,并与实验结果进行了比较,两者在定性上符合较好,准三维无粘流体模型与其它的无粘膜型一样也仅能用来分析无外阻尼或外阻尼较小的部分充液转子系统的稳定性问题。     

充液或部分充液转子的动力稳定性

本文讨论了具有内外阻尼的高速充液转子的动力稳定性。首先通过对旋转流体的平面流场的求解，导出充液转子作简谐运动时流体对转子的动压力，由此导出转子的运动方程；讨论了充液转子的动力稳定性，给出了稳定性解析判据和稳定性边界。结果表明，存在转速门槛值，低于该转速时，充液转子可存在稳定区；当高于该转速时，系统永远失稳，这一结论复盖了已有文献的结果。     

管内液膜流动失稳现象的直接数值模拟

为分析液膜流动时垂直液膜的重力加速度分量和表面张力对扰动波振幅的影响,通过FTM模拟了液膜在重力作用下流动失稳过程。结果表明:重力分量和表面张力系数越大,扰动振幅衰减得越快;并且表面张力系数越小,扰动振幅波动的频率越高。此外还分析了促进液膜流动失稳的两个因素,即密度比和雷诺数Re。当增大密度比时,液膜会出现两种极端的变化:一种是液膜不再完全覆盖底面,会形成锲状液膜,随着速度变大和振幅脉动,当锲状液膜前锋出现新的扰动波时,新的扰动会与初始扰动发生干涉产生二次波动;另一种是增加密度比时,液膜将会在剪切力的作用下产生飞脱的现象,在Re数较大时,液膜流动失稳会在剪切力的作用下产生K-H(开尔文-赫姆霍兹)不稳定性,且初始扰动对形成开尔文-赫姆霍兹波浪具有诱导作用。     

气流作用下同轴带电射流的不稳定性研究

通过对气体驱动同轴电流动聚焦的实验模型进行简化,开展了电场力和惯性力共同作用下同轴带电射流的不稳定性理论研究.在流动为无黏、不可压缩、无旋的假设下,建立了三层流体带电射流物理模型并得到了扰动在时间域内发展演化的解析形式色散关系,利用正则模方法求解色散方程发现了流动的不稳定模态,进而分析了主要控制参数对不稳定模态的影响.结果表明,在参考状态下轴对称模态的最不稳定增长率最大,因此轴对称扰动控制整个流场.外层气流速度越高,气体惯性力越大,射流的界面越容易失稳.内外层液-液同轴射流之间的速度差越大,射流越不稳定.表面张力对射流不稳定性起到促进作用.轴向电场对射流不稳定性具有双重影响:当加载电场强度较小时,射流不稳定性被抑制;当施加电压大于某一临界值时,轴向电场会促进射流失稳.临界电压的大小与界面上自由电荷密度和射流表面扰动发展关系密切.这些结果与已有的实验现象吻合,能够对实验的过程控制提供理论指导.     

流动肥皂膜气体界面及其在柱状汇聚激波作用下演化的实验研究

在Richtmyer-Meshkov(RM)不稳定性实验研究中,形成两种不同密度流体的初始扰动界面是前提和关健.本文提出了一种流动肥皂膜气体界面生成方法,其工作原理是由细丝构成的导流框从激波管实验段穿过,肥皂液从导流框的上端注入并在重力作用下在导流框中形成流动肥皂膜,膜的两测可以分别充入不同密度的气体从而形成稳定的气体...     

电场作用下无黏聚焦射流的时间不稳定性研究

基于电场作用下的流动聚焦实验建立了简化的理论模型,开展了带电同轴液气射流的时间不稳定性分析.在无黏假设下,得到了扰动在时间域内发展演化的解析形式的色散关系,分析了主要控制参数对不稳定模态的影响.结果表明,只有轴对称扰动和第一类非轴对称扰动在时间域内是增长的;液气界面的表面张力对轴对称扰动有着双重影响而对非轴对称扰动起抑制作用;外层气体的流速以及密度的增加均能促进射流的失稳.这些结论与实验结果是定性一致的.结果也表明,在不考虑初始界面电荷密度时,单一的轴向电场能抑制射流的失稳.      

“C.R.Johnston直接法”求解弹性地基支撑充液管中的非线性波

输油管道液体流动、血管中血液流动等都可抽象为充液弹性管流体流动模型,对其固液耦合非线性波动问题进行深入研究具有重要的理论价值和潜在的应用前景。本文以充有理想流体、受弹性地基力作用的无限长充液管为研究对象,考虑管壁材质为橡胶类或软组织材料,利用"C.R.Johnston直接法"研究了充液管中的非线性波动问题,得到了系统的孤立波解,并与约化摄动法所得孤波图形进行了比较分析。图形对比表明,"C.R.Johnston直接法"求解所得图形孤波波幅较高,传播速度较快。相关文献曾指出:如果在支配方程中保留高次项,那么所得结果就会更精确,对应孤波解波幅较高,传播速度也较快。由此说明"C.R.Johnston直接法"是一种求解过程简单,求解结果精度较摄动法更高的方法。在给定参数的情况下,只需初始条件满足w(0)≤2.466200,对于任意的波速c,都可以得到对应的精确孤立波解。     

U型充液管道的流固耦合分析

基于流固耦合原理,在ADINA软件中建立了U型充液管道的有限元模型,对其进行了流固耦合模态分析,研究了壁厚对管道固有频率的影响;并对简支U型管道的充液过程进行了数值仿真,研究了充液加速度和壁厚对管道位移及应力的影响。结果表明:随着模态阶数的增加,管道的固有频率逐渐增大;在无耦合时计算的管道固有频率大于流固耦合状态时的结果,管道固有频率随壁厚的增加呈非线性增加;在充液过程中U型管道的最大应力出现在弯头处;在充液过程中管道的位移与应力随充液加速度的增大而增大,随壁厚的增大而减小;但无论充液加速度多大,充液后管道的最终位移基本相同。     

采用不同黏性处理方法的宽无叶扩压器不稳定流动研究

径向无叶扩压器的全局稳定性可能受到核心主流失稳,出口回流与壁面边界层分离等因素影响,对于宽无叶扩压器,无黏核心主流与壁面边界层流动对不稳定扰动诱发的作用机理是当前研究的重点.本文首先通过数值计算获得了大宽度比孤立无叶扩压器平均流动,然后基于小扰动理论和周向均质假设,分别对欧拉方程与 Navier-Stokes 方程进行线性化,建立了基于无黏核心流动的稳定性分析方法,以及基于涡黏性与分子黏性的混合稳定性分析方法;通过与实验结果的对比,验证了混合稳定性分析方法预测所得流动失稳频率和全局直接模态的准确性;最后基于伴随方法获得了特征值的结构敏感性,揭示了不同黏性处理条件下宽无叶扩压器内全局不失稳扰动的源发区域.在只考虑核心主流的无黏条件下,宽无叶扩压器内流动不稳定扰动来源于流场中部,为二维的离心失稳;在同时考虑核心主流与边界层的作用时,宽无叶扩压器不稳定扰动不仅来源于扩压器流场中部的核心主流,壁面回流对于不稳定扰动的产生了重要影响.      

采用不同黏性处理方法的宽无叶扩压器不稳定流动研究

径向无叶扩压器的全局稳定性可能受到核心主流失稳,出口回流与壁面边界层分离等因素影响,对于宽无叶扩压器,无黏核心主流与壁面边界层流动对不稳定扰动诱发的作用机理是当前研究的重点.本文首先通过数值计算获得了大宽度比孤立无叶扩压器平均流动,然后基于小扰动理论和周向均质假设,分别对欧拉方程与Navier-Stokes方程进行线性化,建立了基于无黏核心流动的稳定性分析方法,以及基于涡黏性与分子黏性的混合稳定性分析方法;通过与实验结果的对比,验证了混合稳定性分析方法预测所得流动失稳频率和全局直接模态的准确性;最后基于伴随方法获得了特征值的结构敏感性,揭示了不同黏性处理条件下宽无叶扩压器内全局不失稳扰动的源发区域.在只考虑核心主流的无黏条件下,宽无叶扩压器内流动不稳定扰动来源于流场中部,为二维的离心失稳;在同时考虑核心主流与边界层的作用时,宽无叶扩压器不稳定扰动不仅来源于扩压器流场中部的核心主流,壁面回流对于不稳定扰动的产生了重要影响.     

Experimental study of extreme thrust on a tidal stream rotor due to turbulent flow and with opposing waves

Time-varying thrust has been measured on a rotor in shallow turbulent flow at laboratory scale. The onset flow has a turbulence intensity of 12% at mid depth and a longitudinal turbulence length scale of half the depth, about 5 times the vertical scale, typical of shallow flows. The rotor is designed to have thrust and power coefficient variations with tip speed ratio close to that of a full-scale turbine. Three extreme probability distributions give similar thrust exceedance values with the Type 1 Pareto in mid range which gives 1:100, 1:1000 and 1:10 000 exceedance thrust forces of 1.38, 1.5 and 1.59 times the mean value. With opposing waves superimposed the extreme thrust distribution has a very similar distribution to the turbulent flow only. Exceedance forces are predicted by superposition of a drag force with drag coefficient of 2.0 based on the wave particle velocity only and with an unchanged mean thrust coefficient of 0.89. These values are relevant for the design of support structures for marine turbines.     

Implementation of physical boundary conditions into computational domain in modelling of oscillatory bottom boundary layers

This paper discusses the importance of realistic implementation of the physical boundary conditions into computational domain for the simulation of the oscillatory turbulent boundary layer flow over smooth and rough flat beds. A mathematical model composed of the Reynolds averaged Navier–Stokes equation, turbulent kinetic energy (k) and dissipation rate of the turbulent kinetic energy (ε) has been developed. Control‐volume approach is used to discretize the governing equations to facilitate the numerical solution. Non‐slip condition is imposed on the bottom surface, and irrotational main flow properties are applied to the upper boundary. The turbulent kinetic energy is zero at the bottom, whereas the dissipation rate is approaching to a constant value, which is proportional to the kinematic viscosity times the second derivative of the turbulent kinetic energy. The output of the model is compared with the available experimental studies conducted in oscillatory tunnels and wave flume. It is observed that the irrotational flow assumption at the upper boundary is not realistic in case of water tunnels. Therefore, new upper boundary conditions are proposed for oscillatory tunnels. The data of wave flume show good agreement with the proposed numerical model. Additionally, several factors such as grid aspect ratio, staggered grid arrangement, time‐marching scheme and convergence criteria that are important to obtain a robust, realistic and stable code are discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

The turbulent/non-turbulent interface at the outer boundary of a self-similar turbulent jet

The flow at the outer boundary of a submerged self-similar turbulent jet at Re=2᎒³ is investigated experimentally by means of combined particle image velocimetry (PIV) laser-induced fluorescence (LIF) measurements. The jet fluid contains a fluorescent dye so that the LIF data can be used to discriminate between the jet fluid and the ambient fluid. The axial velocity, Reynolds stress, and vorticity are determined relative to the jet boundary. The results are compared against the conventional profiles, and the results of a direct numerical simulation of the turbulent far-wake behind a flat plate. The results show a sharp transition between rotational and irrotational fluid at the fluid interface, and the existence of a layer of irrotational velocity fluctuations outside the turbulent region.     

Finite element analysis of lift build-up due to aerofoil indicial motion

In this paper the problem of impulsively started aerofoil or suden change of incidence of an aerofoil in incompressible potential flow is investigated. The essence of solution lies in the representation of a timely and spatially varying wake in a largely irrotational potential flow field. This is achieved by representing the wake through velocity potential difference, which seems to be the only way of imposing a velocity difference condition in the finite element context with velocity potentials as the basic unknowns. Superposition is employed to meet various boundary conditions, which is justified by the linearity of the problem. The finite element solutions are compared with those from singularity method.     

Calculation of three-dimensional weakly expanding flow in jets and channels

The results are given of a calculation of laminar flow in a channel of square section and the motion of a turbulent jet from a cruciform nozzle in an ambient flow. To calculate the secondary flows, the field of the transverse velocity is decomposed into irrotational and solenoidal components. The results of the calculation of the flow in the channel are compared with the calculations of other authors and experimental data. To calculate the flow in the turbulent jet, a one-parameter turbulence model is used, and the influence of the inhomogeneity of the distribution of the longitudinal component of the velocity on the components of the Reynolds stress tensor is taken into account. The results of calculation of the flow in the jet behind a cruciform nozzle are compared with experimental data.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 36–44, July–August, 1984.     

低雷诺数沟槽表面湍流/非湍流界面特性的实验研究

湍流/非湍流界面是流动中湍流和无旋流的边界,其相关研究在加深对湍流与无旋流之间的物质、动量和能量交换的理解有重要意义.本文采用时间解析的二维粒子图像测速技术,分别对零压梯度光滑、顺流向锯齿形沟槽表面平板在不同雷诺数下对湍流/非湍流界面的几何特征及动力学特性进行了实验研究.实验雷诺数为$Re_{\tau } =400\sim1000$.本文采用了湍动能准则对湍流/非湍流界面进行了识别,并分析界面高度分布、分形特征及界面附近的条件平均速度和涡量.结果表明在不同雷诺数下, 无论是光滑壁面还是沟槽壁面,界面平均高度在0.8 $\sim$ 0.9$\delta_{99} $附近. 对于沟槽壁面而言,减阻时对应的界面高度的概率密度分布与光滑壁面基本一致, 均遵循正态分布,而当阻力增大时, 界面高度分布偏离正态分布出现正的偏度. 在本实验情况下,界面分形维度、跨界面速度跳变均会随着雷诺数增大而增大. 此外,不同壁面情况下无量纲条件平均涡量在界面附近的分布相近,而界面附近无量纲速度梯度最大值近似为常数.      

EXPERIMENTAL STUDY ON PROPERTIES OF TURBULENT/NON-TURBULENT INTERFACE OVER RIBLETS SURFACES AT LOW REYNOLDS NUMBERS1)

湍流/非湍流界面是流动中湍流和无旋流的边界,其相关研究在加深对湍流与无旋流之间的物质、动量和能量交换的理解有重要意义.本文采用时间解析的二维粒子图像测速技术,分别对零压梯度光滑、顺流向锯齿形沟槽表面平板在不同雷诺数下对湍流/非湍流界面的几何特征及动力学特性进行了实验研究.实验雷诺数为$Re_{\tau } =400\sim1000$.本文采用了湍动能准则对湍流/非湍流界面进行了识别,并分析界面高度分布、分形特征及界面附近的条件平均速度和涡量.结果表明在不同雷诺数下, 无论是光滑壁面还是沟槽壁面,界面平均高度在0.8 $\sim$ 0.9$\delta_{99} $附近. 对于沟槽壁面而言,减阻时对应的界面高度的概率密度分布与光滑壁面基本一致, 均遵循正态分布,而当阻力增大时, 界面高度分布偏离正态分布出现正的偏度. 在本实验情况下,界面分形维度、跨界面速度跳变均会随着雷诺数增大而增大. 此外,不同壁面情况下无量纲条件平均涡量在界面附近的分布相近,而界面附近无量纲速度梯度最大值近似为常数.     

On the role of the liquid flow characteristics on low-Weber-number atomization processes

At low Weber numbers, the aerodynamic forces due to the interaction between gas and liquid do not influence liquid atomization processes. In these situations, atomization processes depend on issuing liquid flow characteristics only. According to the literature, the atomization efficiency is best when the issuing liquid flow shows a high turbulence level. Some injectors are based on this concept and promote the production of turbulence by imposing deflection of the flow inside the nozzle. However, many studies indicate that the level of turbulence does not solely control the atomization efficiency. By conducting a numerical and experimental study on the behavior of cavity nozzles, it is found that internal flow deflection to produce turbulence also produces a non-axial flow component at the nozzle exit whose effect on the atomization process is of paramount importance. Indeed, the results show that the surface energy produced during the atomization process is linearly dependent on the sum of the turbulent kinetic energy and the non-axial kinetic energy at the nozzle exit. This sum represents the energy available for the atomization process, and the influence of the injection pressure as well as of the nozzle geometry on this energy is investigated.     

High-Velocity Laminar and Turbulent Flow in Porous Media

We model high-velocity flow in porous media with the multiple scale homogenization technique and basic fluid mechanics. Momentum and mechanical energy theorems are derived. In idealized porous media inviscid irrotational flow in the pores and wall boundary layers give a pressure loss with a power of 3/2 in average velocity. This model has support from flow in simple model media. In complex media the flow separates from the solid surface. Pressure loss effects of flow separation, wall and free shear layers, pressure drag, flow tube velocity and developing flow are discussed by using phenomenological arguments. We propose that the square pressure loss in the laminar Forchheimer equation is caused by development of strong localized dissipation zones around flow separation, that is, in the viscous boundary layer in triple decks. For turbulent flow, the resulting pressure loss due to average dissipation is a power 2 term in velocity.     

Flow field studies on a micro-air-vehicle-scale cycloidal rotor in forward flight

This paper examines the flow physics and principles of force production on a cycloidal rotor (cyclorotor) in forward flight. The cyclorotor considered here consists of two blades rotating about a horizontal axis, with cyclic pitch angle variation about the blade quarter-chord. The flow field at the rotor mid-span is analyzed using smoke flow visualization and particle image velocimeV are compared with flow fields predicted using 2D CFD and time-averaged force measurements acquired in an open-jet wind tunnel at three advance ratios. It is shown that the experimental flow field is nearly two dimensional at μ = 0.73 allowing for qualitative comparisons to be made with CFD. The incoming flow velocity decreases in magnitude as the flow passes through the retreating (upper) half of the rotor and is attributed to power extraction by the blades. A significant increase in flow velocity is observed across the advancing (lower) half of the rotor. The aerodynamic analysis demonstrates that the blades accelerate the flow through the lower aft region of the rotor, where they operate in a high dynamic pressure environment. This is consistent with CFD-predicted values of instantaneous aerodynamic forces which reveal that the aft section of the rotor is the primary region of force production. Phase-averaged flow field measurements showed two blade wakes in the flow, formed by each of the two blades. Analysis of the blades at several azimuthal positions revealed two significant blade-wake interactions. The locations of these blade-wake interactions are correlated with force peaks in the CFD-predicted instantaneous blade forces and highlight their importance to the generation of lift and propulsive force of the cyclorotor.