亚格子尺度湍流特性研究

采用大涡模拟方法模拟了雷诺数ReH=18,400的后台阶湍流流动,研究了亚格子尺度湍流动能和湍流耗散的特性。给出了后台阶湍流流动的流场结构以及亚格子湍动能和亚格子湍耗散的空间分布结果,比较了大涡模拟预报湍流粘性以及等效计算粘性。研究表明,亚格子尺度湍动能和亚格子湍耗散随着流动在空间的发展而呈现减弱趋势,回流区内亚格子湍动能和耗散较弱;在台阶截面(y/H=1处)亚格子湍动能和耗散最大。亚格子湍动能小于脉动动能统计量,亚格子粘性小于等效湍流模型粘性预报结果。     

横风作用下移动点湍流脉动特性风洞试验研究EI北大核心CSCD

为了研究移动点历经的湍流脉动风特性,设计了一套能够实现测量探针以均匀速度在流场中移动的试验系统,测量了不同风速-车速比(风速与移动点速度的比值)下的湍流脉动特性,提出了半椭圆湍流积分尺度模型,分析了与静止点观测结果相比的湍流脉动风速谱及能谱的变化。研究结果表明,移动点测得的湍流脉动特性与静止点测量结果存在明显的不同。对于某一固定来流风速,随着移动点速度的增加,纵向湍流积分尺度减小,相比Balzer及Cooper积分尺度模型,半椭圆模型的描述更加准确。基于半椭圆模型的脉动速度谱具有较高的精度,移动点历经的湍流谱能量因运动而重新分布,并表现出明显的Doppler效应。随着观测点移动速度的增加,脉动速度谱整体向更高的频率偏移,高频区域中的谱值增大,低频区域中的谱值减小,且能量向更高的波数区传递。     

横风作用下移动点湍流脉动特性风洞试验研究

为了研究移动点历经的湍流脉动风特性,设计了一套能够实现测量探针以均匀速度在流场中移动的试验系统,测量了不同风速-车速比(风速与移动点速度的比值)下的湍流脉动特性,提出了半椭圆湍流积分尺度模型,分析了与静止点观测结果相比的湍流脉动风速谱及能谱的变化。研究结果表明,移动点测得的湍流脉动特性与静止点测量结果存在明显的不同。对于某一固定来流风速,随着移动点速度的增加,纵向湍流积分尺度减小,相比Balzer及Cooper积分尺度模型,半椭圆模型的描述更加准确。基于半椭圆模型的脉动速度谱具有较高的精度,移动点历经的湍流谱能量因运动而重新分布,并表现出明显的Doppler效应。随着观测点移动速度的增加,脉动速度谱整体向更高的频率偏移,高频区域中的谱值增大,低频区域中的谱值减小,且能量向更高的波数区传递。     

预处理情形下斜管湍流横向射流大涡数值模拟

采用带预处理情形下的大涡模拟方法,结合高阶WENO格式,针对椭圆斜管湍流横向射流进行了数值模拟。计算结果显示在选取截面上,三个方向上平均速度分布与试验值较为接近。对计算结果进行分析,描述流场中发卡涡拟序结构及其运动演化过程,阐述发卡涡对流向速度分布的影响,讨论流场中雷诺应力与发卡涡运动演化之间的关系。     

不同SST模式在细长体绕流模拟中的应用比较

分别运用SST（Shear-Stress Transport）k-ω、SST-DES、SST-SAS湍流模型对亚音速条件下细长旋成体大攻角绕流流场及气动力特性进行了数值研究,通过侧向力轴向演化、物面压力分布、涡粘度变化、计算网格适应性及对流场细节的捕捉等方面比较了3种湍流模式的计算结果。数值实验表明:在相同计算条件下,3种模式对细长体后部的模拟结果差异较大,SAS模型能捕捉到更多的小尺度涡及尾涡系的非定常特性;相同横截面处涡粘度处于同一量级且大小关系为SAS      

湍流入口条件下建筑非定常风场的大涡模拟

考虑湍流入口条件对绕德州理工大学标准模型的三维非定常风场进行了数值模拟;比较了稳态入口边界条件下时均风速剖面形状对建筑表面压力分布的影响;分别采用旋涡法、谱合成法和预前模拟法生成湍流入口风速脉动;并将相应的大涡模拟结果与实测数据和风洞试验数据进行了对比分析。结果表明:时均风速剖面形状对时均流场的影响几乎可以忽略,但对瞬态流场的计算有很大影响,工程应用中建议根据当地实测数据修正风速剖面公式;采用由预前模拟法生成的湍流作为入口边界条件进行大涡模拟得到的建筑表面风压非定常特性与实际情况最为吻合;通过高精度插值算法,预生成的入口湍流时程数据能在位于相同地貌下不同建筑绕流的大涡模拟中反复使用,在一定程度上弥补了预前模拟耗时的缺陷。     

叶轮式介质搅拌磨单相湍流场的数值模拟

采用计算流体动力学对叶轮式介质搅拌磨中的单相流湍流流场进行数值模拟,分析不同转速下的流体速度、速度梯度、剪切力、湍流强度、湍动能输运等流场特性,并对该搅拌磨磨腔内单相流(水)的流体速度进行实测。结果表明,磨腔中的流体能量不断分散并重新分配,产生随机分布的涡量,流场能量主要集中在靠近叶轮搅拌器及腔筒内壁的区域内,为有效研磨区;在一定范围内增大搅拌器转速,流体速度及速度梯度增大,流体之间剪切力增大,研磨效果改善;黏度越小的湍流场,湍流强度越高,湍动能在传输过程中耗散越小,能量利用效率越高;实测所得的流体速度变化情况与模拟结果相似,两者的平均速度值相差较小,表明模拟方法可行。     

基于统计方法的NOPD耗能机理定量分析

非阻塞性微颗粒阻尼（NOPD）是在传统颗粒阻尼和冲击阻尼技术基础上发展起来的新型阻尼技术。本文从湍流物理模型出发,基于统计方法定量分析了NOPD技术的耗能机理。经分析认为,高频振动中的NOPD颗粒群其运动状态和湍流运动相似,故引入Kolmogorov的局部各向同性假设,得到NOPD的结构函数表达式及能谱密度表达式。研究结果表明,同种材料,相同颗粒直径情况下,能量耗散率随颗粒群体积比的增加而增大;相同颗粒群体积比时,能量耗散率随颗粒直径的增加而增大。统计方法的引入,为NOPD的工程应用提供了一种有效的定量分析方法。     

基于LES和DES的定日镜结构风致响应分析

定日镜作为典型的风敏感结构,设计中必须考虑其动力特性和风致响应。选用大涡模拟(large eddy simulation, LES)和分离涡模拟(detached eddy simulation, DES),结合一种新的湍流脉动流场产生方法 (discretizing and synthesizing random flow generation, DSRFG)模拟风场的湍流边界条件,计算得到了0°风向角下0°、30°、60°镜面仰角下定日镜的流场分布和风荷载时程数据。建立了定日镜整体结构的有限元模型,进行了定日镜整体结构在不同镜面仰角下的风致响应分析。结果表明,通过与风洞试验结果对比,LES和DES能较好地预测出顺风向等效风荷载,但LES的结果更接近试验值;随着仰角增大,定日镜下部的共振峰值能量逐渐减小,上部的共振峰值能量逐渐增大;定日镜下部的最不利工况为仰角0°时,风振系数为3.1,中上部的最不利工况发生在仰角为60°时,风振系数分别为2.0、3.4;LES和DES能较好地模拟出流场中的紊流与涡旋,且随着仰角增大,尾流区变得狭长。结合风洞试验,为定日镜以及相似结构的数值模拟以及抗风...     

台风“海葵”近地风脉动特性实测研究

基于10、20和30 m高度处台风"海葵"影响下的上海浦东地区近地风现场实测数据,分析了阵风因子、湍流度、峰值因子以及湍流积分尺度的变化规律。结果表明,各向阵风因子随高度和风速的增大而减小;不同高度处各向湍流度随风速的增加而减小;峰值因子基本上随平均风速的增大略有减小但是变化不甚明显且离散性较大,观测高度对峰值因子随时距变化的影响较小;各向的湍流积分尺度均有随平均风速的增大而增大的趋势,并且随着平均风速的增大,其离散度也略有增大,不同高度处纵向和横向湍流积分尺度均随时距的增大而增大。     

Regional statistics in confined two-dimensional decaying turbulence

Two-dimensional decaying turbulence in a square container has been simulated using the lattice Boltzmann method. The probability density function (PDF) of the vorticity and the particle distribution functions have been determined at various regions of the domain. It is shown that, after the initial stage of decay, the regional area averaged enstrophy fluctuates strongly around a mean value in time. The ratio of the regional mean and the overall enstrophies increases monotonously with increasing distance from the wall. This function shows a similar shape to the axial mean velocity profile of turbulent channel flows. The PDF of the vorticity peaks at zero and is nearly symmetric considering the statistics in the overall domain. Approaching the wall, the PDFs become skewed owing to the boundary layer.     

Calculation of the statistical characteristics of a turbulent flow under the action of external forces

A study is made of the statistical characteristics of isotropic velocity and scalar fields in supply of the kinetic energy of turbulence from the energy of the average flow. Two models based on the distributions of the kinetic turbulence energy and the intensity of scalar-field pulsations by wave numbers and length scales are used for calculation of the statistical characteristics of turbulent velocity and scalar fields. The calculation results are compared to the data of the direct numerical modeling performed under the same initial conditions. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 1, pp. 148–154, January–February, 2006.     

Kelvin-Wave Cascades in Turbulent Superfluid 4He at Very Low Temperatures

There has been much interest recently in the mechanism by which superfluid (quantum) turbulence can decay in liquid ⁴He at very low temperatures, where mutual friction has a negligible effect. As in classical turbulence, energy must probably flow from larger to smaller length scales, and it has been suggested that on the smallest scales the relevant motion is a Kelvin wave on a quantized vortex with wave number greater than the inverse vortex spacing. By considering the behaviour of a simple model it is shown by computer simulations how energy can flow to shorter length scales (higher wave numbers) in a system of Kelvin waves, and how this process can lead to a remarkably simple Kelvin-wave energy spectrum. A discussion is included of the relevance of this model to the decay of superfluid grid turbulence at a very low temperature.     

BER evaluations for multimode beams in underwater turbulence

Abstract

In underwater optical communication links, bit error rate (BER) is an important performance criterion. For this purpose, the effects of oceanic turbulence on multimode laser beam incidences are studied and compared in terms of average BER (<BER>), which is related to the scintillation index. Based on the log-normal distribution, <BER> is analysed for underwater turbulence parameters, including the rate of dissipation of the mean squared temperature, the rate of dissipation of the turbulent kinetic energy, the parameter that determines the relative strength of temperature and salinity in driving index fluctuations, the Kolmogorov microscale length and other link parameters such as link length, wavelength and laser source size. It is shown that use of multimode improves the system performance of optical wireless communication systems operating in an underwater medium. For all the investigated multimode beams, decreasing link length, source size, the relative strength of temperature and salinity in driving the index fluctuations, the rate of dissipation of the mean squared temperature and Kolmogorov microscale length improve the <BER>. Moreover, lower <BER> values are obtained for the increasing wavelength of operation and the rate of dissipation of the turbulent kinetic energy in underwater turbulence.     

The mixing and decay of a turbulence field characterized by two significant dynamic length scales

Abstract

The kinetic equation governing the probability density function of the fluid elements in velocity space and describing a multi‐length scale turbulence field is employed to analyze an initially uniform and homogeneous turbulence which is characterized by two different significant length scales. The mixing and decay of this turbulence field have been analyzed and the generation and the subsequent behavior of the apparent length scale have been also investigated.     

Turbulent regime of thermogravitational convection in a closed cavity

We have performed a numerical analysis of the nonstationary turbulent natural convection in a closed region with heat-conducting walls of finite thickness and a heat source located at the cavity base under the conditions of convective-radiative heat exchange with the environment. Typical distributions of the thermohydrodynamic parameters (streamlines, temperature field, field of the kinetic energy of turbulence, and dissipation field of the kinetic energy of turbulence) in a fairly wide range of Grashof numbers 10⁷ ≤ Gr ≤ 10⁹ have been obtained. Results characterizing the scales of influence of the nonstationarity factor and the relative heat conductivity coefficient of the material of the surrounding walls on the heat transfer intensity are presented. A correlation for determining the average Nusselt number on the heat source surface has been established.     

The role of coherent vortices near the turbulent/non-turbulent interface in a planar jet

The role of coherent vortices near the turbulent/non-turbulent (T/NT) interface in a turbulent plane jet is analysed by a direct numerical simulation (DNS). The coherent vortices near the jet edge consist of large-scale vortical structures (LSVSs) maintained by the mean shear and intense vorticity structures (IVSs) created by the background fluctuating turbulence field. The radius of the LSVS is equal to the Taylor micro-scale R(lsvs)≈λ, while the radius of the IVS is of the order of the Kolmogorov micro-scale R(ivs)~η. The LSVSs are responsible for the observed vorticity jump at the T/NT interface, being of the order of the Taylor micro-scale. The coherent vortices in the proximity of the T/NT interface are preferentially aligned with the tangent to the T/NT interface and are responsible for the viscous dissipation of kinetic energy near the T/NT interface and to the characteristic shape of the enstrophy viscous diffusion observed at that location.     

Scintillation index of optical spherical wave propagating through biological tissue

Effects of the tissue turbulence on the propagation of an optical spherical wave are analysed. For this purpose, scintillation index of an optical spherical wave which is propagating in a soft tissue is formulated and evaluated in weakly turbulent soft tissue. Scintillation index of the optical spherical wave is examined against the changes in the tissue parameters which are the tissue length between the optical spherical wave source and the detector, random variations in the refractive index of the tissue and the outer scale of the tissue turbulence. According to our graphical outputs, it is observed that increase in the random variations of the refractive index of the tissue results in an increase in the scintillation index at a certain realization of the turbulence spectrum. On the other hand, larger outer scales and longer tissue lengths yield larger scintillations. The variation of the scintillation index of the optical spherical wave versus the wavelength is also investigated. It is found that at small tissue lengths, wavelength has almost no effect on the scintillations; however, when the tissue length reaches a certain value, shorter wavelengths give rise to larger intensity fluctuations.     

Energy and vorticity dynamics in decaying isotropic turbulence

The mechanics of energy and vorticity decay associated with an exact solution to the von Karman-Howarth equation is described. Three distinct modes of decay are found to occur depending on the magnitude of the product of turbulent Reynolds number and skewness factor. Vortex stretching is shown to be responsible for transferring energy to wave numbers in the dissipation range. Vorticity accumulated in this process is dissipated by viscosity once the turbulence scale is reduced sufficiently by vortex stretching. Several comparisons to experimental data of the computed values of energy and microscale are given, with generally favorable result.     

Length scales and size distributions in dynamic fragmentation

The shatter of a cherished wine glass on impact with the kitchen tile, the spallation in the high-energy collision of atomic nuclei, the fragmentation of the Shoemaker-Levi comet on passage of the Roche limit of the Jovian gravitational field, collectively span vast length scales, yet are each examples of dynamic fragmentation with a number of commonalities. In the above examples, as well as many other dynamic fragmentation events, the consequence is the breakage of the body into some number of fragments that are distributed over size. At the heart of a satisfactory theory is the prediction of the number of fragments and the statistical distribution of these fragments over size. A theory based on energy principles is found to provide length scales that govern both the characteristic fragment size and the distribution spread. Fundamental failure and fracture properties of the material are central in determining the nature of the fragment size distribution. Fragment size distributions can range from relatively tight exponential functions to power-law relations spanning a number of decades in fragment size. The fragment distribution and the dynamic fracture processes leading to power-law distributions bear striking similarities to hydrodynamic turbulence. Onset of fracture asymptotes to a range of length scales in which destruction is self-similar and fractal, requiring that consequences, including the fragment size distributions, exhibit a power-law dependence on the length scale. The theory is described and supporting experimental evidence is provided.