近壁区理论耗散率k-ε模型的研究

提出在湍流边界层近壁区采用共振三波的理论模型描述湍流相干结构,根据理论模型计算了ε的分布。并且在传统k-ε模式基础上依照理论ε值计算了平均速度分布。在粘性作用层理论值与直接数值模拟符合很好。表明该理论方法不仅有益于对湍流机制的了解,而且可能为湍流的近壁模型化开辟一条新的途径。     

近壁区理论耗散率κ-ε模型的研究

提出在湍流边界层近壁区采用共振三波的理论模型描述湍流相干结构，根据理论模型计算了ε的分布。并且在传统κ－ε模式基础上依照理论ε值计算了平均速度分布。在粘性作用层理论值与直接数值模拟符合很好。表明该理论方法不仅有益于对湍流机制的了解，而且可能为湍流的近壁模型化开辟一条新的途径。     

湍流边界层近壁区多个相干结构的数值模拟

从流动稳定性理论中的一般共振三波概念出发,提出一种湍流边界层近壁区多个相干结构的理论模型,采用高精度差分格式和Fourier谱展开相结合的方法,求解三维不可压Navier-Stokes方程,直接数值模拟近壁湍流多个相干结构的演化问题.并将得到的湍流边界层近壁区多个相干结构的数值演化特性与实验观察到的特性进行了比较.     

湍流边界层拟序结构的大涡模拟研究

采用动力亚格子模型,利用大涡模拟方法模拟了雷诺数为13000的充分发展槽道湍流流动。从瞬时速度和脉动 速度场、脉动速度相关、均方根脉动涡量分布、以及瞬时涡量场等多个方面,对湍流边界层流动的拟序结构进行了分析, 包括近壁区小尺度湍流结构和瞬态过程,如条纹结构、喷射和扫掠过程、以及近壁旋涡结构等。     

粗糙壁面湍流边界层流动和拟序结构的大涡模拟研究

采用LES方法和虚拟粗糙元模型,,模拟了Rer=180有粗糙壁面的充分发展槽道湍流流动.从流向平均速度、脉动速度均方根、雷诺剪切应力、脉动速度相关,以及瞬时近壁准流向涡结构等多个方面,对粗糙壁面湍流边界层流动和拟序结构进行了分析.结果表明,粗糙壁面处雷诺剪切应力、脉动速度均方根以及近壁条带平均距离和准流向涡尺度都大于光滑壁面,且随着粗糙元高度的增加这种趋势更加明显.本工作为进一步研究颗粒在近壁区域的弥散规律奠定了基础.     

三角肋条大涡模拟与减阻机理研究

已有实验表明三角肋条具有较好的表面减阻效果,但雷诺时均湍流模型难以模拟与揭示肋条减阻机理。本文采用大涡模拟(LES)对下壁面安装肋条的槽道流进行模拟,通过减阻率与实验值的对比以及平均速度剖面和脉动速度均方根等与直接数值模拟(DNS)结果的对比确认本文计算的准确性。根据计算的流场细节分析表明:肋条通过增加近壁区的边界层厚度,抑制流向涡的发展,减少近壁活动,使得近肋条壁面的剪切力比平板的剪切力小而获得减阻。     

超声速湍流边界层密度场特性

本文采用基于纳米示踪平面激光散射技术的密度场测量方法,对Ma=3.0零压力梯度平板湍流边界层的密度场特性进行了实验研究,分析了边界层密度场的平均特性和脉动特性,并利用泰勒假设将空间信号转换为时域信号,对密度脉动的频谱特性进行了分析.研究发现,随着高度增加,边界层平均密度逐渐增大,但密度脉动仅在对数区内逐渐增大,在边界层外层却逐渐减小.密度脉动的概率密度函数在对数区内呈正态分布.从频谱特性可以发现,湍流边界层内部密度脉动具有丰富的脉动频率,最高达到MHz量级;在边界层近壁区和外层,密度脉动以低频分量为主;在对数区,高频分量与低频分量所占的比例基本相同.结合速度和密度同时测量发现,密度脉动与质量流量脉动在概率密度分布和频谱特性方面具有非常好的一致性,但与速度脉动相关性不大.超声速湍流边界层内部强烈密度脉动,及其与速度脉动的明显差别,是可压缩湍流边界层与不可压湍流边界层的显著区别之一.     

横向粗糙元壁面槽道湍流的大涡模拟研究

采用大涡模拟和浸没边界法相结合对不同高度和不同间距横向粗糙元壁面槽道湍流进行了模拟,得到了光滑壁面和粗糙壁面湍流的流向平均速度分布,雷诺剪切应力,脉动速度均方根和近壁区拟序结构。结果发现横向粗糙元降低了流向平均速度,增大了流动阻力,粗糙壁面湍流的雷诺剪切应力大于光滑壁面。粗糙元降低了流向脉动速度,增强了展向和法向脉动速度。粗糙元高度越高,对湍流流动影响越大,而粗糙元间距对湍流统计特性的影响不大。粗糙壁面仍然存在着和光滑壁面类似的条带结构。     

非Kolmogorov大气湍流温度谱标度指数的测量与分析

 阐述了非Kolmogorov湍流谱理论以及湍流谱标度指数的测量与计算方法。在近地面多个地点对大气湍流温度起伏进行了多次的实验观测,结果表明：实际大气湍流温度谱标度指数多数不等于-5/3,并且通常在-2到-1之间变化。分析了湍流温度谱标度指数与湍流发展程度的相关性,利用小波分析方法展现了不同湍流强度下湍流温度脉动能量在各尺度之间的分配状态,发现湍流温度谱标度指数的绝对值在一定程度上随湍流强度的增加而增大。     

超音速湍流脉动流场气动光学效应分析

对高超音速飞行器在大气中飞行时所产生的湍流脉动气动光学效应进行了理论分析与计算.根据CFD计算流场时所使用的湍流模型及其控制方程,推导出流场的折射率脉动方差控制方程.用统计方法,求出该脉动流场的系综平均光学传递函数及相位均方差,从不同角度描述了湍流脉动的气动光学效应.计算结果表明,气动光学传输函数的幅度响应函数具有低通特征,使所获得的图像发生像模糊,而相位响应函数则导致红外成像相位非线性偏移.此外,在相同飞行高度下,马赫数越高,图像模糊越严重.电弧风洞实验结果验证了本文理论分析的正确性.     

Rotation effect on near-wall turbulence statistics and flow structures

Turbulent flow in an axially rotating pipe, involving complicated physical mecha- nism of turbulence, is a typical problem for the study of rotating turbulent flow. The pipe rotation induces two effects on the flow. One is the stabilizing effect due to the centrifu- gal and Coriolis forces, which accounts for the relaminarization of the turbulence[1—3] and the reduction of the friction coefficient at the pipe wall. The behavior is also related to the wall streaks inclining to the azimuthal di…     

A parallel solution – adaptive method for three-dimensional turbulent non-premixed combusting flows

A parallel adaptive mesh refinement (AMR) algorithm is proposed and applied to the prediction of steady turbulent non-premixed compressible combusting flows in three space dimensions. The parallel solution-adaptive algorithm solves the system of partial-differential equations governing turbulent compressible flows of reactive thermally perfect gaseous mixtures using a fully coupled finite-volume formulation on body-fitted multi-block hexahedral meshes. The compressible formulation adopted herein can readily accommodate large density variations and thermo-acoustic phenomena. A flexible block-based hierarchical data structure is used to maintain the connectivity of the solution blocks in the multi-block mesh and to facilitate automatic solution-directed mesh adaptation according to physics-based refinement criteria. For calculations of near-wall turbulence, an automatic near-wall treatment readily accommodates situations during adaptive mesh refinement where the mesh resolution may not be sufficient for directly calculating near-wall turbulence using the low-Reynolds-number formulation. Numerical results for turbulent diffusion flames, including cold- and hot-flow predictions for a bluff-body burner, are described and compared to available experimental data. The numerical results demonstrate the validity and potential of the parallel AMR approach for predicting fine-scale features of complex turbulent non-premixed flames.     

Effect of surface roughness element on near wall turbulence with zero-pressure gradient

In the present paper,we present an investigation on the effect of roughness elements onto near-wall kinematics of a zeropressure-gradient turbulent boundary layer.An array of spanwisely-aligned cylindrical roughness elements was attached to the wall surface to regulate the near-wall low-speed streaky structures.With both qualitative visualization and quantitative measurement,we found that the regularization only occurs in the region below the height of the roughness elements.Statistical analysis on the probability distribution of the streak spanwise spacing showed that the mean spanwise streak spacing is dominated by the roughness elements;however,the latter's effect is in competition with the intrinsic streak generation mechanisms of smooth wall turbulence.Below the top of the roughness elements,local streamwise turbulent fluctuation intensity can be reduced by about 10%.We used POD analysis to depict such regularization effect in terms of near-wall structure modulation.We further found that if the spanwise spacing of roughness elements increased to be larger than the mean streak spacing in the smooth wall turbulence,there is no streak-regularization effect in the buffer region,so that the near-wall streamwise turbulent fluctuation intensity doesn't reduce.     

A theoretical model of turbulent fiber suspension and its application to the channel flow

A theoretical model of turbulent fiber suspension is developed by deriving the equations of Reynolds averaged Navier-Stokes,turbulence kinetic energy and turbulence dissipation rate with the additional term of fibers.In order to close the above equations,the equation of probability distribution function for mean fiber orientation is also derived.The theoretical model is applied to the turbulent channel flow and the corresponding equations are solved numerically.The numerical results are verified by comparisons with the experimental ones.The effects of Reynolds number,fiber concentration and fiber aspect-ratio on the velocity profile,turbulent kinetic energy and turbulent dissipation rate are analyzed.Based on the numerical data,the expression for the velocity profile in the turbulent fiber suspension channel flow,which includes the effect of Reynolds number,fiber concentration and aspect-ratio,is proposed.     

Velocity,temperature, and Reynolds-stress scaling in the wall region of turbulent boundary layer on a permeable surface

A finite total number of flow parameters in the wall region of a turbulent boundary layer points to universal behavior of turbulent shear stress as a function of mean-velocity gradient and turbulent heat flux as a function of both mean-velocity and mean-temperature gradients. Combined with dimensional arguments, this fact is used to reduce the momentum and heat equations to first-order ordinary differential equations for temperature and velocity profiles amenable to general analysis. Scaling laws for velocity and temperature in boundary layer flows with transpiration are obtained as generalizations of well-known logarithmic laws. Scaling relations are also established for shear stress and rms transverse velocity fluctuation. The proposed method has substantial advantages as compared to the classical approach (which does not rely on fluid-dynamics equations [1–3]). It can be applied to establish scaling laws for a broader class of near-wall turbulence problems without invoking closure hypotheses.     

A one-equation turbulence model for recirculating flows

A one-equation turbulence model which relies on the turbulent kinetic energy transport equation has been developed to predict the flow properties of the recirculating flows. The turbulent eddy-viscosity coefficient is computed from a recalibrated Bradshaw’s assumption that the constant a₁ = 0.31 is recalibrated to a function based on a set of direct numerical simulation (DNS) data. The values of dissipation of turbulent kinetic energy consist of the near-wall part and isotropic part, and the isotropic part involves the von Karman length scale as the turbulent length scale. The performance of the new model is evaluated by the results from DNS for fully developed turbulence channel flow with a wide range of Reynolds numbers. However, the computed result of the recirculating flow at the separated bubble of NACA4412 demonstrates that an increase is needed on the turbulent dissipation, and this leads to an advanced tuning on the self-adjusted function. The improved model predicts better results in both the non-equilibrium and equilibrium flows, e.g. channel flows, backward-facing step flow and hump in a channel.     

Generalized wall functions for turbulent flows with strong adverse pressure gradient

The generalized wall functions for turbulent flows with strong adverse pressure gradients are derived on the basis of the asymptotic theory of near-wall turbulence. The generalized wall functions have a correct asymptotic behavior in the limit of zero friction velocity and can be applied to computations of flows under a strong adverse pressure gradient and with separation or reattachment. Calculations of a turbulent boundary layer in a strong adverse pressure gradient with the aid of the developed modified k-ɛ model of turbulence and comparison with the experimental data validate the advantages of the generalized wall functions over traditional wall functions based on the logarithmic law of the wall.     

A mixed-time-scale low-Reynolds-number one-equation turbulence model

In this paper, a new low-Reynolds-number (LRN) one-equation turbulence model for eddy viscosity is proposed. A mixed time scale, representing a combination of three time scales: two time scales made of strain-rate parameter S and vorticity parameter Ω and the turbulent time scale k/?, is introduced into this model. The proposed model is derived from an LRN k?? two-equation model where the mixed time scale has been proved to be very effective for predicting local flows over complex terrains. In the transport equation of the model, the mixed time scale is included in the production and the dissipation terms. The new model is evaluated in channel flows at various Reynolds numbers, boundary layer flows with or without pressure gradient and backward-facing step flows with different expansion ratios and Reynolds numbers. Then the grid convergence of the model is investigated. Finally, the model performance for different values of the weighting constant C_s in the mixed time scale is assessed. The results show that the proposed model reproduces the correct wall-limiting behaviour of turbulent quantities and performs well in the near-wall region of turbulent flows. The model could be expected to be adopted in hybrid Reynolds averaged Navier–Stokes/large eddy simulation methodology for complex wall-bounded flows at high Reynolds numbers.     

The use of distributed pressure receivers based on elastic piezoelectric composite materials for measuring the hydrodynamic noise produced by a near-wall turbulence

Receiving electroacoustic transducers with sensing elements made of an elastic piezoelectric composite material are described. The parameters of a composite material that exhibits a bulk piezoelectric effect are presented. Results obtained by measuring the turbulent noise in a hydrodynamic channel with the use of piezoelectric composite receivers are reported. The results are compared with those of the noise measurements by a miniature piezoceramic receiver and are considered in the light of the known models of near-wall turbulence.