旋转圆管湍流的大涡模拟数值研究

采用大涡模拟（LES）方法,并结合动力学亚格子尺度应力（SGS）模型,通过数值求解柱坐标系下的滤波Navier-Stokes方程,研究了绕管轴旋转圆管内的湍流流动特性.为验证计算的可靠性,以及动力学SGS模型对于旋转湍流的适用性,将大涡模拟计算所得的结果,与相应的直接模拟（DNS）结果和实验数据进行了对比验证,吻合良好.进一步对旋转圆管湍流的物理机理进行了探讨,研究了湍流特性随旋转速率的变化规律.当旋转速率增加时,湍流流动有层流化的发展趋势.基于湍动能变化的关系,分析了旋转效应对湍流脉动生成的抑制作用.     

Flow field investigation in rotating rib-roughened channel by means of particle image velocimetry

The turbulent velocity field over the rib-roughened wall of an orthogonally rotating channel is investigated by means of two-dimensional particle image velocimetry (PIV). The flow direction is outward, with a bulk Reynolds number of 1.5 × 10⁴ and a rotation number ranging from 0.3 to 0.38. The measurements are obtained along the wall-normal/streamwise plane at mid-span. The PIV system rotates with the channel, allowing to measure directly the relative flow velocity with high spatial resolution. Coriolis forces affect the stability of the boundary layer and free shear layer. Due to the different levels of shear layer entrainment, the reattachment point is moved downstream (upstream) under stabilizing (destabilizing) rotation, with respect to the stationary case. Further increase in rotation number pushes further the reattachment point in stabilizing rotation, but does not change the recirculation length in destabilizing rotation. Turbulent activity is inhibited along the leading wall, both in the boundary layer and in the separated shear layer; the opposite is true along the trailing wall. Coriolis forces affect indirectly the production of turbulent kinetic energy via the Reynolds shear stresses and the mean shear. Two-point correlation is used to characterize the coherent motion of the separated shear layer. Destabilizing rotation is found to promote large-scale coherent motions and accordingly leads to larger integral length scales; on the other hand, the spanwise vortices created in the separating shear layer downstream of the rib are less organized and tend to be disrupted by the three-dimensional turbulence promoted by the rotation. The latter observation is consistent with the distributions of span-wise vortices detected in instantaneous flow realizations.     

MODELLING AND CALCULATION OF THE TURBULENT BOUNDARY LAYER ON A ROTATING CYLINDER SURFACE WITH STRONG SUCTION 1)

提出了湍流边界层的一种简单、快速计算方法, 用以求解强吸气作用下旋转圆筒表面边界层流动. 首先, 理论分析了同心圆筒间的旋转流体运动, 外筒静止、内筒旋转且为多孔吸气条件. 强吸气情况下旋转流动主要表现为内筒壁面附近的边界层流动, 基于这一事实得到了周向速度分布的解析表达式. 其次, 通过引入新参数扩展Cebeci-Smith代数湍流模型, 使其能考虑流线曲率、壁面吸气、低Reynolds数效应等因素. 针对这些因素的综合影响, 采用解析修正和经验参数对模型进行调整. 同时, 基于Reynolds应力湍流模型的仿真结果, 校准代数湍流模型中的经验参数. 最后, 给出基于广义Cebeci-Smith湍流模型的旋转壁面边界层流动的迭代算法, 该算法适用于需要特殊迭代过程的轴向及周向流动均匀情况. 计算了不同旋转速度和吸气强度组合工况下的边界层流动, 其周向速度和湍流强度分布与基于Reynolds应力湍流模型的计算结果非常接近. 并且表明, 当Reynolds应力湍流模型数值模拟预测内筒边界层为稳定层流时, 该方法也再现了相同初始条件下的层流边界层.     

强吸气旋转圆筒壁面湍流边界层建模及计算

提出了湍流边界层的一种简单、快速计算方法,用以求解强吸气作用下旋转圆筒表面边界层流动.首先,理论分析了同心圆筒间的旋转流体运动,外筒静止、内筒旋转且为多孔吸气条件.强吸气情况下旋转流动主要表现为内筒壁面附近的边界层流动,基于这一事实得到了周向速度分布的解析表达式.其次,通过引入新参数扩展Cebeci-Smith代数湍流模型,使其能考虑流线曲率、壁面吸气、低Reynolds数效应等因素.针对这些因素的综合影响,采用解析修正和经验参数对模型进行调整.同时,基于Reynolds应力湍流模型的仿真结果,校准代数湍流模型中的经验参数.最后,给出基于广义Cebeci-Smith湍流模型的旋转壁面边界层流动的迭代算法,该算法适用于需要特殊迭代过程的轴向及周向流动均匀情况.计算了不同旋转速度和吸气强度组合工况下的边界层流动,其周向速度和湍流强度分布与基于Reynolds应力湍流模型的计算结果非常接近.并且表明,当Reynolds应力湍流模型数值模拟预测内筒边界层为稳定层流时,该方法也再现了相同初始条件下的层流边界层.     

Break-Up of Aerosol Agglomerates in Highly Turbulent Gas Flow

Agglomerate aerosols in a turbulent flow may be subjected to very high turbulent shear rates which through the generation of lift and drag can overcome the adhesive forces binding the constituents of an agglomerate together and cause it to break-up. This paper presents an analysis of the experimental measurements of the breakup of agglomerates between 0.1?C10???m in size, in a turbulent pipe flow followed by an expansion zone with a Reynolds numbers in the range 10⁵ to 10⁷. The analysis shows that even in wall bounded turbulence, the high turbulent shear stresses associated with the small scales of turbulence in the core can be the main source of breakup preceding any break-up that may occur by impaction at the wall. More importantly from these results, a computationally fast and efficient solution is obtained for the General Dynamic Equation (GDE) for agglomerate transport and breakup in highly turbulent flow. Furthermore the solution for the evolution of the aerosol size distribution is consistent with the experimental results. In the turbulent pipe flow section, the agglomerates are exposed continuously to turbulent shear stresses and experience more longer term breakup than in the expansion zone (following the pipe flow) where the exposure time is much less and break-up occurs instantaneously under the action of very high local turbulent shear stresses. The validity of certain approximations made in the model is considered. In particular, the inertia of the agglomerates characterised by a Stokes Number from 0.001 for the smallest particles up to 10 for 10???m particles and the fluctuations of the turbulent shear stresses are important physical phenomena which are not accounted for in the model.     

Numerical study of the turbulent flow in strongly curved stationary and rotating U‐ducts

A large‐eddy simulation (LES) study is undertaken to explore the complex flow of developing turbulent flow through stationary and rotating U‐ducts with strong curvature. Three flow cases are investigated: stationary (non‐rotating), positive and negative rotational cases. Stationary and positive rotational cases are shown to have similar flow characteristics in terms of the mean velocity variations, although the predicted separation zone is nearly doubled in size for positive rotational case. Unlike the positive rotation for which the laminarization effects are observed, turbulence is significantly enhanced for the negative rotation mainly due to the existence of strong secondary flow. Turbulence is found to be highly anisotropic throughout the duct apart from the far downstream regions of the bend for the negative rotational case. The stress–strain relation seems to be completely invalid in the U‐duct apart from the bend region. Copyright © 2009 John Wiley & Sons, Ltd.     

Modeling rotational effects in eddy-viscosity closures

A new approach to sensitize turbulence closures based on the linear eddy-viscosity hypothesis to rotational effects is proposed. The principal idea is to ‘mimic' the behavior of a second moment closure (SMC) in rotating homogeneous shear flow; depending on the ratio of the mean flow to the imposed rotational time scales, the model should be able to bifurcate between two stable equilibrium solutions. These solutions correspond to exponential or algebraic time dependent growth or decay of turbulent kinetic energy. This fundamental behavior of SMCs is believed to be of importance also in the prediction of non-equilibrium turbulence. A near-wall turbulence model which is based on the linear eddy-viscosity hypothesis is modified in the present study. Wall proximity effects are modeled by the elliptic relaxation approach. This closure has been successfully applied in the computation of complex, non-equilibrium flows in inertial frames of reference. The objective of the present study is to extend the predictive capability of the model to include flows dominated by rotational effects. The new model is calibrated in rotating homogeneous turbulent shear flow and subsequently tested in three different cases characterized by profound effects of system rotation or streamline curvature. It is able to capture many of the effects due to imposed body forces that the original closure is incapable of. Good agreement is obtained between the present predictions and available experimental and DNS data.     

Turbulent flow in a conical diffuser: A review

This is a review of experimental studies of turbulent flow in a conical diffuser by eight Ph.D. students, eleven M.Sc. students, one M.Eng. student, and myself in the past 29 years. During this time, two conical diffusers were constructed: the first was of cast aluminum construction, and the second was of plastic fabrication. These two diffusers were basically the same in geometry except that the pipe section was constructed as an integral part of the plastic diffuser to avoid the lip at the junction of the inlet pipe and the diffuser. The conical diffuser had a total divergence angle of 8°, an area ratio of 4:1, and an inlet diameter of 0.1016 m (4 in.).

The flow at the inlet of the diffuser was usually fully developed pipe flow, but sometimes it was boundary layer grown on the pipe wall. Hot-wire and pulse-wire anemometry together with computer facilities were used to obtain the results of complex flow present in the conical diffuser. Mean velocity profiles were obtained throughout the diffuser, which in turn were used to obtain strain rates and their principal direction. Turbulence moments up to fourth order were measured. The results were used to assess momentum, turbulent kinetic energy, and shear stress equations. Other features such as instantaneous flow reversals in the wall region, relative strength of large eddies, extra strain rate, and the production of kinetic energy also were investigated to find the dynamical picture in the diffuser flow.     

THE RENORMALIZATION-GROUP ANALYSIS FOR THE STATISTICAL PROPERTIES OF RAPIDLY ROTATING TURBULENCE

利用重正化群方法对强旋转湍流场统计性质予以研究, 通过重正化微扰展开, 对高波数速度分量进行逐 阶平均.计算结果显示当旋转角速度Ω → ∞时, 用以表征高波数速度分量对低波数速度分量影响的重正化黏性将趋于0, 这表明在强旋转条件下科氏力将抑制湍流速度分量之间的非线性相互作用, 从而阻碍湍流的能量级串效应, 当Ω → ∞时湍流的能量级串效应消失, 导致湍流脉动消失, 流动将层流化.理论计算结果还显示对于强旋转湍流, 时域-空域联立Fourier的湍流速度分量存在二维化趋势, 球面平均能谱函数有标度关系E(k) ∝ k^-3.     

Turbulent flow of quadrangle mode in interdisk midplane between two shrouded co-rotating disks

Particle image velocimetry (PIV) was employed to study the flow patterns, time-averaged velocity field, and turbulence properties of the flow in the interdisk midplane between two shrouded co-rotating disks at the interdisk spacing to disk radius ratio S = 0.1 and rotating Reynolds number Re = 2.25 × 10⁵. A quadrangle core flow structure rotating at a frequency 75% of the disks’ rotating frequency was observed. The flow in the region outside the quadrangle core flow structure consisted of four cellular flow structures. Five characteristic flow regions—the hub-influenced region, solid-body rotation region, buffer region, vortex region, and shroud-influenced region—were identified in the flow field. Circumferential and radial turbulence intensities, Reynolds stresses, turbulence kinetic energy, correlation coefficients, as well as the Lagrangian integral time and length scales of turbulent fluctuations were analyzed and presented. Features of the turbulence properties were found to be closely related to the rotation motion of the inner and outer characteristic flow structures. The circumferential components of the turbulence properties exhibited local minima in the buffer region and maxima in the solid-body rotation and vortex regions, while the radial components of the turbulence intensity, turbulent normal stress, and Lagrangian integral turbulence time scale exhibited maximum values in the buffer region and relatively low values in the regions near the hub and the shroud.     

Experimental study and mathematical simulation of the characteristics of a turbulent flow in a straight circular pipe rotating about its longitudinal axis

The vorticity formed in the cross section of a turbulent flow in a straight circular pipe rotating about its longitudinal axis decreases the values of the turbulent stresses, turbulence energy, and dissipation rate along the pipe. The results of laboratory experiments and calculations by the second-order closure model of turbulent transfer are presented. On the whole, the model using a system of transport equations yields better agreement with experimental data than the models with algebraic relations for second-order moments. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 2, pp. 103–116, March–April, 1998.     

Predicting Noninertial Effects with Linear and Nonlinear Eddy-Viscosity, and Algebraic Stress Models

Three types of turbulence models which account for rotational effects in noninertial frames of reference are evaluated for the case of incompressible, fully developed rotating turbulent channel flow. The different types of models are a Coroiolis-modified eddy-viscosity model, a realizable nonlinear eddy-viscosity model, and an algebraic stress model which accounts for dissipation rate anisotropies. A direct numerical simulation of a rotating channel flow is used for the validation of the turbulence models. This simulation differs from previous studies in that significantly higher rotation numbers are investigated. Flows at these higher rotation numbers are characterized by a relaminarization on the cyclonic or suction side of the channel, and a linear velocity profile on the anticyclonic or pressure side of the channel. The predictive performance of the three types of models are examined in detail, and formulation deficiencies are identified which cause poor predictive performance for some of the models. Criteria are identified which allow for accurate prediction of such flows by algebraic stress models and their corresponding Reynolds stress formulations.     

The turbulent wake of a circular cylinder rotating about the streamwise axis

This paper presents measurements in the turbulent wake of a circular cylinder rotating with its axis normal to the free-stream velocity; in other words, the axis of rotation was parallel to the streamwise direction. All three mean velocities and six Reynolds stresses were obtained at three positions downstream of the cylinder, with and without rotation of the free-stream. Most emphasis is given to the latter results because of the better flow quality. The ratio of the circumferential velocity of the cylinder to the free-stream velocity — the swirl number — had a maximum value of 0.6. Measurements for two combinations of the free-stream and angular velocities showed the velocity deficit in the wake to be a multi-valued function of the swirl number, implying that the rotation affected the separation of the cylinder's boundary layer in a complex manner. In the turbulent wake, the rotation did not significantly alter the magnitudes of the normal stresses, but caused large changes to the shape of the profiles of the axial and cross-stream normal stresses. Eventually, the primary (cross-stream) shear stress became almost entirely positive, but there was no corresponding change to the (cross-stream) gradient of the streamwise mean velocity. Despite these alterations to the turbulence, the rotationally-activated generation terms in the Reynolds transport equations never dominated the terms that are common to the wakes of rotating and non-rotating cylinders.This work was supported by the Australian Research Council. Most of the data acquisition software was written by Mr J. J. Smith.     

Heat Transfer Influenced by Turbulent Airflow Inside an Axially Rotating Diffuser

The paper presents a study of heat transfer between the turbulent airflow and the inner wall surface of an axial diffuser rotating around its longitudinal axis. Heat transfer was assessed through the measurement of a time-dependent temperature field of the diffuser inner wall surface. Measurements of the instantaneous flow velocity components were performed by a laser–Doppler anemometry system, which delivered information on mean velocity components as well as on the turbulence intensity. A significant increase of all three mean velocity components was observed near the rotating diffuser wall in comparison with a non-rotating diffuser. Temperature field measurements were carried out by means of infrared thermography. The experiment showed a significant dependence of the temperature field on the turbulent flowfield induced by diffuser rotation. A strong influence of the flow separation and reattachment on the temperature distribution was observed, while rotation was found to suppress the occurrence of flow separation from the diffuser wall. Properties of the velocity field such as turbulent kinetic energy were directly coupled with the temperature distribution in order to gain the information on how to enhance or reduce heat transfer by changing the integral parameters of the diffuser (e.g. rotation frequency or amount of flow).     

Turbulence modeling for the axially rotating pipe from the viewpoint of analytical closures

A single-point model eddy viscosity model of rotation effects on the turbulent flow in an axially rotating pipe is developed based on two-point closure theories. Rotation is known to impede energy transfer in turbulence; this fact is reflected in the present model through a reduced eddy viscosity, leading to laminarization of the mean velocity profile and return to a laminar friction law in the rapid rotation limit. This model is compared with other proposals including linear redistribution effects through the rapid pressure-strain correlation, Richardson number modification of the eddy viscosity in a model of non-rotating turbulence, and the reduction of turbulence through the suppression of near-wall production mechanisms. PACS 47.27.Eq, 47.32.-y     

Effect of system rotating on turbulent boundary layer flow

This paper experimentally investigated the effect of rotating on the turbulent boundary layer flow using hot-wire. The experiments were completed in a rotating rig with a vertical axis and four measured positions along the streamwise direction in channel, which focuses on the flow flied in the rotating channel. The rotating effects on velocity profile, wall shear stress and semi-logarithmic mean velocity profile are discussed in this paper. The results indicated that: due to the Coriolis force induced by rotating, the phenomenon of velocity deficit happens near the leading side. The velocity deficit near the leading side, do not increase monotonically with the increase of Ro. The trend of the velocity deficit near the leading side is also affected by the normal component of pressure gradient, which is another important force in the cross-section of the rotating channel. The wall shear stress near the trailing side is larger than that on the leading side, and the semi-logarithmic mean velocity profile is also different under rotating effects. The phenomenon reveals that the effect of rotation penetrates into the logarithm region, and the flow near the leading side tends to turn into laminar under the effect of rotation. The rotation correction of logarithmic law is performed in current work, which can be used in the wall function of CFD to increase the simulating accuracy at rotating conditions.     

An experimental study of two flows through an axisymmetric sudden expansion

Two turbulent separated and reattaching flows produced by a sudden expansion in a pipe have been studied. The first was produced by a simple axisymmetric sudden enlargement from a nozzle of diameter 80 mm to a pipe of diameter 150 mm. The second was the flow at the same enlargement with the addition of a centerbody 90 mm downstream of the nozzle exit. Detailed measurements of velocity and skin friction (made primarily using pulsed wires) and of wall static pressure are presented. Without the centerbody the flow structure is similar to that observed in other sudden pipe expansions and over backward-facing steps. A turbulent free shear layer, bearing some similarity to that of a round jet, grows from separation and then reattaches to the pipe wall downstream. Reattachment is a comparatively gradual process, the shear layer approaching the wall at a glancing angle. The introduction of the centerbody causes the shear layer to curve towards the wall and reattach at a much steeper angle. Reattachment is much more rapid; gradients of skin friction and pressure along the wall are many times those without the centerbody. The high curvature of the shear layer strongly influences its turbulent structure, locally suppressing turbulence levels and reducing its growth rate.     

大涡模拟研究展向旋转槽道湍流流动

本文用大涡模拟预测了以不同转速做展向旋转的槽道湍流流动,统计平均的流向速度型在壁面附近与已有实验数据符合很好,在通道中部的预测差异也能给出合理解释,对比不同转速的计算结果,表明展向旋转通道的湍流应力和壁面摩擦力在压力面附近提高、在吸力面附近降低,这些高阶湍流统计量的变化规律可以结合湍流应力输运方程加以解释,漩涡识别技术显示了近壁条带结构,其形态和猝发率受旋转附加力的影响发生改变,进而影响壁面摩擦速度的数值和分布,进一步考察垂直流动方向的截面内速度分布,发现旋转引起了垂直壁面方向的流动,形成正负相间排列的流向涡对,并随着转速的增加向压力面靠近。      

Transition from vortex to wall driven turbulence production in the Taylor–Couette system with a rotating inner cylinder

Axisymmetrically stable turbulent Taylor vortices between two concentric cylinders are studied with respect to the transition from vortex to wall driven turbulent production. The outer cylinder is stationary and the inner cylinder rotates. A low Reynolds number turbulence model using the k‐ω formulation, facilitates an analysis of the velocity gradients in the Taylor–Couette flow. For a fixed inner radius, three radius ratios 0.734, 0.941 and 0.985 are employed to identify the Reynolds number range at which this transition occurs. At relatively low Reynolds numbers, turbulent production is shown to be dominated by the outflowing boundary of the Taylor vortex. As the Reynolds number increases, shear driven turbulence (due to the rotating cylinder) becomes the dominating factor. For relatively small gaps turbulent flow is shown to occur at Taylor numbers lower than previously reported. Copyright © 2002 John Wiley & Sons, Ltd.     

Rotation correction of wall function and velocity structure under the couple effects of buoyancy force and rotating effects at the entry section of the smooth straight channel

This paper studies the effect of rotation on the turbulent boundary-layer flow in a rotating duct with a square cross section by using hot-wire. The experiments were conducted with the Reynolds numbers, based on the duct's hydraulic diameter (D = 80 mm) equaling 19,000. The rotation numbers (Ro) studied ranged from 0 to 0.362. Hot-wire measurements of the flow field were made at four cross sections of the rotating duct. The effects of rotation on velocity profile, semi-logarithmic mean velocity profile, and wall shear stress are discussed in this paper. Results obtained show the velocity deficit about the leading surface of the rotating duct, created by the secondary flows induced by the Coriolis force, to not increase monotonically with the increase in the Rotation number. Results obtained also show the effects of rotation to penetrate into the logarithm region, and the flow near the leading surface tends to laminarize. In this study, a correction factor is developed for logarithmic law to account for the effects of rotation, which can be used in CFD studies of rotating ducts that use wall functions.