金属丝壁面加热湍流标度律的实验研究

通过在固壁表面的平板湍流边界层沿流向平行放置若干通电加热的金属细丝,在平板表面形成沿展向周期性分布的温度场,利用该温度场引起的空气热对流,在湍流边界层近壁区域产生一组沿湍流边界层展向周期分布的流向涡结构。对壁湍流小尺度结构标度律统计特性的研究表明,金属丝加热后形成的规则流向涡结构将壁湍流各种尺度湍涡结构不规则的脉动有序地组织起来,增强了湍流小尺度结构的层次结构相似性,减小了壁湍流中小尺度结构的间歇性和奇异性,抑制了壁湍流中奇异的湍涡结构。     

Mechanism of compressor airfoil boundary layer flow control using nanosecond plasma actuation

The flow control effects of nanosecond plasma actuation on the boundary layer flow of a typical compressor controlled diffusion airfoil are investigated using large eddy simulation method. Three types of plasma actuation are designed to control the boundary layer flow, and two mechanisms of compressor airfoil boundary layer flow control using nanosecond plasma actuation have been found. The plasma actuations located within the laminar boundary layer flow can induce a small vortex structure through influencing on the density and pressure of the flow field. As the small vortex structure moves downstream along the blade surface with the main flow, it can suppress the turbulent flow mixing and reduce the total pressure loss. The flow control effect of the small vortex structure is summarized as wall jet effect. Differently, the plasma actuation located within the turbulent boundary layer flow can act on the shear layer flow and induce a large vortex structure. While moving downstream, this large vortex structure can suppress the turbulent flow mixing too.     

Effect of local forcing on a turbulent boundary layer

An experimental study is performed to analyze flow structures behind local suction and blowing in a flat-plate turbulent boundary layer. The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from a thin spanwise slot at the wall. The Reynolds number based on the momentum thickness is about Re _θ =1700. The effects of local forcing are scrutinized by altering the forcing frequency (0.011 ≤ f ⁺≤ 0.044). The forcing amplitude is fixed at A ₀=0.4. It is found that a small local forcing reduces the skin friction and the skin friction reduction increases with the forcing frequency. A phase-averaging technique is employed to capture the large-scale vortex evolution. An organized spanwise vortical structure is generated by the local forcing. The cross-sectional area of vortex and the time fraction of vortex are examined by changing the forcing frequency. An investigation of the random fluctuation components reveals that turbulent energy is concentrated near the center of vortical structures. Received: 17 March 2000/Accepted: 3 April 2001     

壁面加热湍流相干结构的子波分析实验研究

通过在平板湍流边界层沿流向固壁表面平行放置若干条通电加热的金属细丝,在平板表面形成沿展向周期性分布的温度场,利用该温度场引起的空气热对流,在湍流边界层近壁区域产生一组沿湍流边界层展向周期分布的大尺度流向涡结构,改变了平板湍流边界层中不同尺度结构及其能量分布。采用对壁湍流多尺度结构的子波分析表明,在湍流边界层近壁区域产生规则的流向涡结构将壁湍流各种尺度湍涡结构不规则的脉动有序地组织起来,抑制了壁湍流各种尺度湍涡结构脉动,特别抑制了能量最大尺度湍涡结构的脉动,减小由于湍流脉动引起的在湍流边界层法向和展向的动量和能量损耗,从而减小了湍流的阻力。     

Flow structures,nonlinear inertial waves and energy transfer in rotating spheres

We investigate flow structures, nonlinear inertial waves and energy transfer in a rotating fluid sphere, using a Galerkin spectral method based on helical-wave decomposition (HWD). Numerical simulations of flows in a sphere are performed with different system rotation rates, where a large-scale forcing is employed. For the case without system rotation, the intense vortex structures are tube-like. When a weak rotation is introduced, small-scale structures are reduced and vortex tubes tend to align with the rotation axis. As the rotation rate increases, a large-scale anticyclonic vortex structure is formed near the rotation axis. The structure is shown to be led by certain geostrophic modes. When the rotation rate further increases, a cyclone and an anticyclone emerge from the top and bottom of the boundary, respectively, where two quasi-geostrophic equatorially symmetric inertial waves dominate the flow. Based on HWD, effects of spherical confinement on rotating turbulence are systematically studied. It is found that the forward cascade becomes weaker as the rotation increases. When the rotation rate becomes larger than some critical value, dual energy cascades emerge, with an inverse cascade at large scales and a forward cascade at small scales. Finally, the flow behavior near the boundary is studied, where the average boundary layer thickness gets smaller when system rotation increases. The flow behavior in the boundary layer is closely related to the interior flow structures, which create significant mass flux between the boundary layer and the interior fluid through Ekman pumping.     

Natural convection in a cavity with time-dependent flux boundary

Numerical simulations have been carried out to investigate the unsteady natural convection flow in a cavity subjected to a sidewall heat flux varying sinusoidally with time. With all walls non-slip and the upper and lower boundaries and the other sidewall adiabatic, the heating and cooling produces an alternating direction natural convection boundary layer that discharges hot fluid to the top and cold fluid to the bottom of the cavity, generating a time-varying thermal stratification in the cavity interior. Scaling analysis has been conducted for different flow regimes based on the forcing frequency, with the characteristic time scales being the forcing period and the boundary layer development time. The scaling relations are then verified using the simulations, with the results showing overall good agreement with the derived scaling relations.     

EXPERIMENTAL STUDY ON QUANTITATIVE MEASUREMENT OF THREE-DIMENSIONAL STRUCTURE OF HAIRPIN VORTEX BY STEREO-PIV 1)

发卡涡是湍流相干结构研究中最为关注的内容,实现发卡涡三维结构的定量测量并进行流体动力学分析,对深入研究湍流相干结构、实现湍流精准控制等具有重要意义.本研究通过对合成射流装置进行合理控制,使得层流边界层中产生了规则的人造发卡涡结构,进而用体视图像粒子测速仪(Stereo-PIV)锁相实验技术对发卡涡结构所在的三维空间流场进行了定量测量,并得到了一个完整周期内形成的发卡涡三维结构的空间流场. 结果发现,重构所得的三维发卡涡结构质量较高, 实验技术和方案具有可行性.发卡涡结构所在空间流场情况,符合目前人们对于发卡涡、高低速条带、喷射和扫掠事件的常规认识. 此外,对近壁二次流向涡、展向涡量集中区域的展向涡头和强剪切区域、与低速喷射流体相关的汇聚流动和发散流动等有了更细致的认识.同时, 也探讨了"基于二维脉动流场的相关特征去重构发卡涡三维流场"的可行性.为进一步定量探究发卡涡结构的形成演化、不同涡结构的融合及二次诱导等壁湍流相干结构问题提供思路.     

基于Stereo-PIV技术的三维发卡涡结构定量测量研究

发卡涡是湍流相干结构研究中最为关注的内容,实现发卡涡三维结构的定量测量并进行流体动力学分析,对深入研究湍流相干结构、实现湍流精准控制等具有重要意义.本研究通过对合成射流装置进行合理控制,使得层流边界层中产生了规则的人造发卡涡结构,进而用体视图像粒子测速仪(Stereo-PIV)锁相实验技术对发卡涡结构所在的三维空间流场进行了定量测量,并得到了一个完整周期内形成的发卡涡三维结构的空间流场. 结果发现,重构所得的三维发卡涡结构质量较高, 实验技术和方案具有可行性.发卡涡结构所在空间流场情况,符合目前人们对于发卡涡、高低速条带、喷射和扫掠事件的常规认识. 此外,对近壁二次流向涡、展向涡量集中区域的展向涡头和强剪切区域、与低速喷射流体相关的汇聚流动和发散流动等有了更细致的认识.同时, 也探讨了"基于二维脉动流场的相关特征去重构发卡涡三维流场"的可行性.为进一步定量探究发卡涡结构的形成演化、不同涡结构的融合及二次诱导等壁湍流相干结构问题提供思路.      

一种模拟动边界绕流的锐利界面浸入边界法

When the structural wall moves over a fixed grid, the structure coverage will change, resulting in many dead and emerging elements. To avoid the influence of malformation and reconstruction of body-fitted grids on the calculation efficiency and accuracy of the fluid-structure interaction problems with coupled boundary movement on the fixed grid, an improved numerical method for describing the interaction between an immersed rigid body and fluid based on a sharp-interface is proposed. In this method, both the fluid and solid are regarded as pure fluid domains in the whole computational domain, and the solid boundary is divided into several Lagrangian grid points. The flow parameter or velocity is reconstructed by interpolation at the interface element, which is then directly used as the boundary condition of the flow field, thus reflecting the influence of the wall boundary conditions. The method constructs the calculation structure of “virtual point, force point and vertical foot point”, and the velocity of the virtual point is obtained by bilinear interpolation. Then, the velocity of the force point is calculated by forcing the solid boundary to meet the no-slip condition, and the equations of the coupling system based on the immersion boundary method are finally solved to realize the numerical simulation of the flow with a complex moving boundary. The numerical program for this immersed boundary method is established using C++, then the accuracy and reliability of the proposed method are validated by comparison with the literature and experimental results of the basic numerical example of flow around a cylinder. Furthermore, the effects of the structural shape and the angle of attack on the trailing vortex structure, the vortex shedding frequency, and the lift/ coefficient characteristics of the flow around the elliptical cylinder have been analyzed. The anti-symmetric S-type, “P+S” Ⅰ-type and “P+S” Ⅱ-type trailing vortex shedding modes, as well as the variation laws of the vortex structure size, vortex shedding frequency and lift-drag coefficients ratio with axis ratio and angle of attack, are captured. The critical angle of attack (25°) corresponding to the maximum lift-drag ratio is determined as 25°.     

The effects of forced boundary conditions on flow within a cubic cavity using digital particle image thermometry and velocimetry (DPITV)

Buoyancy-driven convection within a cavity, whose sidewalls are heated and cooled, is a problem of great interest, because it has applications in heat transfer and mixing. Most studies to date have studied one of two cases: the steady-state case or the development of the transient flow as it approaches steady state. Our main concern was to study the response of the cavity to time-varying thermal boundary conditions. We therefore decided to observe the flow phenomena within a convection cavity under sinusoidal thermal forcing of the sidewalls. To map the flow properly, it is necessary to have simultaneous kinematic and thermal information. Therefore, the digital particle image thermometry and velocimetry (DPITV) is used to acquire data. Implementing this technique requires seeding the flow with encapsulated liquid crystal particles and illuminating a cross section of the flow with a sheet of white light. Extraction of the thermal and kinematic content is in two parts. For the first, the liquid crystals will reflect different colors of the visible spectrum, depending on the temperatures to which they are subjected. Therefore, calibrating their color reflection with temperature allows for the extraction of the thermal content. For the second part, the kinematic information is obtained through the use of a digital cross-correlation particle image velocimetry technique. With the use of DPITV, the flow within a convection cavity is mapped and studied under steady forcing and sinusoidally forced boundary conditions at the Brunt-Väisälä frequency. For the sinusoidally forced case, three cases are studied. In the first, the heating between the two walls is in phase. In the second, the heating between the two walls is 180° out of phase. In the third, the heating between the two walls is 90° out of phase. For steady forcing, the thermal plots show that the flow develops a linearly stratified profile within the center of the cell. At the sidewalls, however, owing to forcing, hot/cold thermal boundary layers develop at the left/right walls. These hot/cold thermal boundary layers then turn around the upper-left/lower-right corners and develop into intrusion layers that extend across the top and bottom walls. The vorticity and streamlines show that the bulk of the fluid motion is concentrated around the walls, whereas the fluid within the center of the cell remains stationary. For the sinusoidally forced cases, the thermal plots show the existence of many thermal “islands,” or pockets of fluid where the temperature is different with respect to its surroundings. The vorticity plots show that the center of the cell is mostly devoid of vorticity and that the vorticity is mainly confined to the sidewalls, with some vorticity at the top and bottom walls. For the 0° forcing, the streamlines show the development of two counterrotating rollers. For the 180° forcing, the streamlines show the development of only one roller. Finally, for the 90° forcing, the streamlines show the development of both a two-roller and a one-roller system, depending on the position within the forcing cycle.     

Near field vortex dynamics in axially forced,laminar, co-flowing jets: a descriptive study of the flow configurations

An experimental study is presented of the vortex structures that appear in the shear layer of laminar, co-flowing air jets subjected to strong axial forcing. A set of flow visualisation experiments has been performed to elucidate the nature of the different structures and the mechanisms leading to their appearance and further interactions. The axial forcing sets the axisymmetric instability to prescribed values of amplitude and frequency (and thus wavelength) and produces a strong effect in the lateral spreading of the inner jet. It is shown that the near field development of the flow can be explained via inviscid vortex dynamics arguments, involving three vortex structures. Due to the strong axial forcing, all these vortices already appear as developed concentrations of vorticity in the surroundings of the nozzle exit. An azimuthal perturbation is added to the flow in the form of a lobed nozzle exit, in order to lock the azimuthal organisation of the vortices. The results are discussed and some representative configurations are examined. Each configuration appears for a given range of the forcing parameters. A tentative model of the near-field vortex dynamics is developed, but quantitative measurements are still necessary.     

二维扩压叶栅非定常分离流控制途径探索

二维扩压叶栅非定常黏性数值模拟结果表明,在一定攻角范围内,叶片前缘点附近的周期性吹吸气激励能有效控制混乱的非定常分离流．详细研究了非定常激励频率、幅值、位置对流场的影响．满足一定条件的非定常激励能够使流动由无序变为有序,时均气动性能提高。     

Large-Eddy Simulations of Forced Three-Dimensional Impinging Jets

Large-eddy simulations of the flow field around twin three-dimensional impinging jets were carried out to simulate the near-ground hover configuration of a vertical takeoff and landing (VTOL) aircraft. Both the impinging jet and the upwash caused by the collision of the wall jets are modeled in this study. The evolution of the vortical structures in the impinging jet flow field, due to the introduction of axisymmetric and azimuthal perturbations at the jet exit, has been investigated. The vortical structures formed in the jet shear layer due to azimuthal forcing, show significant three-dimensional vortex stretching effects when compared to the structures formed during axisymmetric forcing. Breakdown of the large-scale structures into smaller vortices also occurs much earlier during azimuthal forcing. When compared to the upwash formed during axisymmetric forcing, the azimuthally perturbed jet forms an upwash that is less coherent and results in a weaker upload or lift-off force on the aircraft undersurface. Comparison with available experimental data indicates good agreement for the centerline velocity decay, the wall pressure variation and the phase speed of the vortical structures.     

Some features of a turbulent wing–body junction vortical flow

Laser-Doppler velocimeter measurements of a wing/body junction flow field made within a plane to the side of the wing/wall junction and perpendicular both to a 3:2 elliptical nose—NACA 0020 tail wing, and a flat wall are presented. Reynolds number of the approach boundary layer was, Re_θ = 5940, and free-stream air velocity was, U_ref = 27.5 m/s. A large vortical structure residing in the outer region redirects the low-turbulence free-stream flow to the vicinity of the wing/wall junction, resulting in thin boundary layers with velocity magnitudes higher than free-stream flow. Lateral pressure gradients result in a three-dimensional separation on the uplifting side of the vortex. Additionally, a high vorticity vortical structure with opposite sense to the outer-layer vortex forms beneath the outer-layer vortex. Normal and shear stresses increase to attain values an order of magnitude larger compared to values measured in a three-dimensional boundary layer just outside the junction vortex. Bimodal histograms of the w fluctuating velocity occur under the outer-layer vortex near the wall due to the time-dependent nature of the horseshoe vortex. In such a flow the shear-stress angle (SSA) highly lags the flow-gradient angle (FGA), and the turbulence diffusion is highly altered due to presence of vortical structures.     

海岸城市上空热力内边界层的基本特征

海岸热力内边界层内大气在热力驱动下产生垂向对流运动,对海岸带的大气运动规律产生深刻的影响.在海岸城市地区,受城市冠层复杂结构的影响,热力内边界层呈现诸多独特的性质.采用大涡模拟的方法,研究了海岸城市热力内边界层内的流动特征.将海岸城市建筑物拟形为一系列有序排列的立方体块,并基于浸入边界方法考虑其对大气流动的影响.和常见的区域尺度数值研究中将城市下垫面假设为均质属性平面的方法相比,模拟结果的精度明显提高,海岸热力内边界层内各种尺度的动力过程也能在一定程度上得以描述.模拟计算了海岸城市热力内边界层、中性海岸城市边界层和开阔的自然海岸热力内边界层共3种情况,对比研究了城市摩阻和海岸热力作用对海岸城市上空大气运动的影响.研究结果表明:在空间上,边界层中各紊流特征量均在街谷尺度呈现出有规律而非均匀分布的特点;在强度上,热力作用与城市构筑物的摩阻作用相互促进,使得紊流强度显著提升并大于两种作用线性叠加的结果.此外,还发现有城市冠层海岸的热力内边界层发展远快于相同热力条件下的自然海岸带,边界层的发展规律也有显著的不同.      

Radial variation of adiabatic and diabatic spiral vortex flow in a wide annular gap

Transitions occurring after the onset of spiral vortex flow in a wide concentric annular gap of radius ratio 0.8, formed by a stationary outer cylinder and a rotatable inner cylinder, have been studied experimentally. By isothermal heating of the annular surface, it was possible to consider diabatic as well as adiabatic conditions. At an axial Reynolds number of 500 and for a range of Taylor numbers up to 10⁷, power spectra and auto-correlograms were taken at two radial positions near to the inner and outer annular surfaces; these are compared with previous results taken at mid-gap under adiabatic conditions. Measurements of turbulence intensity across the gap were made also. Probability histograms and signal traces for diabatic flow near to the outer annular surface are presented. It has been shown that the vortex transitions affect the thermal boundary layer and, consequently, the heat transfer rates at the outer surface. A positive radial thermal gradient was seen to have little effect on the flow. The imposed axial flow was shown to be stabilising under adiabatic conditions but destabilising under diabatic conditions.     

Bound vortex boundary layer control with application to V/STOL airplanes

Effectiveness of the bound vortex boundary layer control is assessed with reference to airfoils modified with a leading edge rotating cylinder. Results of the test program and the numerical models suggest the following:

• The surface singularity method in conjunction with the boundary layer correction scheme is capable of predicting useful information concerning bound vortex boundary layer control. The predicted pressure distributions are in good agreement with experiment almost up to the point of complete separation from the the airfoil surface except near the trailing edge where more accurate results of the flow field would require the modelling of the separated flow region using the full Navier-Stokes equations.

• The concept of bound vortex boundary layer control appears to be quite promising. With cylinder rotation. the flow never separated completely from the upper surface. The higher rates of rotation promoted reattachment of the partially separated flow giving a significant improvement in the maximum lift and stall characteristics.     

数值研究平板方舵激波-湍流边界层干扰

数值研究了平板方舵激波-湍流边界层干扰流场。模拟出了分离激波与弓型激波砬撞后形成的“λ”激波结构;消晰地显示了分离区中的旋涡结构,发现流场中会出现二次分离涡,并从理论上分析了流场对称面涡心形态与非定常的关系,得到了涡心为不稳定螺旋点或出现极限环是非定常流动特征的新结论。     

湍流一般机理及其应用

本文综述了湍流机理发展中最重要的一些文章,包括:Brown-Roshko的混合剪切层中大尺度涡的发展;Perry和Chong的湍流边界层中A形涡结构的湍流机理;以及笔者提出的关于3维湍流场中涡结构的涡量脉动对流和扩散并存的一般机理。在一般机理基础上建立了判别流场的准则,列举了在固壁边界附近的猝发现象并展望它的未来应用。      

VISCOUS FLOWFIELD BASED ON DISCONTINUOUS BOUNDARY ELEMENT METHOD AND VORTEX METHOD

基于非协调边界元方法和涡方法的联合应用, 模拟了二维和三维黏性不可压缩流场. 计算中利用离散涡元对漩涡的产生、凝聚和输送过程进行模拟, 并将整体计算域分解为采用涡泡模拟的内部区域和用涡列模拟的数字边界层区域. 计算域中涡量场的拉伸和对流由Lagrangian涡方法模拟, 用随机走步模拟涡量场的扩散. 内部区域涡元涡量场速度由广义Biot-Savart公式计算, 势流场速度则采用非协调边界元方法计算. 非协调边界元将所有节点均取在光滑边界处, 从而避免了法向速度的不连续现象; 而对于系数矩阵不对称的大型边界元方程组,引入了非常高效的预处理循环型广义极小残余(the generalized minimum residual, GMRES)迭代算法, 使得边界元法的优势得到了充分发挥, 同时, 在内部涡元势流场计算中对近边界点采用了正则化算法, 该算法将奇异积分转化为沿单元围道上一系列线积分, 消除了势流计算中速度及速度梯度的奇异性. 二维、三维流场算例证明了所用方法的正确性, 也验证了该算法可以大幅度提高模拟精度和效率.