Effect of periodic blowing/suction through sequentially located annular slots on the turbulent boundary layer on a body of revolution

The effect of local periodic forcing in the form of blowing/suction through sequentially located annular slots on the features of the turbulent boundary layer formed on an axisymmetric body of revolution in an incompressible flow is studied experimentally. The Reynolds number based on the momentum thickness of the boundary layer ahead of the annular slot is 1362. The dimensionless amplitude of the forcing signal A ₀ is set to be 0.4. The frequency of the forcing signal in the law-of-the-wall units is f ⁺ = 0.0048. Beginning from the distance upstream from the slot equal approximately to half-thickness of the displacement thickness of the boundary layer δ* and further downstream to 18 δ*, a stable decrease in local friction is observed; the maximum value of this decrease reaches 50 %. Each next slot favors reduction of friction, though the effectiveness of blowing/suction becomes noticeably lower in the downstream direction.     

Frequency Response of Near-Wall Coherent Structures to Localized Periodic Blowing and Suction in Turbulent Boundary Layer

We experimentally investigate the frequency response of near-wall coherent structures to localized periodic blow- ing and suction through a spanwise slot in a turbulent boundary layer by changing the frequency of periodic disturbance at similar velocities of free stream. The effects of blowing and suction disturbance on energy redistri- bution, turbulent intensity U^＋ rums over y^＋ and waveforms of phase-averaged velocity during sweeping process are respectively discussed under three frequencies of periodic blowing and suction in near-wall region of turbulent boundary layer, compared with those in a standard turbulent boundary layer. The most effective disturbance frequency is figured out in this system.     

超声速边界层中壁面抽吸对流动分离的抑制作用

当超声速或高超声速来流经过压缩折角时，由于壁面的位移效应，折角附近往往出现较强的逆压梯度，进而很可能导致流动分离，并伴随着激波与边界层干扰问题的出现.在工程应用中，流动分离会带来诸多不利因素.一个抑制流动分离的有效措施是在折角的上游引入定常的壁面抽吸单元.基于大Reynolds数渐近理论框架下的三层结构理论，文章研究了壁面抽吸抑制层流边界层分离的机理.研究发现，只要抽吸元被安置在折角上游O（R-3/8L）范围内，决定抑制效果的关键参数是抽吸的流量，而与抽吸元的位置无关；同时改变抽吸元的宽度和抽吸速度而保持抽吸流量不变并不影响其对分离区的抑制效果.      

吸气槽道形状对扩压叶栅性能的影响

数值模拟了低速条件下吸气槽道宽度、角度变化对采用附面层吸除技术的大转角扩压叶栅气动性能影响。结果表明,附面层抽吸具有显著降低叶栅损失,改善流动,增加负荷及扩压能力等优点;吸气量相同时,槽道宽度增加可进一步改善角区流动并减小叶栅两端部损失,吸气角度变化则对吸气槽道出口压力有较大影响,为非均匀槽道宽度设计及工况变化时有效控制吸气量提供了设计自由度。     

Turbulent passive scalar transport under localized blowing

The effect of blowing through a localized slot on the wall turbulence dynamics and heat transfer process is analyzed by direct numerical simulations in a fully developed turbulent channel flow. The severity parameter is mild and there is no flow separation induced by the blowing. The shear stress transport and temperature energy budget is discussed in detail. The wall shear and flux decreases immediately downstream the slot in a similar manner but the Reynolds analogy does not hold over the slot. The physical process is governed by the production and pressure redistribution over the slot in a complex manner. The turbulent transport and especially the advection play an essential role in the heat transfer mechanism.     

On the lower bound of net driving power in controlled duct flows

We examine mathematically the lower bound of the net driving power (i.e., the summation of pumping and actuation powers) of a controlled duct flow under a constant flow rate. The net power in a duct with arbitrary cross-section in the presence of the inertial term, blowing/suction from the wall, and arbitrary body forces can be decomposed into four terms: (1) dissipation due to the velocity profile of the Stokes flow (in other words, pumping power for the Stokes flow); (2) dissipation due to deviation of the mean velocity from the Stokes flow profile; (3) dissipation due to velocity fluctuations; and (4) correlation between the wall-pressure of the Stokes flow and the time-averaged blowing/suction velocity. Of these, the first three terms are shown to be non-negative, while the sign of the fourth term is indefinite. The fourth term vanishes in cases where the duct has a constant-shape cross-section, such as cylindrical pipes and plane channels. Namely, in such cases, the lower bound of net power is exactly given by the dissipation rate (pumping power) of the Stokes flow at the same flow rate.     

无缝襟翼吹气控制机理和地面效应分析

基于Coanda效应的无缝襟翼吹气控制能大幅度提升机翼升力, 改善大型运输类飞机起降性能, 因此研究起降阶段地面效应对吹气控制的影响十分必要。通过数值模拟方法, 从流场变化的角度分析了无缝襟翼吹气控制机理, 以及有/无襟翼吹气时地面效应对翼型气动性能的影响。襟翼吹气使Coanda表面产生局部低压区, 形成指向Coanda表面的压力梯度, 进而引起射流上方的主流偏转和加速, 使整个翼面近壁区产生顺时针方向的速度增量; 翼面压力面的压力增大, 吸力面的吸力增强, 其中主翼上翼面吸力增强是翼型升力增加的主要来源。无吹气时, 地面效应使翼型上/下翼面附近的流速均降低, 上/下翼面的压力均有所提高, 整体上使翼型升力降低。有地面效应时的襟翼吹气增强了下翼面对来流的阻滞作用, 进一步提高了下翼面的压力; 襟翼吹气使上翼面气流加速, 可抵消地面效应引起的上翼面气流减速, 一定程度上减小了地面效应引起的上翼面吸力损失。      

涡轮静叶尾缘开槽冷却特性的数值研究

本文用三维数值模拟的方法,对某具有尾缘开槽冷却的涡轮静叶栅进行了详细的气热耦合计算与分析,研究了具有尾缘开槽冷却形式的涡轮静叶片的冷却特性,并结合叶片表面温度分布和静压分布对比分析了叶片在不同吹风比下该冷却方式对流场气动性能的影响。气热耦合计算结果表明:此种尾缘开槽的冷却形式可以对叶片尾缘高温区起到有效的冷却效果,同时使主流通道的气动喉口向上游迁移,在冷却槽口区域引起主流的压力波动。     

MHD boundary layer flow of Casson fluid passing through an exponentially stretching permeable surface with thermal radiation

This article numerically examines the boundary layer flow due to an exponentially stretching surface in the presence of an applied magnetic field. Casson fluid model is used to characterize the non-Newtonian fluid behavior. The flow is subjected to suction/blowing at the surface. Analysis is carded out in presence of thermal radiation and prescribed surface heat flux. In this study, an exponential order stretching velocity and prescribed exponential order surface heat flux are accorded with each other. The governing partial differential equations are first converted into nonlinear ordinary differential equations by using appropriate transformations and then solved numerically. The effect of increasing values of the Casson parameter is to suppress the velocity field. However the temperature is enhanced when Casson parameter increases. It is found that the skin-friction coefficient increases with increasing values of suction parameter. Temperature also increases for large values of power index n in both suction and blowing cases at the boundary. It is observed that the thermal radiation enhances the effective thermal diffusivity and hence the temperature rises.     

Numerical approach to new tangential slot effect on film cooling effectiveness over asymmetrical turbine blade

The focus of this numerical study is to conceive a new basic film cooling configuration in order to increase film cooling effectiveness, especially at the leading edge zone between the injection holes where cooling is mostly needed. The new configuration, resulting from the tangential slot configuration and especially adapted to the leading edge of an asymmetrical blade, is compared to the uniform slot configuration. Three alternatives geometries were proposed and numerically tested to find the configuration that provides the best film cooling effectiveness. The simulation is conducted at a fixed density ratio of 1.0 and a blowing ratio of 0.7. A new parameter, Rc, is defined to measure the rate of blade coverage by the film cooling. The outcomes of the numerical results indicate that the three proposed configurations allow better thermal protection because of their higher film cooling coverage. At suction side, the new configurations provide a better film cooling coverage than the baseline case. The minimal improvement is at approximately 34%, with a light superiority of case 1. At pressure side, the use of the tangential slot is especially interesting for the allowed minimum adiabatic effectiveness values between 0.3 and 0.5.     

壁面局部吹吸边界层感受性的数值研究

目前理论、实验以及数值模拟主要研究自由来流中的小扰动与壁面局部粗糙相互作用激发边界层感受性问题. 但是, 针对自由来流湍流与壁面局部吹吸相互作用诱导边界层感受性的相关报道甚少. 本文采用直接数值模拟和快速傅里叶变换的方法, 数值研究了二维平板壁面具有局部吹吸的边界层感受性问题. 结果发现, 在二维边界层内能找到一组被激发产生的Tollmien-Schlichting(T-S)波波包的包络序列以及从波包中能够分离出一组稳定的、中性的和不稳定的T-S波, 证明了二维边界层内感受性现象的存在性. 经数值计算获得了T-S波波包传播的群速度; 并建立了自由来流湍流强度、壁面局部吹吸强度和长度与二维边界层感受性之间的关系, 获得了与Dietz感受性实验相类似的结论. 另外, 还发现在自由来流湍流与壁面局部吹、吸相互作用下能诱导二维边界层内产生相位相反的T-S波. 依据这一理论机理来优化设计局部吹吸装置, 不但能促使层流向湍流转捩的提前, 也可以延迟转捩过程的发生, 达到控制湍流运动的目的.     

Effects of thermal radiation on Casson fluid flow and heat transfer over an unsteady stretching surface subjected to suction/blowing

The unsteady flow of a Casson fluid and heat transfer over a stretching surface in presence of suction/blowing are investigated.The transformed equations are solved numerically by using the shooting method.The exact solution corresponding to the momentum equation for the steady case is obtained.Fluid velocity initially decreases with the increase of unsteadiness parameter.Due to an increasing Casson parameter the velocity field is suppressed.Thermal radiation enhances the effective thermal diffusivity and the temperature rises.     

Stability and three-wave interaction of disturbances in supersonic boundary layer with mass exchange on the wall

The interaction of disturbances in a boundary layer of the compressible gas is considered in the linear and nonlinear approximation (the weakly nonlinear theory of stability) in the presence of mass exchange (gas blowing or suction) on the surface. The regimes of moderate (the Mach number M = 2) and high (M = 5.35) supersonic velocities of the flow are considered. The suction from the surface is shown to lead to a considerable variation of the linear evolution of disturbances: the vortex disturbances of the first mode and the acoustic disturbances of the second mode are stabilized, the rate of variation is determined by suction intensity. The nonlinear interactions in three-wave systems between the vortex waves in asymmetric triplets at M = 2 and between the waves of different nature (acoustic and vortex waves) ?? in the symmetric ones at M = 5.35 are considered. The planar acoustic wave is the excitation wave in the latter, which excites the three-dimensional subharmonic components of the vortex nature. It is shown that one can delay considerably the transition region with the aid of suction, thereby one can reduce the skin-friction drag. In the gas blowing regime, strong deformations of the mean fields of boundary layers occur, which lead to the destabilization of the vortex and acoustic waves in the linear region and activate the nonlinear processes in transition region. One can expect that this will lead to the acceleration of tripping in supersonic flow.     

Flow control of the semicircular airfoil with a vortex cell at slot suction of air and its blowout into the near wake

In the vicinity of a semicircular airfoil with slot suction of air provided with a 0.2-diameter (chord) vortex cell installed on the backside of the wing, at low speeds and zero angle of attack the pattern of the unsteady separated air-flow undergoes substantial changes, those changes being accompanied with the displacement of flow separation point toward the trailing edge. The slot suction of air and its blowout into the near wake in such an airfoil is organized using a discharge channel with a fan; from this channel, the air jet is discharged into atmosphere tangentially to the airfoil base, with the pressure drop in the fan being equal to twice the pressure head. Under such conditions, the integral force characteristics of the wing show dramatic changes: the lift force, initially being ultra-low negative, becomes positive, and the drag decreases two-fold. The static pressure decreases by two or three times on the upper arch of the profile, and it increases by two times on the lower part of the airfoil, the level of pressure pulsations decreasing by more than ten times.     

涡轮叶片端壁槽缝气与离散气膜孔冷却特性实验研究

真实发动机涡轮叶片端壁为曲面造型,并且其冷却受槽缝气、泄漏流和离散气膜冷却多种冷却气叠加影响,同时又受到主流二次流影响,因此呈现复杂冷却特性。为研究接近真实发动机涡轮叶片端壁构型和工况下的气膜冷却特性,本文采用高速风洞(主流雷诺数为37万)及压敏漆(PSP)技术,研究了槽缝气、泄漏流以及离散气膜对曲面端壁的气膜冷却效率的影响,并针对不同冷气流量比对端壁气膜冷效的影响规律进行了对比分析。结果表明:端壁表面气膜冷效随着槽缝气流量比增大而增大,当流量比增大到1.71%时,槽缝气膜几乎可以覆盖整个端壁表面;与槽缝气相比,端壁表面的离散气膜冷气覆盖范围较为有限,端壁压力面侧下游区域气膜覆盖较差;在喉部之前,随着流量比增大,离散气膜冷效呈现下降趋势;在喉部之后,随着流量比从1.3%增大到1.9%,离散气膜冷效呈现上升趋势;与仅有离散气膜相比,包含槽缝气、泄漏流、离散气膜的全气膜覆盖更为均匀,全气膜冷效的叠加使得端壁冷效相比仅有离散气膜时整体提高了93.4%。     

Control of turbulent boundary layer on a wing section by combined blowing/suction

The possibility of exerting control over the turbulent boundary layer on NACA 0012 wing section with the aim of improving the wing aerodynamic characteristics in incompressible flow is experimentally examined. Control over the boundary layer was exercised in the range of the angles of attack from ?12 to 12° by implementing combined steady-state blowing/suction of air through a finely perforated surface located, respectively, on the windward and leeward side of the wing and presenting part of streamlined surface.     

Modification of the wake behind a circular cylinder by using synthetic jets

Experimental tests were conducted to control the flow around a cylinder by means of unsteady blowing (synthetic jet) through a single slot disposed on the wall of the model. The flow Reynolds number (based on the diameter of the model) wasR _D=10₅. The efficiency of the synthetic jet is quantified in terms of delaying separation and modifying the drag coefficient. The investigations were of three types: measurements of the mean pressure distribution, wall visualizations of the separation line position and measurements of the mean flow-field in the wake.     

缝隙抽吸对液膜撕裂特性影响的试验研究

本文在空气-水膜两相流动试验装置上,利用高速摄影机对空心静叶缝隙抽吸下的静叶出口边液膜撕裂特性进行了试验研究,结果表明:缝隙抽吸可以将静叶表面上的大部分流动液膜去除掉,水份在叶片出口边之前形不成完整的流动液膜,仅表现为溪状流动,静叶出口边的撕裂形式主要为丝线状撕裂,撕裂形成的水滴数目和直径将明显地减小。