Periodic Wall Blow/Suction Perturbation Evolution in Turbulent Boundary Layer

Time sequence signals of instantaneous longitudinal and normal velocity components at different longitudinal and normal positions in a turbulent boundary layer have been finely measured simultaneously by IFA300 constant temperature anemometer and double-sensor hot-wire probe with sampling resolution higher than the frequency that corresponds to the smallest time scale of Kolmogorov dissipation scale before/after introducing artificial periodic blow/suction perturbation. The period-phase-average technique is applied to extract the periodic waveforms of artificial perturbation from instantaneous time sequence signals of longitudinal and normal turbulence background. Experimental investigation is carried out on the attenuation characteristics of periodic perturbation wave with different frequency along longitudinal direction and normal direction in a turbulent boundary layer. The amplitude distributions of longitudinal and normal disturbing velocity component for different perturbation frequencies are measured at different downstream and normal positions in turbulent boundary layer. The amplitude growth rate of artificial periodic perturbation wave is calculated according to flow instability theory. The experimental results are compared and in consistent with the theoretical and numerical results.

Thermal Radiation Effect on the MHD Turbulent Compressible Boundary Layer Flow with Adverse Pressure Gradient, Heat Transfer and Local Suction

The combined effect of magnetic field, thermal radiation and local suction on the steady turbulent compressible boundary layer flow with adverse pressure gradient is numerically studied. The magnetic field is constant and applied transversely to the direction of the flow. The fluid is subjected to a localized suction and is considered as a radiative optically thin gray fluid. The Reynolds Averaged Boundary Layer (RABL) equations with appropriate boundary conditions are transformed using the compressible Falkner Skan transformation. The nonlinear and coupled system of partial differential equations (PDEs) is solved using the Keller box method. For the eddy-kinematic viscosity the Baldwin Lomax turbulent model and for the turbulent Prandtl number the extended Kays Crawford model are used. The numerical results show that the flow field can be controlled by the combined effect of the applied magnetic field, thermal radiation, and localized suction, moving the separation point, xs , downstream towards the plate’s end, and increasing total drag, D . The combined effect of thermal radiation and magnetic field has a cooling effect on the fluid at the wall vicinity. The combined effect has a greater influence in the case of high free-stream temperature.

In uence of Endwall Boundary Layer Suction on the Flow Fields of a Critically Loaded Di usion Cascade

Boundary layer suction is an e ective method used to delay separations in axial compressors. Most studies on bound?ary layer suction have focused on improving the performance of compressors,whereas few studies investigated the influence on details of the flow fields,especially vortexes in compressors. CFD method is validated with experi?mental data firstly. Three single?slot and one double?slot endwall boundary layer suction schemes are designed and investigated. In addition to the investigation of aerodynamic performance of the cascades with and without suction,variations in corner open separation,passage vortex,and concentration shedding vortex,which are rarely seen for the flow controlled blades in published literatures,are analyzed. Then,flow models,which are the ultimate aim,of both baseline and aspirated cascades are established. Results show that single?slot endwall suction scheme adjacent to the suction surface can e ectively remove the corner open separation. With suction mass flow rate of 0.85%,the overall loss coe cient and endwall loss coe cient of the cascade are reduced by 25.2% and 48.6%,respectively. Besides,this scheme increases the static pressure rise coe cient of the cascade by 3.2% and the flow turning angle of up to 3.3° at 90% span. The concentration shedding vortex decreases,whereas the passage vortex increases. For single?slot suction schemes near the middle pitchwise of the passage,the concentration shedding vortex increases and the passage vortex is divided into two smaller passage vortexes,which converge into a single?passage vortex near the trailing edge section of the cascade. For the double?slot suction scheme,triple?passage vortexes are presented in the blade passage. Some new vortex structures are discovered,and the novel flow models of aspirated compressor cascade are proposed,which are important to improve the design of multi?stage aspirated compressors.

Aerodynamic performance of high-turning curved compressor cascade with boundary layer suction

The impact of boundary layer suction on the aerodynamic performance of a high-turning compressor cascade was numerically simulated and discussed.The aerodynamic performance of a curved and a straight cascade with and without boundary layer suction were comparatively studied at several suction flow rates.The results showed that boundary layer suction dramatically improved the flow behavior within the flow passage.Moreover,higher loading over the whole blade height,lower total pressure loss,and higher passage throughflow were achieved with a relatively small amount of boundary layer removal.The integration of curved blade and boundary layer suction contributed to better aerodynamic performance than the cascades with only curved blade or boundary layer suction used,and the more favorable effect resulted from the weakening of the three dimensional effects of the boundary layer close to the endwalls.

Effect of Nonlinear Thermal Radiation on Boundary Layer Flow of Viscous Fluid over Nonlinear Stretching Sheet with Injection/Suction

The present study reveals the effect of nonlinear thermal radiation and magnetic field on a boundary layer flow of a viscous fluid over a nonlinear stretching sheet with suction or an injection. Using suitable similarity transformations, governing partial differential equations were reduced to higher order ordinary differential equations and further these are solved numerically using of Keller-Box method. Effect of flow controlling parameter on velocity, temperature and nanoparticle fluid concentration, local skin friction coefficient, local Nusselt number and local Sherwood numbers are discussed. It is found that the dimensionless velocity decreases and temperature, concentration are increased with the increasing of magnetic parameter. The temperature profile is an increasing function of thermal radiation when it is increasing.

Boundary Layer Stagnation-Point Slip Flow and Heat Transfer towards a Shrinking/Stretching Cylinder over a Permeable Surface

In this paper, the boundary layer stagnation-point slip flow and heat transfer towards a shrinking/stretching cylinder over a permeable surface is considered. The governing equations are first transformed into a system of non-dimensional equations via the non-dimensional variables, and then into self-similar ordinary differential equations before they are solved numerically using the shooting method. Numerical results are obtained for the skin friction coefficient and the local Nusselt number as well as the velocity and temperature profiles for some values of the governing parameters, namely the velocity slip parameter (α), the thermal slip parameter (β), the curvature parameter (γ) and the velocity ratio parameter (c/a). The physical quantities of interest are the skin friction coefficient and the local Nusselt number measured by f’’(0) and –θ’(0), respectively. The numerical results show that the velocity slip parameter α increases the heat transfer rate at the surface, while the thermal slip parameter β decreases it. On the other hand, increasing the velocity slip parameter α causes the decrease in the flow velocity. Further, it is found that the solutions for a shrinking cylinder (c/ac/a>0) case. Finally, it is also found that the values of f’’(0) and –θ’(0) increase as the curvature parameter γ increases.

Boundary layer flow over a moving surface in a nanofluid with suction or injection

An analysis is performed to study the heat transfer characteristics of steady two-dimensional boundary layer flow past a moving permeable flat plate in a nanofluid. The effects of uniform suction and injection on the flow field and heat transfer characteristics are numerically studied by using an implicit finite difference method. It is found that dual solutions exist when the plate and the free stream move in the opposite directions. The results indicate that suction delays the boundary layer separation, while injection accelerates it.

采用附面层抽吸(BLS)控制流动分离的数值模拟

采用数值模拟的方法,研究了附面层抽吸(BLS)对两种具有不同分离特征的叶栅流场的气动影响。通过比较抽吸前后极限流线谱和叶栅损失的变化,对抽吸前后的流场进行定性分析。结果表明,采用附面层抽吸技术可以明显改善大转角、大分离叶栅的气动性能。其中,对以闭式分离为主要特征的叶栅流场,其最佳抽吸位置为主分离区的起始位置,即鞍点附近;对以开式分离为主要特征的叶栅流场,其最佳抽吸位置位于主分离区的上游。

BOUNDARY LAYER AND VANISHING DIFFUSION LIMIT FOR NONLINEAR EVOLUTION EQUATIONS

In this paper, we consider an initial-boundary value problem for some nonlinear evolution equations with damping and diffusion. The main purpose is to investigate the boundary layer effect and the convergence rates as the diffusion parameter α goes to zero.

BL模式在超声速湍流边界层中应用的检验

湍流模型是目前解决工程中湍流问题不可缺少的工具。然而它终究是建立在简化与假设的基础上的,其准确性必须经过实际的检验。对超声速边界层,由于缺乏系统的实验数据,检验工作很难做。本文以BL模型为例,通过可压缩湍流边界层的三个典型算例,与直接数值模拟(DNS)的结果作对比。分析了其成功及不足之处,以及影响其精度的若干因素,并检验了用BL模型预测转捩位置的有效性。

Influence of coupled boundary layer suction and bowed blade on flow field and performance of a diffusion cascade

Based on the investigation of mid-span local boundary layer suction and positive bowed cascade, a coupled local tailored boundary layer suction and positive bowed blade method is developed to improve the performance of a highly loaded diffusion cascade with less suction slot. The effectiveness of the coupled method under different inlet boundary layers is also investigated.Results show that mid-span local boundary layer suction can effectively remove trailing edge separation, but deteriorate the flow fields near the endwall. The positive bowed cascade is beneficial for reducing open corner separation, but is detrimental to mid-span flow fields. The coupled method can further improve the performance and flow field of the cascade. The mid-span trailing edge separation and open corner separation are eliminated. Compared with linear cascade with suction, the coupled method reduces overall loss of the cascade by 31.4% at most. The mid-span loss of the cascade decreases as the suction coefficient increases, but increases as bow angle increases. The endwall loss increases as the suction coefficient increases. By contrast, the endwall loss decreases significantly as the bow angle increases. The endwall loss of coupled controlled cascade is higher than that of bowed cascade with the same bow angle because of the spanwise inverse ‘‘C＂ shaped static pressure distribution. Under different inlet boundary layer conditions, the coupled method can also improve the cascade effectively.

Computational Study on the Boundary Layer Suction Effects in the Supersonic Compressor Flow with a Bowed Blade

Flow control methodologies have been widely used to eliminating flow separation and increasing the blade load in axial compressor.Aiming at revealing the flow mechanism of coupled bowed blading and boundary layer suction in a supersonic compressor cascade,a cascade with a diffusion coefficient of 0.62 is numerically presented.First of all,according to the available experimental data,the numerical method was validated;then,different bowed blading effects on flow field in detail were investigated;at last,based on the flow physics of purely bowed blading,the positively bowed blade was coupled with boundary layer suction on blade suction surface,whereas the negatively bowed blade was coupled with endwall suction.For coupled control method,influence mechanism on flow field,especially on the shock structure was revealed,and different aspect ratios of coupled control method were investigated as well.Results showed that the coupled positively bowed blading and suction surface suction can eliminate the flow separation effectively.Compared with that of baseline supersonic cascade,the total pressure loss coefficient of the coupled scheme was reduced by 37.4%at most.At mid-span,the shock moved downstream and the single shock was separated to a dual-shock structure since the positively bowed blading reduced the static pressure of mid-span.The coupled negatively bowed blading and endwall suction also effectively enhanced the performance of cascade by removing the corner separation,with the loss coefficient reduced by as much as 41.9%.However,the suction coefficient of optimal coupled negatively bowed blading scheme reached 10.5%,which is too high for practical use.After coupled control,the 3 D shock structure became“C”shaped distribution along spanwise because of the difference in influence mechanism of negatively bowed blading on different spanwise location.Due to the opposite influence effect of positively and negatively bowed blading,the shock structure in the two different schemes of cascades were different and showed opposite variation trends as aspect ratio increased.

MHD free convective flow past semi-infinite vertical permeable wall

In this paper, the basic equations governing the flow and heat transfer of an incompressible viscous and electrically conducting fluid past a semi-infinite vertical per- meable plate in the form of partial differential equations are reduced to a set of non-linear ordinary differential equations by applying a suitable similarity transformation. Approx- imate solutions of the transformed equations are obtained by employing the perturbation method for two cases, i.e., small and large values of the suction parameter. From the numerical evaluations of the solution, it can be seen that the velocity field at any point decreases as the values of the magnetic and suction parameters increase. The effect of the magnetic parameter is to increase the thermM boundary layer. It is also found that the velocity and temperature fields decrease with the increase in the sink parameter.

Magneto Hydrodynamics Stagnation Point Flow of a Nano Fluid over an Exponentially Stretching Sheet with an Effect of Chemical Reaction, Heat Source and Suction/Injunction

A numerical investigation is carried out on the effects of heat source suction and viscous dissipation on Magneto hydrodynamics boundary layer flow of a viscous, steady and incompressible fluid. The flow is assumed to be over on exponentially stretching sheet. The governing system of partial differential equations has been transformed into ordinary differential equation using similarity transformation. Keller box method is simulated on the dimensionless system of differential equations. The skin friction coefficient and the heat and mass transfer rates are very significant parameters that are computed, analysed discussed in detail.

Effects of Boundary Layer Suction on Aerodynamic Performance in a High-load Compressor Cascade

This article is aimed to experimentally validate the beneficial effects of boundary layer suction on improving the aerodynamic performance of a compressor cascade with a large camber angle. The flow field of the cascade is measured and the ink-trace flow visualization is also presented. The experimental results show that the boundary layer suction reduces losses near the area of rnidspan in the cascade most effectively for all suction cases under test. Losses of the endwall could remarkably decrease only when the suction is at the position where the boundary layer has separated but still not departed far away from the blade surface. It is evidenced that the higher suction flow rate and the suction position closer to the trailing edge result in greater reduction in losses and the maximum reduction in the total pressure loss accounts to 16.5% for all cases. The suction position plays a greater role in affecting the total pressure loss than the suction flow rate does.

叶栅吸力面吸气位置与角区分离的关联性分析

高压压气机后面级的叶片吸力面三维角区内易堆积大量的低能流体。为了指导吸气槽设计,达到量化分析叶栅吸力面吸气槽位置与角区分离的关联性的目的,以一高负荷压气机叶栅为研究对象,采用计算流体力学仿真软件讨论了全叶高吸气槽轴向位置变化对叶栅性能的影响,并以角区分离起始位置到叶栅前缘之间的距离(ZCS)作为量化标准。研究表明:在设计工况附近,总压损失随着吸气槽轴向位置从前向后移动表现出了先减小后增大的趋势,且叶片吸力面吸气槽在不同工况下存在最佳轴向位置。在0°迎角时,当吸气槽距离分离点间的轴向距离为0.33倍的ZCS时为最佳,可使总压损失系数降低10.9%。这种量化结果借助角区分离结构实现了吸气槽轴向位置设计的标准化指导。

端壁造型与附面层抽吸耦合流动控制对压气机叶栅的性能影响

为改善轴流压气机性能,抑制角区分离,采用数值模拟方法探究端壁造型(PEW)与附面层抽吸(BLS)两种流动控制措施在不同工况下对高负荷压气机叶栅的气动性能影响。使用神经网络作为代理模型,采用多目标遗传算法进行全局寻优,选取得到的5种最优端壁造型结构,与3种不同弦长位置的抽吸缝进行组合。结果表明:兼顾设计工况及近失速工况叶栅气动性能,采用吸力面70%弦长位置抽吸与通道前中部下凹、近压力面上凸的造型组合最优。设计工况下,附面层抽吸抑制了弦长中部转捩泡的生成,有效降低了尾迹损失,此时最优组合减小了12.7%的总压损失。近失速工况下,端壁造型通过控制端区横向二次流,缓解堵塞,抑制了角区分离进而降低尾迹损失,此时最优组合减小了15.5%的总压损失。

激波串与进气道肩部分离泡相互作用

针对高超声速进气道激波串与肩部分离泡相互作用时的流动振荡问题,在来流马赫数为6的激波风洞中,采用高速纹影拍摄结合壁面动态压力测量,研究了有/无抽吸情况下激波串与肩部分离泡的相互作用过程。结果表明:当激波串前移至肩部附近时,有抽吸进气道也会产生大尺度的分离泡,进而有/无抽吸进气道内的激波串均会与肩部分离泡形成耦合振荡,并造成严重的脉动压力。在激波串的推动下,分离泡能够自由地越过肩部凸拐角,使得其自身的低频振荡特性能够显现。激波串内的压力波动会显著改变分离泡的形态,而分离泡形态的变化又会影响激波串内的压力,两者相互耦合从而维持这种低频振荡。无抽吸进气道具有相同的低频耦合振荡特性;而抽吸缝阻碍了上下游的信息传递,使得有抽吸进气道的分离泡低频振荡显著,而激波串振荡具有一定的宽频特性。经分离激波振荡范围和进气道入口速度无量纲后,有/无抽吸进气道低频耦合振荡的St均处于0.011~0.021,与经典分离泡的低频振荡特性相当。

某开口回流式低速风洞固定地板结构优化

针对低速风洞固定地板对地效试验数据质量和试验效率的影响,采用CFD计算方法优化设计固定地板的吸除缝及其整流前缘的结构参数。基于非接触测控方式,设计固定地板随动密封装置;在低速风洞中,通过试验对优化后固定地板上方的风洞落差系数及附面层厚度进行测量。结果表明:在常用风速下,优化后固定地板模型区域的附面层厚度能够满足地面效应实验的要求。

Enhancing aerodynamic performances of a high-turning compressor cascade via boundary layer suction

Experimental investigation was carried out to study the effect of suction positions and suction flow rates on the aerodynamic performance of a compressor cascade with a large camber angle. The ink-trace flow visualization was conducted and the flow fields of the cascade were also measured. Three types of boundary layer suction configurations are compared,i.e. the suction surface suction,the endwall suction and the compound suction. Experimental results show that the large amount of suction flow rate gains more losses reduction than the small amount for a certain proper suction configuration,but the speed of loss decline slows down as the suction flow rate goes on increasing. Boundary layer suction on the suction surface obviously enhances the ability of the boundary layer around the midspan to withstand the negative pressure gradient of the flow passage. The range of the suction surface corner is also decreased. If the suction slot locates in the corner separation region where severe separation has happened,the flow separation will be terminated at the slot and redevelop downstream. And boundary layer suction on the endwall mainly influences the endwall corner region in remarkably delaying,lessening and reorganizing the corner separation. While the whole flow field is remarkably improved at both midspan and the corner region in the compound suction schemes. At higher suction flow rates,the aerodynamic performance of the compressor cascade is better than that with boundary layer suction simply on the suction surface or on the endwall. When the suction flow rate is 1.5% of the inlet mass flow,the compound suction scheme achieves a maximum loss reduction of 17% compared with the cascade without boundary layer suction.