Unsteady Flow Variability Driven by Rotor-stator Interaction at Rotor Exit

Numerical investigation of the unsteady flow variability driven by rotorstator interaction in a transonic axial compressor is performed. Two models with close and far axial gap between rotor and stator rows are studied in the simulation. Particular attention is attached to the analysis of mechanisms involved in driving rotor wake oscillation, rotor wake skewing and flow angle fluctuation at rotor exit. The results show that smaller axial gap is favorable to enhance the interaction in the region between two adjacent rows, and the fluctuation of the static pressure difference between two sides of rotor wake is improved by potential field from down stator, which is the driving force for rotor wake oscillation. The interaction between rotor and stator is weakened by increasing axial distance, rotor wake shifts to suction side of rotor blade with 5%-10% of rotor pitch, the absolute value of flow angle at rotor exit is less than that in the case of close interspace for every time step, and the fluctuation amplitude is also decreased.     

Alleviation of spike stall in axial compressors utilizing grooved casing treatment

This article deals with application of grooved type casing treatment for suppression of spike stall in an isolated axial compressor rotor blade row. The continuous grooved casing treatment covering the whole compressor circumference is of 1.8 mm in depth and located between90% and 108% chord of the blade tip as measured from leading edge. The method of investigation is based on time-accurate three-dimensional full annulus numerical simulations for cases with and without casing treatment. Discretization of the Navier–Stokes equations has been carried out based on an upwind second-order scheme and k-w-SST(Shear Stress Transport) turbulence modeling has been used for estimation of eddy viscosity. Time-dependent flow structure results for the smooth casing reveal that there are two criteria for spike stall inception known as leading edge spillage and trailing edge backflow, which occur at specific mass flow rates in near-stall conditions. In this case, two dominant stall cells of different sizes could be observed. The larger one is caused by the spike stall covering roughly two blade passages in the circumferential direction and about 25% span in the radial direction. Spike stall disturbances are accompanied by lower frequencies and higher amplitudes of the pressure signals. Casing treatment causes flow blockages to reduce due to alleviation of backflow regions, which in turn reduces the total pressure loss and increases the axial velocity in the blade tip gap region, as well as tip leakage flow fluctuation at higher frequencies and lower amplitudes. Eventually, it can be concluded that the casing treatment of the stepped tip gap type could increase the stall margin of the compressor. This fact is basically due to retarding the movement of the interface region between incoming and tip leakage flows towards the rotor leading edge plane and suppressing the reversed flow around the blade trailing edge.     

变轴向间隙下压气机近失速工况时弯曲静叶改善流场的实验研究(英文)

The outlet flow fields of a low-speed repeating-stage compressor with bowed stator stages are measured with five-hole probe under the near stall condition when the rotor/stator axial gap varies. The performances of the straight stator stages are investigated and compared to those of the bowed stator stages. The results show that using bowed stator stages could alleviate the flow separation at both upper and low corners of the suction surface and the endwalls, and decrease the losses along the flow passage as well as the outlet flow angle. As the rotor/stator axial gap decreases, although the diffusion capacity of the compressor increases obviously, the outlet flow field in the straight stator stages deteriorates quickly. By contrast, little changes occur in the bowed stator stages, indicating that as the rotor/stator axial gap decreases, improved performance is achieved in the bowed stator stages.     

Numerical analysis of broadband noise reduction with wavy leading edge

Large Eddy Simulation(LES) is performed to investigate the airfoil broadband noise reduction with wavy leading edge under anisotropic incoming turbulence. The anisotropic incoming turbulence is generated by a rod with a diameter of 10 mm. The incoming flow velocity is 40 m/s and the corresponding Reynolds numbers based on airfoil chord and rod diameter are about 397000 and 26000, respectively. The far-field acoustic field is predicted using an acoustic analogy method which has been validated by the experiment. A straight leading edge airfoil and a wavy leading edge airfoil are simulated. The results show that wavy leading edge increases the airfoil lift and drag whereas the lift and drag fluctuations are substantially reduced. In addition, wavy leading edge can significantly change the flow pattern around the leading edge and a pair of counter-rotating streamwise vortices stemming from each wavy leading edge peak are observed.An averaged noise reduction of 9.5 dB is observed with the wavy leading edge at the azimuthal angle of 90°. Moreover, the wavy leading edge can mitigate noise radiation at all the azimuthal angles without significantly changing the noise directivity. The underlying noise reduction mechanisms are then analyzed in detail.     

Application of Tandem Cascade to Design of Fan Stator with Supersonic Inflow

This article proposes a tandem cascade constructed to tackle the thorny problem of designing the high-loaded stator with a supersonic inflow and a large turning angle.The front cascade adopts a supersonic profile to reduce the shock wave intensity turning the flow into subsonic,while the rear cascade adopts a subsonic profile with a large camber offering the flow a large turning angle.It is disclosed that the losses would be minimized if the leading edge of the rear cascade lies close to the pressure side of the front cascade at a distance of 20% pitch in pitch-wise direction without either axial spacing or overlapping in axial direction.The 2D numerical test results show that,with the inflow Mach number of 1.25 and the turning angle of 52°,the total pressure loss coefficient of the tandem cascade reaches 0.106,and the diffusion factor 0.745.Finally,this article has designed and simulated a high-loaded fan stage with the proposed tandem stator,which has the pressure ratio of 3.15 and the efficiency of 86.32% at the rotor tip speed of 495.32m/s.     

Experiments of effects of inlet-air distortion on aerodynamic performance in transonic compressor

The inlet-air distortion which was caused by high angle-of-attack flight was simulated by plugboard.Experiments were conducted on a transonic axial-flow compressor's rotor at 98% rotating speed.The flow-field characteristics and mechanism of performance degradation were analyzed in detail.The compressor inlet was divided into four sectors at circumference under inlet-air distortion.They were undistorted sector,transition sector A where the rotor was rotating into the distortion sector,distorted sector and transition sector B where the rotor was rotating out of the distortion sector.The experimental results show that compared with undistorted sector,there is a subsonic flow in transition sector A,so the pressure ratio is decreased by a large margin in this sector.However, the shock wave is enhanced in distortion sector and transition sector B, and thus the pressure ratio increases in these sectors.Because of the different works at circumference,the phase angle of total pressure changes 90° when the inlet total pressure distortion passes through compressor rotor.In addition,the frequency and amplitude of disturbances in front of the rotor strengthenes under inlet distortion,so the unstable flow would take place in advance.In addition, the position of stall inception is in one of the transition sectors.      

Numerical Simulation of Rotor-aerodynamic Surface Interaction in Hover Using Moving Chimera Grid

Three-dimensional unsteady Navier-Stokes equations are numerically solved to simulate the aerodynamic interaction of rotor, canard and horizontal tail in hover based on moving chimera grid. The variations of unsteady aerodynamic forces and moments of the canard and horizontal tail with respect to the rotor azimuth are analyzed with the deflection angle set at 0° and 50°, respectively. The pressure map of aerodynamic surfaces and velocity vector distribution of flow field are investigated to get better understanding of the unsteady aerodynamic interaction. The result shows that the canard and horizontal tail present different characteristics under the downwash of the rotor. The canard produces much vertical force loss with low amplitude fluctuation. Contrarily, the horizontal tail, which is within the flow field induced by the down wash of the rotor, produces only less vertical force loss, but the amplitudes of the lift and pitching moment are larger, implying that a potential deflection angle scheme in hover is 50° for the canard and 0° for the horizontal tail.     

AN EXPERIMENTAL STUDYOF ROTOR AERODYNAMICS IN GROUNDEFFECT AT LOW SPEEDS

Results of an experimental study of the aerodynamics of a helicopter rotor in ground . effect at low speeds are presented in this paper. Experiments of rotor wake flow visualization and induced velocity measurement near the rotor disc were conducted. At some advance ratio, the forward part of the rotor wake, after impinging on the ground, rolls up and recirculates near the rotor leading edge and large increase in induced velocity appears near the rotor leading edge. When the advance ratio increases, the rolled-up wake develops into a concentrated vortex, ground vortex, under the rotor, and the induced velocity near the rotor leading edge becomes much smaller. The ground vortex diminishes as the advance ratio further increases. Recirculation and ground vortex occur in a very narrow region of the advance ratio. These results explain the irregular variations of rotor forces and moments with advance ratio reported in previous works.     

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.     

A combined application of micro-vortex generator and boundary layer suction in a high-load compressor cascade

In the current study, the effects of a combined application between micro-vortex generator and boundary layer suction on the flow characteristics of a high-load compressor cascade are investigated. The micro-vortex generator with a special configuration and the longitudinal suction slot are adopted. The calculated results show that a reverse flow region, which is considered the main reason for occurring stall at 7.9° incidence, grows and collapses rapidly near the leading edge and leads to two critical points occurring on the end-wall with the increasing incidence in the baseline. As the micro-vortex generator is introduced in the baseline cascade, the corner separation is switched to a trailing edge separation by the thrust from the induced vortex. Meanwhile, the occurrence of failure is delayed due to the mixed low energy fluid and main flow. The synergistic effects between the micro-vortex generator and the boundary layer suction on the performance of the cascade are superior to the baseline at all the incidence conditions before the occurrence of failure, and the sudden deterioration of the cascade occurs at 10.3° incidence. The optimal results show that the farther upstream suction position, the lower total pressure loss of the cascade with vortex generator at the near stall condition. Moreover, the induced vortex with a leg can migrate the accumulated low energy fluid backward to delay the occurrence of stall.     

跨声压气机动叶弯、掠对静叶静压脉动的影响

对一单级跨声压气机采用了三种弯、掠动叶后设计工况下的非定常流场分别进行了数值模拟,分析了动叶弯、掠对下游静叶表面静压非定常脉动的影响.研究结果表明:三种弯、掠动叶都减轻了静叶前缘顶部和根部的静压脉动,使静叶前缘所受到的静压扰动沿径向分布更为均匀,尤其是弯掠动叶的作用最为明显;除去叶片前缘区域,三种弯、掠动叶对下游静叶的顶部压力面和吸力面的扰动都有所减弱,仍然以弯掠动叶的作用最为显著,而在静叶的中部和根部前缘区域之外的表面,动叶的弯、掠对压力扰动的影响不明显.      

动叶尾迹对静子非定常气动性能影响的研究

本文利用南京航空航天大学单级低速轴流压气机试验器,对动叶尾迹对静子非定常气动性能的影响开展了系统的实验研究。通过静叶表面动态压力分布的测量,获得了在不同的动—静叶排轴向间距、不同的工作转速和不同的工作流量下,动叶尾迹对静子非定常气动性能影响的大量基础实验数据     

跨声速压气机中动叶弯和掠对尾迹输运的影响

对一单级跨声速压气机设计工况下采用了3种弯、掠动叶后的非定常流场进行了数值模拟,分析了动叶弯、掠对动叶尾迹的非定常脉动强度以及向下游输运特性的影响.研究结果表明:与原型动叶相比,3种弯、掠动叶都减弱了动叶出口的泄漏涡和尾迹顶部脉动核心的脉动强度,使其在向下游输运过程中耗散较快;3种弯、掠动叶尾迹的中间段的脉动强度都有所加强,并且在输运过程中有向根部迁移的趋势,从而会使静叶的中部和根部受到较强的扰动.      

动叶弯和掠对压气机效率非定常波动的影响

对一单级跨声压气机设计工况下采用了3种弯、掠动叶后的非定常流场分别进行了数值模拟,分析了动叶弯、掠对级效率非定常波动的影响.研究结果表明:非定常计算条件下,动叶弯、掠后的压气机效率仍然获得大幅提高,并且效率的波动幅值同原型压气机相比明显降低,同时动叶改型前后的效率和压比的非定常波动曲线的相位发生了偏差;弯、掠动叶降低了压气机顶部和根部的分离损失,也使根部和顶部的效率波动显著降低.      

单级环境静叶等离子体流动控制机理仿真研究

在轴流压气机等离子体扩稳研究中,针对单转子压气机流动控制的研究较多,而针对单级环境下静叶流动控制的研究却很少.采用静叶轮毂轴向等离子体激励方式,通过数值模拟方法研究单级环境下静叶流场特性,揭示轴流压气机静叶等离子体流动控制扩稳机理.结果表明:等离子体激励器的轴向位置对单级轴流压气机的扩稳效果影响显著,越靠近叶片前缘,扩稳效果越好;布置在静叶通道后半部的等离子体激励器无法提高压气机的稳定性,而在静叶前缘施加轴向等离子体激励时,近轮毂区气流被诱导加速,主流的轴向速度提高,有效抑制了静叶近轮毂区吸力面的流动分离,静叶近轮毂区的堵塞减小,使得单级轴流压气机的稳定性提高.     

转子叶尖开缝对跨声速轴流压气机性能及流场结构影响机理研究

为了研究转子叶尖开缝对跨声速轴流压气机性能和流场结构的影响机理,设计了一种渐缩式射流缝,提出在转子叶尖不同相对位置开缝的流动控制方案,通过数值计算的方法对各开缝方案与压气机的流场进行对比分析.结果表明,开缝位置靠近前缘时压气机总压比提升了1.39％,但综合稳定裕度降低了0.18％;开缝位置靠近尾缘时,压气机总压比提升了...     

前置圆柱列涡轮静叶栅流场特性的数值研究

数值模拟了前置圆柱列涡轮静叶栅的三维流动,详细分析了圆柱列动静状态下叶栅流向的气动特性变化,发现不同时刻上游尾迹扫过下游叶片排不同位置,是造成多级涡轮非定常特性的主要原因。通过圆柱列动静两种状态下叶片表面压力脉动、流场变化特性的比较,发现静叶表面的压力分布存在差异,静叶前缘受上游圆柱列影响较大;随着流动的继续至叶栅流道下游,差异逐渐变小,直至消散;而叶栅前缘的差异主要通过叶片自身具有的大转折角逐步被抹平。     

转子叶片表面非定常气动力的构成及叠加分析

数值模拟了一个带有导叶的单级低速压气机中转子叶片表面的非定常气动力.通过研究转子叶片根部、尖部前缘和尾缘局部位置的非定常气动力,阐明了由尾迹和势扰动两种非定常性诱发的气动力在转子叶片表面的叠加特性.结果表明,上游尾迹的影响波及整个转子叶片弦长范围,而下游静子叶片势扰动仅影响转子叶片尾缘附近的脉动压力,改变上、下游叶片的时序位置,当整个转子叶片表面脉动压力的幅值比最大值减小19.4%时,其叶尖前缘脉动压力的幅值仅比最大值减小9.4%.      

航空发动机压气机静子通道压力信号时频特征

进行发动机整机试验,获取了压气机第一级静子通道内的压力信号.通过时频分析,从压力信号中辨识出了以550 Hz和2200 Hz为中心的时频结构;辨识出了压气机前3级转子叶片通过频率信号及它们的一些和频、差频信号.同时,还观察到在试验发动机减速过程中,放气带打开前,压气机前3级转子的叶片通过频率信号及它们的一些和频、差频信号幅度显著增强,这表明减速过程中压气机第一级转子尖区的流动分离逐渐趋于严重.因此,监控压气机静子通道压力信号时频特征的变化,可以获得特定的压气机转子的流场信息,从而为压气机稳定性控制提供重要依据.      

后置静叶周向弯曲对表面非定常压力脉动的影响

在目前的叶轮机械中,普遍存在着旋转动叶与后置静叶干涉产生的非定常流动现象。这种流动现象会导致静叶表面产生非定常性压力脉动。这种脉动可分为旋转动叶的速度亏损尾迹引起的叶片表面的周期性脉动和气流本身的湍流脉动及其他涡团导致的随机脉动两个方面。采用一工业用单级通风机,对其动静叶的干涉现象进行研究。通过在后置静叶表面沿弦长及叶高方向布置动态压力传感器的方法,研究静叶的周向弯曲（倾斜）角度对静叶表面的非定常力的影响。研究结果表明：采用周向弯曲静叶会改变动叶尾迹所引起的后置静叶周期性脉动的幅值在静叶表面的分布特点,静叶顶部压力脉动的幅值与原始径向静叶相比能够有效地降低,同时与中部的压力幅值相比有一定提高,但整体呈现下降趋势;另一方面,静叶表面随机性的压力脉动强度受周向弯曲的影响较小。