二维扩压叶栅非定常分离流控制途径探索

INVESTIGATION ON WAYS TO CONTROL UNSTEADY SEPERATED FLOW IN 2D COMPRESSOR CASCADE

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.

In recent years, lots of experiment and CFD study have been carried out in order to control post-stall flow of airfoil by using unsteady forcing in external flow field, which showed that unsteady forcing can enhance lift and reduce drag. This paper investigates a new way to control unsteady separated flow in compressor based on learning these research results. But flow environment of airfoil and compressor is different. Airfoil is a single-object system and unsteady forcing must be artificially excited. However, compressor is a multi-object system. In multistage axial compressor, relative moving wake of upstream blades can be deemed to an 'unsteady forcing' to unsteady separated flow field of downstream adjacent blades. This paper uses periodic blowing-suction to simulate this 'unsteady forcing' for researching the mechanism which unsteady forcing controls separated flow. By using a Reynolds-aver aged two-dimensional computation of turbulent, numerical results showed that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing-suction near the leading edge of 2D compressor cascade. This unsteady forcing can modulate the evolution of the boundary layer to promote the coalescence of small vortices when forcing frequency, forcing amplitude and forcing location satisfy some conditions in a certain range of incidence. Thus, most of separated flow becomes organized flow, associated with a significant enhancement of time-averaged aerodynamic performance: Loss coefficient reduced by 13.1% and turning angle increased by 14.3%. The effect of forcing frequency, forcing amplitude and forcing location on flow field was investigated in detail. When forcing frequency is equal to vortex shedding frequency, we obtain the most favorable increase of time-averaged aerodynamic performance. The effective forcing frequency spans a wide smooth spectrum. Forcing amplitude exists a threshold value, which is about equal to 10% (relative main-flow velocity). The optimal forcing amplitude is about 40%. Forcing location at 2% of chord length from leading edge is optimal.