Evolution of the Non-linear and Unsteady Low Reynolds Number Laminar Separation Bubble around the Airfoil with Small angle of attack

摘要

Laminar separation bubble and laminar separation are the typical characteristics of the low speed low Reynolds number flow. There are stronger unsteady and nonlinear effects as the Reynolds number decreasing. Three significant effects are listed below: 1 Decreasing of the lift coefficients, increasing of the drag coefficients, and rapid deterioration of the lift-to-drag ratio. 2 Nonlinear phenomenon of aerodynamic coefficients near the small angle of attack. 3 Static hysteresis effect at the middle angle of attack. Based on the former numerical simulation studies about the laminar separation effect around the E387 and SD8020 airfoil at the low Reynolds number, the author discovered a new kind of trailing-edge laminar separation bubble(TLSB), which is significantly different from the classical long laminar separation bubble(LLSB). Furthermore, in this paper, the unsteady numerical method and wind tunnel PIV flow visualization method were taken to study the flow field of the symmetrical airfoil SD8020 at the small angle of attack with low Reynolds number more deeply and systemically. The unsteady and time-averaged flowfield structure and pressure coefficient, the nonlinear alteration of the unsteady lift coefficient with the angle to attack were given out in this paper. The results of CFD and experiments were good agreement with each other, especially for the Strouhal number (St), proved that our studies were correct. The PIV experiment proved that the classical long laminar separation is the time-averaged results of the unsteady laminar separation flow field. And the time-averaged trailing-edge laminar separation bubble indeed exists. And there are significant differences between the TLSB and LLSB among the shape, inner structure and the evolution law. The details of the time-averaged structures of the two LSBs were given out in this paper. The St number and the amplitude of the lift coefficients pulse between the two LSBs change sharply. And the unsteady and time-averaged pressure distributions and separation flows of the two LSBs are different greatly. The nonlinear effects of the lift coefficients to the angle of attack are caused by the evolutions between the TLSB and the LLSB.