Experimental study on the effect of rough surface on aerodynamic characteristics and flow field of low Reynolds number airfoil

In order to explore the influence of rough surfaces on aerodynamic characteristics and the flow field of the low Reynolds number airfoil and to deeply understand the action mechanism of rough surfaces, the SD8020 airfoil was used for experimental research (Re = 4 × 10⁴). The aerodynamic force of the airfoil was measured in the experiment, and the flow field around the airfoil was observed in detail by using luminescent oil-film, smoke wire flow visualization and hot wire technology. The results show that the lift coefficient of the smooth airfoil has nonlinear characteristics in the range of α = 0°～3°, and the abrupt change of the laminar flow separation bubble structure is the main reason for the nonlinear characteristics of the lift coefficient of the airfoil at low Reynolds number. Too small leading edge roughness (S_a⁺ = 0.00025) does not have significant influence on the flow field, but appropriate leading edge roughness (S_a⁺ = 0.0051, 0.013) can delay the separation of boundary layer, accelerate the reattachment of shear layer, and reduce or even eliminate laminar separation bubbles. So it can significantly reduce aerodynamic drag and increase the lift-drag ratio. The lift-drag ratio of the smooth airfoil is increased by 35.7% and 41.4%, respectively. When S_a ⁺ =0.013, the nonlinear characteristics of the lift coefficient growth is weakened at a small attack angle and the lift coefficient can increase significantly (e. g. the lift coefficient increases by 219.5% when α=2°). The roughness of the leading edge accelerates the growth of the disturbance (manifested as the growth of high-frequency velocity pulsation and T–S wave), rolls the vorticity of the wall into the flow field faster, and forms the vortex structure earlier. The vortex structure can strengthen the normal convection, improve the resistance of the boundary layer to the adverse pressure gradient, and delay the separation. After the boundary layer separation, the vortex structure plays a leading role in the transition process of the separated shear layer, speeding up the transition of flow to turbulence and flow reattachment in advance.