Improvement in aerodynamic performance of NACA0021 airfoil using moving surface boundary layer: A computational study

The NACA0021 being a thick airfoil, possesses soft stall behavior with moderate maximum coefficient of lift (clmax). The present study focusses to improve aerodynamic performance using the moving surface boundary control. The modifications to airfoil include a rotating leading edge cylinder with airfoil aft body. The leading edge rotating cylinder injects momentum in top surface boundary layer thereby, keeping the flow attached in the otherwise adverse pressure gradient. The attached flow over top surface shall contribute to improve the airfoil aerodynamic performance in terms of lift, drag and stall angle. The present computational study focusses on modified airfoil performance for varying velocity ratios (i.e. ratio of tangential velocity of rotating cylinder and free stream velocity) between 0.0 and 1.78 at different airfoil angles of attack. The computational study clearly highlights improvement in modified airfoil aerodynamic performance in terms of coefficient of lift, drag, stall angle and clmax even at velocity ratios less than 1.0. The lift and drag characteristics of modified airfoil are found to be superior to base airfoil for velocity ratios beyond 0.356 and 0.7 respectively. The stall angle of attack for modified airfoil is also seen to increase linearly with velocity ratio. The stall angle of attack nearly doubles from 10° for stationary leading edge to 19° for velocity ratio of 1.78. The extent of momentum injection in boundary layer with varying cylinder speed is analysed by comparing the coefficient of pressure plots. The aerodynamic performance improvement thus achieved by momentum injection by low cost and practically feasible leading edge rotating cylinder in NACA 0021 promises its possible utilization for varied applications i.e. for low speed aircraft wing, wind turbine blades and hydrofoils.