Direct force measurements of a two-dimensional airfoil undergoing dynamic stall

Abstract

A direct force measurement technique is designed to investigate experimentally a two-dimensional airfoil undergoing dynamic stall at low Reynolds number (Re). Two servomotors and two piezoelectric load cells are installed at each end of a NACA 0012 airfoil mounted horizontally across the test section in a wind tunnel. Thus designed, the servomotors can pitch the airfoil sinusoidally about its one-quarter chord at various mean angles of attack and amplitudes, while the load cells give the force responses of the airfoil during oscillations in two directions in the plane normal to the airfoil axis. All experiments are carried out at a Re based on the airfoil chord of 7.7x104. Total five cases covering the range of airfoil pre-stall, light stall, and deep stall are tested. They are Case 1: a = 0o + 5osin(wt) and Case 2: a = 5o + 5osin(wt) in the pre-stall region, Case 3: a = 10o + 5osin(wt) in the light-stall region, and Case 4: a = 10o + 10osin(wt) and Case 5: a = 15o + 10osin(wt) in the deep-stall region. Four relatively low reduced frequencies, k = 0.005, 0.01, 0.02, and 0.04 are tested for each case. Flow visualization is also performed using the same airfoil in a water tunnel at the same Re and k. Phase-averaged lift of the airfoil undergoing dynamic stall is presented. Hysteresis loops of the lift occur both when the airfoil is being pitched to exceed its static stall limit and when it is still within its static stall limit. It is observed that both the dynamic stall angle and the maximum lift increase with increasing k at the same pitching mean angle of attack and pitching amplitude for the stall cases. Flow visualization pictures at various phases of different oscillations are also presented. The time series of the lift response signals are analyzed using fast Fourier transform and wavelet analysis. Nonstationary and nonlinear characteristics of the lift time series of both the light-stall and deep-stall cases are observed.