Fluid forces on a very low Reynolds number airfoil and their prediction

This paper presents the measurements of mean and fluctuating forces on an NACA0012 airfoil over a large range of angle (α) of attack (0–90°) and low to small chord Reynolds numbers (Rec), 5.3 × 103–5.1 × 104, which is of both fundamental and practical importance. The forces, measured using a load cell, display good agreement with the estimate from the LDA-measured cross-flow distributions of velocities in the wake based on the momentum conservation. The dependence of the forces on both α and Rec is determined and discussed in detail. It has been found that the stall of an airfoil, characterized by a drop in the lift force and a jump in the drag force, occurs at Rec ⩾ 1.05 × 104 but is absent at Rec = 5.3 × 103. A theoretical analysis is developed to predict and explain the observed dependence of the mean lift and drag on α.