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 (alpha) of attack (0-90 degrees) and low to small chord Reynolds numbers (Re(c)), 5.3 x 10(3)-5.1 x 10(4), 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 a and Re, 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 Re(c) >= 1.05 x 10(4) but is absent at Re(c) = 5.3 x 10(3). A theoretical analysis is developed to predict and explain the observed dependence of the mean lift and drag on alpha. (C) 2010 Elsevier Inc. All rights reserved.