Noninvasive Measurement Methods for Transient Flows in Wind Tunnels

Abstract

The noninvasive measurement technique proposed in this paper uses the response of a Kevlar wall in a wind tunnel to infer the angle-of-attack time history of an airfoil in the working section undergoing a pitching motion. Recent work has shown that the displacement of a Kevlar wall can be measured accurately via an optical method to obtain the loading on an airfoil in a steady flow, but the feasibility of using this method on time-varying flows has yet to be explored. Based on thin airfoil theory, a model of an airfoil undergoing a sudden change in angle of attack is considered, and the pressure along the wall was found using potential flow theory. The process of relating the pressure field to the wall displacement is presented, and it is shown how the wall response can be found. The membrane response is then used to obtain estimates of the angle-of-attack time history of the airfoil through an inversion process. It was found that the response of the Kevlar wall is dominated by the effects of the circulation around the airfoil and not its wake, and so the unsteady airfoil lift is measured with the proposed method, without the need for a force balance with a fast response time. The limitations and the applications of the proposed measurement method are discussed.