The unsteady normal force on an airfoil in oscillating flow

Abstract

the effects of oscillating flow on the pressure force normal to the chord of a symmetrical airfoil were investigated experimentally employing a remote pressure transducer to measure the instantaneous pressure distribution. An open circuit wind tunnel having a set of the test section was used to produce the oscillating flow. Electrical signals analogous to the free stream velocity and surface pressure were recorded simultaneously on separate tracks of a magnetic tape. The recorded data were converted to digital representation, and numerical techniques utilized to evaluate the spectral composition of the measured pressure distribution, from which the normal force was calculated. It was found that the magnitude of the total normal force at high angles of attack is significantly greater in oscillating flow than is steady flow and is frequency dependent; while a low angles of attack no significant differences were observed. Moreover it was found that higher order harmonics of the fundamental free-stream frequency constitute a significant fraction of the normal force, and these fractions are also frequency dependent. The observed results are not adequately predicted by quasi-steady aerodynamic analysis.