Considerations about the phase averaging method with application to ELDV and PIV measurements over pitching airfoils

Some considerations about the principles of the phase averaging method, often used to decompose a measured signal into mean and fluctuating parts, are presented for periodic flow configurations involving natural and forced unsteadiness. The order of procedure to theoretically determine the number N of cycles required to perform a correct phase averaging decomposition is discussed and a practical criterion is proposed. The use of this criterion is first illustrated in the case of ELDV measurements in and outside the boundary-layer on an airfoil oscillating in pitch at moderate conditions of unsteadiness. The principles of the phase averaging method are then extended to the case of PIV measurements and both theoretical and practical criterions are given. Examples of potential applications of the phase averaging method for quantitative PIV analyses on an airfoil oscillating through dynamic stall and submitted to strong conditions of unsteadiness are also presented. Received: 8 April 1998/Accepted: 6 November 1998