Determining Flow Propagation Direction from In-Flight Array Surface Pressure Fluctuation Data

When characterizing spatial coherence properties of turbulent boundary-layer surface pressure fluctuation data, it is important to determine the local flow direction first. Without flow direction, it is very easy to introduce errors due to misalignment between sensors and the flow. For cases with two-dimensional microphone distributions, a method of determining flow direction from the orientation of the coherent pressure in spatial domain was introduced recently. If the data are analyzed in wavenumber domain, flow information can be obtained by the position and orientation of the convective ridge. In this publication, flow directions determined from a revised spatial domain approach and from two wavenumber domain approaches are considered. It was found that the result from the spatial domain approach and the result from the orientation of the convective ridge are similar for most frequencies, while the result based on the position of the convective ridge differs in the lower frequency range. Tilted convection of coherent structures in the turbulent boundary layer is discussed as a possible cause of these observations. A modification of the analytical model for surface pressure coherence is derived that takes the findings into account.