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Multipoint wall pressure measurements and Laser Doppler Velocimetry (LDV) measurements of the velocity field generated over a 30° forward facing ramp are discussed in this paper. Mean and fluctuating signals are analyzed both in the mean separated region and in the recovering boundary layer flow. We present an analysis based on proper orthogonal decomposition (POD). The choice of a “POD region” as a subset of the sensor array is discussed. In a “POD region” restricted to the mean separated region, we show that the spatio-temporal coherence of the signal is efficiently captured by the POD decomposition. Extended POD is then introduced to extract in an objective way the part of the surface pressure signal, downstream of the mean reattachment point, which is correlated with the pressure signal under the separated region. The properties of this decomposition are demonstrated. We show that the power spectral density of the correlated part always displays a peak in the frequency domain associated with vortex shedding while the high frequency region is dominated by the uncorrelated part. A physical model is proposed to link large scale vortex structures and the correlated surface pressure fluctuations. We prove that the correlated pressure signal downstream of the mean separation length carries the properties of the distorted mixing layer eddies being advected and dissipating into the outer boundary layer flow. Finally, the spectral content of the correlated pressure signal is studied and we show that the first extended mode is able to capture the normalized power spectral density of the correlated pressure accurately. The method proposed here is therefore able to extract the contribution of separated shear layer eddies from the complex pressure trace in the recovering boundary layer near the mean reattachment point. The fast convergence of the POD decomposition in the separated region and the properties of the first extended mode downstream of the reattachment may be used to obtain an efficient reduced description for control or analysis purposes.

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