Experimental investigation of wall-pressure fluctuations on a transiently moving hydrofoil by empirical mode decomposition

In this paper, we describe the experimentally measured wall pressure distribution on a hydrofoil's suction side undergoing a transient pitching motion. The obtained signals are of unsteady, wide-band, noisy, and turbulent nature. These signals are hard to deal with by use of classical methods because they involve nonstationary nonlinear chaotic and turbulent signals. A novel signal processing method based on the Empirical Mode Decom-position (EMD) is proposed. The EMD decomposes signals into frequency sub-band components called Intrinsic Mode functions (IMFs) by separating high frequency content of the signal from low frequency one. As a result, the EMD method allows to distinguish amongst noise several features of the flow such as the appearance of the boundary layer transition point accompanied by high frequency components during the low frequency transient motion. The transition point is first detected by the downstream transducers on the signal partially reconstructed using low frequency IMFs. It is observed to be moving toward the leading edge until Boundary layer separation occurs.