Wall pressure signature of separated flows: A comparison between flat plate and airfoil

Abstract

Wall pressure fluctuations for separated flows formed along a flat plate due to an adverse pressure gradient and near the trailing edge of two different airfoils in pre-stall conditions are analyzed for three distinct experimental setups. First, a spectral analysis reveals that the presence of backflow can be convincingly identified at high frequency, StL=fLb/Uref>3, where Lb is the mean separation length and Uref is the nominal inlet velocity. This feature takes place immediately after the classical vortex shedding at mid-frequencies (0.15<StL<2). Such small scale dynamics within the recirculation region is shown to be associated with a marked bifurcation in the spectrum that corresponded to the passage from mid to high frequency ranges, suggesting that two motions of opposite directions can be detected with a single pair of pressure sensors. Secondly, a time domain analysis highlights localized non-stochastic behaviors that depends on the level of flow separation. Frequency-filtered time series based on the breathing (0.15<StL), vortex shedding (0.15<StL<2) and small scale (StL>3) unsteadiness allow us to look at the main contributors of high-order moments (skewness and kurtosis). We draw the portrait of either an attached (in a mean sense), weakly or massively separated turbulent boundary layers: the low-frequency causes a positively skewed wall pressure fluctuation at the onset of separation that switches to a negative one in the backflow region only for the separated cases. Finally, the unsteady contributions to the kurtosis are primarily driven by the breathing, followed by the vortex shedding in the case of a massive separation, whereas attached APG TBL and weakly separated cases show the opposite effect.