Wall-pressure fluctuations in an axisymmetric boundary layer under strong adverse pressure gradient

Measurements of fluctuating wall pressure in a high-Reynolds-number flow over a body of revolution are described. With a strong axial pressure gradient and moderate lateral curvature, this non-equilibrium flow is relevant to marine applications as well as short-haul urban transportation. The wall-pressure spectrum and its scaling are discussed, along with its relation to the space-time structure. As the flow decelerates downstream, the root-mean-square level of the pressure drops together with the wall shear stress (t(w)) and is consistently approximately 7t(w). While the associated dimensional spectra see a broadband reduction of over 15 dB per Hz, they appear to attain a single functional form, collapsing to within 2 dB when normalized with the wall-wake scaling where t(w) is the pressure scale and U-e/d is the frequency scale. Here, d is the boundary layer thickness and U-e is the local free-stream velocity. The general success of the wall-wake scaling, including in the viscous f(-5) region, suggests that the large-scale motions in the outer layer play a predominant role in the near-wall turbulence and wall pressure. On investigating further, we find that the instantaneous wall-pressure fluctuations are characterized by a quasi-periodic feature that appears to convect downstream at speeds consistent with the outer peak in the turbulence stresses. The conditional structure of this feature, estimated through peak detection in the time series, resembles that of a roller, supporting the embedded shear layer hypothesis (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592-642; Balantrapu et al., J. Fluid Mech., vol. 929, 2021, A9). Therefore, the outer-region shear-layer-type motions may be important when devising strategies for flow control, drag and noise reduction for decelerating boundary layers.