Experimental analysis of the coherent structures and turbulence past a hydrofoil in stalling condition beneath a water-air interface

In this paper, measurements beneath the interface of the flow past a hydrofoil in water during stalling conditions are used to analyse the mean flow and, in more depth, the large eddies and turbulence. The properties of the time-averaged flow are analysed. In addition to the classical wake, with a recirculation area near the low pressure side of the hydrofoil, a breaker develops due to the limited distance between the hydrofoil and the interface, which causes the generation of further vorticity. Vorticity develops in the recirculation area and at the trailing edge as well. The fluctuating streamwise and transverse velocity demonstrate a classical distribution with maxima at the edge of the wake and beneath the breaker. A quadrant analysis of the Reynolds shear stress indicates the main flux of the momentum, which is always directed towards the axis of the wake to reduce the defect velocity of the wake. The spatial correlations for the longitudinal and transverse fluctuating velocity are computed at several points along the axis of the wake at four different locations downstream. The correlation of the transverse fluctuating velocity shows an evident anisotropy, which is more extensive in the transverse direction and is limited in the streamwise direction. In the streamwise direction, the correlation of the transverse fluctuating velocity also assumes negative values. The correlation of the longitudinal fluctuating velocity is significantly isotropic, and the associated length scale increases downstream. The shape of this last correlation can be fitted with a limited number of eddies; hence, a stable and repetitive pattern of eddies is predicted, as shown in the wavenumber longitudinal and transverse spectra as well.