Pressure field and cavitation inception in secondary vortices in a turbulent shear layer

Cavitation inception in a turbulent shear layer starts at multiple points along quasi-streamwise vortices (QSVs) stretched between the primary spanwise structures. Being pressure-dependent, this experimental study characterizes the pressure field in QSVs in a shear layer developing behind a 10mm high backward facing step. The time-resolved volumetric velocity distribution is measured using tomographic particle tracking. Interpolation of data to a grid with 200 µm spacing using a constrained cost minimization process that makes the velocity divergence-free and material acceleration curl-free, and subsequent integration of the material acceleration, provides the pressure distribution. Regions of QSVs are detected using k-means clustering using a series of variables involving velocity gradients. The pressure is indeed lower, and its minima last longer within the QSVs compared to the surrounding flow. These pressure minima are more likely to appear after a period of axial vorticity stretching and before contraction events. The QSVs have a typical diameter of 1 mm and are 2-5 mm long, and contain contraction and stretching bands with a typical size of 1 mm. Accordingly, the regions of low pressure are localized, consistent with the dimensions of cavitation inception events.