Control effect of micro vortex generators on leading edge of attached cavitation

Abstract

In this study, we investigate the effects of micro vortex generators (VGs) installed close to the leading edge of a quasi-two-dimensional NACA0015 hydrofoil under cavitating and non-cavitating conditions. Our aim is to improve physical insight into interaction mechanisms of the boundary layer with the formation and stability of partial cavities. Under non-cavitating conditions, the proposed micro VGs effectively suppress laminar separation. However, under cavitating conditions, even very small micro VGs within the boundary layer promote the formation of counter-rotating cavitating vortices. In comparison with the smooth hydrofoil surface (without micro VGs), the cavitation onset is shifted toward the leading edge. Additionally, classical “fingering structures” and Tollmien–Schlichting waves are no longer present. Since the onset of the cavity does no longer appear at (or close to) the laminar separation line, a novel onset mechanism is observed experimentally. The mechanism consists of stable vortex cavitation, followed by vortex break-down into bubbly structures that are finally accumulated into an attached cavity region. By reduction in the height of the micro VGs, a delayed vortex break-down is found, leading to an increase in the length of the cavitating vortex pattern. This allows for enhanced control on the cavity dynamics, especially with respect to the penetration depth of the re-entrant jet. As a result of our investigation, we conclude that well suited micro VGs show a high potential to manipulate and control cavity dynamics.