A robust inverse design solver for controlling the potential aggressiveness of cavitating flow on hydrofoil cascades

This article presents the development of a new inverse design algorithm capable of generating blade geometries for cavitating cascade flows. With this methodology, we demonstrate the controllability of the pressure distribution in and around the cavity and thereby provide a means to regulate the aggressiveness of blade cavitation phenomena. The solver proposed here uses the Tohoku–Ebara equation of state to model phase change, combined with bespoke preconditioning and multigrid methods designed to handle the system's ill conditioning and cope with the hypersonic flow regime of the mixture, respectively. Blade geometries and the cavitating flow field are calculated simultaneously in a robust and efficient manner, with a blade loading that matches the target distribution. In this article, the accuracy of the cavitating flow solver is first demonstrated for the NACA0015 hydrofoil case and associated experimental data. The inverse design procedure is then applied to a typical axial flow pump cascade: a new blade profile is generated with a topology that successfully reduces the gradient of the pressure jump at cavity closure

Experimental analysis of shock smoothing design strategy for reducing cavitation erosion aggressiveness

This article presents the experimental analysis of cavitation erosion for two cascade hydrofoil profiles. The aim is to evaluate the change in erosive intensity between a conventional smooth blade surface and one generated by the means of inverse design specifically to reduce cavitation aggressiveness. The applied design strategy consists in imposing a reduced amplitude and gradient at the cavity closure pressure jump in order to bring down the potential energy contained in the vapor sheet. The result is a unique geometry that presents a surface kink located at cavity closure, which successfully smoothes the pressure jump according to computational fluid dynamics (CFD) verification analysis. Here, an experimental rig is constructed and equipped with a pressure sensing system and high-speed imaging to capture the flow field. The measurements for both geometries are first compared against a set of steady-state CFD solutions, which demonstrate the reliability of the inverse design solver for generating targeted flow characteristics in non-cavitating and cavitating conditions. Visual recordings also reveal significant changes in the aspect of the vapor sheet between the two blades indicating a shift in its dynamic behavior. Erosion intensity levels are then measured by paint method at identical conditions. The outcome of the experiment is highly conclusive as a marked reduction in paint erosion is observed for the design geometry. The measured data also serve as a benchmark test for predictive cavitation erosion models by comparing the measured erosion distributions for each blade to those obtained numerically from unsteady CFD

Numerical Study of Sheet Cavitation Break-off Phenomenon on a Cascade Hydrofoil

2-D unsteady cavity flows through hydrofoils in cascade which is the most fundamental element of turbomachinery are numerically calculated. In particular, attention was paid to instability phenomena of the sheet cavity in transient cavitation condition and the mechanism of break-off phenomenon was examined. A TVD MacCormack's scheme employing a locally homogeneous model of compressible gas-liquid two-phase media was applied to analyze above cavity flows. The present method permits us to treat the whole cavitating/noncavitating unsteady flow field. By analyzing numerical results in detail, it became clear that there are at least two mechanisms in the break-off phenomena of sheet cavity; one is that re-entrant jets play a dominant role in such a break-off phenomenon, and the other is that pressure waves propagating inside the cavity bring about an another type of break-off phenomenon accompanied with cavity surface waves