Experimental and Numerical Investigation of Large Scale Structures in Cavitating Wakes

Abstract

Cavitation is a design consideration for a broad variety of devices handling liquids. In many cases, unstable operation is caused by cavitation-induced flow instabilities. Complex cavitation characteristics are observed in many types of fluid machinery. Examples range from the high-pressure fuel pumps in the Space Shuttle Main Engine to a variety of hydroturbines. In addition there is an increasing interest in very high performance marine vehicles that must operate in the cavitating regime. Associated with the deleterious effects of performance breakdown, noise, and vibration, there is a possibility of erosion. The purpose of this research is to investigate the two-phase flow structure in the wake of a hydrofoil undergoing unsteady partial cavitation using an integrated experimental/numerical approach. This topic provides both numerical and experimental challenges. A two-dimensional NACA 0015 hydrofoil was selected for study, because of its previous use by several investigators around the world. The simulation methodology is based on a Large Eddy Simulation (LES), using a barotropic phase model to couple the continuity and momentum equations. The complementary experiments were carried out at two different scales in two different water tunnels. Tests at the St. Anthony Falls Laboratory (SAFL) were carried out in a 0.19x0.19m2 water tunnel and a geometrically scaled up series of tests was carried out in the 0.3x0.3 m2 water tunnel at the Versuchsanstalt für Wasserbau (VAO) in Obernach, Germany. The tests were designed to complement each other and to capitalize on the special features of each facility. Nomenclature c = chord length f = frequency l = cavity length p ∝ = upstream pressure pv = vapor pressure U ∝ = upstream velocity α = angle of attack ρ = density σ = cavitation number I.