Modeling Cavitation over Axisymmetric Bodies: VOF Technique versus Boundary Element Method H. Moin 1 , I. Rashidi 2 , Mohammad P. Fard 3 , Mahmoud P. Fard 4 and E. Roohi 5 1,2,3,4 Department of Mechanical Engineering, Ferdowsi University of Mashhad Mashhad, Iran 5 Department of Aerospace Engineering, Sharif University of Technology Tehran, Iran Email: mpfard@um.ac.ir ABSTRACT A computational study of super- and partial-cavitation over axisymmetric bodies is presented using two numerical methods. The first method is based on the VOF technique where the transient Navier-Stokes equations are solved along with an equation to track the cavity interface. The second method is that of the steady boundary element method (BEM) which is a model based on the potential flow theory. The supercavitation results of the two methods for disk and cone cavitators are compared with each other and with those of the available experiments and analytical relations. Two different geometries for a cone with various cone angles are considered. Also, the results of comparison between the two methods for partial cavitation over a sphere, a blunt cylinder, and a cylinder with a spherical head are presented. 1. INTRODUCTION The cavitation phenomenon is known as liquid vaporization that occurs whenever the liquid pressure falls bellow its vapor pressure. This phenomenon is categorized by a nondimensional parameter called cavitation number: ) ( ) ( 2 2 1 ∞ ∞ − = V P P v ρ σ (1) where P v is the vapor pressure, ρ the liquid density, and P ∞ , V ∞ are the main flow pressure and velocity, respectively. The cavitation regimes are classified to incipient-, shear-, cloud-, partial- and super-cavitation depending on the cavitation number [1]. The cavitation occurs around axisymmetric bodies at points where the local pressure drops to the environment vapor pressure. Any sudden change in the body shape may cause a pressure rise or fall and, therefore, may be an inception point for cavitation. Although super-cavitation decreases drag forces extensively, but when maneuvering of the vehicle is necessary, partial cavitation is more preferable [2]. Also, partial cavities are widely used in ventilated systems [2, 3]. During the last decades, numerous studies have been performed in cavitation using various methods [1]. Cavitation models based on the Navier-Stokes equations emerged in early 1990’s. These models are divided into two main categories: interface tracking method and homogeneous equilibrium flow [4]. In interface tracking method, a constant pressure (vapor pressure) is assumed for the cavity region and a wake model is used to predict the shape of the cavity in adaptive grids. In the second category, used in this study, a single-fluid modeling approach is employed for both phases. Various models in this category differ in the calculation of the variable density field. In the Volume-of-Fluid (VOF) model, an advection equation for liquid volume fraction is solved and density is obtained based on the volume fraction of the two phases. Yuan et al. [5] suggested a cavitation model based on Rayleigh relation. Singhal et al. [6], Merkle et al. [7] and Kunz et al. [8] have used different mass transfer models based on semi-analytical equations. A well-known method to solve the advection of a free- surface such as a cavity interface is VOF technique. Frobenius and Schilling [9] and Wiesche [10] used this technique to simulate cavitation over hydrofoils and pump impellers. A review of the reported literature reveals that the VOF method can accurately capture cavity shape and characteristics. In this study, a modified VOF technique based on Youngs’ PLIC algorithm [11] is combined with a mass transfer model of Kunz et al. [8] to simulate cavitation.