Scale Effects on Various Types of Limited Cavitation

Abstract

Scale effects on limited cavitation discussed in this paper are those departures from the classical similarity relations due to variations in size and velocity. These scale effects are an important consideration in the prediction of limited cavitation in a design process. In most cases, the limited cavitation number of a model is different from that of the prototype. Some recently published results have shown that viscous effects and free-stream nuclei can cause significant scale effects. The purpose of this paper is two-fold. First, new experimental data are presented which show scale effects for several types of cavitation. Second, these data are discussed and examined for possible rational explanations. Limited cavitation data are presented and discussed for axisymmetric headforms and three families of hydrofoils. For the hydrofoils, measurement of surface cavitation for both sheet and bubble types in addition to tip-vortex cavitation were obtained over a range of Reynolds numbers. Each family of hydrofoils is geometrically similar in planform and cross section and has chord lengths of 38.1, 76.2, 152.4 and 304.8 millimeters. The cross sections of the three hydrofoil families are: (1) a modified Joukowski hydrofoil section with a maximum thickness of 10.6 percent. (2) a NACA 0015 hydrofoil section and (3) a cambered NACA 0010 section with a maximum camber of 10 percent. These hydrofoil families were tested in the 1.22 m water tunnel at ARL/PSU. It is indicated that scale effects are due to changes in the pressure field (type #1) and to bubble dynamic and nuclei effects (type #2). The various types of cavitation are examined to determine the type of scale effects which appear to be controlling the cavitation process. Approximate analyses are presented for bubble-ring, band, tip-vortex, and fixed-patch or sheet cavitation. The latter appear to be controlled by surface roughness effects.