No abstract

This paper describes a study of the response of a recently developed low-drag partially cavitating hydrofoil (denoted as OK-2003) to periodical perturbations of incoming flow. A two-flap assembly specially designed to simulate sea wave impact on the cavitating hydrofoil generates the perturbations. The design range of cavitation number was maintained by ventilation. Unsteady flow can be simulated over a range of ratios of gust flow wavelength to cavity length. The measurement of time-average lift and drag coefficients and their fluctuating values over a range of inflow characteristics allows a determination of hydrofoil performance over a range of conditions that could be expected for a prototype hydrofoil. Both regular interaction with practically linear perturbations and resonancelike singular interaction with substantial nonlinear effects were noted. The observations are accompanied by a numerical analysis that identifies resonance phenomena as a function of excitation frequency.

An experimental study of air supply to bottom cavities stabilized within a recess under a horizontal surface has been carried out in a specially designed water tunnel. The air supply necessary for creating and maintaining an air cavity in steady and gust flows has been determined over a wide range of speed. Flux-free ventilated cavitation at low flow speeds has been observed. Stable multiwave cavity forms at subcritical values of Froude number were also observed. It was found that the cross-sectional area of the air supply ducting has a substantial effect on the air demand. Air supply scaling laws were deduced and verified with the experimental data obtained.

Partial cavitation reduces hydrofoil friction, but a drag penalty associated with unsteady cavity dynamics usually occurs. With the aid of inviscid theory a design procedure is developed to suppress cavity oscillations. It is demonstrated that it is possible to suppress these oscillations in some range of lift coefficient and cavitation number. A candidate hydrofoil, denoted as OK-2003, was designed by modification of the suction side of a conventional NACA-0015 hydrofoil to provide stable drag reduction by partial cavitation. Validation of the design concept with water tunnel experiments has shown that the partial cavitation on the suction side of the hydrofoil OK-2003 does lead to drag reduction and a significant increase in the lift to drag ratio within a certain range of cavitation number and within a three-degree range of angle of attack. Within this operating regime, fluctuations of lift and drag decrease down to levels inherent to cavitation-free flow. The favorable characteristics of the OK-2003 are compared with the characteristics of the NACA-0015 under cavitating conditions.

No abstract

Profiles of velocity and pressure for the vortex core in turbulent flow were obtained by solving Reynolds equation for the circumferential component of the fluid momentum. The viscous core radius is defined as a function of viscosity coefficient, vortex intensity, and a Reynolds stress component. The obtained velocity profiles are in much better agreement with known experimental data than are the Rankin vortex profiles.

No abstract

Cavities behind a surface irregularity appear in vortices drifting downstream of it. Cavitation can occur there substantially earlier than over smooth surfaces of the same bodies. Cavitation inception and desinence behind surface irregularities have been intensively studied in the course of water tunnel experiments several decades ago, but no corresponding quantitative theoretical (numerical) analysis was reported. This numerical study is aimed at elaboration of a general approach to the prediction of cavitation desinence numbers for various irregularities over various surfaces and on determination of the major factors influencing these numbers in both computations and experiments. The developed multi-level computational method employs diverse models for flow zones of diverse scale. The viscous-inviscid interaction approach is used for the flow around irregularities submerged (or partially submerged) in the turbulent boundary layer. Combinations of the semi-empirical and asymptotic analyses are used for vortices and cavities in their cores. The computational method is validated with various known experimental data.