Laser-doppler anemometry in ship hydrodynamics

Abstract

Since the advent of laser-Doppler anemometry (L.D.A.) as a new tool in experimental fluid mechanics much effort has been devoted to developing practical instrumentation for engineering applications. A non-interfering velocity measurement technique which provides high spatial resolution, directional sensitivity and time resolved information can provide measurements in ship hydrodynamics which hitherto were unobtainable. This paper describes an L.D.A., constructed at Southampton University to take axial velocity measurements in a ship-model towing tank. The optical geometry used is versatile and is readily applicable to cavitation tunnel work. An appendix is included which describes the extension of the instrument to measure three orthogonal components of velocity simultaneously. Results from the wake of a ship model showing mean wake fraction and wake spectra are presented. Three dimensional presentation of the wake power as a function of frequency is also included. Measurements taken in the wake of a circular cylinder towed along the tank verify the periodic vortex shedding at a Strouhal number of 0.2 and demonstrate the real-time measurement capability of the instrument. It is hoped that this paper will promote the use of L.D.A. in ship science research. If further progress in the problems of propeller-hull interactions, wake scaling, bilge vortex convection, cavitation and propeller induced vibration is to be made then time resolved velocity information provided by this instrument is required.

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