An Experimental Investigation of the Effect of Drag Reducing Polymer Additives on Surface Pressure Fluctuations on Bodies of Revolution with Rough Surfaces Moving through Water

Abstract

A sound detector in the surface of a moving body receives not only sound signals radiated from a distant source but also detects pressure fluctuations originating in the turbulent boundary layer of the fluid surrounding the body. The purpose of the present work was to assess the magnitude of the surface pressure fluctuations on a body moving in water and in water with polymer additive under nearly zero pressure gradient conditions. Measurements were made using a single transducer in the surface of an axi-symmetric body. Both smooth and grit-roughened surfaces were used. Mean square pressure fluctuation amplitudes were measured as a function of frequency, non-dimensionalized, plotted, and compared with some results obtained by others in both water and air. It was concluded that the addition of roughness to a smooth surface increases the amplitude at the peak of the spectrum and at all lower frequencies. Polymer additive in the water has just the opposite effect on a rough-surfaced body, decreasing the amplitude at the peak and at all lower frequencies, the reduction increasing monotonically with drag reduction. There was little or no effect at high frequencies attributable to either roughness or polymer additive, but it must be noted that the transducer used was too large to obtain a true measure of amplitude at the highest frequencies. The peak of the spectrum in water appears to have a somewhat higher amplitude than it does in air.