Behavior of Nonbuoyant Jets in a Wave Environment

Abstract

This paper presents experimental results of a turbulent neutrally buoyant jet vertically discharged in a stagnant ambient and of the same jet discharged in a flow field of regular waves, in the intermediate range between deep and shallow water. A number of such studies have been performed on this configuration in the past, but mostly limited to concentration (dilution) measurements. Thus one could argue the need for a detailed analysis of this problem with modern instrumentation. This paper will address the deficiency and will present the results of a new experimental study into wave-induced mixing. Velocity jet components were measured with a backscatter, two-component four-beam laser Doppler anemometer system. For the cases of jets with waves, ensemble-averaged velocities were obtained by phase averaging the measured signals separated by the wave period over about 500 waves. The main results of the present study indicate the following: (1) the entrained flow is higher in the case of jet with waves; (2) oscillating velocity components cannot be described by classic wave motion theories; (3) comparison of the root-mean square of turbulent velocity components indicates the effect of wave presence; (4) in the case of jet-wave interaction, the reduction of the jet momentum shown in literature can be explained by the presence of the turbulent Reynolds normal stresses (which are not as small as in the case of stagnant ambient and, consequently, cannot be ignored) and the wave Reynolds normal stresses; and (5) the analysis of the wave Reynolds shear stresses shows that it is not always right to apply a wave motion theory to obtain the statistical contribution of the wave field; a conclusion particularly interesting in order to enable the improvement of mathematical models present in literature.