Description
This study 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. The measurements were carried out in the wave channel at the laboratory of the Department of Civil and Environmental Engineering – Water Engineering Division of Bari Technical University (Italy). Velocity jet components were measured with a backscatter, two-component four-beam LDA system. For the cases of jet with waves, ensemble-averaged velocities were obtained by phase-averaging the measured signals separated by the wave period over about five hundred 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) For cross sections farther from the nozzle, the experimental values of turbulent Reynolds shear stresses indicate that, for configurations with the presence of wave motion, an inversion occurs of the sign compared to the jet issued in quiescent ambient. Wave Reynolds shear stresses diagrams are similar to that of turbulent Reynolds shear stresses of jets in still water, but their magnitude is smaller. 5) The analysis of the wave Reynolds normal and 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.
The movies show the jet issued in still water and the same jet issued in a wave field with a period T=2 s.
