Three-dimensional modelling of vertical jets in random waves

Abstract

This thesis describes an investigation of flow behaviour of a buoyant or non-buoyant jet discharging vertically into random waves. In the part of experimental study, a round jet nozzle was vertically arranged at the bottom of a random wave flume. The tap water was adopted as the jet fluid for non-buoyant jet while the mixture of ethanol and water was adopted for buoyant jet to generate initial buoyancy. Velocities, concentrations and surface elevations were measured for a number of flow cases. The measured data showed that the centreline velocities of non-buoyant or buoyant jet in random waves decays faster than those in stagnant ambience, corresponding to a significant increase of jet dilution. The jet half-width increases with incident wave period and wave height. Due to the wave effect, the location of peak turbulence shifts towards the jet inlet. The extent of location shift is related to wave period and wave height, which indicates that the jet turbulence is generated by both wave and jet motions. In order to convert those findings into more general forms, two characteristic length scales, i.e., wave-momentum length scale lm and wave-momentum-buoyancy length scale lw were introduced. Several empirical formulas for prediction of jet characteristic variables, such as centreline velocity, dilution and half-width, were derived based on the method of dimensional analysis. In the part of numerical study, a fully nonlinear numerical wave tank (NWT) was first developed for modelling of random wave situations. A novel Langrage-Euler method was introduced to solve the free surface problem. The NWT with the new free surface treatment was verified by several test cases with theoretical solutions or experimental data. A non-buoyant or buoyant jet was then added to the NWT domain and the flow field was computed with turbulence modelled by the large eddy simulation (LES) method. Velocities and concentrations obtained from numerical computation were in fairly good agreement with those from laboratory experiments. As an application of the numerical model, the effect of buoyancy on vertical jets in random waves was investigated through numerical comparison between a non-buoyant and a buoyant jet with a same discharge velocity. It was found that under the buoyancy force the wave effect on jet entraining and mixing is weakened to some extent, corresponding to a slower decay rate of velocity and a narrower jet width in the buoyant jet than in the case for the non-buoyant jet.