Large eddy simulation of diffusion of a buoyancy source in ambient water

The movement of a lump of buoyancy in an ambient fluid is called a thermal. Previous experiments show that thermals display a significant degree of variability. This is because there is always an uncertainty of the discharge condition and also the fluid motion induced by thermals is turbulent. Each case of discharge can be considered as a single realization of the phenomenon. This paper presents numerical simulation of thermals in a water-tank. Emphasis is on the reproduction of the variability as well as the mean flow characteristics of the phenomenon. In the simulation, a three-dimensional large eddy simulation (LES) numerical model has been developed. In the model, the governing equations are split into three parts in the finite difference solution: Advection, dispersion and pressure propagation. The advection part is solved by the QUICKEST scheme. The dispersion part is solved by the central difference method and the pressure propagation part is solved implicitly by using the Gauss-Seidel iteration method. It is found that the LES model can capture the transient characteristics of the turbulent flow. The uncertainty of the initial falling regime of the thermal observed in an experiment has been accounted for in the numerical simulation by introducing a random component to the initial conditions of the buoyancy source to trigger the generation of turbulence. The results of numerical simulation are in good agreement with the experiments. The results show that: (1) Thermal has an appearance of characteristic protuberances. (2) The rear of a thermal is hollow. (3) Variability exists among different realizations of a thermal.