Transitional High Prandtl Number Plumes with an Isothermal Source

Abstract

Plume flows in the transitional range, rising from an iso-thermal source, are numerically investigated for a fluid having a Prandtl number (Pr) of 7.0 to reveal the conditions under which an intermittent puffing behaviour takes place. Simulations have been conducted with varying Reynolds number (Re), planar and axisymmetric configurations, no-slip (all the velocity components are zero) and free-slip (Neumann conditions with zero derivative for tangential velocities and zero normal velocity) conditions at the bottom boundary. It has been found that the intermittent puffing behaviour only occurs in planar plumes. More noteworthy is that with the use of the free-slip condition at the bottom boundary, the intermittent puffing behaviour does not take place. Non-dimensional puffing frequencies have been obtained, and it has been found that the Re variation in the frequencies is small. The mean momentum and buoyancy balance equation terms have also been plotted, and similar trends to those for a turbulent plume have been observed.