University of Manitoba
Winnipeg, Manitoba, Canada
Laboratory experiments of a thermal impinging on a stratified interface are presented. Pure water, representing the thermal, is released from the bottom of a tank filled with two different density fluids. The entrainment of upper layer fluid into the thermal is investigated using flow visualization. These experiments will help further test the importance of the persistence parameter (Cotel and Breidenthal 1997). The entrainment rate of a thermal is predicted to be similar to that of a vortex ring, even though the forcing is different, i.e. momentum for the vortex ring and buoyancy for the thermal.
The entrainment rate is measured and found to obey a Ri$^{3/2}$ power law. This result has also been obtained for stirring grid turbulence (Turner 1973), which can be modeled using a single vortex ring (Linden 1973). In both cases, the persistence parameter has a value of unity and is believed to be the dominant parameter in determining stratified entrainment regimes.
References:
[1] Cotel and Breidenthal, A model of Stratified Entrainment Using Vortex Persistence, Applied Scientific Research 57: 349-366
[2] Turner, Buoyancy effects in fluids. Cambridge University Press, 1973.
[3] Liden, The ineraction of a voertex ring with a sharp density interface: a model for turbulent entrainment. JFM 60: 467-480.