Effects of Varying Ambient Stratification Strengths on the Dynamics of a Turbulent Buoyant Plume: Fid-Bau Portal

Effects of Varying Ambient Stratification Strengths on the Dynamics of a Turbulent Buoyant Plume

Mirajkar, Harish N. / Balasubramanian, Sridhar

This paper presents the results of laboratory experiments conducted to study the effects of varying ambient stratification strengths, characterized by the buoyancy frequency $N$ , on the dynamics of a buoyant plume propagating in a linearly stratified environment. The buoyancy frequency was varied between $N = 0.37 - 1.17 s^{- 1}$ , while the source velocity was constant at $W_{0} = 26 cm s^{- 1}$ , giving a source bulk Richardson number of ${Ri}_{b} = (π / 4)^{0.25} \sqrt{g^{'} d_{j} / W_{0}^{2}} = 0.09 - 0.31$ . Bulk parameters such as maximum height, $Z_{m}$ , spreading height, $Z_{s}$ , and radial propagation of the plume, $R_{f}$ , were measured to characterize the plume and understand the flow dynamics. Experimentally, it was observed that the maximum height and spreading height of a plume decreases with increasing $N$ , a trend attributed to the reduction in the momentum and buoyancy flux due to the vigorous entrainment between the plume and ambient fluid. The radial propagation of the plume, $R_{f}$ was insensitive to the variations in $N$ and $R_{f} \propto t^{0.75}$ for all values of $N$ . An empirical parameterization involving ${Ri}_{b}$ and $N$ was developed that gives the characteristic time for a plume to reach a steady-state spreading and maximum height. Further, the transient density profiles show that the mixing-layer thickness decreases with increasing $N$ , thereby rendering the entrainment to occur in a shallow layer of fluid. Using empirical relations, it is shown that the entrainment coefficient is larger for higher values of $N$ , thus becoming a function of the Richardson number. The results are expected to be useful in understanding the mixing and convective process in various environmental flows.