Entrainment in Pulsing Plumes

Abstract

Turbulent axisymmetric lazy plumes produced by discharging saline fluid downwards into a less dense uniform environment from a round pipe are exam- ined experimentally. The plumes development is con- trolled by the source flow rate Q0, momentum M0 and buoyancy F0. This study investigated plumes where the flow rate Q0(t), momentum M0(t) and buoyancy F0(t) are sinusoidal functions of time. The pulsing flow is generated by a programmable ISMATEC gear pump. The maximum frequency f of this pulsing plume is of O(U0/D), where D/U0 is the eddy turnover time scale at the source, D is the source diameter, U0 is the average velocity at the source. Experiments with puls- ing plumes were carried out with a flow rate ampli- tude A of up to 80% and with Strouhal number St =fD/U0 ranging from 0.012 to 1.2. The bulk dilution and entrainment measurements were made by using the experimental approach of Hunt and Kaye (2001). Average local entrainment is obtained via the integral relationship for Q(z) and M(z) from the model estab- lished by Morton et al. (1956) for continuous sources. The influence of the amplitude of source flow rate and Strouhal number St on the entrainment coefficient α is examined, found to be very small over the entire range of source conditions considered.