Entrainment in pulsing plumes

Turbulent axisymmetric lazy plumes produced by discharging saline fluid downwards into a less dense uniform environment from a round pipe are examined experimentally. The plume development is controlled by the source flow rate $$Q_0$$ , momentum $$M_0$$ and buoyancy $$F_0$$ . This study investigated plumes where the source flow rate, momentum and buoyancy 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 $$\mathcal {O}(U_0/D)$$ , where $$D/U_0$$ is the eddy turnover time scale at the source, D is the source diameter, and $$U_0$$ is the average velocity at the source. Experiments with pulsing plumes were carried out with a flow rate amplitude A of up to $$80\%$$ and with Strouhal number $$St=fD/U_0$$ ranging from 0.012 to 1.2. The bulk dilution and entrainment measurements were made using the experimental approach of Hunt and Kaye (J Fluid Mech 435:377–396, 2001). Average entrainment is obtained via the integral relationship for Q(z) and M(z) from the model established by Morton et al. (Proc R Soc Lond A 234(1196):1–23, 1956) for continuous sources. The influence of the amplitude and Strouhal number St on the entrainment coefficient $$\alpha$$ is examined, which was found to be very small over the entire range of source conditions considered.