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.