An experimental study of large area source turbulent plumes

A series of experiments was performed to measure the rate of entrainment of ambient fluid into a negatively buoyant turbulent plume emerging from a large area source with low initial momentum flux, a so called ‘lazy plume’. Immediately below the source, the plume contracts until it reaches a neck of diameter approximately half the source diameter. Beyond the neck it expands, eventually forming a classic pure plume with linear radial growth rate. The variation of volume flow rate, Q, with vertical distance from the source, z, was measured in the near source region, between the source and the neck for different source values of Γ0, equivalent to the source Richardson number. Experiments were run for 105 < Γ0 < 5 × 107. Our results indicate that the volume flux in the plume increases linearly from the source to the neck, that is, Q ∝ z. This scaling is in contrast to pure plumes where the flow rate increases as Q ∝ z5/3. Our experimental results collapsed onto a single line when scaled, giving an empirical expression for the near source flow rate of the form Q∝Γ01/3z. We discuss this result in relation to the classic entrainment model for turbulent plumes and show that modifications to existing closure schemes are required for lazy plumes.