Helium plumes at moderate Reynolds number

We present an experimental investigation of the turbulent entrainment in non-Boussinesq steady plumes, focusing on three helium releases issued from an axisymmetric source, and with increasing Reynolds number. Two-dimensional instantaneous velocity fields, measured using particle image velocimetry (PIV), are exploited to compute first- and second-order velocity statistics focusing on the near field, i.e., up to a distance of a few tens of source radii. Flow visualizations and velocity statistics profiles are investigated to describe the flow transition from a quasilaminar zone, governed by Rayleigh-Taylor instability, to a more distinctly turbulent region. The vertical evolution of the integral fluxes, the Richardson number, and the entrainment coefficient are presented enlightening the influence of an increasing Reynolds number at the source. The plume characteristic density is reconstructed based on the assumption, sustained by recent literature data, of a null-divergence flow. Making use of the entrainment decomposition, we investigate the contribution of the different physical mechanisms involved in the mixing process at varying distances from the source, with a focus on the effect of the local density ratio. Our results show that the variations of the entrainment coefficient in the turbulent region are primarily affected by the near-field generation of turbulent kinetic energy and by a rising contribution of buoyancy effects. Both features do not exhibit a clear dependence on local variations of the density ratio.