The internal structure of forced fountains

We study the mixing processes inside a forced fountain using data from direct numerical simulation. The outer boundary of the fountain with the ambient is a turbulent/non-turbulent interface. Inside the fountain, two internal boundaries, both turbulent/turbulent interfaces, are identified: (i) the classical boundary between upflow and downflow which is composed of the loci of points of zero mean vertical velocity; and (ii) the streamline that separates the mean flow emitted by the source from the entrained fluid from the ambient (the separatrix). We show that entrainment due to turbulent fluxes across the internal boundary is at least as important as that by the mean flow. However, entrainment by the turbulence behaves substantively differently from that by the mean flow and cannot be modelled using the same assumptions. This presents a challenge for existing models of turbulent fountains and other environmental flows that evolve inside turbulent environments.

Automated summary

Using direct numerical simulation, we investigate the mixing processes inside a forced fountain. The fountain has a turbulent/non-turbulent interface with the ambient at the outer boundary. Two turbulent/turbulent internal interfaces are identified within the fountain: (i) the classical boundary between upflow and downflow determined by points of zero mean vertical velocity; and (ii) the streamline separating the mean flow from the entrained fluid emitted by the source and the ambient (the separatrix). Our findings demonstrate that entrainment through turbulent fluxes across the internal boundary is equally important to entrainment by the mean flow. However, entrainment by turbulence behaves differently and cannot be modeled using the same assumptions, posing a challenge for existing models of turbulent fountains and environmental flows inside turbulent environments.