The internal structure of forced fountains

TL;DR

This paper investigates the internal mixing processes within forced fountains through direct numerical simulation, revealing complex turbulent interfaces and highlighting the importance of turbulent fluxes over mean flow in entrainment.

Contribution

It identifies and characterizes internal turbulent boundaries in forced fountains, emphasizing the distinct roles of turbulence and mean flow in entrainment processes.

Findings

Entrainment by turbulence is as significant as by mean flow.

Two internal turbulent interfaces are identified: the classical boundary and the separatrix.

Turbulent fluxes cannot be modeled using mean flow assumptions.

Abstract

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: 1) the classical boundary between upflow and downflow which is composed of the loci of points of zero mean vertical velocity; and 2) 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.