  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.
