Vertically distributed wall sources of buoyancy. Part 1. Unconfined

Abstract

We examine a turbulent distributed wall-source plume: the flow resulting from a uniform vertical wall source of buoyancy such as that produced by an evenly heated or cooled vertical wall. The vertically distributed buoyancy source is created by forcing dense salt water solution through a porous wall. Velocity measurements on a vertical plane normal to the wall are first presented examining the full height of the wall in order to identify the region in which the bulk flow has become fully turbulent, self-similar and reached an invariant balance between the fluxes of volume, momentum and buoyancy. Simultaneous velocity and buoyancy field measurements are then presented in this region and an entrainment coefficient of α = 0.068 ± 0.006 is determined. This value is small compared to that of buoyancy-driven unbounded flows, e.g. a free line plume, and we reason this to be due to the presence of a rigid boundary restricting meandering and turbulence production, rather than the effect of the vertically distributed source of buoyancy. Turbulent velocity and buoyancy statistics are presented and, in order to gain physical insights into the flow behaviour, the results are compared to those of other canonical buoyancy-driven free and wall-bounded flows. We show that the bulk mixing of distributed wall-source plumes can be captured by consideration of the characteristic vertical velocities and a constant entrainment coefficient. This mixing is inhibited both by the presence of a rigid boundary and the reduced characteristic velocities (compared to those of wall line plumes).