Application of laser Doppler velocimetry (LDV) to study the influence of heat transfer on the structure of gravity currents

Abstract

Gravity currents are important physical phenomena which have direct implications in a wide range of physical situations including geophysical processes, air conditioning, and building fires where they are responsible for the transport of smoke and hot gases, particularly, along long corridors. Despite recent progress in the field, relatively little is known about the structure of gravity currents under conditions pertinent to building fires. In particular, the impact of heat transfer at boundaries is not well understood. The present investigation is an attempt to address this shortcoming by studying the turbulent structure of gravity currents under both adiabatic and isothermal boundary conditions. For this purpose, a series of experiments was conducted in a rectangular tank with turbulent underflows. Laser Doppler velocimetry was employed to quantify the velocity field and associated turbulence parameters. Experimental results indicated that the mean flow within the head region primarily consisted of an undiluted large single vortex which rapidly mixed with the ambient flow in the wake region. Flows with isothermal wall boundary conditions showed three-dimensional effects whereas those with adiabatic walls exhibited two-dimensional behaviour. Turbulence was found to be highly inhomogeneous and its distribution was governed by the location of large eddies. While all components of turbulence kinetic energy showed minima in the regions where velocity was maximum (i.e. low fluid shear), they reached their maximum in the shear layer at the upper boundary of the flow.