Evolution of a turbulent jet subjected to volumetric heating

Abstract

The goal of this study is to understand the effect of latent heat release on entrainment in cumulus clouds by employing a laboratory analogue consisting of a volumetrically heated turbulent axisymmetric jet. The jet fluid is volumetrically heated in an off-source manner to simulate condensation heat release in clouds. The experimental set-up is similar to Bhat & Narasimha (1996), and the current application of wholefield velocimetry and thermometry has allowed us to probe in detail the velocity and temperature fields within the heat injection zone (HIZ) for the first time, leading to several new results. We are able to demarcate three distinct zones within the HIZ based primarily on the nature of the cross-stream velocity profile, and we present sharp differences in flow properties in these zones. Thermochromic liquid crystal-based temperature visualizations have revealed details about the complex interplay of velocity, local concentration and temperature leading to a physically coherent understanding of this flow. We also provide evidence using linear stochastic estimates (LSE) to show that large eddies are disrupted in the latter part of the HIZ; the disruption of large eddies is linked to the change in the nature of the cross-stream velocity profile. While our results have confirmed certain previously reported observations such as a reduction in scalar width, we have measured significantly larger r.m.s. values within the HIZ than previously reported, which is corroborated by direct numerical simulation results. We focus on the bulge in the scalar and velocity width at the start of the HIZ and link it to the excess deceleration of the centreline velocity there. Ideas proposed by Tso & Hussain (1989) are used to explain the eventual reduction of jet width with buoyancy addition. We also employ LSE to show that eddies are laterally compressed in ordinary jets, and that the surviving ones become more circular with heat addition in accordance with Lumley's (1971) hypothesis. Our primary conclusion differs from Bhat & Narasimha (1996) in that we measure a mass flux that exceeds that of an unheated jet throughout the HIZ. Further, we show that a step-increase in momentum flux corresponds to a step-increase in mass flux.