Buoyant point-source scalar plumes in a turbulent boundary layer

Abstract

This thesis investigates the dispersion of buoyant plumes in a turbulent boundary layer. While previous studies have primarily focused on heavy or light plumes individually, a systematic comparison of heavy, neutral, and light plumes has not been conducted to the author's knowledge.

To address this research gap, a novel experimental setup is developed to release gases of varying densities in a boundary layer wind tunnel, utilising iso-butylene as a tracer. By changing the density of tracer gas systematically, it is possible to investigate the dispersion of neutral, light and heavy plumes. Further, the wind tunnel floor is altered to emulate a flow in an urban setting. Downstream of the point source, the streamwise and vertical velocity components are measured using an $\times$-wire anemometer, while the concentration of the tracer gas is measured using a Photo-Ionisation Detector.

It is observed that the mean plumes of heavy and light gases can be described by a Gaussian model, akin to those observed in a neutral plume. The wall-normal spread of heavy plumes is smaller, whereas the light plumes are wider. The behaviour of higher-order central moments (up to the 6th moment) is found to possess Gaussian characteristics as well. An engineering model for vertical scalar fluxes is proposed here that describes the scaling of these fluxes, offering an alternative to the common gradient-diffusion model.

The spread of plumes in a rough wall boundary layer is also investigated. The impact on plume characteristics by density is also observed in the rough-wall case, although the differences are less pronounced compared to the smooth-wall case. In the outer region of the roughness, the plume profile can be described using a truncated-Gaussian model. In contrast, no universal equation describes the mean concentration profile within the roughness canopy.