Measurements of buoyant plumes in a turbulent boundary layer

This study presents a novel experimental setup designed to release a buoyant scalar plume and measure concentration and velocity simultaneously. Tracer gas is released from a point source in a low-Reynolds-number boundary layer (\(Re_\tau \approx 1600\)). The buoyant plume released consists of a mixture of varying proportions of tracer gas and a chemically stable base gas, resulting in density ratios relative to ambient air (\(\rho _s/\rho _\infty\)) of 1.48, 1, and 0.15. The concentration of the tracer gas (iso-butylene) is detected using a photo-ionisation detector (PID) that ionises a small volume of the tracer gas within its chamber. Additionally, an \(\times\)-wire is employed to measure the streamwise and vertical components of velocity. Results of the mean and root-mean-square (RMS) concentration profiles for the positively and negatively buoyant plumes exhibit a Gaussian or reflected-Gaussian behaviour similar to a neutral plume, albeit with altered parameters such as the plume centreline that now vary with \(\rho _s/\rho _\infty\). The data indicate that the half-width of the positively buoyant plume is wider than that of the neutral plume, and the spread of negatively buoyant plumes is thinner. Consequently, the maximum concentration of the negatively buoyant plume is the largest among the three \(\rho _s/\rho _\infty\). Although power laws are fitted to describe the downstream evolution of plume spread and maximum mean and RMS values of concentration, the accuracy of the fit appears to be limited.