Local transport of passive scalar released from a point source in a turbulent boundary layer

• Published in: Journal of fluid mechanics 2018-07, Vol.846, p.292-317
• Main Authors: Talluru, K. M., Philip, J., Chauhan, K. A.
• Format: Article
• Language: English
• Subjects: Atmosphere; Boundary layer; Boundary layers; Computational fluid dynamics; Correlation coefficient; Correlation coefficients; Fluctuations; Fluid mechanics; Influence; JFM Papers; Meandering; Momentum; Probability density functions; Probability theory; Regions; Reynolds number; Shear stress; Spectrum analysis; Studies; Transport; Turbulent boundary layer; Velocity; Water pollution; Width
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2018.280

Simultaneous measurements of streamwise velocity ( $\tilde{U}$ ) and concentration ( $\tilde{C}$ ) for a horizontal plume released at eight different vertical locations within a turbulent boundary layer are discussed in this paper. These are supplemented by limited simultaneous three-component velocity and concentration measurements. Results of the integral time scale ( $\unicode[STIX]{x1D70F}_{c}$ ) of concentration fluctuations across the width of the plume are presented here for the first time. It is found that $\unicode[STIX]{x1D70F}_{c}$ has two distinct peaks: one closer to the plume centreline and the other at a vertical distance of plume half-width above the centreline. The time-averaged streamwise concentration flux is found to be positive and negative, respectively, below and above the plume centreline. This behaviour is a resultant of wall-normal velocity fluctuations ( $w$ ) and Reynolds shear stress ( $\overline{uw}$ ). Confirmation of these observations is found in the results of joint probability density functions of $u$ (streamwise velocity fluctuations) and $\tilde{C}$ as well as that of $w$ and $\tilde{C}$ . Results of cross-correlation coefficient show that high- and low-momentum regions have a distinctive role in the transport of passive scalar. Above the plume centreline, low-speed structures have a lead over the meandering plume, while high-momentum regions are seen to lag behind the plume below its centreline. Further examination of the phase relationship between time-varying $u$ and $c$ (concentration fluctuations) via cross-spectrum analysis is consistent with this observation. Based on these observations, a phenomenological model is presented for the relative arrangement of a passive scalar plume with respect to large-scale velocity structures in the flow.