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Local transport of passive scalar released from a point source in a turbulent boundary layer
Published online by Cambridge University Press: 04 May 2018
Abstract
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.
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