Simultaneous measurements of the velocity and scalar concentration fields have been
made in the plume emitting from a two-dimensional line source at the wall. The source
is one obstacle height, h, downstream of a two-dimensional square obstacle located on
the wall of a turbulent boundary layer. These measurements were made in two fluid
media: water and air. In both media particle image velocimetry (PIV) was used for the
velocity field measurements. For the scalar concentration measurements laser-induced
uorescence (LIF) was used for the water flow and Mie scattering diffusion (MSD) for
the air flow. Profiles of the mean and root-mean-square streamwise and wall-normal
velocity components, Reynolds shear stress and mean and root-mean-square scalar
concentration were determined at x = 4h and 6h downstream of the obstacle in the
recirculation region and above it in the mixing region. At these streamwise stations
the scalar fluxes, uc and vc, were also
determined from the simultaneous velocity and
scalar concentration field data. Both of these fluxes change sign from negative to
positive with increasing distance from the wall in the recirculating region at 4h.A conditional analysis of the data was carried out by sorting them into the eight
categories (octants) given by the sign combinations of the three variables:
±u, ±v and ±c. The octants with
combinations of these three variables that correspond to types
of scalar concentration flux motions that can be approximated by mean gradient
scalar transport models are the octants that make the dominant contributions to uc
and vc. However, in the recirculating zone, counter-gradient transport type motions
also make significant contributions. Based on this conditional analysis, second-order
mean gradient models of the scalar and the momentum uxes were constructed; they
compare well to the measured values at 4h and 6h, particularly for the streamwise
scalar flux, uc.Additional measurements of the velocity and concentration fields were made further
downstream of the reattachment location in the wake region of the air flow. The mean
velocity deficit profile determined from these measurements at x = 20h compares quite
well to a similarity solution profile obtained by Counihan, Hunt & Jackson (1974).
Their analysis was extended in the present investigation to the concentration field.
The similarity solution obtained for the mean concentration compares well to profiles
measured at x = 12h, 15h, and 20h, up to about three obstacle heights above the wall.
J056021 Turbulence Characteristics of the Concentration Field Dispersed from a Point Source in a Turbulent Boundary Layer
