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EXPERIMENTS ON BUOYANT PLUME DISPERSION IN A LABORATORY CONVENTION TANK
Citation:
Weil, J. C., W H. Snyder, R E. Lawson Jr., AND M. S. Shipman. EXPERIMENTS ON BUOYANT PLUME DISPERSION IN A LABORATORY CONVENTION TANK. BOUNDARY-LAYER METEOROLOGY 102(3):367-414, (2002).
Impact/Purpose:
This task objective is the development and improvement of state-of-the-science meteorology models and contributing process parameterizations for use in advanced air quality simulation model systems such as the Community Multi-scale Air Quality (CMAQ) modeling system and for other modeling studies and situations involving transport and dispersion of pollutants. Components of this work include: (a) improved meteorological and transport modeling, (b) improved meteorological modeling physics, (c) physical modeling of flows- building wakes, complex terrain, urban canyons, (d) modeling of transport and dispersion of specialized situations and (e) develop AERMOD (AMS/EPA Regulatory MODel).
Description:
Buoyant plume dispersion in the convective boundary layer (CBL) is investigated experimentally in a laboratory convection tank. The focus is on highly-buoyant plumes that loft near the CBL capping inversion and resist downward mixing. Highly- buoyant plumes are those with dimensionless buoyancy fluxes F* >/~ 0.1, , where F* = Fb/(Uw2/*zi), Fb is the source buoyancy flux, U is the mean wind speed, w* is the convective velocity scale, and zi is the CBL depth. The aim is to obtain statistically-reliable mean (C) and root-mean-square (rms, sigma c) concentration fields and plume characteristics as a function of F* and the dimensionless distance
X = w*x/( U zi), where x is the distance downstream of the source.
The experiments show the following. I) For 3 - X - 4 and F* >/- 0.1 , the crosswind-integrated concentration (CWIC) fields exhibit distinctly uniform profiles below zi with a CWIC maximum aloft, in contrast to the nonuniform profiles obtained earlier by Willis and Deardorff (1987). 2) The lateral dispersion (sigma y) variation with X is consistent with Taylor's (1921) theory for F* - 0.1 and a buoyancy-enhanced dispersion, sigma y/zi oc F*1/3 X 2.3, for F* = 0.2 and 0.4. 3) The entrapment-plume fraction above zi - has a mean value (E) that follows a systematic variation with X and F*, and a variability (sigma e/E) that is broad (~0.2 to 2) near the source but subsides to ~2.5 far downstream. 4) Vertical profiles of the concentration fluctuation intensity (sigma c/C) are uniform for z
These are the first experiments to obtain all components of the lateral and vertical dispersion parameters (rms meander, relative dispersion, total dispersion) for continuous releases in a convective tank. They also are the first tank experiments to demonstrate agreement with field observations of: 1) the scaled ground-level concentration along the plume centerline, and 2) the lateral dispersion sigma y/zi of buoyant plumes.