This paper describes the effects of intermediate and large freestream levels of turbulence (intensity and integral length scale) on the dispersion of fine particles in the near wake regions of long flat plates placed normal to the freestream. The work is particularly relevant to the transport of airborne contaminants in complex flows, especially like those encountered in many ambient and occupational atmospheric environments. An experiment was designed based on the release of fine aerosol particles upstream of the obstacle and the measurement of the concentration decay for the entrained aerosol in the near wake region after the source generation has been suddenly interrupted. Five grids were employed for generating specific levels of freestream turbulence characterized by the parameter, Λ(f) (defined as l(f)k(1/2)(f)/UD, where l(f) and k(f) are the length scale and the kinetic energy of the free stream turbulence, and U and D are the freestream velocity and the plate width, respectively). The investigation covered the ranges 1500≤Re≤11,000 and 0.0015≤Λ(f)≤0.071. The dimensionless residence time, H (defined as τU/D, where τ is the time constant for the observed decay of the aerosol concentration in the near wake cavity), appeared to be independent of the freestream turbulence for values of Λ(f) between 0.0015 and 0.013, for which the average H-value was about 7.2±1.5. For larger freestream turbulence, corresponding to Λ(f) greater than 0.018, H increased up to about 10. A visualization procedure was used to capture some of the qualitative features of the dispersion process for aerosols in the near wake regions of the plates and provide some support for the physical mechanisms suggested by this work. In particular, by comparing the results with those of other studies, we note the marked contrast in the effects of the freestream turbulence for aerosol flows about two- and three-dimensional bodies, respectively. Copyright (C) 1999 Elsevier Science B.V.
- Aerosol dispersion
- Bluff bodies