The effect of freestream turbulence on the transport of particles in the vicinity of a blunt flow obstacle

This paper describes the effects of intermediate and large freestream level s of turbulence (intensity and integral length scale) on the dispersion of fine particles in the near wake regions of long flat plates placed normal t o the freestream. The work is particularly relevant to the transport of air borne contaminants in complex flows, especially like those encountered in m any ambient and occupational atmospheric environments. An experiment was de signed based on the release of fine aerosol particles upstream of the obsta cle and the measurement of the concentration decay for the entrained aeroso l in the near wake region after the source generation has been suddenly int errupted. Five grids were employed for generating specific levels of freest ream turbulence characterized by the parameter, Lambda(f) (defined as l(f)k (f)(1/2)/UD, where l(f) and k(f) are the length scale and the kinetic energ y of the free stream turbulence, and U and D are the freestream velocity an d the plate width, respectively). The investigation covered the ranges 1500 less than or equal to Re less than or equal to 11,000 and 0.0015 less than or equal to Lambda(f)less than or equal to 0.071. The dimensionless reside nce time, H (defined as tau U/D, where tau is the time constant for the obs erved decay of the aerosol concentration in the near wake cavity), appeared to be independent of the freestream turbulence for values of Lambda(f) bet ween 0.0015 and 0.013, for which the average H-value was about 7.2 +/- 1.5. For larger freestream turbulence, corresponding to Lambda(f) greater than 0.018, H increased up to about 10. A visualization procedure was used to ca pture some of the qualitative features of the dispersion process for aeroso ls 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 ef fects of the freestream turbulence for aerosol flows about two- and three-d imensional bodies, respectively. (C) 1999 Elsevier Science Ltd. All rights reserved.