National Aeronautics and Space Administration


Date of this Version



Aerosol Science and Technology 2:543-544 (1983)


U.S. Government Work


The preferential deposition of material on the windward side of a bluff aerodynamic object is sometimes taken as evidence that the dominant mechanism of mass transport is the inertial impaction of large particles1 rather than the convective difSuon of vapors or small particles (e.g., Fuchs, 1964). However, although the latter mechanism is able to deliver material efficiently to the leeward portion of a bluff object (through the turbulence and recirculation), it is also true that below a particular Reynolds number (~105 for a cylinder in crossflow) even convective diffusion delivers more than half of the material to the windward portion of a cylinder. To quantify this we have reexamined the available experimental data for the angular distribution of the dimensionless (Nusselt) transfer coefficient2, Nu(θ; Re), and calculated the Re dependence of the windward surface fraction (the fraction of the total mass collected for - 90˚<19 <90˚) over the Re range 2 X 101 < Re < 4 X l105. The results (Figure I), obtained from polar integrations of data from eight sources reveal that, first, for Reynolds numbers up to about 2 x 103 more than 70% of the total transfer occurs on the windward surface and, second, only at Re values above about 105 does the windward percentage fall beneath 50%. Thus, unless the windward fraction of the total transport to a circular cylinder in crossflow is significantly larger than, say, 72% or the Reynolds number is much larger than 105, preferential windward surface mass transfer is not sufficient evidence of the dominance of inertial capture mechanisms over diffusional ones.