Rooftop-Mounted Stacks

David Sumner, Ph.D., P.Eng., Professor

Department of Mechanical Engineering, University of Saskatchewan

The flow around small-aspect-ratio rooftop-mounted stacks is influenced by the rooftop boundary layer and the flow over the building and neighbouring structures.  Kármán vortex shedding from stacks, which can become synchronized with vortex structures in the stack jet, can lead to problems with flow-induced vibrations.  At low jet-to-cross-flow velocity ratios, stack downwash brings pollutants towards the building rather than disperse them higher into the atmosphere.  Most of the literature on stacks has been concerned with atmospheric dispersion and the plume trajectory, with relatively little emphasis on the local flow field near the stack and the jet exit.  Stack design guidelines, in general, do not incorporate significant physical insight on the flow field of the stack, its wake, and the jet.

This research area followed on logically from work on the flow around a finite circular cylinder. The emphasis has been on the effect of the jet-to-cross-flow velocity ratio on the mean aerodynamic forces, Kármán vortex shedding, and the time-averaged turbulent wake velocity and streamwise vorticity fields. Using thermal anemometry, a seven-hole pressure probe, and force measurements, with an emphasis on measurements in the cross-stream plane, the local flow field has been classified into three flow regimes depending on the velocity ratio.  A physical understanding of the flow field is being developed, based on our current understanding of the finite circular cylinder.

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Last updated: October 14, 2009