Fluid modeling simulation of stack-tip downwash for neutrally buoyant plumes

Abstract

Downwash of neutrally buoyant effluent on the immediate lee side of a circular stack was modeled using a wind-tunnel simulation. Both subcritical and supercritical turbulent flows were simulated, where the criticality refers to Reynolds numbers below and above the critical Reynolds number, Re c (⋍2 × 10 5) , where the boundary layer on the cylinder becomes turbulent and the drag coefficient drops sharply. Subcritical Reynolds numbers are typically attained by small-diameter stacks in relatively light winds; supercritical ones are attained by large-diameter stacks in strong winds. The downwash characteristics differ markedly in the two regimes. In the subcritical regime, downwash begins when the ratio of effluent speed to wind speed ( W/ U) is about 1.5; in the supercritical regime, downwash begins at W/U ⋍ 1.1. The downwash in the immediate lee of the stack at W/ U = 0.3 extends to 7 diameters down from the stack top in the subcritical regime and to 4 diameters down in the supercritical regime. Graphs and simple formulas are presented describing the plume trajectories and plume widths for 0.4⩽ W/ U⩽2.0 and for downwind distances from the source to 30 stack diameters. Practical recommendations are made for modeling the downwash of neutrally buoyant plumes.