Highly buoyant bent-over plumes in a boundary layer

Abstract

Highly buoyant plumes, such as wildfire plumes, in low to moderate wind speeds have initial trajectories that are steeper than many industrial waste plumes. They will rise further into the atmosphere before bending significantly. In such cases the plume's trajectory will be influenced by the vertical variation in horizontal velocity of the atmospheric boundary layer. This paper examined the behavior of a plume in an unstratified environment with a power-law ambient velocity profile. Examination of previously published experimental measurements of plume trajectory show that inclusion of the boundary layer velocity profile in the plume model often provides better predictions of the plume trajectory compared to algebraic expressions developed for uniform flow plumes. However, there are many cases in which uniform velocity profile algebraic expressions are as good as boundary layer models. It is shown that it is only important to model the role of the atmospheric boundary layer velocity profile in cases where either the momentum length (square root of source momentum flux divided by the reference wind speed) or buoyancy length (buoyancy flux divided by the reference wind speed cubed) is significantly greater than the plume release height within the boundary layer. This criteria is rarely met with industrial waste plumes, but it is important in modeling wildfire plumes.

Wildfire and Weather Radar: A Review

“…Reid and Vines (1972) presented the turret tracking method, determining proxies for vertical and horizontal velocities. In subsequent papers an automated approach was opted for in echo top time series, but at a cost of information on the pulsing nature of plumes in cross-flows (Dowdy et al, 2017;Fromm et al, 2012;McRae et al, 2015;Morton, 1956;Morton & Ibbetson, 1996;Rosenfeld et al, 2007;Tohidi & Kaye, 2016). Vortex features of different scales and forms have been observed in the Doppler imagery captured by radar.…”

Fire Whirls

“…Similar to Tohidi & Kaye (2016), Q, M , and F can be written in the form of specific fluxes asQ = Q/(2πρ 0 ),M = M /(2πρ 0 ), andF = F/(2πρ 0 ) (Lee & Chu 2012). One can map the conservation equations with these specific fluxes and integrate them using the standard entrainment model that is incorporated with the fire whirl radius b A and the mean plume buoyancy force per unit volume γ , which can be shown as…”

Comprehensive wind tunnel experiments of lofting and downwind transport of non-combusting rod-like model firebrands during firebrand shower scenarios

“…Dynamic similarity is, also, not possible in wind tunnels as pool fires need to be created and then the wind speed scaled to match both the Froude number and plume to wind velocity ratio [51]. For small-scale laboratory fires, even in the largest available wind tunnels, this would lead to very low wind speeds which would lead to a boundary layer that is not in the fully developed turbulent regime.…”

“…This means that a jet will bend horizontally at a lower height compared to a turbulent plume with the same source momentum flux but with an additional buoyancy flux. However, the overall structure of the two flows is similar and the effect of buoyancy on the large scale dynamics can be accounted for in re-scaling [51]. The density of the hot air in the plume will make only a very small difference to the flight trajectory of a firebrand as the buoyancy force acting on the firebrand is negligible.…”