Drag on circular cylinders with porous outer layers in turbulent cross-flow

Abstract

Abstract The effects of free-stream turbulence intensity and porosity on the drag on cylinders with porous outer layers in cross-flow was investigated experimentally. This work is motivated by the need to better model spotting -- a forest fire propagation mechanism in which burning branches and other debris (termed firebrands) are transported away from the main fire by the prevailing wind and ignite new fires. Multiple levels of background turbulence were studied by using no grid, passive grids, and an active grid to generate turbulence intensities of 0.4%, 1.7%, 2.7% and 12.4%. The porous char-layer on the outer surface of firebrands was mimicked by wrapping wire meshes (of 10, 20 and 40 pores-per-inch or PPI) around the cylinders, each to three different fractions (1/16, 1/8 and 1/4) of the cylinder's outer diameter. The drag on one smooth cylinder and nine cylinders with porous outer layers was measured for Reynolds numbers in the range 7000 to 17000, at the aforementioned four turbulence intensities. The results showed that i) the free-stream turbulence intensity and PPI of the wire meshes affect the Reynolds number dependence of the drag coefficient; ii) the drag coefficient increases with free-stream turbulence intensity when it is relatively low (0.4%-2.7%), then decreases at high intensities (12.4%), and this decrease is more pronounced as the PPI increases; iii) the drag coefficient increases with the thickness of the porous layer, and asymptotes to an effectively constant value after a critical thickness of about 1/8 of the cylinder's diameter; and iv) the drag coefficient exhibits a non-monotonic dependence on the PPI of the wire mesh. These results demonstrate the importance of free-stream turbulence intensity and the (porosity and depth of the) porous outer layers of cylinders when modeling the drag on firebrands, or in any other application in which cylinders have porous surfaces.