Flow over cylinders with porous outer layers: influences of free-stream turbulence and porous layer depth and porosity*

A better understanding of the mechanisms affecting the propagation of forest fires, which have increased in frequency and intensity over the past 10-15 years, is essential for their effective mitigation and control. One such mechanism is spotting, in which burning leaves, branches, and other debris, termed firebrands, are transported away from the main fire by the wind and ignite new fires. Although spotting has been the subject of prior experimental and numerical studies, the influences on firebrand trajectory of both free-stream turbulence and the porous char layer that forms on their outer surface have not been accounted for. As a first step of a more comprehensive study aimed at improving predictions of firebrand trajectories, wind tunnel experiments were conducted to quantify the effects of free-stream turbulence and porous outer layers on the flow over stationary, non-burning cylinders. Passive and active grids were used to generate flows of different turbulence intensities, in which force-balance and hot-wire anemometry measurements were made. The dependence of the drag on the cylinder, as well as the flow in its wake, on the free-stream turbulence and the porous outer layer (depth and porosity) will be presented.