The critical velocity for aggregate blow-off from a built-up roof

A new experimental technique is presented for calculating the critical conditions under which loose gravel on a roof becomes airborne. The critical condition for gravel blow-off from the top of a roof depends on the building geometry, particle properties, and the wind conditions. A series of two-dimensional wind tunnel tests were run to measure the critical condition for particle removal. The experimental results demonstrate that the critical condition for blow-off, parameterized in terms of a particle densimetric Froude number, is a function of the dimensionless particle size (d⁎) and the building geometry. Results for buildings without a parapet show that the critical particle densimetric Froude number has a power-law relationship with the dimensionless particle size as Frd2=8.1(d⁎)−0.44 for the range of parameters tested. For buildings with a parapet, the densimetric Froude number for the critical condition depends on both Reynolds number and parapet height to building height ratio. The experimental results indicate that buildings without a parapet are not always the most prone to blow-off, and that under certain conditions, a small parapet height can increase the risk of gravel removal.As the critical Froude number is dependent on the Reynolds number, raw data from small scale experiments cannot simply be scaled up by using a Froude number. Further, it is demonstrated that the current approach of using the Shields Diagram (or equivalent data) to scale results from small to large scale are also flawed as the motion initiation mechanism is different. Therefore, existing design guides should be re-visited and full-scale experiments should be conducted in order to fully analyze the risk of blow-off.

► A new experimental technique was used to measure the critical conditions for gravel blow off.
► Existing methods for scaling laboratory results up to full scale are inappropriate.
► The critical velocity is strongly dependent on the Reynolds number and parapet height.
► Full scale testing is required to quantify the conditions for blow off and the role of the parapet.