The AA. report detailed results with respect to the aerodynamic roughness of different gravel surfaces from wind tunnel experiments. With the philosophy that aerodynamic roughness of gravel surfaces is a function of gravel size, spacing and the near
(PDA) that employs phase Doppler technique in a wind tunnel. The primary objectives are to define the probability distribution of the speed and angle of impacting and entrained particles, and establish the relationship between impact and entrainment