A Wind Tunnel Investigation of the Shear Stress with A Blowing Sand Cloud

In a blowing sand system, the wind provides the driving forces for the particle movement while the moving particles exert the opposite forces to the wind by extracting its momentum. The wind-sand interaction that can be characterized by shear stress and force exerted on the wind by moving particles results in the modification of wind profiles. Detailed wind profiles re-adapted to blown sand movement are measured in a wind tunnel for different grain size populations and at different free-stream wind velocities. The shear stress with a blowing sand cloud and force exerted on the wind by moving particles are calculated from the measured wind velocity profiles. The results suggest that the wind profiles with presence of blowing sand cloud assume convex-upward curves on the u(z)-ln(z) plot compared with the straight lines characterizing the velocity profiles of clean wind, and they can be better fitted by power function than log-linear function. The exponent of the power function ranging from 0.1 to 0.17 tends to increase with an increase in wind velocity but decrease with an increase in particle size. The force per unit volume exerted on the wind by blown sand drift that is calculated based on the empirical power functions for the wind velocity profiles is found to decrease with height. The particle-induced force makes the total shear stress with blowing sand cloud partitioned into air-borne stress that results from the wind velocity gradient and grain-borne stress that results from the upward or downward movement of particles. The air-borne stress increases with an increase in height, while the grain-borne stress decreases with an increase in height. The air-borne shear stress at the top of sand cloud layer increases with both wind velocity and grain size, implying that it increases with sand transport rate for a given grain size. The shear stress with a blowing sand cloud is also closely related to the sand transport rate. Both the total shear stress and grain-borne stress on the grain top is directly proportional to the square root of sand transport rate. So, the profound effect of the moving particles on the airflow must be considered in modeling the blown sand movement. With the presence of sand movement, the boundary layer with a blowing sand cloud is no longer a constant air shear layer.