Aeolian sand transport: a wind tunnel model

Wind sand transport is an important geological process on earth and some other planets. Formulating the wind sand transport model has been of continuing significance. Majority of the existing models relate sand transport rate to the wind shear velocity based on dynamic analysis. However, the wind shear velocity readapted to blown sand is difficult to determine from the measured wind profiles when sand movement occurs, especially at high wind velocity. Moreover, the effect of grain size on sand transport is open to argument. Detailed wind tunnel tests were carried out with respect to the threshold velocity, threshold shear velocity, and transport rate of differently sized, loose dry sand at different wind velocities to reformulate the transport model. The results suggest that the relationship between threshold shear velocity and grain size basically follow the Bagnold-type equation for the grain size d>0.1 mm. However, the threshold coefficient A in the equation is not constant as suggested by Bagnold, but decreases with the particle Reynolds number. The threshold velocity at the centerline height of the wind tunnel proved to be directly proportional to the square root of grain diameter. Attempts have been made to relate sand transport rate to both the wind velocity and shear velocity readapted to the blown sand movement. The reformulated transport model for loose dry sand follows the modified O'Brien-Rindlaub-type equation: Q=f(1)(d)(1-R-u)(2)(rho/g)V-3, or the modified Bagnold-type equation: Q=f(2)(d)(1-R-1)(0.25)(rho/g)U-*(3). Where Q is the sand transport rate, the sand flux per unit time and per unit width, in kg m(-1) s(-1); p is the air density, 1.25 kg m(-3); g is the acceleration due to gravity, 9.81 m s(-2); R-u = V-t/V; R-1 = U-*1/U-*; V is the wind velocity at the centerline of the wind tunnel, in m s(-1) I; V, is the threshold velocity measured at the same height as V, in m s(-1); U-* is the shear velocity with saltating flux, in in s(-1); U-*1 is threshold shear velocity, in m s(-1); f(1)(d) = 1/(475.24 + 93.62d/D);f(2)(d) = 1.41 + 4.98exp(-0.5(ln(d/1.55D)/0.57)(2)); d is the grain diameter, in mm; and D is the reference grain diameter, equals 0.25 mm. The Bagnold's equation that asserts for a given wind drag the rate of movement of a fine sand is less than that of a coarse sand is not supported by the reformulated models. (C) 2003 Elsevier Science B.V. All rights reserved.