Saltation and wind-flow interaction in a variable slope wind tunnel

In a 15 m long horizontal wind tunnel, we studied the internal boundary layer over fixed arrays with roughness (z0) similar to the roughness of saltation of a sand bed. Utilizing turbulence spires the logarithmic overlap-layer is at least 75 mm thick some 5 m downwind of the entry. This permits a precise determination of u∗ from wind profile data. sand bed in a variable slope wind tunnel with a 6 m long working section we studied how slope, grain size, and friction speed influence bed roughness. Owen (1964) proposed that z0 = C · u∗2/2g where C is a constant, but we found that C depends in a complex manner on grain size and friction speed. C increases from very low values near the threshold to an apparently limiting value at large friction speeds. For an almost uniform 125 μm sand, the limiting value of C is 0.06. The limiting value decreases, however, with particle size: for a uniform 544 μm sand, C only approaches 0.03 for friction speeds as high as 1 m/s. With the measured values of friction speed corrected for the effects of slope, our results appear to show that the limiting values of C do not depend on the slope angle, even though the slope affects the individual shapes of particle trajectory. C is perhaps independent of slope angle because the roughness height, and, therefore, C, is more directly related to the amount of momentum extracted from the air by the particles (i.e. the shear stress) rather than to the manner of extraction (particle trajectory shapes).