Field measurements of the flux and speed of wind‐blown sand

Abstract

A field experiment was conducted to measure the flux and speed of wind-blown sand under known conditions in a natural setting. The experiment, run at Pismo Beach, California, involved a tract 100 m long (parallel with the wind) by 20 m wide. The site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary-layer, temperature probes to determine atmospheric stability and wind vanes to determine wind direction. From these measurements, wind friction speeds were derived for each experimental run. In order to measure sand saltation flux, a trench 3 m long by 10 m wide (transverse to the wind direction) by 0-5 m deep was placed at the downwind end of the tract and lined with 168 collector bins, forming an 'egg-box' pattern. The mass of particles collected in each bin was determined for four experimental runs. In order to assess various sand-trap systems used in previous experiments, 1 2 Leatherman traps, one Fryberger trap and one array of Ames traps were deployed to collect particles concurrently with the trench collection. Particle velocities were determined from analysis of high-speed (3000 and 5000 frames per second) motion pictures and from a particle velocimeter. Sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined. Fluxes for each run were calculated using various previously published expressions, and then compared with the flux derived from the trench collection. Results show that Bagnold's (1941) model and White's (1979) equation most closely agree with values derived from the trench. Comparison of the various collector systems shows that the Leatherman and Ames traps most closely agree with the flux derived from the trench, although these systems tended to under-collect particles. Particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories. Results show that particle velocities from the velocimeter are in the range 0-5-7.0 m s-', compared to a wind friction velocity of 0.32-0.43 m s-l and a wind velocity of 2.7-3.9 m s-' at the height of the particle measurements. Descending particles tended to exceed the speeds of ascending particles by -0.5 m s?. study the flux of wind-blown sand, including field measurements (e.

Electrical Activity and Dust Lifting on Earth, Mars, and Beyond

“…First, electric forces affect the trajectories of saltating particles, as first suggested by Schmidt et al (1998) and Zheng et al (2003). Indeed, the presence of electric forces might explain the puzzling discovery that the height to which saltating particles bounce does not increase with wind speed (Greeley et al, 1996;Namikas, 2003). This is in direct contradiction with saltation theory (Bagnold, 1941;Owen, 1964), which posits that the saltation layer height should increase markedly with wind speed.…”

“…Classical saltation theory predicts that z 50 increases strongly with shear velocity (Bagnold, 1941;Owen, 1964), which the numerical model also predicts when electric forces are not included (black circles). However, field measurements [squares (Namikas, 2003) and triangles (Greeley et al, 1996)] show that z 50 remains approximately constant with shear velocity. The inclusion of sand electrification in the numerical model (red circles) apparently resolves the discrepancy between theory and measurements.…”

Reconstructing the vertical distribution of the aeolian saltation mass flux based on the probability distribution of lift-off velocity

“…Many studies have revealed that there is an exponential decay of mass flux with height (Dong et al, 2002b;Greeley et al, 1996;Rasmussen and Sørensen, 1999). The experiment of Dong et al (2002b) shows the sand mass flux decreases exponentially with height.…”

“…The experiment of Dong et al (2002b) shows the sand mass flux decreases exponentially with height. The field measurement of Greeley et al (1996) shows that there is a clear log-linear decrease in sand flux with height. Rasmussen and Sørensen (1999) used the isokinetic trap to measure the sand mass flux in a field, and there is an exponential behavior between mass flux and height.…”

Experimental investigation of the concentration profile of a blowing sand cloud

“…Although high-speed multi-flash photography and photoelectric cell methods have been used to measure movement velocities of individual particles (Zou et al, 1994;Greeley et al, 1996), these methods prove to be only good for fairly large particles (Greeley and Iversen, 1985). It is also difficult from photographic images to interpret the velocity of sand particles at very low heights such as below 0.5 cm where the grains are too crowded, and at higher height, such as over 7 cm, where the moving grains are too few (Zou et al, 2001).…”