Vortex Threshold: Experimental Results at Martian Atmospheric Pressures

Details Vortex Threshold: Experimental Results at Martian Atmospheric Pressures Vortex Threshold: Experimental Results at Martian Atmospheric Pressures

Dec 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.p51a0334b&link_type=abstract

American Geophysical Union, Fall Meeting 2002, abstract #P51A-0334

Other

0343 Planetary Atmospheres (5405, 5407, 5409, 5704, 5705, 5707), 3307 Boundary Layer Processes, 3346 Planetary Meteorology (5445, 5739), 5409 Atmospheres: Structure And Dynamics, 5445 Meteorology (3346)

Scientific paper

Many examples of Martian dust devils and tracks left by their passage have been identified in Viking and Mars Orbital Camera images and inferred from lander data (Viking and Mars Pathfinder). Recent surveys suggest that dust devils may be common phenomena on Mars and, unlike Earth, could contribute significantly to the global dust budget. Previous studies have noted the apparent paradox that Martian airborne dust is abundant and only a few microns in diameter yet experiments at Mars pressures suggest current Martian ambient wind speeds are insufficient to lift such fine particles from the surface; speeds of the order of 10s or even 100s of m/s are required. Local wind speeds within terrestrial dust devils are typically much greater than ambient wind speeds, but we have no in-situ measurements of the velocity structure of Mars dust devils and so cannot directly quantify their ability to entrain material. However, by using laboratory simulations we can directly measure the ability of a vortex to lift material of known size and density under a variety of atmospheric pressures. We have constructed a vortex generator consisting of a large vertical cylinder containing a rotor comprising four vertical blades and capable of speeds up to 4500 RPM. Beneath the cylinder is a 2.4 by 2.4 m tabletop which can be covered in particles for threshold tests or instrumented with pressure transducers to measure the pressure structure of the vortex. The distance between the cylinder and the tabletop and the height of the blades within the cylinder can be varied to generate a wide range of geometries and intensities of vortices. Recently, the apparatus has been operated at the NASA-Ames Research Center Mars Surface Wind Tunnel facility to simulate Martian atmospheric conditions. We have measured vortex `saltation' threshold using many types of particles ranging in density from walnut shells (1.1 kg/m-3) to steel grit (7.6 kg/m-3) with particle sizes from 2 to 2000 microns and using atmospheric pressures ranging from 10 mbar (representing current Mars atmospheric conditions) to ambient. As expected, vortex threshold was more difficult to achieve with lower pressure conditions. Only the `optimum' particles (those with low densities and particle sizes ranging from 70 to 350 micron) reached full `saltation' at 10 mbar pressure before the apparatus speed limit was reached. Our results suggest that vortex threshold is directly analogous to boundary layer shear threshold for sand-sized particles at pressure from 65 mbar to ambient. We have used this result to equate vortex and boundary layer results in the sand-sized particle regime and hence to compare vortex threshold data with boundary layer results for smaller particles and lower pressures. We used empirical boundary layer expressions for threshold (corrected for particle size and particle Reynold's number). In all cases, vortex action appears more efficient than boundary layer winds at lifting small dust-sized particles and at lifting all particles at very low pressure. We conclude that Martian dust devils are more efficient mechanisms for particle entrainment than boundary layer winds, not merely because they have enhanced local wind speeds but also through another intrinsic mechanism. We suggest that a lift force caused by the passage of the low-pressure core of the dust devil over the particles would have such an effect and present examples of experimental `pressure-well' measurements at low pressures to support this.

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