Bruce R. White

Magnus effect in saltation

High-speed motion pictures (2000 frames/s) of saltating spherical glass microbeads (of diameter 350–710 μm and density 2·5 g/cm 3 ) were taken in an environmental wind tunnel to simulate the planetary boundary layer. Analysis of the experimental particle trajectories show the presence of a substantial lifting force in the intermediate stages of the trajectories. Numerical integration of the equations of motion including a Magnus lifting force produced good agreement with experiment. Typical spin rates were of the order of several hundred revolutions per second and some limited experimental proof of this is presented. Average values and frequency distributions for liftoff and impact angles are also presented. The average lift-off and impact angles for the experiments were 50° and 14° respectively. A semi-empirical procedure for determining the average trajectory associated with given conditions is developed.

An experimental study of Froude number effect on wind-tunnel saltation

The simulation of the natural process of saltation in a wind tunnel is considered. The pioneering interactive boundary-layer analysis of Owen and Gillette [11] concluded that an independence Froude number criterion did apply to the problem and they estimate, based on wind-tunnel Froude number data ranging from 35 to 80, an independence Froude number value of about 20 for saltating flows to be free of facility constraints imposed on the saltation. The present experimental flows had Froude numbers ranging from about 6 to 1000. Analysis of friction speed variation as a function of downstream position suggests a more conservative critical Froude number value of 10 be used. Also, there appears to be an additional requirement, for most of our data, that tunnel’s downstream length-to-heigt ratio be greater than 5. Therefore, a maximum Froude number of 10 and minimum tunnel length-to-height ratio of 5 is suggested to insure accurate saltation tunnel simulation.

Applications of spaceborne radar laboratory data to the study of aeolian processes

Aerodynamic roughness (z0) is an important parameter in studies of sand and dust transport, as well as atmospheric circulation models. Aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles. Because field measurements require complex logistics, z0 values have been obtained for very few localities. If radar can be used to map z0, estimates can be obtained for large areas. In addition, because aerodynamic roughness can change in response to surface processes (e.g., flooding of alluvial surfaces), radar remote sensing could obtain new measurements on short timescales. Both z0 and the radar backscatter coefficient σ0 are dependent on topographic roughness at the submeter scale, and correlation between these two parameters was developed based on radar data obtained from aircraft (AIRSAR). The Spaceborne Radar Laboratory (SRL) afforded the opportunity to test the correlation for data obtained from orbit. SRL data for sites in Death Valley, California; Lunar Lake, Nevada; and Gobabeb, Namibia, were correlated with wind data and compared with previous radar z0 relations. Correlations between σ0 and z0 for L band (λ=24 cm) HV (H, vertically and V, vertically polarized modes) L band HH, and C band (λ=5.6 cm) HV compare favorably with previous studies. Based on these results, maps of z0 values were derived from SRL data for each site, demonstrating the potential to map z0 for large vegetation-free areas from orbit using radar systems.

Aeolian behavior of dust in a simulated Martian environment

The behavior of aeolian dust, particles 1-2 microns (μm) in diameter, was analyzed in a simulated Martian environment. Three main areas have been investigated: (1) characterizing spectral and aeolian properties of Martian particles and natural wind-blown terrestrial dust in order to identify a suitable surrogate Martian dust, (2) emplacement of the material in the simulated Martian environment, and (3) experimental procedures and testing in the Martian Surface Wind Tunnel (MARSWIT). The first phase of the study involved choosing a terrestrial dust that closely resembled Martian dust. Data accumulated from various sources suggest that Martian dust is derived from the weathering of basaltic parent material, nontronite clay being a good candidate composition. A commercial clay, Carbondale Red Clay (CRC), was determined to be an appropriate surrogate Martian dust. Using a compressed air-dust ejection system, an air-entrained dust cloud was generated which settled to cover the test section in the MARSWIT. This method best replicated the natural acrodynamic settling process presumed to exist on Mars. Low-pressure experiments were performed in the MARSWIT facility located at the NASA Ames Research Center, Moffett Field, California. Two separate wind tunnel floors were used for these experiments; one provided an aerodynamically smooth-surface flow, and the other was an aerodynamically rough-surface flow. Initial and momentary particle movement was recorded at friction velocities as low as 2 m/s. However, the dust never reached fluid saltation threshold because individual particles less than 10 μm typically do not saltate, but pass into suspension. An estimated dust flux of 3.7 x 10 -7 g/cm 2 s was determined for a friction velocity of 2 m/s. This flux could suspend about 10,000 metric tons of dust per second over the surface of Mars. The dust behavior on the smooth surface was markedly different than on the rough surface under Martian conditions. Application of the processes described above to the Martian environment suggests that on rocky surfaces dust suspension would first occur, whereas smooth plains would require higher wind speeds for dust entrainment.

Windblown sand on Venus: Preliminary results of laboratory simulations

Abstract Small particles and winds of sufficient strength to move them have been detected from Venera and Pioneer-Venus data and suggest the existence of aeolian processes on Venus. The Venus wind tunnel (VWT) was fabricated in order to investigate the behavior of windblown particles in a simulated Venusian environment. Preliminary results show that sand-size material is readily entrained at the wind speeds detected on Venus and that saltating grains achieve velocities closely matching those of the wind. Measurements of saltation threshold and particle flux for various particle sizes have been compared with theoretical models which were developed by extrapolation of findings from Martian and terrestial simulations. Results are in general agreement with theory, although certain discrepancies are apparent which may be attributed to experimental and/or theoretical-modeling procedures. Present findings enable a better understanding of Venusian surface processes and suggest that aeolian processes are important in the geological evolution of Venus.

Slope effect on saltation over a climbing sand dune

Abstract Results from a three-year study are presented that integrate field work (in Israel), physical modeling (wind-tunnel testing at UC Davis), and numerical solutions of grain trajectories to model and explain sand transport over a climbing dune. Field grain-size analyses of surface- and saltation-trap materials taken along various positions of the slope suggest that only smaller particles ( u ∗ , equal 30 cm/s) were able to climb a 20 degree slope in an escarpment normal to the prevailing strong wind direction. Numerical solutions of the particle trajectories are in good agreement with field measurements and confirm that particle motion is diminished at the base area of the slope and that the motion of larger-sized particles is completely terminated which causes an accumulation of these particles. The results suggest that the transport of the majority of the larger particles (> 230 μm) is by saltation. The model is numerically extended to a general series of slopes, particle sizes, and friction speeds with similar tends exhibited.

Higher-than-predicted saltation threshold wind speeds on Titan

Titan, the largest satellite of Saturn, exhibits extensive aeolian, that is, wind-formed, dunes, features previously identified exclusively on Earth, Mars and Venus. Wind tunnel data collected under ambient and planetary-analogue conditions inform our models of aeolian processes on the terrestrial planets. However, the accuracy of these widely used formulations in predicting the threshold wind speeds required to move sand by saltation, or by short bounces, has not been tested under conditions relevant for non-terrestrial planets. Here we derive saltation threshold wind speeds under the thick-atmosphere, low-gravity and low-sediment-density conditions on Titan, using a high-pressure wind tunnel refurbished to simulate the appropriate kinematic viscosity for the near-surface atmosphere of Titan. The experimentally derived saltation threshold wind speeds are higher than those predicted by models based on terrestrial-analogue experiments, indicating the limitations of these models for such extreme conditions. The models can be reconciled with the experimental results by inclusion of the extremely low ratio of particle density to fluid density on Titan. Whereas the density ratio term enables accurate modelling of aeolian entrainment in thick atmospheres, such as those inferred for some extrasolar planets, our results also indicate that for environments with high density ratios, such as in jets on icy satellites or in tenuous atmospheres or exospheres, the correction for low-density-ratio conditions is not required.

Assessment of aerodynamic roughness via airborne radar observations

The objective of this research is to assess the relationship among measurements of roughness parameters derived from radar backscatter, the wind, and topography on various natural surfaces and to understand the underlying physical causes for the relationship. This relationship will form the basis for developing a predictive equation to derive aerodynamic roughness (z0) from radar backscatter characteristics. Preliminary studies support the existence of such a relationship at the L-band (24 cm wavelength) direct polarization (HH) radar band frequencies. To increase the confidence in the preliminary correlation and to extend the application of the technique to future studies involving regional aeolian dynamics, the preliminary study has been expanded by: 1) defining the empirical relationship between radar backscatter and aerodynamic roughness of bare rocks and soils, 2) investigating the sensitivity of the relationship to microwave parameters using calibrated multiple wavelength, polarization, and incidence angle aircraft radar data, and 3) applying the results to models to gain an understanding of the physical properties which produce the relationship. The approach combines the measurement, analysis, and interpretation of radar data with field investigations of aeolian processes and topographic roughness.

Windblown dust on Mars: laboratory simulations of flux as a function of surface roughness

Abstract Experiments were conducted to determine the flux of dust (particles (u ∗ =322 cm/s ) ; for a moderately rough surface (z0=0.010 cm corresponding to 0.10 cm on Mars), flux averaged 1.5×10 −7 g/cm 2 /s ; for a rough surface (z0=0.015 cm corresponding to 0.15 cm on Mars), flux averaged 5×10 −7 g/cm 2 /s . Although the results are preliminary, flux varied widely as a function of wind speed and roughness, suggesting that raising dust into suspension on Mars is complex. Nonetheless, using these results as a guide, 9000 Mt of dust could be raised into the atmosphere of Mars per second from only 5% of the surface.

The effect of slopes on sand transport — numerical modelling

Abstract Most aeolian sediment transport models and experiments have been conducted on flat horizontal surfaces. Very little numerical or analytical research has been carried out for aeolian sand transport on slopes. The current work presents numerical modelling of saltating sand for a climbing dune which has a slope angle of 20 °. The model utilised wind fields predicted from the FAA complex terrain model for two dimensional escarpment and field data and wind-tunnel measurements for flow over a scaled model. Results suggest that only particles smaller than 0.230 mm are able to climb this slope under a wind shear velocity of 30 cm s −1 . The field observations and measurements confirm the numerical results.