Balme, Matthew R.

Eddlemon, Eric E.

Greeley, Ronald

Haan, Frederick L.

Iversen, James D.

Neakrase, Lynn D. V.

Sarkar, Partha

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDust devils on Earth and Mars: Extension of particlethreshold laboratory simulationsConference or Workshop ItemHow to cite:Neakrase, Lynn D. V.; Greeley, Ronald; Haan, Frederick L.; Sarkar, Partha; Iversen, James D.; Balme, Matthew R.and Eddlemon, Eric E. (2006). Dust devils on Earth and Mars: Extension of particle threshold laboratory simulations.In: 37th Lunar and Planetary Science Conference, 13-17 Mar 2006, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2006/Copyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukDUST DEVILS ON EARTH AND MARS: EXTENSION OF PARTICLE THRESHOLD LABORATORY SIMULATIONSLynn D. V. Neakrase1, Ronald Greeley1, Frederick L. Haan, Jr2, Partha Sarkar2, James D. Iversen2, Matthew R. Balme3, Eric E. Eddlemon41Dept. of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85287-1404, USA (neakrase@asu.edu)2Dept. Aerospace Engineering, 2271 Howe Hall, Iowa State University, Ames, IA 50011, USA3Dept. of Earth Sciences, Open University, Walton Hall, Kilton Keynes, MK7 6AA, UK4Mars Surface Wind Tunnel Facility, NASA Ames Research Center, Mail Stop 242-6, Moffett Field, CA 94035, USAIntroduction.  Dust devils are proposed aseffective mechanisms for lofting large quantities ofdust into the martian atmosphere.  Previous work[1] showed that vortices entrain dust more easilythan simple boundary layer winds. However, theseprevious lab studies that used the Arizona StateUniversity Vortex Generator (ASUVG) werelimited to vortices of 0.01 to 0.10 m in diameter.Utilizing a newer facility at Iowa State University,the Tornado/Microburst Simulator (ISUTMS)[2],allowed larger vortex sizes (0.60 to 1.20 m indiameter) to be studied.  Natural dust devils onEarth are generally a few meters to 10s of meters indiameter and occasionally reach upwards of 100 min size [3]. On Mars, however, dust devils expandto much larger sizes from <10 m to 500 m indiameter [4,5]. The goal of this study was toexamine vortex particle threshold in the laboratoryusing the ISUTMS to study vortices with a range ofdiameters larger than was possible with theASUVG and then apply the results to scalingaspects of terrestrial and martian dust devils.The two vortex systems used in this study aredesigned differently yet provide similar Rankine-like vortices.  The main observational difference isthat the ASUVG vortex 'wanders' within an~60cm-radius zone because the system is notconfined as in the ISU case. The ISUTMS employsa recirculatory system resembling a Ward-modelvortex generator [2] to produce an incredibly stablewith almost no wander. This allows the system tobe accelerated to tornadic conditions (i.e., v > 27m/s).Approach and Methods.  Building on previouswork [1], we investigated how dust devils liftsand at laboratory scales. We employed similartechniques outlined in [1] for determiningthreshold using the ASUVG and the ISUTMS.Threshold was designated as the point at whichsediment begins to move under wind action.Movement can be designated as rolling, saltating,or suspended in the flow.  Vortex action is differentfrom boundary layer winds as small particles aremore easily entrained, probably because of anadded lift provided by the low pressure in the coreof the vortex, which overcomes interparticle forces(e.g. cohesion, statics, etc.)[1].For both systems, threshold values weredetermined by slowly ramping up the velocitiesuntil particles began to move on the test surface.Particle layers were thick (> 5mm) for eachexperiment to reduce testbed surface effects.  Thetwo mechanical systems behave differently becauseof differences in design. Therefore, thresholdappears different in both cases, which could havean effect on the comparison of the data.Results and Implications. The preliminary datashow better-constrained values of threshold forthe ISUTMS system (less scatter).  The lowerscatter is probably due to higher vortex stabilityin the ISUTMS experiments. Threshold valuesfor the ISUTMS also display values that arelower than those published in [1].  This alsomight be due to the instability (wander) of theASU vortices.  Greeley et al. [1] noted thatthreshold is attained at two distinct points:"continuous threshold" where entrainment isapproximately constant over time and a lowervalue deemed "intermittent" in which bursts ofentrainment occur. Intermittent thresholdcoincides more with initial movement ofparticles on the test surface, which makes it amore likely analog to threshold determined withthe ISUTMS.  Due to the wander instability,"continuous" threshold is prohibited until highervelocities are attained potentially skewing thedetermination of the true threshold velocities.Figure 1. A plot showing the extension of threshold data for 125μmsilica sand using the ASUVG "continuous" data (blue), the ASUVG"intermittent" data (green), and the ISUTMS data (red) collected lastsummer.Lunar and Planetary Science XXXVII (2006) 1196.pdfExtrapolating these results to martianconditions (also discussed in [1]), vortex thresholdvelocities should behave similarly.  Figure 2,shows threshold velocities as a function of radiusfor both terrestrial and martian laboratory runs.These data are used to suggest where the ISUTMSdata should plot if that system could be run at lowambient pressures.Figure 2. A plot showing ASUVG data plotted for both Earth and Marscases, as well as the data collected from the ISUTMS in Summer 2005.The pink ellipse suggests where ISUTMS data would plot if that systemcould be run under martian conditions.Future Work.  Based on this preliminary work,threshold curves as a function of particle size andrelative strength of the vortex (P as a percentageof ambient) will be generated for both vortexgenerators. The relationship between P andtangential velocity for the ASUVG was discussedin [6,7] and suggested that there could be a simplerelationship between terrestrial and martianconditions, demonstrated in Figure 3.  Intermittentambient pressures could be used to simulate theappropriate ranges in P (% ambient) for bothterrestrial and martian cases. The efficiency of theASUVG at low ambient pressures increases andallows stronger Ps to be investigated.Appropriate ranges as reported in [6,7] for naturaldust devils are ~0.2-1.0% ambient for dust devilson Earth and Mars. Pressure wells from theISUTMS are important to verify if velocitiescorroborate these relationships as vortex sizesapproach those of natural dust devils.  Inconclusion, the ISUTMS has enabled the firstextension of threshold laboratory data to naturaldust devils and will be useful in the future forexamining the intermediate ranges in vortex sizesbetween the laboratory and natural dust devils.Vortices at different ambient pressures appear tobehave very similarly to each other suggestingthat dust devil mechanics work the same for Earthand Mars.  Future work with the ASUVG shouldfocus on studying the range of ambient pressuresthat produce P in the correct percentage range.Extrapolating ISUTMS data to martian dust devilssuggests that larger vortices should display lessvariation in tangential velocity than what isobserved over smaller ones.  Overall this suggeststhat smaller, tightly wound dust devils arestronger and exhibit more variation that the largerones.Figure 3. A plot showing all ASUVG data for all tested ambientpressures 10 to 1000 mbar. Maximum tangential velocities as dictatedby the pressure wells are plotted showing the relationship with Pmeasured in percentage of ambient pressure. (after [6,7])References. [1] Greeley et al. (2003) J.Geophys. Res., 108(E5), 5041. [2] Sarkar et al.,10th Am. Conf. on Wind Eng, (2005); [3]Sinclair (1966) PhD. [4] Metzger et al. (1999)GRL, 26(18), 2781-2784. [5] Fisher et al.,(2005)JGR 10.1029/2003JE002165.  [6] Neakrase etal. (2005) AGU Fall Mtg., #P11-0147(Abstract). [7] Neakrase et al. (2006),GRL inprep.Lunar and Planetary Science XXXVII (2006) 1196.pdf

Dust devils on Earth and Mars: Extension of particle threshold laboratory simulations

Dust devils on Earth and Mars: Extension of particle threshold laboratory simulations

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