7/28/2019 EGU Poster 2012 Flagstaff Edit

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Derek W.T. Jackson1, Thomas A.G. Smyth1, Mary C. Bourke2 and J. H. Meiring Beyers31Centre for Coastal & Marine Research, School of Environmental Sciences , University of Ulster , Northern Ireland2Planetary Science Institute, Tucson, Arizona, USA 3Klimaat Consulting & Innovation Inc., Guelph, Canada

.

High Resolution Computational Fluid Dynamic Modelling of

Airflow over Dunes in Proctor Crater, Mars.

Methods

Introduction

Derek W.T. Jackson, Thomas A.G. Smyth, Mary C. Bourke and J. H. Meiring Beyers

Contactd j k @ l t k

Recent high resolution Hi-RISE images present an opportunity to

examine the patterns of dune and inter-dune areas inside Proctor

Crater, Mars. Multiple dune crest and ripple formations are found

orientated in distinct directions. To date, these have been used to infer

directional components of the wind regimes present with the boundary

layer over these features. Using a generated Digital Terrain Surface

alongside these clearly defined bedform features compels us to

examine the possible wind flow behaviour that has been responsible for

geomorphological patterns.

Multiple length scales are observed showing progressively smaller

bedform features superimposed on larger dunes, giving rise to complex

but regular topographical patterns that may be indicative of multi-

directional (and magnitude) wind regimes. There is therefore a need to

understand the airflow behaviour at a relevantscale over these features

to investigate if the formational pattern and orientation of the bedforms

correspond to localised wind flow forcing.

Using computational fluid dynamics modelling (OpenFOAM CFD code)

with a RNG k- solver over a 3-D surface mesh, we present preliminary

findings within Mars Proctor Crater (Fig. 1) to examine a dunefield area

of 4.4km x 3.0km, and a computational cell resolution of 5m x 5m.

Three wind directions were considered; Primary (WSW), Secondary

(ESE) and Tertiary (ENE) winds (Fig.2). Each simulation had a

logarithmic inlet with an input of 20ms-1 at 30m from the surface and a

roughness parameter of 0.05m. Kinematic viscosity was assumed to be

0.0019 m2/s. Output was presented in the form of surface flow vectors

(at 0.5 m from the surface) and superimposed over 3-D and Hi-Rise

imagery to allow direct comparisons with local bedforms. A cross-

sectional slice of the boundary layer over the dunes was also presented

to show detached and other flow characteristics under each of the wind

direction scenarios.

ResultsIsolating a particular 1km x 1km area (Fig. 2) where three main dune

ridges are in view and inter-dune trough topography is imaged, results

reveal a distinct relationship between steered airflow and localised

bedform orientation, clearly mapping orthogonally onto much of the

crestal ridges present at multiple scales.

Wind direction and magnitude

Under Primary (WSW) winds (Fig. 2 i), flow vectors were strongly

steered orthogonally against the smaller inter-dune dune ridges in a

SW orientation, coincident with re-attached flow coming off the main

dune crests. Winds within the troughs of the main dunes were reduced

in magnitude by 25-35% from their crestal velocities.

Under Secondary (ESE) winds (Fig. 2 ii), patterns of flow detachment

are less evident as wind direction is generally parallel to the main dune

crests. As a result wind velocity undergoes less of a reduction between

crest and trough. Under Tertiary flow (Fig. 2 iii) conditions (ENE)

steered winds inside the dune troughs travelled from a SE to Edirection, and were orthogonal to microdunes (ripples) located on the

stoss side of the main dunes as well the mesodune ridges inside the

dune troughs. Steered winds were reduced in magnitude by up to 60%

compared to crest velocities during tertiary winds.

Detached and attached flow

Within the three wind direction scenarios, detached flow was evident in

the Primary and Tertiary (fig 3 i and iii) simulations only whilst the

Secondary winds (fig 6 iii) contained largely attached flow only

behaviour. This has been dictated by the main dunes crestal

configuration relative to incident wind direction. These detached/un-

detached flow behaviours in turn induce re-attachment zones or

maintain attached wind flow which has morphological implications for

localized ripple and dune migration patterns.

This work has important implications for the reconstruction of the

formative winds for aeolian dunes within craters on Mars [1] and can

help lend further support to studies examining recent activity of Martian

dune migration[2].

References:

[1] Fenton, L.K. et al. (2005) Jour. Geophys. Res. Vol.110,

doi:10.1029/2004JE002309.

[2] Bridges, N.T. et al., (2012) Geology, 40, 31-34

Fig. 1 (a) Location of study site within the main area of Proctor Crater (474126.36S 295453.54E)(b) the northern section of the dune field (c) and the area of the computational domain over which the

CFD model was run .

(a) (b) (c)

Fig. 2 Surface flow vectors at 0.5m elevation overlain ontop of the HiRise imagery for (i) Primary, (ii) Secondary and

(iii) Tertiary flow directions. Inside the large dune troughsthe crest of smaller bedforms are aligning orthogonally with

local airflow directional vectors. Note that significant flow

velocity reduction takes place in the troughs of the largedunes in all directional scenarios. A cross-sectional line in

annotated on each image and boundary layer slice isshown in Fig.3.

Fig. 3 Cross-section through the local boundary layer

during each directional scenario. During Primary winds (i)flow becomes Detached, (ii) secondary winds flow is

Attachedand with (iii) Tertiary winds the flow is again

Detached.

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