Airflow over Barchan dunes: field measurements, mathematical modelling and wind tunnel testing
Wiggs, G. F. S.
- Publisher: University of London
arxiv: Physics::Geophysics | Condensed Matter::Soft Condensed Matter | Physics::Fluid Dynamics
There are few empirical measurements of velocity, shear velocity, sand transport, morphological change on the windward slopes of dunes.This thesis compares field measurements on a barchan
dune in Oman with calculations using a mathematical model (FLOWSTAR) and measurements
in a wind tunnel.
All three techniques demonstrate similar patterns of velocity, confirming the acceleration of flow
up the windward slope, deceleration between the crest and brink and significant flow
deceleration upwind of and at the toe of the dune. The FLOWSTAR model is unable accurately
to predict airflow at the brink and its predictions near the surface are highly susceptible to small-scale
terrain irregularities. The measurements of shear velocity in the field and those predicted
by the FLOWSTAR model reflect observations of previous studies including the widely reported
upwind reduction in shear velocity. Such a reduction in shear velocity upwind of the dune should
result in a reduction in sand transport and sand deposition. This is not observed in the field.
Wind tunnel modelling using a near-surface pulse-wire probe suggests that the field and
FLOWSTAR methods of shear velocity derivation are inadequate. The wind tunnel results
exhibit no reduction in shear velocity upwind of or at the toe of the dune. This maintenance of
upwind shear stress may be caused by concave (unstable) streamline curvature, which is not
taken into account by the field and FLOWSTAR techniques. From this hypothesis, a new model
of dune dynamics is developed relying on the establishment of an equilibrium between windward
slope morphology, streamline curvature and streamwise acceleration.