Numerical experiments on the hydrodynamic structure of a barchan dune constrained by dense atmospheres: Comparative applications to Venus and Titan

Abstract

Venus and Titan, two very different terrestrial bodies in the solar system possessing extremely hot and extremely cold near-surface dense atmospheres, respectively, have been identified as having surface dune distributions associated with aeolian sand transport. Previous studies on planetary dunes have rarely involved a detailed investigation of flow behavior over dunes under such extreme environmental conditions. This study takes the highly migratory elementary barchan dune as the research object, and aiming at the realistic thermophysical environment of the wind field near the surfaces of Venus and Titan, a computational fluid dynamics model of the turbulent boundary layer on the surface that considers the real gas effect and planetary gravity constraints was constructed to carry out numerical experiments and comparative analysis of the hydrodynamic behavior of sand dunes induced by different planetary atmospheric environments. The predicted results show that there are notable differences in the leeward secondary flow structures of the barchan dunes induced by the real gas flows of Venus and Titan. Under the conditions of a 0-km elevation and the same incident flow speed, the flow reattachment length of the Venusian dune is slightly smaller than that of Titan's dune. This may be caused by differences in the thermo-hydrodynamics of planetary atmospheric fluids and differences in the turbulent flow represented by the flow Reynolds number. In essence, for Venusian dunes with high Reynolds number turbulence, the kinematic viscosity, as the only variable parameter, has no significant impact on the flow reattachment length. At the same wind intensity conforming to the in-situ data, for both Venus and Titan, the dune areas where the wind can cause fine sand of the same size to take off and subsequently be eroded tend to be located on the crest of the dune. The maximum dimensionless friction velocity, which is used to characterize the erosion intensity, decreases linearly with increasing elevation on Venus. This indirectly clarifies the previous argument about the relationship between planetary atmospheric density and aeolian geomorphology scales in a new light. The quantitative comparisons of the eroded extent and intensity on dune surfaces suggest that the induced effect of the atmospheric environment on Titan at 0 km is the closest to that on Venus at 11 km. This research can provide inspiration and a theoretical reference for the study of comparative planetology in the field of aeolian geomorphology.