Numerical Studies On The Evolution Of Wind-blown Sand On Mars

The movement of wind-blown sand plays an important role in land surface processes and atmospheric radiation on Mars. The study of wind-blown sand will be not only beneficial for recognizing the history of and predicting the evolution of Martian land surface and atmosphere, but also for studies of Aeolian processes on other extraterrestrial planets.This thesis put forward a numerical model of sand saltation, which adapts to various Aeolian surroundings. The model provides true and timely initial conditions for lift-off grains, which are no longer depending on semi-empirical or empirical results from existing theories or measurements as previous models did, with the evolution of wind-blown sand. In contrast to previous models, new model has three advantages: an improved stochastic sand-bed collision model that includes the influences of impact parameters on the speed restitution of saltating sand was employed to solve the sand-bed collision process analytically; the speed-angle dependent statistics was conducted considering the correlation between speeds and angles of saltating sand; the surface roughness of a quiescent sand bed was expressed as a function of wind speed, particle diameter, air density and viscosity. The new model was verified through comparisons of simulated results and terrestrial measurements, which demonstrates an accurate modeling of wind-blown sand needs both real and timely initial lift-off conditions for saltating sand but not semi-empirical or empirical ones.With the help of the model, the evolutions of wind-related characteristics(wind profile, fluid shear stress, grain shear stress and aerodynamic roughness) and backward saltation in wind-blown sand on both Mars and Earth were studied in this thesis for the first time, in cases of different wind speeds and particle diameters. Also, the evolutions of saltating speed, saltating trajectory and transport rate on the two planets under different wind speeds and particle diameters were investigated, and the simulated Martian results were compared with previous studies.Thus, some new results are obtained: different distribution laws of lift-off and incident velocities of saltating sand between on Mars and Earth, the reverse of surface shear stress with the increase of wind speeds on Mars, a higher proportion of backward saltation on Mars than that on Earth, and the rise of incident angle with the increase of wind speeds for medium and coarse sand on the two planets. Besides, some uniform expressions for physical quantities(impact threshold, the height and the corresponding speed of Bagnold focus, aerodynamic roughness and saltation sand transport rate) in wind-blown sand, which could be applied on both Mars and Earth, are found. Furthermore, it reveals that all Aeolian-related physical quantities in wind-blown sand are significantly affected by particle size except for lift-off angle. The averaged lift-off angles show weak sensitivity about the change of environment and are approximately equal on the two planets. Additionally, above analyses indicate that a reasonable description of sand-bed collision process is the key to simulate wind-blown sand accurately.