| Dust storms are an extreme weather phenomenon that usually occur in arid and semi-arid regions.The two-phase flow motion of wind and sand on the gravelly Gobi surface during dust storms has been an issue of great interest in the physics of wind and sand.A systematic and comprehensive study of the evolution of wind and sand and the underlying physical mechanisms is important to elucidate the occurrence of dust storms and the prevention and control of wind and sand disasters in the source area.Due to technical limitations,field observations and wind tunnel experiments are difficult to capture large-scale particle motions,and the wind-sand motion processes in the Gobi region are generally difficult to observe due to the influence of experimental instruments,time-varying wind fields and turbulence instability,especially the description of near-surface wind field structures with different gravel cover is very difficult.Therefore,this thesis adds additional control equations for sand particles to the basic control equations for unpressurizable fluids,and analyzes the wind-sand two-phase flow motion law and dust release process in the incoming dust storm under single(multiple)gravel cover by constructing a three-dimensional field numerical simulation with a fluid-particle collisional mutual feedback model.It is found that the modified results based on the 3D field fluid-particle collisional feedforward model can simulate the wind-sand two-phase flow motion on the Gobi surface more accurately.The simulation results further indicate that:(1)Under the influence of single gravel,the wind speed of the sand-holding wind is reduced more obviously compared with the net wind;the sand-holding wind on the leeward side of the gravel can restore to the wind speed on the incoming side more quickly.The gravels can influence the vertical height range of 0.04m～0.07 m,which is the most active area for sand movement.At 5%～10% cover of the gravel bed,the sand movement is consistent with that of single gravel,i.e.,the wind velocity of the sand-holding wind is greater than that of the net wind.20% cover of gravel is a special turning point,where the sand-holding wind and the net wind are nearly identical in their blocking benefits.When the cover increases to 30%,the minimum value of the sand-holding wind speed is higher than the minimum value of the net wind speed,and the blocking benefit continues to weaken;the sand-holding wind and the net wind speed stabilize when the cover reaches 40%～50%.(2)During different incoming dust storms,the changes of wind-phase velocity with height are all consistent with the logarithmic function,and their velocities decrease with the increase of gravel cover,and form a Bagnold focus near the surface of the Gobi where the gravel cover is greater than 40%.Compared with the wind phase,the velocity reduction effect of the sand phase is not obvious.In addition,above 4cm from the gravel bed surface,the velocities of both wind and sand phases remain basically the same.(3)When the incoming flow velocity is greater than 10m/s,1.5m～2.1m height range from the ground to form an obvious intersection of the two-phase flow velocity,the intersection of the upper and lower performance of the “ Wind is faster than sand ” and“ Sand is faster than wind ” phenomenon,and below this range is the zone of weak wind-phase sand carrying capacity.And the phenomenon of “ Sand is faster than wind ”is especially obvious in the intense collision area below 4cm.(4)During the dust storm,the near-surface dust concentration is “ Pulsating ” along the mean line,which is a cyclic process of surface dust deposition and re-release,while the gravel cover size directly affects the dust deposition distribution,and can effectively suppress dust release when the cover is greater than 40%.In summary,the results of the simulations in this thesis all indicate that gravel cover of 40% or more can prevent dust movement,but it is possible that it can only effectively inhibit the initiation of near-source dust,when there is a distant source of wind and sand flow,the particulate matter in the hostage wind can still drive the surface sand movement,and then form a more intense dust storm,resulting in a greater overall dust release on the Gobi surface than on the desert surface. |
