Numerical Researches On The Emergence And Development Of Aeolian Sand Ripples Under The Coupling Action Of Wind Field,Sand Grain And Dynamic Bed Surface

On Earth,Mars and many other planets with atmosphere,due to the wind erosion,granular surface becomes a frequently appearing landform.On this kind of bed surface,the aeolian sand ripple is the mostly observed surface pattern.It is widely distributed,easy to form and can develop rapidly.Therefore,since the seminal work of aeolian particle transportation,the sand ripple has always been considered as a typical characteristic in atmospheric,geological and planetary meteorological researches.Within a particle transport system,the bed grain size,the wind strength and the near-wall wind field all affect the characteristic of the sand ripple.As bridges that connect the sand ripple morphology with the local wind field and particle properties,these factors should be focused during the research of aeolian sand ripples.However,due to the limitation of the observation methods and the complex of the sand ripple system,current researches haven't brought out any systematic work that contains all these aspects.In the other hand,the aeolian sand transport is always accompanied by sand ripples,which leaves us a question that “will the sand ripple influence the particle transport?” In the wind tunnel researches of recent years,more and more attentions have been paid on this question,but further studies on this phenomenon is still needed.Based on Reynolds averaging and large eddy simulation,respectively,this paper establishes two dynamic bed surface numerical models,which can direct simulate the emergence and development aeolian sand ripples within the aeolian sand particle transport system.Using these models,following works have been done:By simulating the growth of aeolian sand ripples,this paper reproduces the entire three-dimensional development from the flat bed surface to the sand ripple surface.The obtained sand ripples are similar to those in nature and they directly reveal the temporal variation rules of sand ripple wavelength and amplitude.With different particle size distributions,we study the development of sand ripples and found that on the bed surface composed of bi-disperse particles,no matter whether the coarse particles can be splashed or not,they tend to accumulate on the crest of sand ripples.This phenomenon causes a bigger initial wavelength and slows down the development rate of sand ripples within the nonlinear stage.What's more,for the first time,we directly reveal the spanwise development of sand ripples by a particle tracking numerical simulation.It was observed that the particles were more inclined to deposit at the break points of sand ripples.In order to obtain a proper theory that reflects the particle transport upon ripple surface,we first studied the variation of the wind profile during the sand ripple formation process and found that the hydrodynamic roughness would change with the sand ripple morphology.By analyzing how the roughness changes,this paper reveals that the morphological deformation of sand ripples after the linear stage will cause a linear increase of the dynamic threshold with time.By running numerical cases with specific pre-rippled bed surfaces and combining the simulation results with theoretical analysis,we finally derived the scaling law between the sand flux on the ripple surface and the wind strength.We found that the sand flux is more sensitive to the ripple bed surface while the wind strength is small.To find the factors that from the particle transport system affecting the initial wavelength of sand ripples,the numerical simulation results corresponds to the model with or without mid-air collisions were compared.From this comparation,we directly proved that mid-air collisions control the sand ripple wavelength.Analyzing the particle trajectories on different pre-rippled surfaces,we revealed the mechanism that influences the sand ripple wavelength and obtained the scaling law between the initial wavelength and the wind strength.We studied the influences from the complex wind field on the development of aeolian sand ripples.It was found that the complex wind field can accelerate the sand ripple formation.Since complex wind field enhances the effects from particle relaxation time,additional effects are brought to the sand ripple initial wavelength result.We also found that for saturated particle transport,the fluid shear stress on the ripple surface is no longer directly related to the dynamic threshold.As sand ripple grows,it will gradually decrease with time.This is because the normal stress on the stoss slope of sand ripples brings additional momentum loss.With complex wind field,the fluid shear stress on the stoss slope or on the crest is always larger than the dynamic threshold shear stress of particles,which not only leads a faster ripple migration rate but also makes the ripple index closer to the measurement results than that derived from the averaged fluid field model.