新月形沙丘不对称性的起源：数值模拟的启示
Origins of barchan dune asymmetry: insights from numerical simulations
摘要：
新月形沙丘形成于单向风和低沙可用性的地区，通常呈现不对称的形状，一个分支延伸到顺风。几个因素已被确定为地球和火星新月形沙丘不对称的潜在原因：不对称的双峰风况，地形，流入不对称和沙丘碰撞。然而，动态和潜在范围的新月形形态下出现的每一个特定的情况下，导致沙丘的不对称性是远远没有被理解。在目前的工作中，我们使用沙丘模拟，以调查的形成和演变的不对称barchans。我们发现，当一次风和二次风之间的辐散角大于$90^{\circ}$时，双峰风场会导致边缘延伸，而当二次风和一次风之间的输送率之比大于25%时，延伸的边缘会演变成自沙丘。在恒定风向条件下的倾斜表面上沙丘形成的计算也导致新月形的不对称性，但是单独从表面倾斜没有得到seif沙丘。沿倾斜地表迁移的不对称新月体横向迁移，横向迁移速度与地形坡度成正比。当新月形新月形新月此外，短暂的不对称新月形延伸的肢体也出现在沙丘之间的碰撞或由于不对称的流入。我们的研究结果可以用于定量推断当地的风制度或空间异质性的行星沙丘领域托管不对称barchans的运输条件。
英文摘要：
Barchan dunes --- crescent-shaped dunes that form in areas of unidirectional winds and low sand availability --- commonly display an asymmetric shape, with one limb extended downwind. Several factors have been identified as potential causes for barchan dune asymmetry on Earth and Mars: asymmetric bimodal wind regime, topography, influx asymmetry and dune collision. However, the dynamics and potential range of barchan morphologies emerging under each specific scenario that leads to dune asymmetry are far from being understood. In the present work, we use dune modeling in order to investigate the formation and evolution of asymmetric barchans. We find that a bimodal wind regime causes limb extension when the divergence angle between primary and secondary winds is larger than $90^{\circ}$, whereas the extended limb evolves into a seif dune if the ratio between secondary and primary transport rates is larger than 25%. Calculations of dune formation on an inclined surface under constant wind direction also lead to barchan asymmetry, however no seif dune is obtained from surface tilting alone. Asymmetric barchans migrating along a tilted surface move laterally, with transverse migration velocity proportional to the slope of the terrain. Limb elongation induced by topography can occur when a barchan crosses a topographic rise. Furthermore, transient asymmetric barchan shapes with extended limb also emerge during collisions between dunes or due to an asymmetric influx. Our findings can be useful for making quantitative inference on local wind regimes or spatial heterogeneities in transport conditions of planetary dune fields hosting asymmetric barchans.
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