关键词: 金字塔沙丘, 流场, 动力学, 莫高窟

Abstract: The paper, based on wind tunnel experiment and sedimentary structure determination to the pyramidal dunes on top of Mogao grotto of Dunhuang, aims at re establishing the flow field of pyramidal dune surface and further discussing the dynamics processes of dune forming and evolution.The pyramidal dunes on top of Mogao grotto are formed in condition of wind blowing from three directions including NW, NE and SW. NE and SW wind directions have blow out the dune's main arm in E-NW direction, and WN wind direction has formed the secondary arm of it. The dynamics processes of pyramidal dune are that main arm deforms then produce secondary arms. The acting of special "assembling circulation" and "diffusing circulation" of pyramidal dune is the key force for dune's top forming. In the forming of pyramidal dune, the effective angle between flow direction and dune ridge must be more than 30°. Under this condition, an inverse accelerating flow velocity belt appears at leeward side of dune, which makes sand materials develop upward from the two sides of dune ridge. Otherwise, the dune will develop longitudinally. In the views of eolian landforms and evoluting processes, it is thought that some internal relations are existed among complicate sand dunes. One combination of multi direction wind regime can form a landform suitable to surface circulation in the processes of main and secondary arms (ridges) evoluting. When wind regime changes, the type of sand dune will evolute to another type suitable to the relative surface circulation. In return, landform is also the important factor determining the environmental wind regime. The integrated research combining sedimentary structure determining and wind tunnel experiment is suggested to be a more all round, systematic and further method than before single process research.

Key words: Pyramidal dune, Flow field, Dynamics, Mogao grotto