横向沙丘背风侧沙粒风蚀起动的风洞模拟

Wind tunnel simulation of the threshold wind velocity in the lee-side of transverse dunes

通过风洞实验,探讨了横向沙丘背风侧“二次流”的沉积学和形态-动力学意义。在不同迎风坡坡度的横向沙丘模型背风侧,我们观测了不同位置沙粒的起动风速以及在临界状态下沙粒的运动特征。结果表明,沙丘背风侧的颗粒起动风速不仅与其距沙丘顶部的距离有关,也与沙丘迎风坡坡度有密切关系。根据沙丘背风侧颗粒运动特征,可以将其划分为向后运动区域、晃动或摆动区域以及向前运动区域,产生这一现象的原因是在沙丘背风坡气流分离、反向涡和气流重新辐合共同作用的结果。在所有的观测结果中,迎风坡坡度为15°的沙丘具有最大的沙粒起动风速和最远的气流重新辐合距离,其原因尚需进一步研究。

Secondary flow plays an important role in the development of dunes. To investigate the sedimentological and form-dynamical significance of secondary airflow in the leeward side of transverse dunes, six models of transverse dunes with different stoss slopes were simulated in a wind tunnel. In the lee-side of different transverse dunes, the threshold wind velocities were measured by a pitot tube and the corresponding particle movement characteristics were described at various locations. The results showed that the threshold wind velocity in the leeward side of transverse dunes changes as a function of both the horizontal distance downwind and the stoss slope, and there are three main modes of particle movement that occur at the threshold velocity： forward movement （movement in the direction of the open-field wind）, backward movement （ movement in the opposite direction to the open-field wind）, and oscillating movement （alternating forward and backward movements）. Generally, the leeward area can be classified into three regions based on these different modes of particle movement. From upwind to downwind, it can be classified as the backward-movement region （with a typical width of 5 to 11 h）, the oscillating-movement region （0.2 to 0.7 h）, and the forward-movement region. This phenomenon is result from the separation （ especially the reverse vortex） and re-attachment of airflow in the lee-side of various transverse dunes. According to these results, the flow reattachment distance in the lee side of transverse dunes can be determined. Finally, we think that there is a further research need on the 15° stoss slop because it had the maximum threshold wind velocity and re-attachment distance.