Experimental study on the origin of lobe-cleft structures in a sand storm

Abstract

Lobes and clefts are characteristic structures at the front of sand storms. In this paper, their original formation mechanism and geometric features are studied experimentally and theoretically. A rotatable lock-exchange tank is utilized to avoid the strong local disturbances existing in the conventional horizontal apparatus, and the original lobe size is selected as the dominant spanwise wavelength by a statistical method instead of the mean lobe size used in the literature. It is shown that at the initial formation stage, the measured lobe sizes for different tank geometries and density differences (or Atwood numbers) have a 1/3 scaling law with the Grashof number (Gr) as predicted by a Rayleigh–Taylor (RT) model, especially at moderate Gr range, and substantially depend on the diffusion effect represented by the Schmidt number. Furthermore, using turbulent Schmidt number and eddy viscosity for atmosphere in the RT model predicts large scale dominant lobes, whose sizes agree qualitatively with those observed in a real sand storm, revealing that the underlying control mechanism for these lobe and cleft structures is intrinsically related to the RT instability.