Laboratory Studies of Low Vortices in a Low Froude Number Flow Pasting over an Isolated Obstacle

The present paper mainly focuses on the generation and development of lee vortices for the low Froude number density stratified flow passing over an isolated obstacle. A series of laboratory experiments were carried out by towing-tank equipments to simulate the airflow passing over a large terrain. Based on the similarity principle, the Froude number of the stratified flow, the stratified condition, and the rotation parameter are chosen in the specified scope. Our studies show that the lee vortices can appear in the non-rotational and rotational cases. For the non-rotational fluid, the lee vortices generate only under the condition of strong stratification and the small Froude number (0.1 < F_r <0.3). However, the lee vortices can be formed under the condition of relatively strong stratification and the proper Froude number (0.1< F _r <0.5) for the rotational fluid. The results of the divided experiments including non-rotational and rotational cases show that the formation mechanism of lee vortices is different for non-rotational flow and rotational flow. The lee vortices are created by the tilting term of perturbation velocity for the non-rotational flow, but for the rotational flow, the β-effect associated with the horizontal convergence and divergence of the horizontal perturbation velocity is more important.