NUMERICAL STUDY OF VERTICAL SHEAR AND STRATIFICATION EFFECTS ON THE EVOLUTION OF A VORTEX PAIR

The objective of this study is to investigate with a numerical model how coexisting vertical shear and stratification affect the evolution of a vortex pair. Our results show that the Richardson number (the ratio of stratification forces to shear forces) is an important parameter in the evolution. When the Richardson number is large, the vortex pair evolves more or less symmetrically, the left and right vortex decaying equally, or nearly equally, rapidly. In this case, the effect of the shear is small, and stratification dominates the evolution. When the Richardson number is sufficiently small, however, the evolution is dramatically different. Under these conditions, the vortices evolve asymmetrically, and the vortex with rotational sense opposite to that of the mean shear decays while the vortex with the same rotational sense as the mean shear survives. This “solitary vortex” then decays slowly with time. The transition from solitary to nonsolitary vortex evolution occurs over Richardson numbers between 0.5 and 4 for the Froude number range 0.5 to 4.0. © 1990 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.