Numerical Experiments of Vortices in the Wakes of Large Idealized Mountains

Abstract

The Purdue Mesoscale Model (PMM) is applied to study the flow past large idealized mountains under a low Froude number. Results show that for Reynolds numbers in the range of 4 < Re < 1000, as long as the flow is symmetric to the central line of a symmetric mountain, two vortices remain stably attached to the mountain. For Re≤100, the size of the attached vortices after 120 hours of integration increases linearly with the increase of Re, but the size decreases slightly with Re for Re > 100. Results also show that small perturbations in the oncoming wind, the inclination of the oncoming wind and major axis of the mountain, the mountain shape, and the Coriolis force all can contribute to atmospheric vortex shedding. The Reynolds number is not a good indicator of whether a vortex will stay or break away from the mountain in the atmosphere. When the earth's rotation is included, the simulated pressure field and wind increase considerably on the left-hand side (facing downstream) of the mountain, which is quite different from that of an irrotational flow, although the pattern of vortex shedding is similar. It is also found that the Reynolds number and ? effect can change the propagating speed but not the period of vortex shedding. On the other hand, the shape and size of the mountain and asymmetry of the oncoming wind can strongly influence the character of vortex shedding.