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Fig. 19. Regions of positive and negative vorticity for `high' and `low' Re. Rn denotes regions of negative vorticity, Rp denotes regions of positive vorticity, and Ro denotes regions of negligible vorticity. (A) For a wing in a fluid moving from left to right at Re>64, an attached leading edge vortex with negative vorticity is formed. A trailing edge vortex of positive vorticity is formed and shed from the wing. This asymmetry in the time rate of change of the first moment of positive and negative vorticity produces lift. (B) For a wing in a fluid moving from left to right at an Re between 8 and 32, an attached leading edge vortex with negative vorticity and an attached trailing edge vortex of positive vorticity are formed. This `near-symmetry' in the time rate of change of the first moment of positive and negative vorticity reduces lift.





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