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An experimental and theoretical study is presented of the inlet-vortex (or ground-vortex) phenomenon. The experiments were carried out in a water tunnel using hydrogen-bubble flow visualization. The theoretical study is based on a secondary-flow approach in which vortex filaments in a (weak) shear flow are viewed as convected (and deformed) by a three-dimensional irrotational primary flow; the latter being calculated numerically using a three-dimensional panel method. Two basic mechanisms of inlet-vortex generation are identified. The first of these, which has been alluded to qualitatively by other investigators, is the amplification of ambient (i.e. far-upstream) vorticity as the vortex lines are stretched and drawn into the inlet. Quantitative calculations have been carried out to illustrate the central features connected with this amplification. I n contrast with what has been supposed, however, there is another mechanism of inlet-vortex formation, which does not appear to have been recognized previously and which does not require the presence of ambient vorticity. It is thus shown that an inlet vortex can arise in an (upstream) irrotational flow, for an inlet in cross wind. In this situation, the vortex is accompanied by a variation in circulation along the length of the inlet. The ratio of inlet velocity to upstream veIocity is an important parameter for both mechanisms.
Siervi et al. (Mon,) studied this question.