Figure 1 is a sketch of an “aerodynamic seal”. This type seal has potential application where it is necessary to control leakage of fluid through a clearance space bounded by one moving wall. For mechanical reasons it is necessary to maintain a clearance, in this case equal to the throat width t, between the inner wall I and the outer wall O. To prevent flow from the high pressure chamber H from passing into the low pressure chamber L, pump P supercharges fluid from H and forces it through nozzle J. The resulting fluid jet, which may be aided in following the curved surface by the Coanda effect, passes through the throat t and into diffuser D which recovers part of the entering velocity head. In the ideal case, with no losses and one-dimensional flow, the per cent recovery of inlet velocity head may be made as near to 100% as desired, since the diffuser recovery coefficient becomes asymptotic to 1-0 with increasing area ratio. Conversely, the dumping loss may be made as near to zero as desired. The attractiveness of this proposal hinges on diffuser performance. The pump is necessary to make up for the velocity head lost at the diffuser exit and for flow losses in the system. The pressure at the diffuser throat should be approximately that of the low pressure chamber for optimum performance. For large pressure ratios it may be necessary to stage two or more seals in series.