Abstract
Two aspects of modeling the impact of a characteristic thick boundary layer developing along the inner duct surface with inviscid elements are explored. First, it is argued that the trajectory of shed-vortex filaments adjacent to the walls depends on the boundary-layer speed reduction and the duct contraction selected to achieve decelerating through-flow control. Self induction associated with the forced contraction carries the tip-vortex filaments toward the duct surface, producing locally large surface speeds. This interaction calculation is combined with other components of a simple inviscid model of ducted-propulsor performance: axisymmetric duct and hub boundary-element approximations coupled with a lifting-line representation of each blade row. Parameters of the overall model are selected to bring about correlation with the circumferential-mean pressure distribution on the duct and component loads acting on the blades of the ERG Pumpjet, a strongly decelerating ducted propulsor.
A second impact of the developing axial flow-speed reduction near the passage walls is a local mismatch between the flow angle and rotor surface slope. Parametric numerical estimates using an inviscid lifting-surface design code illustrate the degree of change in the rotor blade-tip geometry needed to achieve flow alignment.
The rotor for the ERG Pumpjet has a large-diameter hub with significant crosssection area. An adjustment in shaft thrust to compensate for the increased pressure acting in a hub gap aft of the rotor produces a corrected axial force level more in line with other decelerating ducted propulsors.
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