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A Theoretical Investigation of a Circular Lifting Jet in a Cross-Flowing Mainstream.

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Final rept. Jul 69-Dec 70,

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Finite-element potential-flow-modeling theoretical techniques are described which predict, from first principles, both the rolled-up geometry and the path of a round lifting jet convergent into a cross-flowing mainstream, as on VTOL or direct lift-assisted STOL aircraft. Starting with a straight-cylinder geometry, point vortex elements are perturbed using a pre dictor-corrector stepping method to give a first estimate of the bent-back shape, using assumed circulation values. A collocation scheme is next used to revise the circulation values, and after three or four iterations, a final exit-plane pressure distribution may be calculated. The fan-induced total pressure rise is simulated by injecting vortex rings at a chosen position in th duct which feeds the jet. Since the scope of the method is entirely non-viscous, separations toward the rear of real jets and the associated pressure changes are not simulated and base-pressure type of pressures cannot be expected. Nevertheless, for forward speed ratios of 0.1, 0.2, 0.3 and 0.4, the low-pressure contours at each side of the jet do show an increasing rearward shift, just as is found experimentally. Somewhat surprisingly, the simulated plumes were more stable at higher velocity ratios. At lower forward speeds, there was a tendency to flap, rather like a hose end when freed. It is anticipated that, if viscous effects were simulated, these motions might damp out. Author

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  • Aerodynamics
  • Fluid Mechanics

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