NASA NASA-NGR-17-002-043

NASA NASA-NGR-17-002-043 1968-JAN-01 Wnd tunnel nvestgatons of vortex breakdown on slender sharp edged wngs

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Systematic wind tunnel investigations of vortex breakdown have been conducted on sharp-edged delta and modified delta wings with sweep angles from 45° to 85° at Reynolds numbers of about 1 x 106, utilizing a schlieren system for flow visualization. Vortex breakdown positions are presented as a function of angle of attack and sweep. Results agree well with the limited amount of previously published data. For the very slender wings (75° to 85° sweep), a result not anticipated by extrapolation of previous data.

Lift measurements show that Pohlhamus' leading-edge suction analogy provides an excellent means for predicting lift for wings with sweep angles between 65° and 80°, at angles of attack below vortex breakdown. For very low sweep ( 65°) and very high sweep ( 80°) less than theoretical lift is achieved. The reasons for these discrepancies are discussed in some detail in the paper.

Drag due to lift is predicted quite satisfactorily as the streamwise component of a normal force for all models tested. Although no satisfactory method for predicting pitching moments is available, experimental effects of vortex breakdown on pitching characteristics are presented.

Results of tests of the modified delta wings show that vortex breakdown is influenced much more strongly by plan-form changes near the apex than changes near the trailing edge. Cropping the wing tips or changing trailing-edge sweep had practically negligible effects on vortex breakdown. The addition of strakes (double-delta and ogee wings) may greatly delay initial breakdown. It is shown that vortex breakdown and force measurements from the present tests of an ogee wing agree quite favorably with NASA flight test results at a twenty times larger Reynolds number.

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