THE MASS INJECTED TIP— ACTIVE AND PASSIVE 



As implied, tip mass injection involves the ejection of a fluid, linearly down- 

 stream, directly into the tip vortex core. The mass injection process increases the 

 vortex core axial pressure and accelerates the vortex decay through the viscous 



interaction of the irrotational jet and the rotational vortex core. The effective- 



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 ness of this device has been demonstrated repeatedly in air flow studies with 



little or no effect on wing or rotor performance. Although there is no data on the 

 correlation between the water mass injection rates required to delay tip vortex 

 cavitation and the reported air mass rates required to reduce vortex core vorticity, 

 this active concept may prove to be an effective means of delaying tip vortex cavita- 

 tion if the required delivery rates or power are small. 



The above mass injection scheme will obviously require an energy source. A 

 somewhat similar concept--the passive mass injected tip — requires no external energy 

 source and consists of a hole or channel connecting the foil tip pressure and suction 

 sides. The objective is to locate the hole in such a fashion that mass flow is 

 diverted from the tip pressure side directly into the forming tip vortex core on the 



foil suction side. This concept, which evloved out of the literature survey, is 



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 new. However, the idea was obtained from work in the aircraft industry for porous 



wing tips. The success of the passive mass injection tip will depend upon the crit- 

 ical location and orientation of the channel and the magnitude of the diverted mass 

 flow. 



METHODS AND PROCEDURES 

 FACILITY 



The tip vortex delay study was conducted in the DTNSRDC 24-inch variable pres- 

 sure water tunnel (VPWT) . This tunnel is a closed-duct circuit oriented in a vert- 

 ical plane in which water is circulated by a motor-driven impeller located in the 



3 

 lower horizontal leg. The tunnel capacity is 13,600 gal (51.5 m ) and the maximum 



water speed is 44 ft/sec (13.4 m/sec). Tunnel pressure — to 35 psia (0 to 2.38 x 

 10 Pa) — was varied by changing the air pressure on the water surface at the top of 

 the tunnel. The water level is automatically maintained at a constant 3.42 ft 

 (1.04 m) above the centerline of the test section. 



A 27-in. (0.686-m) diameter, semiclosed jet test section was used for the tip 

 vortex experiments in the 24-in. VPWT. This test section had a conic transition 

 to horizontal walls which produced approximately uniform flow over the hydrofoil. 



