A special feature of internal flows with cavitation is the coupling 

 of the fluid motion in various parts of a system by the displacement of a 

 changing volume of cavitation. This unsteadiness may arise deliberately 

 as a part of a test procedure (e.g., a study of propeller-wake inter- 

 action), or it may happen that the cavitation itself is inherently unsteady. 

 (Partial cavitation of about three-quarters chord length provides such an 

 example.) Because of the coupling of the flow with unsteady cavitation, 

 there is an interaction between the internal flow circuit and the basic 

 flow past the cavitating body itself. This interaction may, in fact, so 

 control the whole process that the original basic flow may be entirely 

 masked (as for example in an unsteady hydrofoil test) . The confining 

 effect of a tunnel has been in fact a serious deterrent to measurement of 

 unsteady forces on cavitating hydrofoils in the past. We need ways, then, 

 to correct for the "unsteady" internal effect, as well as for the better- 

 understood steady wall effect. 



It may happen that, because of cavitation and circuit-cavitation 

 interactions, an inherently steady or stable cavitation flow may exhibit 

 self-excited oscillations. Such oscillations occur frequently in pump 

 applications or in more complex systems, such as pump-fed liquid fueled 

 rocket. This is the basis of the notorious "Pogo" longitudinal oscilla- 

 tions of large liquid booster rockets. In fact, some of these Pogo 

 oscillations have reached acceleration levels of 50 g's* and it is 

 customary for all liquid fueled rockets to suffer some extent of this 

 instability. Cavitating pumps are usually thought of primarily in 

 connection with space applications. However, the types of designs and 

 performance demands of modern high-speed naval craft, such as the PHM 

 hydrofoil and surface effect ship propulsion systems, are very similar to 

 those for large booster propulsion. But it is not necessary to go so far 

 afield to find situations where unsteady cavitation-system coupling presents 

 a problem of interpretation. A good example is afforded by the unsteady 

 hull pressure-propeller cavitation measurements of Keller and Weitendorf 

 (1970). In their tests, the pressure at several points on a wall adjacent 

 to a cavitating propeller was measured in a closed tunnel as a function of 



*Dr. S. Rubin, Aerospace Corp. 



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