Cavitation Inception on Rough Bodies 



28 30 32 



FLOW VELOCITY IN KNOTS 



Fig. 7 - Effect of the velocity profile on 

 cavitation inception on a two-dimensional 

 circular-arc roughness (C = -0.5,2 = 0.4 

 in., T = 54^) 



more susceptible to cavitation. The change in viscosity, however, makes a sig- 

 nificant change in Reynolds number. As the temperature increases, the cavita- 

 tion inception velocity decreases significantly on those roughness elements 

 which are particularly sensitive to Reynolds number effects. Figures 9 and 10 

 show how the critical roughness heights of triangles and cylinders vary with flow 

 velocity for the two extreme temperatures. Whereas a 40 °F temperature change 

 decreases the cavitation inception speed for two-dimensional triangular rough- 

 ness elements by only about 0.5 knot, it decreases the inception speed for three- 

 dimensional cylindrical roughnesses by about 2 knots. 



Figures 11 and 12 show the relationships between roughness heights, the 

 boundary layer thickness, and the pressure coefficient of the parent body. It 

 can be seen in both figures that roughness is most critical in small boundary 



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