Borden 



most critical. At flow velocities above 28 knots, all types of roughness must be 

 kept extremely small to avoid cavitation. 



Figures 6, 7, and 8 show the effect of the shape of the boundary layer profile 

 on cavitation inception over three of the roughness geometries: triangles, cir- 

 cular arcs, and cylinders. Cavitation inception over the triangle and circular-arc 

 roughnesses is particularly critical to changes in velocity profile because of the 

 averaging process needed to represent the local flow velocity u^ over the two- 

 dimensional roughness elements. 



An increase in water temperature from 35 to 75 °F increases the vapor 

 pressure head from 0.23 to 0.86 ft of water and decreases the kinematic viscosity 

 from 1.87 to 1.04 x 10"^ ftVsec. The change in vapor pressure is equivalent to 

 a decrease in submergence depth of 0,63 ft, which makes the body only slightly 



24 26 28 



FLOW VELOCITY IN KNOTS 



Fig. 6 - Effect of the velocity profile on 

 cavitation inception on a two-dimensional 

 triangular roughness (C = -0.5, 5 = 0.4 

 in., T = 54"^) " 



192 



