381 



JU- 



u 



^-^"^ "^ — ^ s — ^L.,j\__^' 



PRESSURE GAGE P, 



PRESSURE GAGE Pj 



PRESSURE GAGE P 



FOIL ANGLE (K=0) 



CAMERA PULSE TRACE 



FIGURE 22. Surface pressure 

 fluctuations for K = 0, 

 V^ 11.5 m/s, P^ = 76.2 kPa, 

 «"= 3.25°. 



SIDE VIEW 



TOP VIEW 



FIGURE 23. Alternate spanwise 

 cloud cavitation shedding for 

 K = 0, V^ = 11.5 m/s, P^ = 76.2 

 kPa, a ="3.25. 



lation supports McCroskey's conclusion that unsteady 

 viscous effects on fully wetted oscillating airfoils 

 are less important than unsteady potential flow 

 effects, if the boundary layer does not interact 

 significantly with the main flow. 



Six series of oscillating foil experiments were 

 carried out in this test program to study the 

 leading edge sheet cavity growth and collapse. 

 A simplified mathematical model was developed to 

 explain experimental results for leading edge sheet 

 cavitation inception. The mathematical model 

 utilizes Giesing's method for calculating the 

 unsteady potential flow. A significant delay in 

 unsteady cavitation inception was both predicted 

 and measured. A further delay in cavitation 

 inception was also observed and predicted with 

 increasing pitch amplitude. It is shown that 

 unsteady cavitation inception is a function of: 



(1) the ratio of dynamic to static angular 

 pressure gradients 



(dC /da) / (dC /da) 

 p u p s 



and. 



(2) the phase shift between the foil angle 

 and the dynamic pressure response. 



Due to the phase lag in pressure response a signifi- 

 cant delay in unsteady cavitation inception is 

 predicted theoretically and observed experimentally. 

 Additionally, the angle at which cavitation inception 

 occurs increases with increasing pitch amplitude. 

 This effect results from a change in the phase angle. 



It is well known that even in a steady condition 

 the cavitation inception process is extremely complex. 

 The theoretical prediction is still very difficult. 



