370 



-e,-^ A A- 



EXP X/C 



D 0.033 



i 0.10 



O 0.25 



a = 3.25 + 1.0 Sin (<Jt 

 V, = 13.1 m/s 



5 10 15 20 25 



FREQUENCY, HZ 



FIGURE 6. Mean pressure coefficients deduced from 

 oscillating tests. 



test program always initiated near the foil leading 

 edge. 



Let (dCp/dajg denote the static pressure gradient 

 with respect to foil angle at a given location on 

 the foil. Similarly, let (dCp/da)^ denote the 

 dynamic pressure gradient with respect to foil angle 

 at the same location on the foil with the reduced 

 frequency, K, as the parameter. To simplify the 

 writing, they will be referred to as the "static" 

 and "dynamic" angular pressure gradients respectively. 

 Let g (k) be the ratio of dynamic angular pressure 

 gradient versus static angular pressure gradient 

 at a given location on the foil, namely 



5{K) = (dC /da 

 P 



) / 



(dC /da) 

 P s 



(9) 



This ratio S (K) and the phase angle (fi for several 

 locations and reduced frequencies have been calcu- 

 lated and are given in Table 2. The static angular 

 pressure gradient (dC /da) at a given location is 

 approximated for mean foil angles of 3.3 to 4.3° 

 since leading edge cavitation inception typically 

 occurred within this range. As seen in Table 2, 

 for a given reduced frequency, the amount of 

 reduction in dynamic pressure ratio (5) remains 

 almost a constant value in the range of 0.004 < 

 X/C < 0.06 which covers the foil region over which 

 leading-edge cavitation occurs. Consequently, if 

 the foil is oscillated around the mean foil angle 

 cXq, the shape of the pressure distribution in the 

 neighborhood of the suction peak and the peak 

 location are essentially the same for both zero 



1.5 , 



1.0- 



S .5- 



r\ 



I \ 



1 



^^V - - CpsmIn'3-5) * Cp3„,^(3.25) 



o 



THEORY AT Oj = 3.5 DEG 

 THEORY AT Og = 3.25 DEG 

 EXP. AT a = 3.25 DEC 



.05 



.10 



.15 



X/C 



.20 



.25 



FIGURE 7. Static pressure distributions at 

 foil angles of 3.5 and 3.25 deg. 



