368 



5 5- 



a - 3.25 + 0.5 Sin a.'t 



X/C EXP. THEORY 



0.033 O - - - 



0.10 A 



0.25 O 



-Q DO □ g -TT 



1.0 1.5 



REDUCED FREQUENCY. K 



1.0 1.5 



REDUCED FREQUENCY. K 



FIGURE 5a. Magnitude of dynamic pressure response at 

 ai = 0.5 deg. 



FIGURE 5c. Magnitude of dynamic pressure response at 

 ai = 2.0 deg. 



E 



a 



Ol = 3.25 + 1.0 Sin "it 



X/C EXP. THEORY 



0.033 D • - - 



0.10 A - — 



0.25 O 



^% 



4^ 4*-A 





©-' 



bv^ 



p. © 



c?-' 



1.0 1.5 

 REDUCED FREQUENCY, i; 



FIGURE 5b. Magnitude of dynamic pressure response at 

 ai = 1.0 deg. 



plotted in Figure 6. Aside from some sc^atter in 

 the data, they are seen to be independent of 

 frequency (or reduced frequency) . The steady 

 pressure distribution calculated theoretically at 

 3.25° is given in Figure 7. A suction peak appears 

 at around 1.8 percent of the chord length aft of 

 the leading edge. Reasonably good agreement between 

 the theoretical prediction and the three experimental 

 measurements should be noted. Experimental data 

 confirm the basic assumption made in Eq. (2) that 

 the total pressure coefficient Cp(t) is the sum of 

 the dynamic pressure coefficient Cpu(t) plus the 

 static pressure coefficient Cpg at the mean foil 

 angle, i.e. 



Cp(t) 



ps 



C (t) 

 pu 



We will now proceed to examine the possible 

 relationship between the dynamic pressure coeffi- 

 cient Cpu(t) and the static pressure coefficient 

 Cps. Let the instantaneous foil angle be expressed 

 as in Eq. (1). The dynamic pressure response is 

 then given by 



-C (t) 



pu 



AC 



pu 



sin (cut + (j)) 



(5) 



Here |ACpu| and (|) are the amplitude and phase angles. 

 They are functions of reduced frequency K and 

 location X/C. They can be obtained either from 

 experimental measurements or theoretical calculations. 

 In the following study, Giesing's program will be 

 used to compute these variables. In our oscillating 

 tests, the mean foil angle was always maintained at 

 a = 3.25°. The type of cavitation observed in our 



