475 



behavior recorded on tape . An example of the 

 recorded signals is shown in Figure 8. 



As light sources, two 1,000 Watt spotlights were 

 used. To get a proper background without reflections 

 the hydrofoil was painted with a red matte paint. 



A test was performed with black and white film 

 (Kodak 2479 RAR Film) . The result was not very 

 good, the contrast between hydrofoil and cavitation 

 being too small. Color film (Kodak Vide News Film) 

 was then used, with very good results. 



Evaluation of Films and Pressure Signals 



The pressure pulse generated by a cavity is related 

 to the volume acceleration of the cavity and thus 

 it is desirable to measure the cavity volume as a 

 function of time. With complex cavities this is 

 not very simple. An estimate of the cavity volume 

 could be obtained if both cavity extent (area) and 

 thickness were filmed synchronously. This is 

 possible by the use of optical systems reflecting 

 the two pictures into the same frame [Lehman (1966)]. 

 No such attempts were made. Most photographs were 

 taken in order to measure the cavity area on the 

 suction side of the hydrofoil. To obtain information 

 about the cavity thickness some photographs were , 

 however, taken from the free end of the hydrofoil. 

 A method of estimating the relative thickness, 

 synchronous with the cavity area, was to measure 

 the length of a cavity shadow generated by the 

 directed light. The method, which was calibrated 

 by use of spherical bubbles, was rather rough, but 

 some general information of thickness behavior was 

 obtained. 



The photographs were studied by use of an analysis 

 projector permitting single-frame projection on a 

 focusing screen, where the area of the cavities 

 could be measured by summing up elements in a 

 pattern. For identification of cavitation events 

 on the films and noise recordings the synchronization 

 flash was the primary starting point. To increase 

 the accuracy of identification of events far from 

 the flash easily identifiable events, such as 

 single bubble collapses, were used as reference 

 points. 



is used: 



5 = oscillation angle 

 f = oscillation frequency 

 In the figures the reduced frequency 



Tif c 

 _ !^ _ osc 

 c - 2U U 



where 



" = 2^fosc 



c = chord length of the hydrofoil 



U = water velocity 

 After some introductory tests the following con- 

 ditions of hydrofoil oscillation were selected from 

 high-speed filming: 



Results 



Primary results are presented as pressure signals 

 from cavitating and non-cavitating hydrofoils, 

 measurements of cavity area, and sketches of the 

 cavitation pattern at various oscillation parameters. 



Presentation of Results 



Experiments 



The experiments with an oscillating hydrofoil 

 presented in this paper are the first of this kind 

 carried out at SSPA and they are to be regarded as 

 introductory in several respects. 



Only one hydrofoil was used. The following 

 flow parameters were held constant during the tests: 



Relative gas content (at atmospheric pressure) 



of the tunnel water was 25^ 



Water velocity in test section = U = 5.0 m/s 



Cavitation number at the center of test section 



where 



Pq - Pv 



ipu2 

 2 



0.76 



Pq = surrounding pressure = 11.850 Pa 



P^ = vapor pressure of water (20°C) = 2.338 Pa 



p = density of water = 998 kg/m^ 

 The following oscillation parameters were varied 

 in the experiments (see Figure 7) : 



ttQ = mean angle of attack of the hydrofoil 



In Figures 9 - 14 a survey of pressure signals 

 and cavitation patterns at various oscillation 

 conditions is shown. All pressure signals shown 

 in these and other figures are from the hydrophone 



(HI) near the leading edge of the hydrofoil. For 

 each condition some oscillation periods are shown. 

 The length, Tog^, = 1/fQgQ, of an oscillation period 

 is identified by the markings of maximiom angle of 

 attack, Omax- '^^^ figures show primarily cavitating 

 conditions (cavitation number = 0.76) but in some 

 cases signals from the corresponding non-cavitating 

 condition is sketched (without the fine structure, 

 which is apparatus noise) . The pressure scale is 

 given as a number of Pascal (Pa) per scale unit 



(su) defined at the top of the figures. The time 

 scale is 5.15 ms/scale unit in all signal examples 

 in Figures 9-13. For one of the oscillation periods 

 the number of the oscillation period (relative to 

 the synchronization flash) is shown in a circle, 

 and for this period some additional data is given 

 to the right. In the cavitation sketches are shown 

 the maximum area extent, the maximum chordwise 



cavity length, J,^ 



and the cavitation extent at 



