363 



by Tanibayashi (1962) . He concluded that the 

 occurrence of cloud cavitation in nonuniform flow 

 cannot be predicted on the basis of uniform flow 

 experiments. In earlier work by Tanibayashi and 

 Chiba (1968) , it was concluded from experiments 

 with an oscillating two dimensional foil that an 

 unsteady flow was required for the formation of 

 cloud cavitation. However, unlike the earlier 

 results of Ito, no distinct critical reduced 

 frequency was found. Since these latter results 

 were for nominally hemispherical travelling bubbles, 

 instead of a leading edge sheet, it remains to be 

 established whether the type of cavitation in the 

 growth phase is of importance to cloud cavitation 

 formation. 



Chiba and Hoshino (1976) carried out extensive 

 measurements of induced pressures on a flat plate 

 above a propeller. On the basis of comparing 

 results with and without a wake field and with and 

 without cavitation, they determined that strong 

 pressure impulses were detected on the flat plate 

 and these correlated with the presence of cloud 

 cavitation. 



Strong pressure fluctuations of very short 

 duration have also been detected by Meijer (1959) 

 on the surface of a cavitating two dimensional foil. 

 He attributed these pressure fluctuations to a 

 stagnation point at the rear of the sheet cavity 

 passing over a pressure gage. Chiba (1975) has 

 attempted to correlate cavity collapse on a two 

 dimensional oscillating foil with the response from 

 a pressure gage mounted in the foil. He concluded 

 that, as expected, when the shed vapor collapses 

 large pressure impulses occur. The essential 

 points for both of these experiments are that foil 

 mounted pressure gages can be used in the presence 

 of cavitation and when correlated in time with 

 photographs can assist in the interpretation of the 

 physical processes involved. This technique was 

 also used in interpreting the results to be reported 

 here. 



Two other oscillating foil experiments have also 

 been reported, Miyata (1972), Miyata et al. (1972), 

 and Radhi (1975) , that demonstrate the importance 

 of the reduced frequency on the whole cavity 

 inception, growth, and collapse process. Both have 

 shown that for the particular conditions of their 

 experiments, inception could be delayed. The 

 greatest suppression occurred for reduced frequencies 

 in the range of 0.4 to 0.5. Both of these experi- 

 ments will be discussed later in more detail within 

 the context of the results to be reported in this 

 paper. 



All of the experiments reviewed above describe 

 various aspects of cavitation instabilities that 

 are associated with the cavitation performance of 

 oscillating foils and propellers in a wake. This 

 cavitation performance appears to be uniquely 

 related to the unsteady flow field that exists 



over the cavitating surface. In the sections that 

 follow analytical and experimental results will be 

 presented in an effort to provide a better under- 

 standing of how these various results are related 

 and of the associated physical processes involved. 



2. EXPERIMENTAL APPARATUS AND TEST PROCEDURE 



Foil and Instrumentation 



The foil was machined from 17-4 PH stainless steel 

 to a rectangular wing of Joukowski section with the 

 trailing edge modified to eliminate the cusp. To 

 simulate the viscous effects at the leading edge 

 as close to a prototype as possible, the model was 

 designed with a chord length of 24.1 cm and a span 

 of 77.5 cm. The maximum thickness to chord ratio 

 is 10.5 percent. The foil surface was hand finished 

 within 0.38 pro RMS surface smoothness. 



Pressure transducers were installed at a distance 

 of 7.96, 24.1, and 60.3 mm from the leading edge. 

 These locations correspond to 3.3, 10, and 25 

 percent of chord length from the leading edge. 

 Kulite semiconductor pressure gages of the diaphram 

 type were mounted within a Helmholtz chamber con- 

 nected to the foil surface by a pinhole. With this 

 arrangement one could measure the unsteady surface 

 pressures due to foil oscillation and high frequency 

 pressure fluctuations inside the boundary layer 

 over a pressure range of ±207 KPa (±30 PSI) and a 

 calibrated frequency range of to 2 kHz. In order 

 to increase the spatial resolution in measuring 

 the local pressure fluctuations inside the boundary 

 layer, the diameter of the pinholes installed on 

 the foil surface were kept at 0.31 mm (0.012 inches), 

 (see Figure 1) . This arrangement also reduces the 

 danger of cavitation damage to the pressure 

 transducers. Extreme care was taken to fill the 

 Helmholtz-type chamber through the pinhole under 

 vacuum with deaerated water to minimize the possible 

 occurrence of an air bubble trapped inside the 

 chamber. If a gas bubble was present within the 

 gage chamber, the resonant frequency of the chamber 

 would be reduced below its 3880 Hz value. For 

 example, with the above procedures for filling the 

 gage chamber at a pressure of 3.4 KPa, a bubble of 

 0.6 mm diameter at atmospheric pressure is produced. 

 This bubble will lower the chamber's resonant 

 frequency to 1100 Hz. The danger of becoming a 

 Helmholtz resonator was not observed in our dynamic 

 calibration tests up to 2000 Hz. The calibration 

 procedure used here was developed by the National 

 Bureau of Standards, Hilten (1972), modified to 

 the extent that water rather than silicone oil was 

 the fluid medium. Since it was very important to 

 determine the relative phase difference between the 

 foil angle and the pressure gage signals, all 

 amplication and recording equipment was selected to 

 minimize the introduction of unwanted phase shifts. 



iZ 



r 



Ps ( /c = 0.033) 



^3 C'/c = 0.10) 

 P, ("/c = 0.25) 



— <V- 



0.31 ram 



^ 



^ 



PITCH AXIS LOCATION 

 C = 241 im 



HELMHOLTZ TYPE 

 CAVITY 



KULITE PRESSURE GAGE 



FIGURE I. A sketch of the foil 

 and three pressure gage locations. 



