364 



Photographic Instrumentation 



All photographs used to document the inception and 

 cavity instability processes were taken with two 

 35 mm cameras. Illumination was provided by strobe 

 lights having a light duration of 10 microseconds. 

 With the camera shutter open, the first frame of a 

 sequence was taken when a foil position indicator 

 triggered the strobe lights. Each succeeding 

 exposure was taken 10 and 1/25 foil oscillations 

 after the preceeding exposure. An electrical pulse 

 from a light detector was recorded on a channel of 

 the same magnetic tape that was used to record the 

 foil position, pressure gage responses, and a time 

 code. Oscillograph records then allowed a direct 

 correlation between these events. Both top and 

 spanwise photographs were taken simultaneously by 

 exposing the film with one set of flash lamps. In 

 order to focus the camera lens in the same region 

 as the location of the pressure gages when viewing 

 in the spanwise direction, the camera was elevated 

 at an angle of 4° and directed slightly downstream 

 by an angle of 10". 



High-speed 16 mm movies were taken at a rate of 

 9,300 frames per second to assist in the interpre- 

 tation of the 35 mm pulse camera sequential 

 photographs . Adequate exposure for these photographs 

 was achieved by using high intensity tungsten 

 filament flash bulbs of 25 millisecond duration. 



Test Section 



The closed jet, test section of the 36-inch water 

 tunnel was modified by the insertion of sidewall 

 liners to provide two flat sides as shown in Figure 

 2. On each end of the foil a disc was, attached. 

 This disc rotated in a sidewall recess. Thus the 

 foil could be rotated without gap cavitation 

 occurring between the end of the foil and the 

 sidewall of the tunnel. One sidewall assembly was 

 fitted with clear plastic windows to permit side 

 view photography. 



The foil was oscillated by a mechanism whose 

 conceptual design is shown in Figure 3. With this 

 type of design the foil mean angle (a ) can be 

 adjusted statically and the amplitude of foil 

 oscillation (aj) can be continuously adjusted 

 between 0° < aj < 4° while in operation. The 

 oscillation frequency is continuously variable 

 between 4 Hz < f < 25 Hz. Air bags, shown in 

 Figure 3, were installed to reduce the fluctuating 

 torque requirements on the motor drive system. 



PNEUMATIC 

 AIR BAGS 



FOIL SHAFT 



VIEWING PORTS 



FOIL DISC 



777777777T77777777P 



CONNECTING 

 ROD 



ECCENTRIC CRANK 

 DRIVEN BY VARIABLE 

 SPEED D.C. MOTOR 



SIDEWALL 



FIGURE 2. Schematic of closed jet test section. 



FIGURE 3. Conceptual design of foil oscillation 

 mechanism. 



Water Tunnel Resonant Frequencies 



The study of cavity dynamics in a water tunnel gives 

 rise to a fundamental question, namely, the effect 

 of tunnel compliance on transient cavity flows. If 

 the tunnel was perfectly rigid and if there were 

 no free surfaces other than that of the cavity 

 itself, then an infinite pressure difference in an 

 incompressible medium would be required to create 

 a changing cavity volume. To make sure that this 

 kind of tunnel effect would not be present in our 

 model tests, a hydraulically operated piston having 

 a frequency range of to 45 Hz was initially 

 oscillated in a test section opening to simulate 

 the maximum expected change of cavity volume. A 

 sharp peak of futndamental tunnel resonance was 

 observed at 4.7 Hz. Consequently, all of the foil 

 oscillation experiments reported here were carried 

 out at frequencies either above or below this 

 resonant frequency. 



Data Reduction 



Due to the installation of two sidewall liners in 

 the test section, the tunnel velocity was corrected 

 according to the area- ratio rule. The tape recorded 

 time histories of foil angle and pressures were 

 digitized using a Raytheon 704 minicomputer and 

 reduced using algorithms implemented on the DTNSRDC 

 CDC-6000 series digital computers. The time histo- 

 ries were recorded on one inch magnetic tape at 

 15 inches per second (38 cm/s) using IRIG standard 

 intermediate band, frequency modulation techniques. 

 During digitization, these data were filtered using 

 eight-pole Butterworth low pass filters that have 

 a -3 db signal attenuation frequency at 40 Hz. 

 They were then sampled at 125 hertz. The run 

 lengths used in the data reduction were nominally 

 40 seconds. For the oscillating foil data the 

 computer output consists of values of mean and 

 standard deviations, sine wave amplitudes and 

 frequencies, and transfer function magnitudes and 

 phases. Mean and standard deviation values were 

 obtained from the stationary foil data. For the 

 transfer functions, the system input was foil angle, 

 where the pressures were responses to this input. 

 For the dynamic runs, foil angle was sinusoidal; 



