Hsu and Yu 



pa^rticle motion resulting from the flexure of the rubber sheet was a 

 backward- rotating 3:1 ellipse as compared with the forward- rotating 

 circle of deep water waves. Again, the boundary condition was 

 approximately satisfied for small amplitude waves. 



3.1.3. Wind-wave research channel (unsteady- state, true air- 

 water interface) 



The physical features of the Stanford facility were reported 

 by Hsu [ 1965] . The channel Is approxlm.ately 6 ft high and 3 ft 

 wide and has a usable test section length of 75 ft. At the downwind 

 end, there are a beach to absorb wave energy and a centrifugal fan 

 to produce the wind in the channel. At the other end, the air is 

 drawn vertically through a system of filters and then carried hori- 

 zontally on to the water surface at the beginning of the test section 

 by a converging elbow. A hydraulically-drlven, horizontal- displace- 

 ment, wave- generating plate is located 17 ft upstream of the test 

 section. This distance is sufficient to allow generated waves to 

 become fully established prior to being subjected to wind action. 

 Sinusoidal waves, ranging In frequency from 0. 2 to 4 cps , can be 

 generated. The maximum wind speed Is approximately 70 fps with 

 a nominal water depth of 3 ft in the channel. A limitation of the 

 present facility is that the wind and the propagating waves move in 

 the same direction, 



3.2. Measurements of Wave-Induced Perturbation Pressures 



Because the flow field was steady in the moving wavy-boundary 

 experiment (Sec. 3.1.1), two conventional, but small (1/32 in. O.D), 

 pitot-statlc probes were used for all the velocity and pressure measure- 

 ments. The reference probe was located in the free- stream while the 

 other probe could be moved to any distance from the moving boundary 

 by a traversing mechanism. Realizing that the traversing probe 

 nciust be aligned with the local flow direction, we mounted this probe 

 in a special rotating device. These probes were connected to a 

 Pace P-90 differential pressure transducer through a manifold 

 system which provided selective readings of dynamic pressure or 

 pressure differential between the two probes. 



The pressure measurements in the flexible wall experiments 

 (Sec. 3.1.2) were made through static holes in the flexible surface. 

 Essentially, a length of metal tubing in the form of a loop was used 

 to connect the static hole and the pressure transducer. The loop 

 served to cancel the unwanted pressure gradient generated by the 

 motion of the tubing. 



Because the thickness of critical layer was small in the wind- 

 wave channel experiment (Sec. 3,1.3), the measurements of pertur- 

 bation pressure were obtained by use of a specially-designed wave 

 following system. Again, the perturbation pressure is the difference 

 between the pressure at the air-water interface and that in the free 



16 



