296 



The vertical profiles for air velocity taken for increasing F along the 

 center line of the tunnel were found to fit the form: 



l/n 



U _ Z - d ; 



U ' \ 6 



where <5 is the thickness of the boundary layer as defined by the value of 

 z' where u(z') = 0.99U^. Over a wide range of fetch, and for 6.1<Uoo<13.6 

 mps the data taken in the channel fit Eq. (l) where n = h.^. This similarity 

 distribution is shown in Figure 6. Several typical values of 5 are given 

 in Table I. The shape corresponding to Eq. (l) with n = i^.5 is frequently 

 found in wind tunnel data for flow over moderately rough surfaces. 



Air Flow Close to the Waves 



One of the purposes of this study is to examine the nature of the air 

 flow close to the water boundary. A key problem in this work centers around 

 the question of separation of flow to the leeward of the wave crests (Ursell 

 (1956)). Furthermore, recent theories for energy transfer to the waves from 

 the air place strong emphasis on the behavior of the region where air 

 velocity equals the phase speed of waves. For waves generated in the tunnel, 

 this zone is very close to the water surface, much closer than can be reached 

 with a fixed probe. That is, the present equipment can only measure average 

 velocities in the air within about 1 cm of the crests of the highest waves. 

 To observe the nature of the air motion near U = c^, and to trace the 

 presence of separation, the probe must be placed much closer to the oscillating 

 water surface than the fixed probe will permit. Therefore, a moving probe 

 has been designed which will follow the significant waves and maintain ap- 

 proximately a constant level above the water surface . The schematic picture 

 of the design, worked out by one of the authors, is shown in Figure 7* The 

 probe is maintained at a constant level above the water by a servo-driven 

 mechanism activated by the depth gauges. This system is currently under 

 construction, and we expect to begin obtaining data from it sometime next 

 year. 



IV. PROPERTIES OF THE WAVES 



Over a wide range of air flow which follows the patterns described in 

 sec. Ill, only small gravity waves and capillaiy ripples were generated on 

 the water standing in the channel. Although the air reached speeds greater 

 than 12 mps, breaking of waves, in the sense of forming white caps, was not 

 observed. At high air velocities droplets of spray were observed being shed 

 from crests of the larger waves, but the waves did not become sharp crested 

 as seen in "fully developed" seas. 



Up to wind speeds of about 3 mps, taken about 20 cm above the water, no 

 waves appeared on the water surface . However, very small oscillations of 

 the entire water surface could be observed in this range of air flow by 

 watching variations in reflected light on the water. Above 3 mps, ripples 

 began to form near the leading edge of the water. These small disturbances 



