Laboratory Investigations on Air-Sea Interactions 



cannot be regarded as an adequate, steady-state simulation of the 

 wind-generated wave problem, because the critical layer in the 

 experiments is of zero thickness and, hence, the critical level lies 

 on the stationary boundary. All of the above experiments, with the 

 exception of Thijsse's indicated a smaller sheltering coefficient than 

 that anticipated by Jeffreys, who expected the pressure distribution 

 to be out-of-phase with the wave (in accordance with Miles' inviscid 

 theory). The resulting small sheltering coefficient may be attributed 

 to either viscous or finite wave -amplitude effects. 



For a more realistic steady- state simulation of wind-generated 

 waves and demonstration of the importance of the critical-layer 

 mechanism of energy transfer, the wavy boundary must be moving 

 with a speed equal to the wave celerity and opposite to the direction 

 of mean free-stream. An Important advantage in this simulation Is 

 that the flow field is steady. Consequently, measuring techniques 

 are greatly simplified. 



The first successful moving, wavy boundary experiment and 

 Its resultant presentation of the normal pressure distribution on the 

 boundary were reported by Zagustln, et^ al. [ 1966, 1968] . Subse- 

 quently, Ott, et al. [ 1968] extended the Zagustln Investigation and 

 used refined experimental procedures to achieve better experimental 

 accuracy. Small amplitude waves with a length of 3 ft and amplitude 

 of 0.65 In. were used. Because of the limited capability of the 

 experimental facility, C/Ugp was limited to approximately 0.75 (U^^ 

 Is the air velocity at the edge of the boundary layer). 



3. 1.2. Flexible wall with progressive waves (unsteady- state) 



Kendall [1970] described a series of experiments on wind- 

 wave simulation In a low turbulence wind tunnel. The wavy wall was 

 the floor of the constant pressure test section of the tunnel. The 

 surface of the wavy wall was composed of neoprene rubber sheet 

 which was constrained to form a series of sinusoidal waves (length = 

 4 In. and height = 0.25 In.). The rubber sheet was supported from 

 beneath by a series of ribs which were connected to Individual circu- 

 lar eccentric cams. Each cam was positioned with proper phase 

 difference on a common cam shaft expending the length of test section. 

 Rotation of the cam shaft caused each rib to execute a reciprocating 

 vertical motion and thus a progressive wave form was produced. 

 Reversing the direction of rotation of the cam shaft produced waves 

 traveling In the opposite direction, giving - 0.5 < C/U < 0.5. 



The boundary conditions for the two methods of wind-generated 

 wave simulation described above deviate slightly from those of a true 

 air -water Interface. If the fluid particle velocity In a wave motion 

 Is small compared to the wave celerity (true for small amplitude 

 waves), the moving wavy boundary simulation approximately satisfied 

 the boundary conditions. In the flexible wall experiment the surface 



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