Air-Sea Interactions ; Program at IMST 



4. Reproduction of Interfacial M<rtioris 



The problem of obtaining laboratory waves statistically- 

 similar to those encountered over the oceans has been thoroughly- 

 studied these last years, with the view, either to perform more 

 realistic structural tests, or to experimentally investigate the 

 mechanism of wave generation by wind. The works of Veras [ 1963] , 

 Hidy and Plate [ 1965] , Hsu [ 1965] , Gupta [ 1966] , and of the 

 Waterloopkundig Laboratorium [ 1966a, b] can be cited among many 

 others. The unsymmetrical, randomly varying and three-dimen- 

 sional waves existing in nature can be simulated only at the cost of 

 building large laboratory facilities. The so-called "wind-wave 

 tunnels" reach one hundred meters in length and several meters in 

 width, with smooth and parallel side walls and an efficient absorbing 

 beach at the end. 



The main characteristics of waves naturally generated by 

 wind along our 40 meters long tank have been forecast from the 

 proceeding references and are shown by Fig. 2(c). It is clearly 

 possible to generate gravity waves of appreciable amplitude, and 

 thus to cover a nonnegligible range of Froude numbers. However, 

 the "wave age," i.e. the ratio of the celerity of propagation, C, 

 of dominant waves to the wind velocity, U, remains low, especially 

 at the highest velocities. The same is true of the ratio C/U (where 

 U* is the friction velocity in the boundary layer), which is known 

 as an important parameter in the wave generation process. As a 

 matter of fact, these two ratios control the relative magnitude of 

 normal and tangential stresses exerted by wind on water, with Im- 

 portant consequences upon the various energy exchange mechanisms 

 (see II. 1). It Is therefore necessary to have the possibility to act 

 on these parameters, by controlling the wave height and celerity 

 independently of wind velocity. This will be done by means of a 

 wavemaker set at the beginning of the water channel and conveniently 

 randomly actuated. It is known that the combined action of such a 

 device and of wind blowing will result, after some distance, in a 

 satisfying wave pattern. 



The details of that part of the equipment are sketched by 

 Fig. 8. A new type of wavemaker, comprising a fully submerged 

 wave plate connected to the tank by means of bellows, has been 

 Imagined. This arrangement allows to realize a fairly snnooth 

 joining of air and water flows, even in the presence of waves. The 

 end of the channel will be equipped with an absorbing beach made 

 from parallel tubes with a 7° slope. A slight water movement 

 (between 0. 1 and 0.01 m/s) necessary for cleaning and temperature 

 controlling purposes, will be Insured byca recirculating 35 HP heli- 

 coldal pump. 



By A. Ramamonjlarlsoa. 



49 



