A 230-ton bridge spans the length of the basin and provides for rotation of up to 45 deg to 

 allow model testing in a complete range of oblique waves. The 15-knot carriage which tra- 

 verses the bridge structure carries instrumentation for measuring and recording the dynamic 

 phenomena associated with a test on an attached free-running model. 



Waves are generated by varying air pressure in a line of enclosed domes along the 

 side of the basin. Thirteen such domes (each 25 ft long) are along the long side, eight along 

 the short bank. Air for each dome is supplied separately by 100-hp blowers and controlled 

 by flapper valves which alternate the flow of air to and from the domes. At the most efficient 

 operating frequency, this system is capable of generating regular waves up to 24 in. (peak- 

 to-trough) in height. 



The present installation has two methods for controlling the wavemaker flapper valves 

 and the resulting generated waves. The long bank of wavemakers is ganged together mech- 

 anically on a single shaft which rotates at a (constant or varying) programmed speed and 

 produces a regular or a "quasi-random" wave train. A similar arrangement can be used 

 along the short bank or, alternately, eight separately controlled electrohydraulic servo actua- 

 tors are available for positioning the wavemaker flapper valves. It is this latter system 

 (shown in Figure 1) which was used in the sea simulation described in this report. 



Each of the eight hydraulic servos will control the angle of an air-controlling flapper 

 valve so as to follow a voltage command signal from the wavemaking control station. These 

 voltage command signals can originate from separate channels of an installed 14-track tape 

 recorder, from a single such channel, or from low-frequency sinusoidal signal source. 



DESIRED CHARACTERISTICS OF A SIMULATED SEAWAY 



In attempting to make the most accurate simulation possible of a random seaway, 

 naval architects are limited by the amount of statistical knowledge available to characterize 

 actual ocean behavior. Assuming a linear wave theory, the most complete (and the only 

 adequate) description of a wave system is the directional spectrum of the sea, which is a 

 measure of the amount of average energy contained in each incremental wave length traveling 

 in each incremental direction. Unfortunately, very little is currently known of the directional 

 distributions of waves at sea although oceanographers are now working extensively in this 

 area. 



One characteristic of wave measurements which are made at sea is the type of random- 

 ness encountered. With the wave system created under wind action which acts on the surface 

 in incremental gusts spread over a large area, it is reasonable to assume that the motion of 

 the surface is closely approximated by a Gaussian random process, a type which is often 

 encountered in physical applications and which contains the "most randomness" for a given 

 level of distrubance. Experimental measurements support this assumption.^ 



Even if all waves in a seaway appear to come from a single direction, or nearly so, 

 there is no general agreement among oceanographers as to the distribution of average energy 



