RIPPLE TANK STUDIES OF THE MOTION OF SURFACE GRAVITY^TAVES 



by 

 Osvald Sibul, Dept. of Engineering, University of California 



Technical Report Series 3, Issue 346, Institute 

 of Engrg. Research, Wave Research Lab. - prepared 

 under contract with the Office of Naval Research. 



INTRODUCTION 



Ocean, waves are generated by the action of wind blowing over the surfaoe of water. 

 The winds are irregular in their intensity, direction and duration and the result- 

 ant waves vary in period, height and direotion of travel. "When we consider also 

 the extreme difficulties one encounters in measuring the waves in nature, and the 

 effect of hydrography on wave characteristics, then it oan be appreciated how 

 greatly the investigator is handioapped in his study of wave action and how much 

 model studies are needed in many instances in order to understand the basic laws 

 of wave motion. One of the most useful units to the engineer in this regard is 

 the ripple tank, which as a laboratory instrument permits the study of wave prob- 

 lems involving refraction, diffraction, reflection and deoay of waves. It is also 

 reliable and handy for demonstration purposes, for it shows, within the field of 

 vision of an engineer, phenomena which in the prototype might oover many square 

 miles* 



Ripple Tank t 



The ripple tank used in this study is located in the Fluid .Meohanios Laboratory 

 of the University of California, Berkeley, California (Fig. l). It is 44^- inohes 

 wide, 20 feet long, and 5 inohes deep. The tank is made of aluminum, except the 

 middle section of the bottom whioh consists of a glass plate, 3/8 inohes thiok, 

 for observation. A point source light consisting of a 26CW mercury-vapor lamp is 

 located underneath the channel at an optioal distance of approximately 10 feet* 

 A mirror set at 45° to the horizontal reflects the light 90° to an observation 

 soreen. A sohematio diagram of the ripple tank is shown in Figure 2* 



The screen oonsisted of a piece of traoing paper stretched to a 60 inch by 60 

 inch wooden frame. It was located directly over the glass seotion of the ripple 

 tank between two guides (one on each side of the tank) and could be raised or 

 lowered by means of strings and pulleys for the purpose of focusing the wave 

 images on the soreen. The theory of the optioal system is based on the faot 

 that when light passes through a disturbed water surface, it will not be uniform 

 in intensity due to the varying angle of refraction at the surfaoe. A wave acts 

 as a lens and concentrates light at the wave crests ; hence the wave orests are 

 represented by bright bands in the photographs* 



Photographic Equipment t 



The camera used was a Bell and Howell "Filmo" 16 mm movie oamera 70-DA f. 1*5 

 with a 15mm focal length lens. The film used was Cine-Kodak Super XX high speed 

 panchromatic safety film. Most of the motion pictures were taken at 64 frames 

 per second (corresponding to an exposure time of l/l25 sec.) with an opening of 

 f. 1.5. The distance between still-water level and the screen varied from 25 

 inches to 60 inches, averaging 40 inohes. The distance between the screen and 

 camera averaged 6 feet, in most cases. All the data observed during the tests 

 are presented in Table I. 



