Wiegel and Delmonte 



circular cylinder which extended from near the bottom through the 

 water surface. For this case the undisturbed water particle motion 

 was not simply a rectilinear back and forth motion, but the water par- 

 ticles moved in an elliptical orbit in a vertical plane, so that they were 

 never at rest. Furthermore, any eddies that formed were affected by 

 the free surface at the interface between the air and water. The hori- 

 zontal component of water particle velocity is given by Equation (3), 

 and the vertical component by 



_ 7T H sinh 2 it (y + d) /L 2 tt t , . 



v = —= : — : — r-Tz sin — — — (8) 



T sinh 2 tt d/L T v ' 



2 2 2 

 In deep water the water particle speed, q = u + v , is given by 



q = * H/T (9) 



at the surface. Thus, the speed remains constant in deep water while 

 the particle continuously changes direction. It is more complicated in 

 transitional and shallow water. An example of the water particle path 

 is shown in Figure 4. There is little reason to expect that eddies 

 formed in such a flow would have the same characteristics as those 

 formed in simple oscillating flow. 



One of the most crucial factors in oscillating flow of this type 

 is the fact that the wake formed during one portion of the cycle be- 

 comes the upstream flow in another portion of the cycle (the paper by 

 Laird, Johnson and Walker, I960, is useful in gaining some under- 

 standing of this problem), and little is known of the water particle mo- 

 tions under these conditions. When eddies, form, they appear to be of 

 about the same size as the pile. In this regard it is interesting to 

 refer to an observation made by Bacon and Reid (1923) in some studies 

 of fluid forces on spheres. They found that if the scale of the turbu- 

 lence was small compared with the diameter of the sphere, Reynolds 

 number was a good criterion, but if the grain were coarse, then 

 Reynolds number no longer served even as an indicator. 



During the first stages of the study by Bidde, immiscible fluid 

 particles with the same specific gravity as the water were made of a 

 mixture of carbon tetrachloride and xylene, with some zinc oxide 

 paste added to make the particles easily visible. The fluid was inject- 

 ed into the water by means of a long glass tube which had a rubber 



768 



