A POSITIVE DISPLACEMENT OSCILLATORY WATER TUNNEL 



by 



Karl E.B. Lofquist 



I, INTRODUCTION 



This report describes a general purpose oscillatory water tunnel 

 able to provide purely sinusoidal motion over wide ranges of period and 

 amplitude. The tunnel was designed to study, under near prototype con- 

 ditions, the effects of bed permeability upon the net movement of sand 

 by offshore wave action. Lofquist (1975) described the unique features 

 of the tunnel special to the study of permeability effects. Subsequent 

 removal of those special features and doubling the depth of flow in the 

 test section have produced the general purpose tunnel described here. 

 This tunnel has no single capability or capacity not found in other 

 existing tunnels (discussed in Section VII). However, the capabilities 

 it does have combined with its modest size and power requirement make 

 this tunnel unusual. 



II. GENERAL DESIGN 



The water tunnel is of U-tube design with the middle horizontal 

 part comprising the test section (Figs. 1 and 2). The vertical end 

 parts of the tunnel are two cylinders with tight-fitting pistons at 

 one end, and two reservoirs open to the air at the other. The pistons, 

 in unison, move the water over the sand bed between the cylinders and 

 reservoirs. The cylinders and reservoirs were designed in pairs to 

 provide two separate channels when the test section was divided 

 longitudinally by a partition. With the partition removed, the cylin- 

 ders and reservoirs act as units. 



A variable -speed motor (Reeves 3/4 horsepower) drives the two 

 pistons in unison by a combination of sprockets and a worm gear that 

 rotates two arms at opposite ends of a common axis. Ball-bearing "pins" 

 attached to the arms at an adjustable distance from the axis move in 

 parallel circles and slide in horizontal slots in a frame constrained 

 to move vertically. The pistons are attached to the upper part of the 

 frame. A constant rate of rotation of the arms drives the frame, or 

 "scotch yoke", and the attached pistons in simple harmonic motion. The 

 yoke and pistons are balanced by a "passive" counterweight. 



The required power input has been minimized and a nearly steady 

 operation at any period of rotation has been made possible by the 

 addition of an "active" counterweight. This weight is supported by a 

 frame that rotates at twice the rate of the arms which drive the yoke 

 and pistons. The displacement of the weight from the axis of its sup- 

 porting frame affects the resonance period (neglecting friction) of the 

 entire system. By adjusting the position of the weight (this can be 



