A horizontal cylinder extending along the entire width of the wave tank was located a distance of approximately 7.7 m from the wave paddle. The support system allowed adjustment of the vertical position of the cylinder to various depths of submergence h, as defined in Figure 1c. Figures 1d and 1e provide schematics of the test cylinder and experimental system. The cylinder had a diameter D = 1.27 cm and a length of 36.8 cm. It was mounted on a 3.2 mm2 sting with an arrangement of 8 strain gauges to measure the in-line and transverse components of the fluid loading. End plates were employed at either end of the cylinder. The end plate was 3 mm-thick Plexiglas, which allowed use of particle image velocimetry. The gap between the end of the cylinder and this end plate was kept at 0.7 mm. On the far side of the cylinder, the end plate had a thickness of 6.4 mm; it was beveled at an angle of 30°. This end plate was mounted in such a fashion that it did not come in contact with the cylinder or the support sting. In order to allow penetration of the sheet of laser light through the cylinder with minimum refraction, a 12 mm-wide thin-wall window was machined in the cylinder. The center of this window was located a distance of 12 mm from the end of the cylinder, which was within the region of spanwise coherent vortex formation, as visualized with a scanning laser sheet across the tank. The window was filled with distilled water.
The entire cylinder apparatus can undergo arbitrary motion provided by high resolution stepper motors (Parker Compumotor AX57-102) which have an angular resolution of 25,000 steps per revolution. The motors are controlled by a microcomputer via Parker PC-23 indexers. The motor displacement for each discrete time step is specified by the Stream Function Generator (SFG) software (Magness, 1990). The software Automated Laboratory Technician (ALT) (Magness and Troiano, 1991) executes the command sequences produced by the SFG software for up to three stepper motors and allows for simultaneous data acquisition through a Data Translation (DT 2801 series) A/D converter. Figure 1a shows a diagram of the experimental system.
During the first three experiments, the cylinder was kept stationary at the submergence depths of 20 mm, 7 mm and 0 mm respectively. During two additional experiments, the cylinder was undergoing a small-amplitude orbital motion obtained by programming a sinusoidal and a cosinusoidal motions in horizontal and vertical directions respectively. The experiments involving moving cylinder are discussed in appendix D.