# Extreme Wave Loading on a Vertical Circular Cylinder
## 2 Experimental Modeling and Data Analysis  
### Experimental Setup  
The aforementioned physical experiments on wave interactions with a vertical cylinder were conducted in a wave flume at Dalian University of Technology, China. The flume is 22 m in length, 0.45 m in width, and 0.65 m in depth, see Figure 1a. A 'piston-type' wave paddle is installed at one end, while a sloping beach acting as a passive absorber is placed at the other end. In the experiments, linear wave generation was used, and the water depth \(h\) was set to 0.35 m.  
In the experiments, a single vertical cylinder of diameter 0.165 m (i.e., radius \(R=0.0825\) m) was located with its front stagnation point 9.3 m away from the wave paddle in the center of the flume. The cylinder was supported from above by a stiff frame via a load cell at the top, permitting measurement of the total horizontal and vertical forces, see Figure 1b. The load cell is of the strain gauge type (KD4), which measures the applied force via the change in resistance incurred by the subsequent strain deforms. The sampling frequency was set to 2000 Hz in this work. It can also be seen from Figure 1b that colorful strips of 3 cm were wrapped around, and a ruler was attached to the cylinder. These, in combination with a high-speed camera, were used for measuring the wave run-ups around the cylinder visually. This paper is focused on the wave loads on the cylinder.  
The height of the cylinder was 0.3 m, and a range of air gaps, i.e., vertical distances from the mean still water level in the flume to the bottom of the cylinder, were considered, aiming to investigate its effect on the impact force. These were 0, 3, 6, 9, and 12 cm, and the value of 0 cm indicates that the bottom of the cylinder was at the still water level of the flume. As mentioned in the Introduction, this is designed to represent the breaker height and, in turn, isolate the slamming wave contribution from the total force-time histories.  
In addition, free surface elevations in the flume were measured using three ultrasonic-type wave gauges, which were located at 2 m, 8 m, and 8.9 m away from the wave paddle and along the wave flume centerline, respectively. The sampling frequency of the wave gauges was set to 50 Hz.  
The schematic diagram of the experimental setup, including the layout of wave gauges, is shown in Figure 2. Also shown in the figure is the Cartesian coordinate system _Oxz_ introduced in this work for the sake of discussion. The origin \(O\) is the plane of the undisturbed free surface, \(x\) = 0 indicates the location of the wave paddle, and \(z\) is positive upwards.  
In addition to the wave-cylinder interaction experiments, a set of experiments in the absence of the model cylinder were also carried out for studying nonlinear wave evolutions and, thus, the contribution of higher-order wave components driving the nonlinear loads  
Figure 1: (**a**) Photograph of the wave flume used. (**b**) Close-up view of the model cylinder.  
Figure 2: A schematic diagram of the experimental setup and a violent wave event on the vertical cylinder showing a thin sheet of water wrapped around the upstream half of the cylinder (top left).  
on a marine structure. It is noted that the arrangement of the wave gauges was slightly different for measuring the undisturbed wave fields in this set of experiments. They were arranged at \(x=2\) m, 6.7 m, and 9.3 m, respectively. The incident wave field characterized in Section 2.2 followed is based on the records at wave gauge 3 (WG3; \(x=9.3\) m), and WG1 (\(x=2\) m) was used to check the repeatability of wave runs with and without the cylinder in place.