In coastal and ocean engineering, understanding the ocean environment and the forces from waves, tsunamis and waterborne debris on structures and floating bodies is an important aspect of designing safe and effective infrastructure. Understanding the impact of these forces on structures becomes an important question in the design process. Answering these questions relies on mathematical modeling. Mathematical and numerical modeling often relies on physical modeling to collect data to further develop the model or equations. The physical parameters in these models can be measured in a wave research laboratory. This research will focus on measuring the physical parameters of displacements, the free surface, and motions. Currently these are measured with motion tracking technologies including optical tracking, position transducers, inertial measurement units (IMU), and wave gauges. Limitations of wave gauges include their intrusiveness and spatial limitations. Limitations of optical tracking systems include visualization constraints. Position transducers are invasive and can only measure single linear motion. There are spatial limitations with the amount instruments that can be used thus the extent of data that can be collected. We seek to resolve these limitations. The focus of this study is to explore methods in which IMUs can be used to reconstruct motion and an optical tracking system can be used for collecting data of the free surface. Two motion capture technologies: Xsens DOT accelerometers and Qualisys Track Manager motion capture system have been utilized. The Xsens DOT accelerometer is an inertial measurement unit (IMU). The Xsens DOT IMU has advantages for tracking acceleration data in the wave laboratory because the DOT is small, waterproof, and self-contained. The sensor can be attached to an object in the water such as a debris piece and will have minimal effect on the results of the experiment. The sensor can be considered non-intrusive and can track acceleration of an object underwater. Qualisys Track Manager is an optical motion capture system which uses cameras to track the movement of objects. QualisysTM can track the position of an object provided the object remains in view of the camera. In this study, we look at the potential to use QualisysTM as a non-intrusive wave gauge.
This study was broken into four separate but related experiments analyzing the motion capture technologies. The first experiment placed the Xsens DOT sensors inside waterborne debris in the large wave flume to track the position of the debris as it moved through a tsunami-like wave and impacted with a model elevated house. On dry land, tests were run with the Xsens DOT sensors moving in sinusoidal motions to determine if the sensors could record acceleration data that could then be integrated to obtain position data. A third set of experiments examined straight, linear motion with the Xsens DOTs. A fourth experiment tested QualisysTM and the DOTs in the wave basin to test the ability of the DOTs to record acceleration data of the wave motion and obtain accurate position data of a particle on the free surface of the water. The goal with QualisysTM was to track the free surface and determine a wave height and water depth from the elliptical orbits described by the water particles under waves.
The results of these experiments conclude that for some cases, the sinusoidal motion recorded by the DOTs can be integrated into position data. For linear motion, the sensors can provide position data within 20% accuracy for the first 0.4 meters of motion or the first 2-5 seconds of movement. QualisysTM has shown mixed results for determining the wave height and water depth. More research involving different wave conditions is needed to conclude more about the applicability of these motion capturing techniques. Finally, the tsunami experiment illustrated that while the Xsens sensors could not yet provide position tracking, the acceleration data spikes occur during debris impact and these sensors have potential for being further developed and used in wave laboratory environments.