Reflection and breaker generation in the Ohmsett Wavetank, Leonardo, New Jersey

The Ohmsett wavetank located in Leonardo New Jersey is the largest wavetank in the world used for oil spill research. The wavetank is 203 m long, 20 m wide, and has a water depth of 2.4 m. We report herein a hydrodynamic investigation to improve the tank performance. Three objective were pursued: 1) Evaluate the reflection in the wavetank based on wave properties that are commonly generated in the Ohmsett tank, 2) Use Computational Fluid Dynamics (CFD) to design a system to dissipate the wave energy and thus reduce wave reflection, and 3) Generate breaking wave conditions in the Ohmsett tank that are reproducible and their mixing energy can be quantified. Regarding the first objective, we evaluated the reflection coefficient, and found that, when the "beach" system in the wavetank is in place, the reflection coefficient in the Ohmsett tank varies from 30% for waves whose period T is 3.0 seconds (all methods) to 60% for waves of T= 2.0 s. This suggests moderate to high reflection in the tank that would "contaminate" the hydrodynamics, as reflection is minimum at sea. For the second objective, we considered a series of twelve screens spaced by 1.0 m (resulting in 12 m from the back wall to the first screen facing the wavemaker). The first six screens (facing the wavemaker) had a porosity of 75% while the second set of six screens had a porosity of approximately 60%. We also considered two situations: One where the screens are completely submerged, and another where the screens were submerged by 0.9 m from the Mean Water Level (MWL). For waves with T=2.0s and height of 0.60 m, the reflection coefficient based on CFD was less than 10% and 5% for the partially and completely submerged screens, respectively. This would result in a majorly reduced reflection coefficient in comparison with the current "beach" setup. For the third objective, we used the frequency sweep method and generated reproducible breakers. The breaker was generated as follows: A train of short-period waves (T = 1.5 s and wavemaker stroke=12.5 cm) was first generated for a duration of 6.0 s (i.e., 4 wave cycles). It was followed by a no-Action duration of 18.5 s, and then a train of T = 2.0 s (and wavemaker stroke =30 cm) was generated for a duration of 10.0 s (i.e., 5 wave cycles). The two wave trains met at around 100 m from the wavemaker, where they resulted in a plunging breaker. The breaker was also uniform across the width of the tank, which is in stark contrast to the breakers obtained currently.