Minimize reflections off lateral boundaries

GWAVE · May 2023

Before I dive into this rabbit hole, I wanted to see if anyone has tried this, and succeeded, or if it's even possible. I'm working on a numerical wave tank, in which a rigid object is to be placed about 1/2 way downstream of a piston wave maker. I've run tests without the object and compared the wave elevations with experimental data and it's near perfect. So my generation, settings, etc. all seem good to go!

After placing the object in the domain, I'm getting reflections off the object, which is expected. However, those reflections are now reflecting off the lateral walls and pollute the wave field making difficult to compare with experimental data. To make matters worse, they eventually hit my wave maker and get amplified to create a standing wave field that was nonexistent in the experiment. The domain length is already quite long, so I can only go so wide before I run out of memory.

I'm wondering if I can modify the lateral boundaries so they act like an outflow boundary? Maybe some type of absorption? The waves propagate down the basin at a normal incident angle. In theory, the only waves that would "exit" the domain would be those that are not parallel to the lateral boundary. If I can get this to work, it would not only save a significant amount of computation time, it will also clean up my wave field. This seems like a common challenge, but I haven't read much about this treatment in the docs.

Thoughts?

Alex · May 2023

The minimum width of the tank should be 3 times the device width.

In addition you should use in the lateral walls:

periodicity
damping layers

GWAVE · May 2023

Thanks for the suggestions Alex! I did find a section in the XML guide on "passive absorption", which I'm assuming is the "damping layer" approach you're referring to. I'm curious why you suggest a periodic boundary as opposed to say a wall?

On another note, are there any references/papers you can highlight off the top of your head to help guide me? I'm assuming others have followed this approach in the past.

rubens · May 2023

Hi @GWAVE ,

Papers of Tagliafierro about wave energy converter (wec) or wind turbines can help you, e.g., the paper below

Numerical Assessment of a Tension-Leg Platform Wind Turbine in Intermediate Water Using the Smoothed Particle Hydrodynamics Method

https://doi.org/10.3390/en15113993

The open-source code DualSPHysics, based on the Smoothed Particle Hydrodynamics method for solving fluid mechanics problems, defines a complete numerical environment for simulating the interaction of floating structures with ocean waves, and includes external libraries to simulate kinematic- and dynamic-type restrictions. In this work, a full validation of the SPH framework using experimental data available for an experimental test campaign on a 1:37-scale floating offshore wind turbine tension-leg platform (TLP) is presented. The first set of validation cases includes a surge decay test, to assess the quality of the fluid–solid interaction, and regular wave tests, which stimulate the mooring system to a large extent. During this phase, tendons (tension legs) that are simulated by MoorDyn+ are validated. Spectral comparison shows that the model is able to capture the surge and pitch dynamic amplification that occurs around the resonant fundamental mode of vibration. This work concludes with a numerical investigation that estimates the response of TLP under extreme events defined using multiple realizations of irregular sea states; the results suggest that the tendon loads are sensitive to the sea-state realization, providing maximum tendon peak forces in a range of ±10% about the mean. Furthermore, it is shown that the load pattern that forms from considering the relative position of the tendons to the incident wave direction leads to higher forces (≈20%).

Best

GWAVE · May 2023

Thanks @rubens. I greatly appreciate the lead. I see in section "3.2.1. Wave Tank Design" they present the idea suggested by @Alex along with some references. I'm going to read up on those to better understand the use of periodic as opposed to wall boundaries. I'll follow up if I have any more questions. Thanks again to the two of you for pointing me in the right direction. It's a big help!

Alex · June 2023

If you need help, share with us (dualsphysics@gmail.com) your XML and we will correct that

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Minimize reflections off lateral boundaries

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Numerical Assessment of a Tension-Leg Platform Wind Turbine in Intermediate Water Using the Smoothed Particle Hydrodynamics Method

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