# Physics Simulation Forum

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 Posted: Wed Sep 25, 2013 11:24 pm

Joined: Wed Sep 25, 2013 11:05 pm
Posts: 3
I would like to create or use a simulator of a rigid body submersed in water with constant flow.
The rigid body is tied with a rope to a point in the middle of the water so it is always near the observer.
The point and the rope don't disturb the water.
I am searching the web because I don't have an idea how to do that. The quality of the presentation is not very important, I just want the physics simulation.

The rigid body is built in a 3d cad and imported to the simulator. I hope I won't need help with this.

I studied maths many years ago and I work with computers. I have management programing experience.
I want results, not learn a lot of physics and SDK programming. But I can learn new things if necessary.

Any ideas?

Thank you
Fernando

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 Posted: Fri Sep 27, 2013 2:32 am

Joined: Fri Nov 30, 2012 4:50 am
Posts: 461
For physically accurate representations, you will need to integrate the pressure over the surface area (which includes drag effects, buoyancy, etc, and results in a force vector), then solve the rope inequality (fixed distance or less from the point, or other attachment point if there are obstacles the rope interacts with). The main issue is finding out what that pressure should be, and depends entirely on your fluid solver. This will not be very conductive to real-time gameplay (some GPU based examples exist but require quite of GPU processing power to be realtime), but would be fine for offline simulations.

If you mean to just make something that looks like it's in the water, there are dozens of ways to go about it. A simple one is to treat buoyancy as negative gravity and drag as a damping factor. Not physically accurate, but looks alright.

Either way though, if you want to do it from scratch you'll need high level math and programming knowledge. If you don't mind spending money to get results with minimal training, perhaps something like Fluent (Ansys, so "custom" pricing, generally >\$10k) or RealFlow (\$2500 for a license, but limited use 30 day trial for free). There are other, sometimes free solutions, but they generally require an investment into programming and math to get them to work correctly.

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 Posted: Fri Sep 27, 2013 5:10 pm

Joined: Wed Sep 25, 2013 11:05 pm
Posts: 3
I want only phisically accurate simulation for offline evaluation of designs.
The rope will be underwater without obstacles, only to keep the body at a fixed distance or less(?).
I don't care about the fluid, I only want the simulation of the rigid body movement. Later I will want the force on the rope.

It's for a hobby so I can't pay a lot for software. But if i Know about a good simulator I can try someone that has it to give me access to his equipment.

I am beginning to learn about this and I realized it can be very complex for great accuracy or good performance but I don't need them. I only need that when I test the real body it looks like the simulation.

I am searching the web and maybe I will try to do the maths and programming with specialized libraries. Any guidance about this?
I hope I won't need to do all that work because I will spend several months without being sure of the final result.

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 Posted: Sun Sep 29, 2013 8:17 am

Joined: Fri Nov 30, 2012 4:50 am
Posts: 461
fernando wrote:
The rope will be underwater without obstacles, only to keep the body at a fixed distance or less(?).
I don't care about the fluid, I only want the simulation of the rigid body movement. Later I will want the force on the rope.

That's like saying you want to drive but don't want a vehicle. The rigid body is 100% dependent on the fluid, and the rope forces on the rigid body.

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 Posted: Sat Oct 05, 2013 11:45 pm

Joined: Wed Sep 25, 2013 11:05 pm
Posts: 3
Yes, there is no motors, only the force of the water.
And the rigid body will have maby some convex shapes to generate turbulence and movement.

I think that I only need to analise the volume around the rigid body position at each moment but probably the simulators will analise all the volume and it will be hard work because the rigid body will move.

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