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-   -   Jumping boat? (https://www.cfd-online.com/Forums/cfx/105251-jumping-boat.html)

 A5729 July 26, 2012 04:44

Jumping boat?

Sorry , this should be my last question :)

I have a hollow cylindrical hull shape for a boat , I'm running a rigid body simulation to show stuff , I have timesteps of 0.025 seconds but after the 7-8th one the boat jumps out of the water by around 1m for the previous timesteps it would move only 1cm or less.

Have i done something fundamentally wrong? Or does CFX not handle flow inside and outside hollow tubes very well?

Thanks in advance for replies

 ghorrocks July 26, 2012 06:04

I've heard of jumping beans but not jumping boats :)

And no need for apologies, the forum is here to ask questions and you are welcome to do so.

It sounds like you have a numeric instability in your simulation. You need to stablise the rigid body motions. I have little experience in rigid body stuff so cannot help you with exactly how to do this.

 A5729 July 26, 2012 06:47

That sounded fancy :) Thanks

Now just hope someone knows how to sort that out. Running simulations at this stage are taking about an hour so trial and error is out of the question.

 A5729 July 26, 2012 07:44

Also how would i go about takingg just the value of an expression without dimensions at then putting dimensions on it?

I have a formula that pumps out values in seconds at the moment and i was it to be in meters per second.

 ghorrocks July 26, 2012 18:53

Have a look at the numerical controls for rigid bodies. There will be stability controls there, that is what you need to play with.

CEL uses units for your own protection - you cannot just convert a time measurement to a velocity. Of course you can just say vel = 1 [m]/time_var and then you have m/s but most of the time this is not somethign you want to do.

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