Jumping boat?

A5729 · July 26, 2012, 04:44

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.

July 26, 2012, 04:44	Jumping boat?	#1
A5729 New Member Angus Bishop Join Date: Apr 2012 Posts: 17 Rep Power: 14	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

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July 26, 2012, 06:04		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	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.

July 26, 2012, 06:47		#3
A5729 New Member Angus Bishop Join Date: Apr 2012 Posts: 17 Rep Power: 14	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.

July 26, 2012, 07:44		#4
A5729 New Member Angus Bishop Join Date: Apr 2012 Posts: 17 Rep Power: 14	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.

July 26, 2012, 18:53		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	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.