Increase in mass during simulation
Hi,
The mass of the system in my problem is increasing. I have used a global rigid body which is turned ON from 0 to 1.57 sec. After 1.57, it is turned OFF. At this junction i.e 1.57 I can see a sudden rise in the mass from 16.374 to 21.39 . I have used output block for part components if there is any mass addition in any of them but found no mass increase in all these components.I want to understand from where this mass increase is possibility as there is no error percentage in mass also. I have attached the image for mass increase.
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Answers
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Hi Gopal,
Can you check the same for the rigid body...?
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Hi,
During the rbody ON period, the mass for the RBDOY comes to be 16.28 which is the required mass and thus no issues. For all the components with parts, I have checked individual masses and could not see any increase in their masses when compared to to the actual. Thus , the problems seems to be either with admas or the rigids. How to go for checking the mass of this component using output block as they have to be part components.
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Hi Gopal,
I dont see a way to output mass for Rigid. But you can output force (F=Ma), ryt?
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Hi,
As the RBODY goes off at 1.57 and after this, I will not be able to get any output over it and all the shell elements get activated. Thus, I think I wont be able to get the rigid body force as is gets switched off. Can anything be done with the mass elements?
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Did you impose any time step in this simulation which results in a mass addition?
Can you share the out files.
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Hi,
I have used AMS in this simulation for time during which RBODY is ON for reducing the simulation time during that phase. Initially I tried on a big model but now I am trying to understand this behaviour in a small model. The mass is again shooting up which causes a sudden rise in KE with very high magnitude. I have attached the images and .out files.
In KE image, you can see increase in KE from 1.0025 e+07 to 1.43771 e+07 at the moment when RBODY gets OFF. i,e at 1.52 sec. I ahve other RBODY involved in model an dthus wants to turn them OFF when the global rigid body gets ON, but do not know how to get the list of all master nodes of other rigid bodies. For this run I made all RBDOY to RBE2 and only global rigid is RBODY.
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Hi,
Also , is there any option for getting the RBDOY master nodes list through hypercrash?0 -
You can get the details of RBODY thorugh Rigid Body Details option available in Hypercrash and you can save them as a text file.
Please see the attached image
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Hi,
Yes I just did the same by taking all the rigid body master nodes to time history and then copying them.
Thank you for response. Have you gone through the image attached above.
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Did you set any ICoG flag for the rigid bodies?. It is advised to set ICoG to 2 to get the most realistic behavior; the center of gravity is then computed taking into account only the slave nodes. If ICoG is set to 1, the master node, with its own mass, is included to compute the center of gravity.
Default is 1, where mass and inertia are added at the master node coordinates; the center of gravity is computed using the master and slave node coordinates, the master node is moved to the computed center of gravity.
Please check this ICoG flag.
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Hi,
For global rigid body, by default it takes ICoG 2 and for the rest of the rbody it is default i.e ICoG:1. When the starter read is completed, in .out file the RBODy initialisation is coming fine with respect to CG , inertia and mass values . Only at the ON-OFF moment, the issue is coming.
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Hi,
This is because the global rigid body is involve din /RBODY/ON and hence by default it will take ICog : 2 irrespective of the defined ICog values in pre-processing.
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Altair Employee
