{"id":318335,"date":"2023-11-19T12:07:46","date_gmt":"2023-11-19T12:07:46","guid":{"rendered":"\/forum\/forums\/topic\/prescribed-motion-acceleration-vs-displacement\/"},"modified":"2023-11-21T10:42:47","modified_gmt":"2023-11-21T10:42:47","slug":"prescribed-motion-acceleration-vs-displacement","status":"closed","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/prescribed-motion-acceleration-vs-displacement\/","title":{"rendered":"LS-Dyna result: displacement vs acceleration"},"content":{"rendered":"

Hello everyone,<\/span><\/p>\n

We are currently working to prescribe some earthquake motions in LS-Dyna (V14.0.0) with LS-PrePost (V4.10.5-23Jun23) and have come across an interesting issue. I’m hoping you can help us with this.<\/span><\/p>\n

In our simulations model, which involve 6721 nodes and 6244 elements, we prescribed all three components of the ground motions as accelerations. However, when we checked the displacement response of a node from the prescribed motion nodeset from d3plot, there was an unexpected drift at the end of the displacement time-history. Interestingly, this drift doesn’t occur if we derive the displacement by integrating the acceleration using alternative methods. This problem happens regardless of the specifics of the input acceleration data, or the target node I choose to compare the integrated acceleration output with the output displacement.<\/span><\/p>\n

In the figure below, I’ve plotted the displacement response, integrated from acceleration, alongside the actual displacement response of node 28289 in the x direction with help of the History tool. This is a random node on the surface of the rigid body system subjected to the motion. As you can see, there is a clear difference between the displacement derived by LS-Dyna and the displacement integrated from LS-Dyna’s acceleration in LS-PrePost –<\/span><\/p>\n

<\/a><\/span><\/p>\n

Interestingly though, when we approach the problem in reverse – starting with output displacements and then performing differentiation twice to obtain the accelerations of a node – the results align much more closely with the output response acclerations –<\/span><\/p>\n

<\/a><\/span><\/p>\n

I’m trying to understand why this issue happens and if there is something we are doing wrong.<\/span><\/p>\n

Another question I have is about prescribing ground motions: should we use displacement or acceleration? And how does the internal algorithm of LS-Dyna originally handles this?<\/span><\/p>\n

 <\/p>\n

Any insights would be greatly appreciated. Thanks in advance for your help!<\/span><\/p>\n

 <\/p>\n

With Regards,<\/span><\/p>\n

Tonmoy.<\/span><\/p>\n","protected":false},"template":"","class_list":["post-318335","topic","type-topic","status-closed","hentry","topic-tag-earthquake","topic-tag-explicit-dynamics","topic-tag-ls-prepost","topic-tag-motion-setup-1"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t