## General Mechanical

#### Transient simulation and basis excitation

• felice
Subscriber
Hello to everyone and thank you for your support. nI am doing a Shock impulse simulation, I have strong non-linearities inside my model, so I can't use the Modal analysis. The model is excited to the fixtures with the shock impulse. Which is the right way to apply the acceleration to the fixed supports of my model?nThank youn
• Chandra Sekaran
Ansys Employee
Because you have nonlinearities you will have to resort to full transient analysis. You can fix the base nodes and apply the acceleration loads to the entire body. This will give you the relative displacements with respect to the fixed base and the resulting stresses etc. On the other hand if you need to include the base motion then you may have to use a large mass approach where you have a very large mass coupled to all the base nodes. Then you apply a force on the large mass (F= M A) that will generate the required base acceleration. n
• peteroznewman
Subscriber
Even though you can't use Modal, you should run a Modal to learn the natural frequencies in your structure before you run the full transient analysis.nYou can also take the shock impulse and compute the shock response spectrum to see what frequencies will be excited the most by your impulse.n
• felice
Subscriber
Hello and thank you for your help. nFor Peter : I did it. I performed a Shock response spectrum with RESP command of APDL (also with Matlab and th differences are minimum) , and I did the simulation with response spectrum approach from Modal simulation. I did also a MSUP simulation. The transient simulation is very important for me because I need to know the beahviour of some contacts during the shock. nFor cs: Do you mean to fix the basis with fixed support and applying the accelerations to the entire system? Is it correct? I thought to apply for each support a Remote displacement that is the double integration of my shock acceleration (in this way I have absolute results). nMy doubt is:nI have a complex model, where there are many components. The test process of shock impulse is conducted with the application of acceleration (with shaker) to the fixed points (screws points). To apply the acceleration to the entire model fixing the base nodes (the same acceleration to all the components - the rapid way inside workbench) is the same of to apply the acceleration to only the fixed points (screws points ). I mean in theoretical term. If the body directly connected to the shaker is not rigid I think that I would have different results with the two approaches. But maybe I lost this lesson during my University course. nThank you.
• peteroznewman
Subscriber
Array nWhat Array said was apply a force on the large massnHave you done the double integration of the shock acceleration? There are some corrections to do along the way, especially for data recorded from accelerometers. Are you using vibrationdata in matlab? That is very useful. If the acceleration is a specification, then there might not be an issue. If you are willing to attach the shock impulse data or definition, I would be interested to look at it. I have matlab. You can put data in a zip file to attach it.n
• felice
Subscriber
Yes, I am using MAtlab but I am using two algorithms founded on the internet. If you do the MSUP with basi excitation (imposing the impulse acceleration), you can verify that the final displacement is the same of the double integration. In Matlab I am using two different algorithms in order to obtain the spectrum of the impulse data (I am working with a Shock impulse with half-sine shape - 10ms_200G). The two algorithms are based on two methods: Kelly-Richman and Smallwood. nSorry but I did not understand the answer to my doubt:nTo apply the acceleration to the entire model (acceleration - all bodies) fixing the base nodes is the same of to apply the acceleration to only the fixed points (screws points ) ?n