Damping example - Nastran In-CAD Tutorial

- [Instructor] In this video, I'm going to run a transient analysis, where I've already set up most of the information and we're going to focus on just changing the damping levels. I'm going to open up this plate, and there's a transient analysis already set up. Go into the environments, go in Autodesk Nastran In-CAD. We'll have a look to see what we've got in here. It's a modal transient analysis already set up. I can edit that just to review it. We can see I'm asking for a displacement and acceleration only. We have our correct carbon steel. Look at the modal status and, same as the modal analysis, just ten normal modes required. We'll go through this in more detail in the subsequent video but we've got .0001 as a time step and then 1000 of them. Applying the load at the corner, you can see the point of application there is .01 force being applied through time and we're inputting this through a table and there's a 10 millisecond impulse. So we go and have a look at that table, edit it, we can see we're putting the data in there and if I just preview it, you can see it's like a little square wave or an angular wave impulse going in there. That's the time dependency and what we call the spatial oppositional dependency. Same contact as before. Now this is what we want to focus on is the damping, so I'm going to focus on the name, two percent critical damping. I want to make sure I have no structural damping and no Rayleigh damping. They can overlay each other. I can also put in a variable damping, but I'm just going to put in a constant damping. We can use either percentage critical, we can use quality factor or we can use structural damping, so two percent, click OK and that's what were going to run with. I'm going to save this off before I run. I'm going to save it again with the modified file name and a particular path where I want the analysis to sit. This is going to include the Nastran input file results and all of the other files associated with Nastran run. I'm going to use a run area and save the file into there. We get kicked out of the environment, so we've got to go back into Autodesk Nastran In-CAD and then we're ready to run, so launch the analysis. This is going to take a little bit more time so I'm going to accelerate the timeline to get through to the result more quickly, and that wraps up the analysis. What I typically want to do to start off with though is to do an XY plot to take a key quantity and make sure it's behaving properly. I'm going to look at the vertical displacement of the corner node, so I'm going to pick the corner node under the loading. I can preview the XY plot, and that's the response. We have initial response and then a decay due to damping. I can copy that off to the clipboard, and that's a useful thing to do. I'm going to be doing three analyses, and we can see that's the level of displacement at that particular point in time, just around about .10 seconds. We want to repeat the analysis, and all we're going to do is define a new damping level. So same sort of idea, I'm going to call that four percent damping input. We have to double check structural damping's off, Rayleigh damping's not on. We just put in our four percent critical damping and I'm going to drag that new damping level into the subcase and it overwrites the previous damping level. Let's just double check it's worked properly. There is four percent damping. I can run the analysis again, again, it will take a little time so I'm just going to accelerate the timeline. I can click OK and we can go view the results. So again I go into XY plot, generate a new XY plot, and exactly the same as before, I want to check the vertical displacement at that corner node. Click that to load it into the buffer window and show that XY plot. I can copy it to the clipboard and click OK. Close that out, and we'll go through this one more time. I'm going to create a new damping, or new damping object. This one's going to be six percent damping. Now, if I label these clearly, they're very useful because now they will stay in the model database once I save that off. Again I drag that over, put it into the subcases and it will overwrite. I always double check and it has the six percent damping that we anticipated. So we run this a final time, click OK, create a new XY plot. Same setup as before, vertical displacement at the corner node. View this XY plot. It's definitely decaying faster. I can copy the data to the clipboard, click OK. This is an Excel spreadsheet I produced, where I just cut and paste this data straight into the Excel spreadsheet. I just modified the titles slightly, cleaning it up. We can clearly see the influence of the damping on there. So at that point, we're just going to save off this database, again, this will save the three damping levels, and then the exercise is complete.