Timestep size vs. tabular load entries

  • Last Post 20 July 2018
ewold posted this 19 July 2018


I am running a 2 way FSI in transient structural and fluent and had a simple question that I have not been able to find the answer to. I have run my simulation with the same time step size twice.The first time, in the table where I input my load in transient structural, I only input a few values and let the program impose ramp functions between the values I set. The second time, I input a load value for every single timestep in my simulation. The results I got in each simulation where drastically different. Why is this the case? Why doesn't the simulation treat the ramping functions the same way as tabular inputs for each timestep? I'd like to increase the timestep by a factor of 10, but I do not want to sacrifice the accuracy of my model because I cannot input load values for all 1000 timesteps. 

Thank you!

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SandeepMedikonda posted this 20 July 2018

Hi ewold,

That is strange when you apply ramped loads, you should still see loads applied in a linear way along the sub-steps unless you are using a stepped (KBC, 1). So here is what I found from the manual:



  You can edit the table of load vs. time and modify this behavior as needed.

  By default you have one step. However you may introduce multiple steps at time points where you want to change the analysis settings such as the time step size or when you want to activate or deactivate a load. An example is to delete a specified displacement at a point along the time history.   You do not need multiple steps simply to define a variation of load with respect to time. You can use tables or functions to define such variation within a single step. You need steps only if you want to guide the analysis settings or boundary conditions at specific time points.   When you add loads or supports in a static or transient analysis, the Tabular Data and Graph windows appear. You can enter the load history, that is, Time vs Load tabular data in the tabular data grid. Another option is to apply loads as functions of time. In this case you will enter the equation of how the load varies with respect to time.   

If you did not enter data at a time point then the value will be either a.) a linearly interpolated value if the load is a tabular load or b.) an exact value determined from the function that defines the load. An “=” sign is appended to such interpolated data so you can differentiate between the data that you entered and the data calculated by the program as shown in the example below. Here the user entered data at Time = 0 and Time = 5. The value at Time = 1e-3, the end time of step 1, is interpolated.  




If this doesn't help? can you post some snapshots of what you are doing and how are the results drastically different?




peteroznewman posted this 20 July 2018


Your posts are missing their images. You can't copy from Word and paste into these posts. You have to save the images separately and add them one-at-a-time inline with the text.

SandeepMedikonda posted this 20 July 2018

Hi Peter, I am not sure whats going on since I am able to see the images in the posts on different computers. Can someone else confirm this?

Also, I tried to save these images separately and tried to do what you suggested but that doesn't work either. I've reached out to the ADMIN, but if you feel my posts don't make sense without these images please feel free to delete them. FYI, I am unable to attach anything to a post as well.



ewold posted this 20 July 2018

Hello everyone,


I was able to resolve the issue. Thanks for your help! It was a problem with the way I was inputting values into the table.