Menu

Discussion Forum

New Discussion
Filter

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Location of a Step Function

Posted Apr 4, 2018, 9:45 a.m. EDT Fluid & Heat, General, Parameters, Variables, & Functions 4 Replies

Baris Gungordu
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Dear experts,

What is the significance of the Location of the step function? In the tutorials provided, COMSOL tends to place it in 0.1 location.

In my simulations, I get different results when I set it to different location values. For a better model, I need to know its significance.

Many thanks.

4 Replies Last Post Apr 6, 2018, 7:37 a.m. EDT
jasonhnicholson
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 7 years ago Apr 4, 2018, 12:54 p.m. EDT
Updated: 7 years ago Apr 4, 2018, 3:12 p.m. EDT

I think you are refering to the step function's time location in the flow past a cylinder model/tutorial. The only point of the step function is to smoothly ramp the inlet flow condition. The flow past a cylinder is a periodic flow field if the simulation time is big enough. The cycle repeats about once every .33-.35 seconds. Thus, if you keep the 7 second time and change the time location of the step function, you should get different results. However, if you adjust the end time accordingly, and look for the cycle beginning and end, you will see the flow field is the same.

Here is a better description of the periodic behavior of flow past a cylinder: DFG benchmark 2D-2 (RE100, periodic).

-------------------
Jason Nicholson
I think you are refering to the step function's time location in the flow past a cylinder model/tutorial. The only point of the step function is to smoothly ramp the inlet flow condition. The flow past a cylinder is a periodic flow field if the simulation time is big enough. The cycle repeats about once every .33-.35 seconds. Thus, if you keep the 7 second time and change the time location of the step function, you should get different results. However, if you adjust the end time accordingly, and look for the cycle beginning and end, you will see the flow field is the same. Here is a better description of the periodic behavior of flow past a cylinder: [DFG benchmark 2D-2 (RE100, periodic)](http://www.featflow.de/en/benchmarks/cfdbenchmarking/flow.html).
Magnus Ringh COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 7 years ago Apr 5, 2018, 3:41 a.m. EDT

Hi,

The value in the Location field is the start location or time for the step. You can set it to, for example, the time at which you want to have a step change, when you call the step function with the time t. The reason for the value of 0.1 in the Flow Past a Cylinder model example is just so that you don't start the simulation with the step change right away.

Best regards, Magnus Ringh, COMSOL

Hi, The value in the **Location** field is the start location or time for the step. You can set it to, for example, the time at which you want to have a step change, when you call the step function with the time t. The reason for the value of 0.1 in the Flow Past a Cylinder model example is just so that you don't start the simulation with the step change right away. Best regards, Magnus Ringh, COMSOL
Baris Gungordu
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 7 years ago Apr 6, 2018, 5:38 a.m. EDT

Hi,

Thanks for the answer. What I am not clear with is the physical significance of the step location. Does it mean the step is located at 10% of the domain? Or, it means it will start at at 0.1 of the time step?

Hi, Thanks for the answer. What I am not clear with is the physical significance of the step location. Does it mean the step is located at 10% of the domain? Or, it means it will start at at 0.1 of the time step?
Magnus Ringh COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 7 years ago Apr 6, 2018, 7:37 a.m. EDT

Hi,

It will start whenever the input value (the time, for example) reaches the value of the step location, so it is not relative but an absolute value of the step's input, whatever that is. It is often set to a small positive value to delay the step a little bit at the start of a transient simulation. You can find all models and apps that use a Step function node by searching for @step in the Search field in the Application Libraries window. In the attached plot of a smoothed step function, the location is set to 10.

I hope this helps.

Best regards, Magnus Ringh, COMSOL

Hi, It will start whenever the input value (the time, for example) reaches the value of the step location, so it is not relative but an absolute value of the step's input, whatever that is. It is often set to a small positive value to delay the step a little bit at the start of a transient simulation. You can find all models and apps that use a Step function node by searching for `@step` in the **Search** field in the **Application Libraries** window. In the attached plot of a smoothed step function, the location is set to 10. I hope this helps. Best regards, Magnus Ringh, COMSOL

Attachments:

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content