# Comparison between simulations for same flow time

harshabharadwaj1
Member

in Fluids

Hello,

I had a question. I want to compare two simulations for the same flow time (4s). I use the Total Time option in the Duration Specification Method box during Calculation.

But one of the simulations converges in the specified time (4s) whereas the other does not. Does it make sense to compare the two even if one of the simulations has not reached steady state?

If not what do you suggest I do?

Thanks for the help in advance.

Tagged:

## Comments

It depends. What are you trying to compare?

Hello,

Thanks for the reply. I am trying to compare the shape and size of the flow at the outlet for different flow velocities and gap distance.

I created a plane near the outlet to get the shape as seen in the image (OutletPlane2.PNG) . This shape changes depending on the inlet velocity and the gap distance (Gap-0.4MM)

Are you running a transient simulation and are you attempting to compare the velocities at t = 4 s?

Is your end goal to compare the two steady state solutions?

Thanks.

Karthik

Hello Karthik,

I am a bit confused with this myself. I am running a transient simulation and I want to compare the different simulations. So from what I understood it made sense to compare the simulations for the same time frame (4s).

But some of the simulations does not reach convergence within 4 seconds. It takes longer. Does it make sense to compare the converged solutions for different times or does it make sense to compare the flow for same time (4s) with unconverged solutions?

I'd suggest that you let all your simulation reach a steady state and then compare results. This way you are sure to compare apples and apples.

Thanks

Karthik

Okay thanks for the help Karthik. But the different flow times will not matter is it?. Under what circumstances are different flow times comparable?

This will not matter if you let your simulations achieve a steady state. After this, your results will be time independent.

Thanks.

Karthik

Okay thanks for the help. I just had one more question. What does it mean when a solution converges at an iteration, but it does not in the next consequent iterations?

Some of the solutions converge intermittently.

What do you mean? Are you talking about a transient simulation run?

Thanks.

Karthik

@Kremella Yes, I am talking about a transient simulation.

Also when do I know if a solution has reached steady state?. If the residuals converge does it mean the solution has reached steady state?

Is there a way I can verify whether steady state has been reached or not?

Please create monitors such as 'Average velocity at the outlet boundaries' or 'Average Temperature at outlet' or Average inlet pressure'. Please plot these against time. When you have a steady solution during your transient run, you should see that these values becomes more or less constant (in an ideal scenario). In most cases, you might be able to achieve a periodic oscillating solution. You should be able to take this solution as approximately (time-averaged) steady state.

I hope this helps.

Thanks.

Karthik

@Kremella Thanks for the help. What if it is already a steady state simulation. Do I still have to check by creating monitors OR as the name 'steady' itself indicates, once the residuals are converged should I just assume it to be steady?

And also a quick clarification. My steady and transient simulations do not match. So I assume that the transient simulations will eventually match my steady results at some point ( 100, 1000 seconds..etc). Is my understanding correct?

Since transient simulations take a long time, is it better off doing a steady simulation for my case?

I really appreciate the help that you have provided me over the month.

Thank you.

You should always create monitors and make sure that your converged solution is steady.

Yes, most transient solutions finally reach a steady state solution. However, there might be cases where you might only get an oscillatory solution.

Steady state solutions are must faster than transient. If you are confident about your modeling and if the physics you are solving has a steady solution, you could work by solving a steady model.

Thanks.

Karthik