## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

#### Convergence issue in PCM simulation Fluent.

Subscriber

I have been trying to perform a PCM melting simulation in FLUENT, but am facing convergence issue. There is normal melting at start of simulation, but then there is a very sudden drop in temperature after which the system becomes completely unstable and goes back up to a very high temperature in the next timestep.

Mesh Quality:

Minimum Orthogonal Quality = 3.52907e-01 cell 323206 on zone 9 (ID: 406547 on partition: 0) at location (-1.57583e-02 2.41255e-03 1.67722e-02)

(To improve Orthogonal quality , use "Inverse Orthogonal Quality" in Fluent Meshing,

where Inverse Orthogonal Quality = 1 - Orthogonal Quality)

Maximum Aspect Ratio = 7.91960e+00 cell 193565 on zone 9 (ID: 276906 on partition: 0) at location (-1.39062e-02 2.66648e-03 1.59572e-02)

Domain Extents:

x-coordinate: min (m) = -2.000000e-02, max (m) = 2.000000e-02

y-coordinate: min (m) = -6.123234e-19, max (m) = 3.664102e-02

z-coordinate: min (m) = 0.000000e+00, max (m) = 5.000000e-01

Volume statistics:

minimum volume (m3): 1.368996e-11

maximum volume (m3): 5.474989e-10

total volume (m3): 3.864102e-04

Face area statistics:

minimum face area (m2): 3.245439e-08

maximum face area (m2): 1.006125e-06

Checking mesh...........................

Done.

• Rob
Ansys Employee
What changes in the simulation result to trigger the sudden temperature problem? n
Subscriber
Not sure. I had not made any changes in the model. this is during a continuous run. n
• Karthik R
Please check your residuals at this point when you see this change in trend. If these residuals look good and very similar to the previous one, this point is telling you something about the physics you are attempting to model. nI'd strongly recommend that you inspect your simulation before and after this spike and you might get a clue as to what might be happening. nKarthikn
Subscriber
Hey, So I checked my residuals, and this is how they look. nThere is a sudden spike in them too. ot sure what to make of this now.. n
• DrAmine
Ansys Employee
Subscriber

This is what I used as my PCM material propertiesn I am sorry but I am not sure what exactly you mean by melting model. I am new to this, could you please elaborate? nThanksnn
• Karthik R
Your model doesn't seem to be converging properly. Also, provide some screenshots of your convergence strategy - Time-step, solution schemes etc.nKarthikn
Subscriber

Your model doesn't seem to be converging properly. Also, provide some screenshots of your convergence strategy - Time-step, solution schemes etc.Karthikhttps://forum.ansys.com/discussion/comment/106180#Comment_106180

n
• DrAmine
Ansys Employee
Definitely high time step. Add material panel from Fluent, I want to check something.n
Subscriber

Definitely high time step. Add material panel from Fluent, I want to check something.https://forum.ansys.com/discussion/comment/106191#Comment_106191

Here you go.nnWhat would be the recommended Time step?.Thanks n
• Karthik R
Please estimate the time-step to make sure that the CFL number is ~1. It depends on the velocity and smallest grid scales.nCFL = del_t * V / del_x ~ 1nI agree with 5 s is too large a time-step.nKarthikn
Subscriber
Changed the settings to adaptive time control with CFL -based. It worked. nThanks everyone for your help!
• GUNA AG
Subscriber