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• # Doubts Regarding Flow Over a Cylinder Results

Posted by on October 16, 2023 at 7:19 pm

Hello everyone,

I have just completed the assignment for flow over a cylinder, and I’ve explored different approaches and methods to understand the differences.

I utilized the “Pseudo Transient Approach” and the “Under Relaxation Factor” and compared residuals and iteration numbers. As expected, the iteration numbers increased with decreasing time step size and under relaxation factor, which doesn’t pose any problems.

However, I encountered difficulties while creating the mesh and achieving convergence with the SIMPLE method. Here are some questions I’d like to address:

1. How can we prepare a 2D domain in Fluent Mesher? Is it possible, or do we need to introduce some thickness into the geometry? (I managed to prepare a 2D domain using the Workbench Mesh.)

2. I attempted to calculate turbulence intensity and reference length, but I have doubts about using the correct numbers. In the assignment, the cylinder diameter was 1 meter, and the inlet velocity was 50 m/s. Consequently, the Reynolds number is 4.975E7, and turbulence intensity, I = 0.0175. So, if turbulence intensity is 1.75% and the reference length is 1 meter, am I correct? If I wish to use turbulence intensity and hydraulic diameter, will the values remain the same? By using the calculated values, the default iteration number decreased from 533 to 441. Yet, there are slight differences in pressure values and streamlines (Figures attached). How can I determine which is closer to the real situation?

3. I wanted to employ the SIMPLE method to utilize under relaxation factors, but I couldn’t achieve a converged solution with “Second Order” Pressure Discretization. Changing it to “PRESTO!” led to convergence, but I’m unsure why this alteration worked. Can you explain the differences? Given the multitude of options, is there a source I can consult to understand the distinctions between these or any other methods?

4. I noticed slight variations in the results of pressure and streamlines when comparing results between the “SIMPLE” algorithm and the “COUPLED” algorithm. How can I determine which one is more stable under specific conditions? Can you recommend a guide or resource to explore this further?

I hope you can see the names of the images. You can differentiate them by their names.

Thank you so much.

Best Regards,

Burak

replied 8 months ago 2 Members · 2 Replies
• 2 Replies
• ### MIDHUN MURALI

Member
October 19, 2023 at 3:50 pm

Haie ,

In fluent mesher we cannot mesh 2d geometry, fluent mesher uses 3d geometry. And regarding turbulence intensity you can the equation i have also attached a screen shot regarding the same which also shows the some common values used . You can also refer to Reynolds Number video in our youtube channel (at 37.28) where turbulence intensity is mentioned .

In case of different schemes :

Standard â€“ The default scheme may have reduced accuracy for flows exhibiting large surface-normal

pressure gradients near boundaries (but should not be used when steep pressure changes

are present in the flow â€“ PRESTO! scheme should be used instead)

PRESTO! â€“ Use for highly swirling flows, flows involving steep pressure gradients (porous

media, fan model, etc.), or in strongly curved domains

* ANSYS (source)

It could be the reason for the deviation but iam not entirely sure. Conducting a grid independency could help in correctly pointing out it could also be due to mesh.

• ### balliburak93

Member
October 21, 2023 at 11:10 am

Thank you Midhun,

I calculated the Turbulence Intensity as 0.0175, but I used 1.75. I thought I needed to multiply it by 100 to get a percentage. So, it was my mistake.