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-   -   I need help in setting up grid for cylinder (http://www.cfd-online.com/Forums/main/94834-i-need-help-setting-up-grid-cylinder.html)

thethirdfarial November 28, 2011 00:58

I need help in setting up grid for cylinder
 
Hello CFD Online forumers.
I would like to ask about the best way to set up grid for flow around cylinder.
I am currently doing a simulation around cylinder where the flow is in axial direction. The aim is to get the flow to transition into turbulent flow. So far i have yet to get the flow to be turbulent no matter how much I change the parameters. When I change the grid into those of plane wall the program works fine and the flow transition into turbulent without problem so I think there must be a problem in the way the grid is set up. I am using fortran for the simulation. Here are the equations I used to setup the grid:


http://i166.photobucket.com/albums/u...upequation.gif

r is the circle radius, and its made so that the grid gets denser the closer it get to the cylinder surface.
RC is the cylinder's radius.
theta in this case is 1.

Here is a sample of the projected grid

http://i166.photobucket.com/albums/u...GridSystem.jpg

The program I am using for this simulation uses LES model with weak compressibility.

So I am quite clueless what's actually happening that the flow won't transition into turbulent flow. Could sin & cos be the problem here?
Can somebody help me?
Thank you

cfdnewbie November 28, 2011 04:32

Did you check the Reynolds number? Can you (based on Re) estimate the transition length? Try turning off the LES model, and see if that helps. I suspect that your model might be too dissipative and that you need to trip the BL or add some noise to it to get it turbulent....

thethirdfarial November 28, 2011 05:57

Thank you for your reply.

For plane wall the critical boundary layer reynolds number (ReDL) for transition is around 1100 so in my case i used ReDL of 1500 and above.
For the flow, the reynolds number is around 60,000.
In my assignment I need the reynolds number to be as low possible since I'm working on the minimum diameter needed to keep turbulent flow in the boundary layer.

I did insert noise at the start of the calculation and it works fine on plane wall.

I haven't tried taking off LES and use other model so far. Is there any other suggestion?


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