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Flow past a cylinder, Reynolds No: 100, Tracking strouhal Number.

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Old   September 26, 2013, 11:12
Default Flow past a cylinder, Reynolds No: 100, Tracking strouhal Number.
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Karan Bansal
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Hello,

I am simulating flow past a cylinder for Re 50, 100, 150 and have to obtain the strouhal number for these flows. (Using Ansys - Fluent)

So far I have tried simulating all the three Reynolds number flows using second order schemes for momentum and both 1st and 2nd order scheme for time.

The problem i am facing is that i can't obtain the correct strouhal number and also the graph for Cl (Coefficient of Lift) vs time is not perfectly periodic. I am getting wiggles at many points and the amplitude varies as well which shouldn't be the case as per the papers i have read and makes sense physically too.

Grid dimensions -
Upstream - 20D
Downstream - 30D

~82,000 elements, structured mesh.

Scheme used -
SIMPLE (Pressure-velocity coupling)
Gradient (Least cell square based)
Pressure(standard)
Momentum(Second order upwind)
Time (simulated for both 1st order and second order)
Under relaxation (Default values in fluent)
Time step (0.05 for first 1000 steps and 0.1 for further time steps)
Max iterations per time step (40)

Density -1 kg/m^3
Inlet velocity - 1 m/s
Diameter - 1 m
Viscosity - 0.02 for Re 50
0.01 for Re 100
0.006666 for Re 150

Outlet - Gauge pressure 0, Normal to boundary
Circular Cylinder Wall - No slip


I can't gather what the problem is. Have been stuck on this for more than a week now. Have modified the grid, refined it, tried changing the schemes etc. But nothing works. Can anyone see what the problem is ? Any help is appreciated.

Thanks
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Old   September 26, 2013, 17:20
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Does it matter if i am using first order time scheme or second order time scheme ?
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Old   September 27, 2013, 03:19
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Quote:
The problem i am facing is that i can't obtain the correct strouhal number and also the graph for Cl (Coefficient of Lift) vs time is not perfectly periodic. I am getting wiggles at many points and the amplitude varies as well which shouldn't be the case as per the papers i have read and makes sense physically too.
Why dont you post some images of your result? An image of the mesh and the computational domain would also help.
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Old   September 28, 2013, 08:30
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Hey,

Thanks for your reply but i found the problem. Life is all good now
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Old   September 30, 2013, 23:36
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Mr Karan
Could you please guide me in this regard.........as i am facing the similar problem and not able to find a satisfactory solution.
Thanks
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Old   October 1, 2013, 00:35
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Hey,

What problems are you facing ? Could you elaborate please !
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Old   October 7, 2013, 10:26
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Thanks Karan
Actually i am simulating a circular cylinder and want to get RMS CL for this cylinder at Re=100, dont know how to get from fluent. as i tried to write a custom field function for this but there is no option for coefficients.kindly if you can help in this regard

Regards
Farhan
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Old   October 7, 2013, 12:37
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Hey Farhan,

Are you using Ansys Fluent for the simulation ? If yes, there is an option in ansys which helps you write the coefficient of lift at every time step in the form of a data file. This data file can be used easily to find the RMS Lift coefficient.

In fluent, when you are setting up the problem, on the left side in the window, under the solution->Monitors you will find Drag coefficient and lift coefficient and residual etc. Here you need to select lift coefficient (double click) and turn on plot, print and write function which will help you write a data file while the simulation runs. You can use this data file for your problem.

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Karan
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