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-   -   Flow past a cylinder, Reynolds No: 100, Tracking strouhal Number. (https://www.cfd-online.com/Forums/fluent/124058-flow-past-cylinder-reynolds-no-100-tracking-strouhal-number.html)

 Karan September 26, 2013 11:12

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

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)
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

 Karan September 26, 2013 17:20

Does it matter if i am using first order time scheme or second order time scheme ?

 flotus1 September 27, 2013 03:19

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.

 Karan September 28, 2013 08:30

Hey,

Thanks for your reply but i found the problem. Life is all good now :)

 gojrian September 30, 2013 23:36

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

 Karan October 1, 2013 00:35

Hey,

What problems are you facing ? Could you elaborate please !

 gojrian October 7, 2013 10:26

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

 Karan October 7, 2013 12:37

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.

Regards,
Karan

 bestniaz September 21, 2015 22:34

[QUOTE=Karan;455537]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.

Dear Karan
How to get the Cl file in CFX module? one more question
If, at beginning, we are unable to monitor the Cd/Cl value in Fluent,,,then its mean we have to run the whole simulation again from the beginning to get the Cd/Cl? data..

 ishan2033 November 13, 2016 13:33

Hello,

Can Anybody please tell me How to plot the Cl vs Time graph to find the Strouhal no. after the analysis. It's Urgent please help

 gojrian November 14, 2016 00:49

If you have monitored your CL then apply FFT on this data set and find the prominent frequency after FFT then non-dimensionalized it to find St.

 ishan2033 November 14, 2016 05:49

Hi Mian,
Thanks for the reply . After using cl-1 history file and FFT on it I get 2 options, first a graph between Magnitude and Frequency and the other is Magnitude vs Strouhal no. but I am not able to understand both the graphs to find the strouhal no.
Also how to find the dominant frequency and how to non dimensionalize it.
Pleas help me i have to submit the report by Friday

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