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

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 September 26, 2013, 11:12 Flow past a cylinder, Reynolds No: 100, Tracking strouhal Number. #1 New Member   Karan Bansal Join Date: Sep 2013 Posts: 9 Rep Power: 12 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 jzc and kpsinghdhania like this.

 September 26, 2013, 17:20 #2 New Member   Karan Bansal Join Date: Sep 2013 Posts: 9 Rep Power: 12 Does it matter if i am using first order time scheme or second order time scheme ?

September 27, 2013, 03:19
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Alex
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 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.

 September 28, 2013, 08:30 #4 New Member   Karan Bansal Join Date: Sep 2013 Posts: 9 Rep Power: 12 Hey, Thanks for your reply but i found the problem. Life is all good now

 September 30, 2013, 23:36 #5 New Member   mian Join Date: Jul 2009 Posts: 3 Rep Power: 16 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

 October 1, 2013, 00:35 #6 New Member   Karan Bansal Join Date: Sep 2013 Posts: 9 Rep Power: 12 Hey, What problems are you facing ? Could you elaborate please !

 October 7, 2013, 10:26 #7 New Member   mian Join Date: Jul 2009 Posts: 3 Rep Power: 16 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

 October 7, 2013, 12:37 #8 New Member   Karan Bansal Join Date: Sep 2013 Posts: 9 Rep Power: 12 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

 September 21, 2015, 22:34 #9 Member   N B Khan Join Date: Jan 2014 Posts: 39 Rep Power: 12 [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..

 November 13, 2016, 12:33 #10 New Member   ishan sharma Join Date: Nov 2016 Posts: 4 Rep Power: 9 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

 November 13, 2016, 23:49 #11 New Member   mian Join Date: Jul 2009 Posts: 3 Rep Power: 16 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.

 November 14, 2016, 04:49 #12 New Member   ishan sharma Join Date: Nov 2016 Posts: 4 Rep Power: 9 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