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

Quote:

Hey,
Thanks for your reply but i found the problem. Life is all good now :) 
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 
Hey,
What problems are you facing ? Could you elaborate please ! 
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 
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 
[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.. 
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 
If you have monitored your CL then apply FFT on this data set and find the prominent frequency after FFT then nondimensionalized it to find St.

Hi Mian,
Thanks for the reply . After using cl1 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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