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  • FLUENT - Unsteady Flow Past a Cylinder - Step 6

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Author: Rajesh Bhaskaran & Yong Sheng Khoo, Cornell University

Problem Specification
1. Create Geometry in GAMBIT
2. Mesh Geometry in GAMBIT
3. Specify Boundary Types in GAMBIT
4. Set Up Problem in FLUENT
5. Solve!
6. Analyze Results
7. Validate the Results

Step 6: Analyze Results

Calculate Strouhal Number

Lift convergence plot can be used to compute the correct value of Strouhal number. Non-dimensionalize the problem and Sr = f*D/U = 0.0859 * 2 = 0.172. The results matches fairly well with the value 0.183 as reported by Williamson

Info
titleCalculating Shedding Frequency

To accurately calculate the shedding frequency, open the cl-history file (saved previously in the same location where the original mesh was read) and plot the data using excel for better data representation and graphing option. Take an average of 10 shedding cycles (e.g 10 CL peak).

Latex

\large
$$
{\rm{Period}} = {{T_2 - T_1} \over {10}}
$$


Latex

\large
$$
{\rm{f}} = {{1} \over {Period}}$$

Latex

\large
$$
{\rm{Sr}} = {fD \over U}
$$

An example of Lift Convergence Plot plotted using excel is shown below:

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https://confluence.cornell.edu/download/attachments/107011458/Excel%20Lift%20convergence.jpg

Display Pressure Contours

Main Menu > Display > Contours
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Under Contours of, choose Pressure.. and Static Pressure Coefficient. Select the Filled option. Increase the number of contour levels plotted: set Levels to 40. Set Levels to 100. Disable Auto Range and Clip to Range from the Options group box. Enter -1.225 and 0.5 for Min and Max, respectively.Click Display.
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https://confluence.cornell.edu/download/attachments/107011458/pressure%20contour%20plotpressure%20coefficient%20contour%20plot.jpg

The contour shows a clear asymmetric pattern in the flow. . The local pressure minima (the green patch downstream) are the center of the vortices.
 

Display Contour of Vorticity Magnitude

Main Menu > Display > Contours



Under Contours of, choose Velocity.. and Vorticity Magnitude. Disable Auto Range and Clip to Range from the Options group box. Enter 0.0001 and 2 for Min and Max, respectively. Select Levels to 50. Click Display.


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https://confluence.cornell.edu/download/attachments/107011458/velocity%20magnitude%20contour%20plotvorticity%20magnitude%20contour%20plot.jpg

This figure shows clear vortex shedding process. Zoom in the view around cylinder. 

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