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Unsteady Results-Flow over a sphere

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Old   March 28, 2019, 10:02
Question Unsteady Results-Flow over a sphere
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mike
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Hi everyone,



i have a question about my unsteady cfd results. I am simulating a time-dependent (unsteady at inlet velocity 20 m/s) flow over a sphere with Star CCM+ which is in the middle of the wind tunnel. Also some pictures are saved in every time-step.



After some time-steps I mean the time= about 0.5 s, the velocity and also vorticity distributions change really minimally, as opposed to the time between 0-0.5s. So that i cant see the turbulent structures on these distributions. The solution is like an end result of a steady state!

(Time-Step is chosen= 0.002s)



I have expected that the periodic fluctating behaviour could be clearly seen.

Is it normal that the flow can be stable in this case? Or is it unrealistic how I define the problem?



Thanks in advance.
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Old   December 9, 2020, 18:23
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Onur
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It would be nice to see the pictures when it looks 'steady'. Im not sure what the diameter of your ball is but if your Re is above 200,000, the turbulence behind the sphere gets very fine. See the picture I attached. 20m/s is pretty high.

You can make some analysis if you would like. Place multiple probes behind the sphere. For each probe you put create a max or a min report and as part select the probe for the report. Finally, create monitor and plot for each report by right clicking(or u can simply select all reports and create a single plot. See if there are fluctuations.

You can also try statistical analysis. Through finding fluctuating velocity of the field (RMS-root mean square velocity).
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Old   January 18, 2021, 01:58
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gago
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First of all what is your Courant number with this setup? Which turbulence model is used? And also mesh size is also very important to capture flow behaviour. Could you please also check the y+ values?
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