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Unsteady Results-Flow over a sphere

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Old   March 28, 2019, 11:11
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   March 28, 2019, 12:06
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Filippo Maria Denaro
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Originally Posted by raskolnikov View Post
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.



What about the diameter and the fluid? What about the details of your numerical formulation?
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Old   March 28, 2019, 12:32
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mike
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Sorry for the missing informations about my model. It is an air flow over a sphere with 220 mm diameter like a soccer ball. The mesh size in the wake is 6 mm.



I think it is a general question for me, because the same thing occured when i tried to simulate a vehicle aerodynamic problem in transient case. I observed, it doesnt come to almost no changes when i looked at velocity distributions after certain time-steps.



So, could it be a problem at the my problem definition or is the time step i have chosen a wrong value? Or Do I have to make the mesh size finer in the wake?



Thanks
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Old   March 28, 2019, 12:41
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Originally Posted by raskolnikov View Post
Sorry for the missing informations about my model. It is an air flow over a sphere with 220 mm diameter like a soccer ball. The mesh size in the wake is 6 mm.



I think it is a general question for me, because the same thing occured when i tried to simulate a vehicle aerodynamic problem in transient case. I observed, it doesnt come to almost no changes when i looked at velocity distributions after certain time-steps.



So, could it be a problem at the my problem definition or is the time step i have chosen a wrong value? Or Do I have to make the mesh size finer in the wake?



Thanks



No, the first and more relevant think is about the mathematical set of equations you are solving. What kind of formulation are you using?

Be aware that your Re is O(10^5). This is a very critical regime, see https://www.grc.nasa.gov/www/k-12/ai...ragsphere.html


This flow is very challenging, requires a proper formulation and a very refined grid
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