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Old   December 9, 2016, 06:32
Default Drag coefficient of a sphere
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Hello everyone,
I would like to ask you about drag coefficient calculations using Fluent.

The problem I am solving is a free flow around a sphere (3D analysis). I want to calculate coefficient of drag in function of Reynolds number. As a starting point I chose Re = 10 000 at which the Cd should be around 0.4 (based on literature). However the result I obtain is Cd=0.24. Below there are some more details. Maybe anyone can spot a mistake?

Ball diameter = 0.01m
Inlet velocity = 16.18 m/s

Domain is modelled without using symmetry plane (BC).
Reference values:
Area = 0.0000785 m2
Velocity = 16.18 m/s

In the drag monitor the sphere wall is selected and the X vector (direction of the flow).

Turbulence model used in solver: standard k- omega. I tried k-epsilon as well and results are almost identical.

Calculations are performed in steady state. I did transient analysis as well (0.1s time step) and the results are also very similar to the steady ones.

I can understand some discrepancies between CFD and measurements but not 2 times too small values.

Any ideas what's wrong?
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Old   December 9, 2016, 17:15
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Flow over a sphere at Re = 10,000 is in the subcritical flow regime where the wake has turbulence, and vortices are shedding from the sphere. The transition from laminar to turbulence is a tricky phenomena to simulate and the k-e and k-w turbulence models are not well suited for this scenario (they diffuse the turbulence so well that you're finding a solution in steady state). If you want to simulate the flow accurately, then investigate large eddy simulations (there are plenty of papers in literature). You should select a small enough time step to accurately resolve the vortices (related to the Strouhal number; say 25 - 50 steps per vortex period).
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Old   December 12, 2016, 05:32
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Thank you for the hint.

Indeed I tried out couple of viscous models and the one which gave me the expected results was the Scale Adaptive Simulation (SAS).
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Old   December 13, 2016, 03:38
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What about reference density? Check all your reference values.
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Old   December 14, 2016, 06:41
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Problem solved with SAS model. Reference density is calculated from the inlet values.
In incompressible flow it looks all right. The question is if this is still valid for the compressible flows with Mach number above 1.0?
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