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Investigation of laminar-turbulent transition

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Old   October 13, 2010, 12:16
Default Investigation of laminar-turbulent transition
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Roland Rakos
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Hi CFX Users,

I am calculating a 3D wing which will invastigated at low - and high Reynolds number. In the first place the laminar-turbulent transition is in the center of my interest, so I would like to see where does the flow change from laminar into turbulent.
How can I see where is the transition zone at the surface of the wing? Which values do show the characteristic of the flow?

Thanks in advance
Roland
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Old   October 13, 2010, 14:04
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Michael P. Owen
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Velocity Invariant Q is useful.
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Old   October 13, 2010, 23:07
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Dynampally Pavitran
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You can plot "cf" at different sections on the wing. Through "cf" plot you can approximate the transition point.
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Old   October 14, 2010, 13:22
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Roland Rakos
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Hello,

Thanks, the "cf" is really correct solution, but I have a little problem. Along the airfoil (from the leading edge to the trailing edge) the "cf" has an strong fluctuation. Value of the the y plus is about 1. The Aspect ratio is about 40 in the boundary layer. What is the reason of the fluctuation of "cf"?

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Old   October 14, 2010, 13:45
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The skin friction may be fluctuating due to the interaction of flow structures (vortex tubes) with the wall. The mere presence of the these type of structures may indicate that the flow has transitioned or is already turbulent. Are you running a steady simulation or is it transient? Also, take care in your approach for the turbulent treatment; if you are studying transition, you should not have any turbulence models enabled (except for the transition model in CFX).
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Old   October 14, 2010, 23:40
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Dynampally Pavitran
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Quote:
Originally Posted by Roland R View Post
Hello,
What is the reason of the fluctuation of "cf"?
Thanks
Roland
Hi,

Apart from flow physics point of view, the other reason for fluctations may be due to the curvature of your 2D profile (if your extruding 2D to 3D), check whether it is smooth.

And if your curvature seems to be fine enough, check whether the nodes are lying on the surface after meshing.

By the way how many cells are there along the airfoil? See that your surface grid captures both geometric and flow features with sufficient resolution
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Old   October 15, 2010, 08:58
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Joshua Counsil
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If your simulation is unsteady, then Cf will change significantly between timesteps. You should use a batch run to produce Cf graphs at each timestep, then average them with a macro.
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