Investigation of laminar-turbulent transition

Roland R · October 13, 2010, 11:16

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

michael_owen · October 13, 2010, 13:04

Velocity Invariant Q is useful.

pavitran · October 13, 2010, 22:07

You can plot "cf" at different sections on the wing. Through "cf" plot you can approximate the transition point.

Roland R · October 14, 2010, 12:22

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"?

Thanks
Roland

csmith_PADT · October 14, 2010, 12:45

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).

pavitran · October 14, 2010, 22:40

Quote:

Originally Posted by Roland R

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

Josh · October 15, 2010, 07:58

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.

October 13, 2010, 11:16	Investigation of laminar-turbulent transition	#1
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	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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October 13, 2010, 13:04		#2
michael_owen Senior Member Michael P. Owen Join Date: Mar 2009 Posts: 196 Rep Power: 17	Velocity Invariant Q is useful.

October 13, 2010, 22:07	Hi	#3
pavitran Member Dynampally Pavitran Join Date: Mar 2010 Location: India Posts: 74 Rep Power: 16	You can plot "cf" at different sections on the wing. Through "cf" plot you can approximate the transition point.

October 14, 2010, 12:22		#4
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	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"? Thanks Roland

October 14, 2010, 12:45		#5
csmith_PADT New Member Join Date: Sep 2010 Posts: 12 Rep Power: 15	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).

October 15, 2010, 07:58		#7
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 17	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.