Can a boundary layer trip wire change the Reynolds number?

midnightblack · January 30, 2015, 17:11

Hi all,

I am wondering, if we have a problem with a simple fixed uniform flow speed against an object (for example airfoil), with the dimensions and shape of the object remaining fixed. Does including a boundary layer trip wire, vortex generators etc actually change the Reynolds number in the analysis of the problem?

Or would you say that at the same Reynolds number as before, we found a way to alleviate the boundary layer effects/create more turbulence in the boundary layer etc.

Thanks for your time!

spl · February 2, 2015, 05:21

Hi,

No the Reynolds no. would remain constant. The trip wire just 'trips' the boundary layer and causes transition giving a turbulent boundary layer at lower flow velocities. It is done to allow wind tunnel tests, where the Reynolds number is lower than the real life application to be more representative of the real case.

And as for vortex generators, they cause the momentum with in the boundary layer to be redistributed to delay flow separation. Again this wouldn't effect Reynolds number as the flow velocity and chord length of the aerofoil remain constant.

I'm not sure what you mean by alleviate boundary layer effects?

Regards

FMDenaro · February 2, 2015, 08:11

Thinking about the question, if you consider the local Re number along the wing (somehow similary to considering the local Re number along a flat plate), the induced perturbation in the velocity can have some effect.
I am not able to say if that effect is also really relevant...

spl · February 2, 2015, 10:55

Hi Filippo

I agree that the trip would create protrusions in the flow. However both trips and vortex generators focus on the flow within the boundary layer. The boundary layer will therefore be effected but if you were to calculate the local Reynolds number you would use the local velocity at the edge of the boundary layer. Therefore, shouldn't the the local Reynolds no. still be unaffected?

FMDenaro · February 2, 2015, 12:20

Quote:

Originally Posted by spl

Hi Filippo

I agree that the trip would create protrusions in the flow. However both trips and vortex generators focus on the flow within the boundary layer. The boundary layer will therefore be effected but if you were to calculate the local Reynolds number you would use the local velocity at the edge of the boundary layer. Therefore, shouldn't the the local Reynolds no. still be unaffected?

Hello,
to tell the truth, I don't know...I was thinking if somehow the trip should be considered as the new starting-edge of the boundary layer...
But I suppose that should not be relevant in a CFD simulation if you consider the cell-Reynolds number of the y+ (which is also a local Re number).

arunintn · February 18, 2015, 03:14

I'm just lost in the "trip wire"

May i know what is that. I understand vortex generator but trip is something that do the same?

spl · February 18, 2015, 04:41

A trip wire is used to "trip" the flow to give a turbulent boundary layer. Its usually used in wind tunnel testing where the Reynolds number of the test is lower than the real case. At the lower test Reynolds number natural transition may occur much later over the model leading to inaccuracies. If you use a trip wire along, or near to the leading edge, the boundary layer will transition just after the trip. This alleviates some of the errors due to scaling.

January 30, 2015, 17:11	Can a boundary layer trip wire change the Reynolds number?	#1
midnightblack New Member Join Date: Jan 2015 Posts: 2 Rep Power: 0	Hi all, I am wondering, if we have a problem with a simple fixed uniform flow speed against an object (for example airfoil), with the dimensions and shape of the object remaining fixed. Does including a boundary layer trip wire, vortex generators etc actually change the Reynolds number in the analysis of the problem? Or would you say that at the same Reynolds number as before, we found a way to alleviate the boundary layer effects/create more turbulence in the boundary layer etc. Thanks for your time!

February 18, 2015, 03:14	About Trip wire	#6
arunintn Member Arun Join Date: Mar 2012 Location: Vellore,TN,India Posts: 43 Rep Power: 14	I'm just lost in the "trip wire" May i know what is that. I understand vortex generator but trip is something that do the same?

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February 2, 2015, 05:21		#2
spl Member spl Join Date: Nov 2014 Posts: 33 Rep Power: 12	Hi, No the Reynolds no. would remain constant. The trip wire just 'trips' the boundary layer and causes transition giving a turbulent boundary layer at lower flow velocities. It is done to allow wind tunnel tests, where the Reynolds number is lower than the real life application to be more representative of the real case. And as for vortex generators, they cause the momentum with in the boundary layer to be redistributed to delay flow separation. Again this wouldn't effect Reynolds number as the flow velocity and chord length of the aerofoil remain constant. I'm not sure what you mean by alleviate boundary layer effects? Regards

February 2, 2015, 08:11		#3
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,896 Rep Power: 73	Thinking about the question, if you consider the local Re number along the wing (somehow similary to considering the local Re number along a flat plate), the induced perturbation in the velocity can have some effect. I am not able to say if that effect is also really relevant...

February 2, 2015, 10:55		#4
spl Member spl Join Date: Nov 2014 Posts: 33 Rep Power: 12	Hi Filippo I agree that the trip would create protrusions in the flow. However both trips and vortex generators focus on the flow within the boundary layer. The boundary layer will therefore be effected but if you were to calculate the local Reynolds number you would use the local velocity at the edge of the boundary layer. Therefore, shouldn't the the local Reynolds no. still be unaffected?

February 18, 2015, 04:41		#7
spl Member spl Join Date: Nov 2014 Posts: 33 Rep Power: 12	A trip wire is used to "trip" the flow to give a turbulent boundary layer. Its usually used in wind tunnel testing where the Reynolds number of the test is lower than the real case. At the lower test Reynolds number natural transition may occur much later over the model leading to inaccuracies. If you use a trip wire along, or near to the leading edge, the boundary layer will transition just after the trip. This alleviates some of the errors due to scaling.