Transition SST - Intermittency/Tu Decay/Y+

Kloz · July 28, 2011, 09:29

Hi, I'm using the Transition SST model in Fluent V13 to model a transitional flow on a flat plate in a wind tunnel. I'm unsure about how to set the intermittency on the velocity inlet. Zero? 1? Any ideas there?
Regarding turbulence intensity, the manual mentions estimating the Tu decay. I have very little decay between my inlet and plate, but it exists nonetheless. How do I go about estimating that decay? Or is just trial and error until I reach the desired turbulence intensity values at the plate?
The manual also states that for the transition models y+<1 is desired. Seems pretty steep a requirement, I'm wondering if the 1-4 range might be sufficient. More importantly, do I also need to have the y+ on the tunnel walls less than 5?
Finally, sorry for the basic question, but what's the best way to identify the laminar and turbulent regions on the plate and the point of transition onset?

Thanks.

Kloz · July 31, 2011, 13:16

Any advice regarding how to set the intermittency on the velocity inlet? It's the biggest question mark at the moment.

sail · July 31, 2011, 23:20

hi,

i reccomand some investigation on the papers of menter and the phd thesis of Langtry. the gamma-retheta model is not as easy as it looks and requires some tuning to get meaningful answers.

y+<1 is a must, anything bigger will move the transition point upstream. it is well documented in the fluent theory manual.

Kloz · August 29, 2011, 12:04

So I've taken a look at Menter and Langtry's paper, 'A Correlation-Based Transition Model Using Local Variables'. I'm trying to replicate their results for one of the test cases they used to validate the model, the Schubauer flat plate experiment from the NACA TN 3489. I'm using the same inlet conditions they specify in the paper as well as the second order upwind scheme but for some reason transition just isn't being triggered. My Cf plot is exactly that of the laminar computation and consistent with the Blasius solution. Not really sure what I'm missing. Do I need to play around with wall roughness or wall material specifications for example? I've also tried upping the freestream turbulence intensity (to 0.5%, 1% & 3% compared to Menter's 0.18%) and playing with the viscosity ratio (10 & 120 compared to Menter's 5) but no luck, still laminar flow. Y+ is <1 all over the plate.

Leonardo.flores · November 19, 2018, 01:54

I am interested on the intermittency setup at the Velocity Inlet (And at the Pressure outlet as well). I tried the same case (a NACA profile at 8 degrees) varying the inlet and outlet intermittency with values of 0, 0.5 and 1... and they gave very different results on lift and drag. What I assume is that the intermittency must be very low, probably zero at the inlet, or it might trigger transition very early at the airfoil... but the paper of Suluksna recommends a value of 1.0... I can not explain why yet.

July 28, 2011, 09:29	Transition SST - Intermittency/Tu Decay/Y+	#1
Kloz New Member Join Date: Jul 2011 Posts: 4 Rep Power: 15	Hi, I'm using the Transition SST model in Fluent V13 to model a transitional flow on a flat plate in a wind tunnel. I'm unsure about how to set the intermittency on the velocity inlet. Zero? 1? Any ideas there? Regarding turbulence intensity, the manual mentions estimating the Tu decay. I have very little decay between my inlet and plate, but it exists nonetheless. How do I go about estimating that decay? Or is just trial and error until I reach the desired turbulence intensity values at the plate? The manual also states that for the transition models y+<1 is desired. Seems pretty steep a requirement, I'm wondering if the 1-4 range might be sufficient. More importantly, do I also need to have the y+ on the tunnel walls less than 5? Finally, sorry for the basic question, but what's the best way to identify the laminar and turbulent regions on the plate and the point of transition onset? Thanks.

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July 31, 2011, 13:16		#2
Kloz New Member Join Date: Jul 2011 Posts: 4 Rep Power: 15	Any advice regarding how to set the intermittency on the velocity inlet? It's the biggest question mark at the moment.

July 31, 2011, 23:20		#3
sail Senior Member Vieri Abolaffio Join Date: Jul 2010 Location: Always on the move. Posts: 308 Rep Power: 17	hi, i reccomand some investigation on the papers of menter and the phd thesis of Langtry. the gamma-retheta model is not as easy as it looks and requires some tuning to get meaningful answers. y+<1 is a must, anything bigger will move the transition point upstream. it is well documented in the fluent theory manual.

August 29, 2011, 12:04		#4
Kloz New Member Join Date: Jul 2011 Posts: 4 Rep Power: 15	So I've taken a look at Menter and Langtry's paper, 'A Correlation-Based Transition Model Using Local Variables'. I'm trying to replicate their results for one of the test cases they used to validate the model, the Schubauer flat plate experiment from the NACA TN 3489. I'm using the same inlet conditions they specify in the paper as well as the second order upwind scheme but for some reason transition just isn't being triggered. My Cf plot is exactly that of the laminar computation and consistent with the Blasius solution. Not really sure what I'm missing. Do I need to play around with wall roughness or wall material specifications for example? I've also tried upping the freestream turbulence intensity (to 0.5%, 1% & 3% compared to Menter's 0.18%) and playing with the viscosity ratio (10 & 120 compared to Menter's 5) but no luck, still laminar flow. Y+ is <1 all over the plate.

November 19, 2018, 01:54		#5
Leonardo.flores Member Leonardo Join Date: Nov 2017 Posts: 37 Rep Power: 9	I am interested on the intermittency setup at the Velocity Inlet (And at the Pressure outlet as well). I tried the same case (a NACA profile at 8 degrees) varying the inlet and outlet intermittency with values of 0, 0.5 and 1... and they gave very different results on lift and drag. What I assume is that the intermittency must be very low, probably zero at the inlet, or it might trigger transition very early at the airfoil... but the paper of Suluksna recommends a value of 1.0... I can not explain why yet.