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Old   July 8, 2013, 04:03
Default aerofoil problem
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hi
i recently picked up an aerofoil from http://www.airfoiltools.com/ and the plots(cl,cd vs alpha ....) are all provided in the website.
i noticed it says the data was acquired at mach=0 Ncit=9
this is the case for all the aerofoils. does any1 know what this means?
thank you.
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Old   July 10, 2013, 06:58
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the graphs that you see in that website is generated using XFOIL which is a vortex panel method code, and Ma=0 is for inviscid flow simulation. Ncrit determines the turbulence level, that is, if Ncrit is 1, too much disturbance is present in the flow. Ncrit 9 is standard and very commonly used.
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Old   July 10, 2013, 07:10
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thanks for the reply.i am new to cfd .i recently meshed an aerofoil from the site and solved it in fluent.For Re=e^6 under sea level conditions the velocity would be around 15m/s so mach number will be around 0.04.i assumed thats y it says mach=0
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Old   July 10, 2013, 07:14
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XFOIL is generally used for quick calculations of aerodynamic coefficients, so most of the time, Ma=0 condition is enough for getting a quick idea about the behavior of the airfoil. I havent tried setting the Ma to lets say 0.5, but i dont think XFOIL can solve it anyways. Re is the dominant input imo.
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Old   July 10, 2013, 08:37
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BTW, xfoil is more reliable than fluent for such reynolds numbers
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Old   July 10, 2013, 08:50
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thanks for the replies guys.but i am curious about the Ncrit .any ideas where i can read more about that?
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Old   July 10, 2013, 09:16
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Ncrit is critical number in e^N theory (the disturbance in linearized bl equations grows e^N times before passing to turbulence). Read x-foil manual:

situation Ncrit ----------------- ----- sailplane 12-14 motorglider 11-13 clean wind tunnel 10-12 average wind tunnel 9 <= standard "e^9 method" dirty wind tunnel 4-8

The conection between Ncrit and turbulence energy level is as follows:

N_crit=-8.43-2.4*ln(Tu%/100%)

Truffaldino

Edit: Take a look e.g. at "the e^N method for transition prediction. Historical review ..."

http://www.google.fr/url?sa=t&rct=j&...p_WbFgEKBaNFeg

or Mark Drela article about principles of x-foil (they have very simplified version of this method)

Last edited by truffaldino; July 10, 2013 at 09:34. Reason: addition
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Old   July 10, 2013, 10:02
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Quote:
Originally Posted by truffaldino View Post
BTW, xfoil is more reliable than fluent for such reynolds numbers
depending on the grid quality and the turbulence model... at low Re numbers, XFOIL results may be better. but thats only because of the turbulence model; since laminar to turbulent transition may occur at low Re numbers and turbulence models may not capture it. k-w SST transition is formulated by Menter for this purpose.
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Old   November 19, 2013, 12:50
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Hey, I've just a small question.

I'm simulating flow around an NACA4412 airfoil in Fluent. I use the k-omega SST model.
To compare my results with experimental data I'd like to knwo which "nCrit" Fluent uses when I use all the default values or where I can find this information?

When defining the BC, I specify for the inlet the Turbulence with "intensity and viscosity ratio" and use the default values "turbulent intensity 5%" and "turbulent viscosity rate 10"...

Thanks for your replies
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Old   November 19, 2013, 13:14
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Fluent does not use Ncrit as it uses turbulence models. Ncrit is used in boundary layer methods as in x-foil which is based on different and, in this case, more reliable (than fluent models) principles.

For connection between turbulent kinetic energy and Ncrit read post number 7 above and see references therein

It is necessary distinguis the three different things

1) Cfd turbulence models for Navier Stokes (fluent k-omega, k-epsilon etc)

2) Viscous inviscid interaction boundary layer models (eg as in Xfoil,l where Ncrit appears)

3) experimental data
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Old   November 20, 2013, 00:56
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dear friends can I have the tutorial for Gambit2.3 and 2.4 plz send a pdf to my mail rnjt_rnjt@yahoo.com
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Old   April 12, 2014, 22:28
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hi there,

I need some help of you expertes .
I am studying wind load on a parabolic solar concentrator at certain angles using Fluent.

I choose to start with the 2D simulation in order to evalate the drag and lift coefficient.

I have some troubles :

1. I don't find a logical drag coefficient value. It seems related to the references values. How can I set up those values in my case ?

2. If the 2D simulation is sufficient to find a good results ?


Thank you for advance

Bassem
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Old   April 14, 2014, 17:17
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What turbulence model are you using?
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Old   April 14, 2014, 23:32
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k-epsilon model. But I am using now a steady case.
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Old   November 10, 2016, 12:44
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Hi guys, could anyone tell me how to change the value for Ncrit in XFoil? Could Ncrit be negative? Thank you .
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Old   November 14, 2016, 13:49
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Load your airfoil in XFOIL. Type OPER. TYPE VPAR. TYPE N <insert the value for N>.

n must be a number larger than 0 and can't be too close to 0 either or XFOIL will not converge. Small values for n mean very turbulent flow. Large numbers mean very laminar flow.
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Old   December 21, 2016, 12:38
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Quote:
Originally Posted by Drunken_SheeP View Post
Load your airfoil in XFOIL. Type OPER. TYPE VPAR. TYPE N <insert the value for N>.

n must be a number larger than 0 and can't be too close to 0 either or XFOIL will not converge. Small values for n mean very turbulent flow. Large numbers mean very laminar flow.
Dear Drunken

Thank you for your information. I just wonder how could we calculate Ncrit? The turbulence level of wind tunnel in my case is around 3%. Then I put into N = −8.43 − 2.4 ln(T). I got around 1.67. So in my case Ncrit is 1.67? That looks quite small.

I am not sure if it is right. At the beginning, I thought the Ncrit is related to the turbulence level at transition point, but now, I use the turbulence level of free stream to calculate Ncrit. I am not sure which turbulence level I should use for this equation.

Cheers

Regards
Weidong Dai
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Old   December 23, 2016, 21:11
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Quote:
Originally Posted by Weidong Dai View Post
Dear Drunken

Thank you for your information. I just wonder how could we calculate Ncrit? The turbulence level of wind tunnel in my case is around 3%. Then I put into N = −8.43 − 2.4 ln(T). I got around 1.67. So in my case Ncrit is 1.67? That looks quite small.

I am not sure if it is right. At the beginning, I thought the Ncrit is related to the turbulence level at transition point, but now, I use the turbulence level of free stream to calculate Ncrit. I am not sure which turbulence level I should use for this equation.

Cheers

Regards
Weidong Dai
Formally, your approach is correct (one should use free stream turbulence level), but there is a big problem.

The turbulence level at which linear (e^N) theory ceases to be valid is about 1%.

For instance, NACA wind low turbulence tunnel has Tu = 0.1%, Delft tunnel 0.6% and for gliders Tu is about .014% (data taken from http://www.google.fr/url?sa=t&rct=j&...p_WbFgEKBaNFeg pp 39-40)
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Old   December 27, 2016, 09:11
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Quote:
Originally Posted by truffaldino View Post
Formally, your approach is correct (one should use free stream turbulence level), but there is a big problem.

The turbulence level at which linear (e^N) theory ceases to be valid is about 1%.

For instance, NACA wind low turbulence tunnel has Tu = 0.1%, Delft tunnel 0.6% and for gliders Tu is about .014% (data taken from http://www.google.fr/url?sa=t&rct=j&...p_WbFgEKBaNFeg pp 39-40)
Thank you, truffaldino. I noticed that X-foil will not be a good reference for me for the moment. I will try to find other ways. Cheers
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