# aerofoil problem

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 July 8, 2013, 04:03 aerofoil problem #1 New Member   TPRPR1 Join Date: Apr 2013 Posts: 13 Rep Power: 13 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.

 July 10, 2013, 06:58 #2 Member   C Join Date: Apr 2013 Posts: 37 Rep Power: 13 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.

 July 10, 2013, 07:10 thanks for the reply #3 New Member   TPRPR1 Join Date: Apr 2013 Posts: 13 Rep Power: 13 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

 July 10, 2013, 07:14 #4 Member   C Join Date: Apr 2013 Posts: 37 Rep Power: 13 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.

 July 10, 2013, 08:37 #5 Senior Member     Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17 BTW, xfoil is more reliable than fluent for such reynolds numbers

 July 10, 2013, 08:50 #6 New Member   TPRPR1 Join Date: Apr 2013 Posts: 13 Rep Power: 13 thanks for the replies guys.but i am curious about the Ncrit .any ideas where i can read more about that?

 July 10, 2013, 09:16 #7 Senior Member     Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17 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) mm.abdollahzadeh, jm_ngs, mahditorabiasr and 2 others like this. Last edited by truffaldino; July 10, 2013 at 09:34. Reason: addition

July 10, 2013, 10:02
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C
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Quote:
 Originally Posted by truffaldino 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.

 November 19, 2013, 11:50 #9 Member   Join Date: Dec 2012 Posts: 32 Rep Power: 13 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

 November 19, 2013, 12:14 #10 Senior Member     Join Date: Jan 2011 Posts: 249 Blog Entries: 5 Rep Power: 17 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

 November 19, 2013, 23:56 tutorial for Gambit2.3 and 2.4 #11 New Member   ok Join Date: Nov 2013 Posts: 5 Rep Power: 12 dear friends can I have the tutorial for Gambit2.3 and 2.4 plz send a pdf to my mail rnjt_rnjt@yahoo.com

 April 12, 2014, 22:28 wind load on parabolic trough #12 New Member   bassem Join Date: Apr 2014 Posts: 6 Rep Power: 12 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

 April 14, 2014, 17:17 #13 Member   DaveyBaby Join Date: May 2013 Posts: 46 Rep Power: 13 What turbulence model are you using?

 April 14, 2014, 23:32 #14 New Member   bassem Join Date: Apr 2014 Posts: 6 Rep Power: 12 k-epsilon model. But I am using now a steady case.

 November 10, 2016, 11:44 #15 New Member   Weidong Dai Join Date: Oct 2016 Location: Edinburgh, Scotland, UK Posts: 9 Rep Power: 9 Hi guys, could anyone tell me how to change the value for Ncrit in XFoil? Could Ncrit be negative? Thank you .

 November 14, 2016, 12:49 #16 New Member   asdfad Join Date: Jul 2012 Location: Norway/Germany Posts: 13 Rep Power: 13 Load your airfoil in XFOIL. Type OPER. TYPE VPAR. TYPE 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.

December 21, 2016, 11:38
#17
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Weidong Dai
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Quote:
 Originally Posted by Drunken_SheeP Load your airfoil in XFOIL. Type OPER. TYPE VPAR. TYPE 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

December 23, 2016, 20:11
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Quote:
 Originally Posted by Weidong Dai 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)

December 27, 2016, 08:11
#19
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Weidong Dai
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Quote:
 Originally Posted by truffaldino 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

 January 23, 2018, 01:32 #20 New Member   RAKESH KUMAR Join Date: Jan 2018 Location: Dhanbad Posts: 2 Rep Power: 0 Hi, i am simulate a model of Darrieus turbine with 3 helical blade using airfoil profile of NACA 4415 in Ansys CFX. I want to calculate torque of turbine with respect to angle of rotation. Also want to calculate lift coefficient and drag coefficient vs angle of attack. Please any help me...

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