# "Section drag-lift coefficient"

 Register Blogs Members List Search Today's Posts Mark Forums Read

 October 25, 2007, 10:48 "Section drag-lift coefficient" #1 Mary Guest   Posts: n/a Hello I am studying the drag-lift coefficient of an airfoil NACA 23012, and don't worry, my question is different than the ones you can find on the whole forum... Is the "section drag coefficient" (found in books such as "Theory of wing sections") the same than the "drag coefficient" (calculated, with CFX for example, on a 3D airfoil) ? (And in fact I have the same question for the lift coefficient...) Any idea? Many thanks in advance Mary

 October 25, 2007, 11:52 Re: "Section drag-lift coefficient" #2 mic Guest   Posts: n/a As far as I remember in CFX Post there's not the "drag coefficient" variable. The drag coefficient of CFX should be for the particle tracking... The drag coefficient of the books for an airfoil should be calculated from the Drag (1/2rhov2CdA) and the Drag is the Force of CFX. Be careful if your airfoil is not aligned with the X and Y axis, you have to project the Force. Is the simulation of CFX done considering only one cell in the 3rd direction? The Naca airfoil is 2D, while CFX needs at least one element in the 3rd direction. Hope this helps!

 October 25, 2007, 12:06 Re: "Section drag-lift coefficient" #3 ag Guest   Posts: n/a Section drag coefficient is the drag coefficient calculated for a 2d cross-section of infinite span. In essence, it is a drag/span type number.

 October 25, 2007, 12:13 Re: "Section drag-lift coefficient" #4 Mary Guest   Posts: n/a You have understood that I have a bad drag coefficient In CFX I can calculate the forces on X and Y axis for the whole airfoil... my airfoil is well aligned... I use your formula, I obtain a Cd : different from the Cd of the book..And I haven't understood in your answer whether they should be equal... I use a good mesh, a good turbulence model, so a good Y+, a good Re...I don't know how to find a good Cd.. In fact I don't know which is the good Cd My lift coefficient, and pressure distribution are almost perfect... Any idea?

 October 25, 2007, 12:23 Re: "Section drag-lift coefficient" #5 Mary Guest   Posts: n/a Thanks Ag So you mean that if my span is 1meter : Cd Cfx = Cd book * 1meter ?? ouinnnnnn it changes nothing

 October 25, 2007, 13:36 Re: "Section drag-lift coefficient" #6 mic Guest   Posts: n/a One question: you say that your airfoil is aligned with X and Y. And your wind? The Drag is the force in the direction of the wind and the lift is perpendicular to it. Is your wind aligned with one of the axis?

 October 25, 2007, 13:37 Re: "Section drag-lift coefficient" #7 ag Guest   Posts: n/a I don't use CFX, so I don't know what CFX is giving you. I just gave the definition of section drag. If you are performing a simulation of a L-meter section of airfoil, with suitable end conditions to simulate an infinitely long airfoil (or alternatively a 2d flow) then the section drag would be the total drag (evaluated over the entire surface) divided by the L-meter span. If you are not in agreement with the experimental values, make sure you are comparing apples to apples. Total drag is composed of the pressure drag and viscous drag - make sure that you're not computing pressure drag in the solver and comparing to total drag in a report, for example.

 October 25, 2007, 14:27 Re: "Section drag-lift coefficient" #8 Mini Guest   Posts: n/a don't worry my wind is on the X axis (like my chord) and it is Y for my lift force Thanks, Like that I have checked that

 October 25, 2007, 14:39 Re: "Section drag-lift coefficient" #9 Mini Guest   Posts: n/a Thanks for your time, and your interesting answer... yes I have a simulation of a 1-meter section of an airfoil with suitable end conditions to simulate an infinitely long airfoil (function of CFX: Symmetry plane at the end of the airfoil) and I am sure that I compute pressure+viscous drag on CFX, and I can check tomorrow in the book if it is really the total drag, but I think so.. I find Cd=0.13 instead of 0.65 Any other ideas ?

 October 25, 2007, 15:07 Re: "Section drag-lift coefficient" #10 ag Guest   Posts: n/a If your lift and pressure distribution look good, but drag is not correct, then my first guess would be that the viscous contribution is not correct. This could be due to having a grid that is too coarse at the wall, incorrect turbulence modeling (are you comparing laminar results to turbulent data, for example), etc. Or there could be an error in the formula you are using to compute drag. Those are the things I would look at first.

 October 26, 2007, 04:42 Re: "Section drag-lift coefficient" #11 Mary Guest   Posts: n/a Check up! : I use the SST model, automatic wall treatment Y+ ~1 I think this is the problem : Reynolds number : -when I calculate it manually, I have a result around 3.10^6 (charateristic length:chord, Reynolds used in the book) -but CFXsolver use an other length and so find Re=1.10^7... The difference shall come from here, no?? If you think so, I am going to ask in the CFX-forum how to change it because I don't know... Thank you very much to give me so much possible error!!!

 October 26, 2007, 12:10 Re: "Section drag-lift coefficient" #12 mic Guest   Posts: n/a As far as I know CFX calculates the Re number considering the inlet velocity, the inlet width, and the average density over the inlet area and the result may be different from the Re number indicated for the airfoil in the book. Regarding the mesh (Hexa or Tetra?) it is important to have 10 nodes in the boundary layer. Hope this helps!

 October 26, 2007, 12:27 Thank you Mic and Ag !! #13 Mini Guest   Posts: n/a Ok for the Reynolds number... I use the Hexa-mesh (it is a good choice?) And I have refined my mesh (my BL is very fine), and now I have a Cd of 0.007 instead of 0.006... It is enough for me Thank you!!

 November 27, 2007, 09:15 Re: "Section drag-lift coefficient" #14 Usman Guest   Posts: n/a Marion i had the answer to your question why did you not ask me. Anyways its too late now! You should work on your dam! Usman

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post archymedes OpenFOAM Pre-Processing 94 July 15, 2016 16:14 n. natik FLUENT 8 March 31, 2015 19:02 mehrdadeng CFX 15 December 9, 2009 22:49 vinz OpenFOAM Running, Solving & CFD 98 October 27, 2008 09:43 Noé CD-adapco 5 July 13, 2004 10:21

All times are GMT -4. The time now is 18:09.