CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Simple Question - or not?

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

Like Tree1Likes
  • 1 Post By truffaldino

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 14, 2011, 17:14
Default Simple Question - or not?
  #1
New Member
 
Florian Ernst
Join Date: Nov 2011
Posts: 3
Rep Power: 14
Florian2 is on a distinguished road
Hi,

I am new to cfd and my question to the members of this Forum seems to be pretty simple:
What shape has an airfoil with maximum lift when drag is abolutly not relevant?
I am searching for this shape for a long time and even thougt about building a small windchannel to get an answer.
Perhaps the answer is extremly simple, but perhaps it is an extremly complicated airfoil consisting of many pieces (e.g. a grid of some kind...)

Let me get more specific:
The air foil has to have max. lift.
The angle of incidence does not matter, air speed does not matter (within "normal" ranges like 5 m/s up to 300 m/s).
So is there any Software out there that could help me? Is there a simple answer?
Florian2 is offline   Reply With Quote

Old   November 14, 2011, 18:12
Default
  #2
New Member
 
raj
Join Date: Nov 2011
Posts: 22
Rep Power: 14
raj.cfd is on a distinguished road
The airfoil needs to be really thin to obtain maximum lift.
raj.cfd is offline   Reply With Quote

Old   November 15, 2011, 05:15
Default
  #3
New Member
 
Florian Ernst
Join Date: Nov 2011
Posts: 3
Rep Power: 14
Florian2 is on a distinguished road
Hi Raj,
thanks for your fast answer!

So a thin airfoil (probably at a high angle) gives maximum lift?
How did you find out that fast?

I always thought the answer would be much more tricky...
Florian2 is offline   Reply With Quote

Old   November 15, 2011, 07:25
Default
  #4
Senior Member
 
truffaldino's Avatar
 
Join Date: Jan 2011
Posts: 249
Blog Entries: 5
Rep Power: 17
truffaldino is on a distinguished road
Thin airfoil is not necessary gives high lift: it could stall at small aoa. The answer is really tricky. I would rather said: strongly cambered airfoil of an optimal thikness (depending on range of Re number) to delay stall.

There is a technique developed by Eppler and Seilig for a rough estimate of airfoil shape with desired characteristics and there is an excellent free software called "x-foil" by Mark Drela from mit (or its version with good GUI called XFLR-5) that has direct and inverse airfoil design tools.

You can see e.g.

http://smartsonde.org/Project/Vehicl...A-2137-802.pdf

http://www.desktop.aero/appliedaero/...lsections.html

Truffaldino
truffaldino is offline   Reply With Quote

Old   November 16, 2011, 13:52
Default
  #5
Member
 
Zachariah Swetky
Join Date: Aug 2011
Posts: 36
Rep Power: 14
swetkyz is on a distinguished road
I might suggest perusing "Theory of wing sections: including a summary of airfoil data" by Abbott and Doenhoff. This should help you to understand how camber and relative thickness play a role in airfoil characteristics.

To give you an idea of what is possible, it is said that the 737 wing has a maximum lift coefficient of about 3.2--and very low drag at low angles of attack.
swetkyz is offline   Reply With Quote

Old   November 16, 2011, 13:54
Default
  #6
Member
 
Zachariah Swetky
Join Date: Aug 2011
Posts: 36
Rep Power: 14
swetkyz is on a distinguished road
P.S. XFOIL, mentioned by Truffaldino, is a great tool!
swetkyz is offline   Reply With Quote

Old   November 17, 2011, 06:32
Default
  #7
Senior Member
 
truffaldino's Avatar
 
Join Date: Jan 2011
Posts: 249
Blog Entries: 5
Rep Power: 17
truffaldino is on a distinguished road
Quote:
Originally Posted by swetkyz View Post

To give you an idea of what is possible, it is said that the 737 wing has a maximum lift coefficient of about 3.2--and very low drag at low angles of attack.
Yes, with junkers flaps and slats we can reach such Cl's. Unfortunately x-foil analyzes only single-piece airfoil. There is another software by M.Drela (i do not remember the name) which is a multiple foil analog of x-foil, but it is not free (can be sent by M.Drela on request).

With blown/ or bl suction foils one can reach even twice of this.
truffaldino is offline   Reply With Quote

Old   November 17, 2011, 15:37
Default
  #8
New Member
 
Florian Ernst
Join Date: Nov 2011
Posts: 3
Rep Power: 14
Florian2 is on a distinguished road
Hi,

thanks for all answers so far.
@swetkyz: 3.2 sounds a little small; so what would the lift coefficient be when the angle is at its maximum?
@truffaldino: I tried xfoil allready - but as you said it is only possible to calculate single foils.
What you mean with "blown/ or bl suction"? Is this the technique of sucking in air on top of the wing?

I think multi-foil wings will be my choice; probably the give max lift (when calculated properly).

Again THANKS A LOT for your help!!!
Florian2 is offline   Reply With Quote

Old   November 17, 2011, 17:29
Default
  #9
Member
 
Zachariah Swetky
Join Date: Aug 2011
Posts: 36
Rep Power: 14
swetkyz is on a distinguished road
You may find "Simulation and Optimization of Flow Control Strategies for Novel High-Lift Configurations" interesting if you are willing to work with slats and flow control. This paper is by Meunier, and it was published in the AIAA Journal in 2009.
swetkyz is offline   Reply With Quote

Old   November 17, 2011, 17:29
Default
  #10
Senior Member
 
truffaldino's Avatar
 
Join Date: Jan 2011
Posts: 249
Blog Entries: 5
Rep Power: 17
truffaldino is on a distinguished road
Quote:
Originally Posted by Florian2 View Post
Hi,

What you mean with "blown/ or bl suction"? Is this the technique of sucking in air on top of the wing?

Again THANKS A LOT for your help!!!
Yes, this is a technique of sucking the boundary layer on the top to delay separation. Some people say that blowing is even more effective.

You can take a look at a book by Bryan Thwaites on incompressible aerodynamics. He has examples of wing with bl suction up to 30% thick with Cl up to 6 or 9.

As for max Cl without suction devices, Antonov-70 has max Cl=7.2 with slats and flaps only: you can look

http://www.youtube.com/watch?v=PdqMHJJDiFM.


As for software: there is a fast software MSES/MISES by Mark Drela (this is a multiple airfoil x-foil). But it you have to ask M.Drela for acopy.

You could also try Fluent, of course, if you are at large reynolds numbers and flow is almost fully turbulent: Just use S-A turbulence models with multiple airfoils.
In general, when you are at low Reynolds numbers, PDE based CFD software like Fluent cannot handle transition and separation bubbles (in spite of a lot of publicity of new sst transitional models) without a huge user input and comparison with experiments.
swetkyz likes this.
truffaldino is offline   Reply With Quote

Old   November 17, 2011, 17:48
Default
  #11
Senior Member
 
truffaldino's Avatar
 
Join Date: Jan 2011
Posts: 249
Blog Entries: 5
Rep Power: 17
truffaldino is on a distinguished road
I have also found a nice video on youtube on bl succion airfoil used by J.Y. Cousteau as a sail which has 5 to 6 times max Cl of an ordinary sail:

http://www.youtube.com/watch?v=X5E0HDBM4xA
truffaldino is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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 Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Simple Outlet Question m.nichols19 OpenFOAM 3 June 25, 2010 02:44
Simple Question Regarding Symmetry Planes Atella CFX 3 April 11, 2010 07:44
Simple Question Regarding Symmetry Planes Atella Main CFD Forum 0 April 9, 2010 11:58
Question of Anil Date's SIMPLE method universez Main CFD Forum 0 November 18, 2009 21:12
Simple Question Regarding Continuity Closure RC FLUENT 2 March 25, 2004 06:30


All times are GMT -4. The time now is 05:37.