CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Negative Pressure Drag Coefficient of main airfoil in Multi-Element rear wing

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

Like Tree1Likes
  • 1 Post By Exkalabur22

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   August 25, 2014, 04:25
Default Negative Drag Coefficient of main airfoil in Multi-Element rear wing
  #1
New Member
 
Masoud Arianezhad
Join Date: Mar 2014
Posts: 4
Rep Power: 11
Masoud.A1 is on a distinguished road
I'm simulating a multi-element wing (so far in 2D) for race cars . Both main and flap are S1223 airfoil with 0.5% chord trailing edge gap. I've been using both SST-K-Omega & Realizable K-E with enhanced wall treatment in "Steady" mode.
1)Y+~1
2)Pressure-Velocity Coupling Scheme: SIMPLE & sometimes Coupled
Turbulent Intensity for both Velocity Inlet & Pressure Outlet boundary conditions= 1%
3)Turbulent Viscosity Ratio for both Velocity Inlet & Pressure Outlet boundary conditions=10%
4) Velocity Inlet=66.67 m/s
5) AoA of main airfoil= 0 -- AoA of flap= 30 deg
6) Overlapping= 0 mm
7) Gap between airfoils=8 mm
8) Boundaries: Left side, top & bottom are "Velocity Inlet" bc. & Right side is Pressure Outlet. & two airfoils are walls with no slip.
9) for Reference values I chose calculate from Inlet & for the Area & Length I put the projected length of the System Chord on X axis. (Unit depth in Z direction)
10) In Monitors (force Monitors) I didn't change the axis of Cl & Cd and left them as they are (Cl= (0,1) & Cd=(1,0))

Now the problem: As you can see in the Force Report (attached photo), the Pressure Drag & finally the total drag coefficient of main airfoil is negative. I get this result in all turbulent models and in all different arrangements of airfoils even when I simulate it in Transient mode.

QUESTION:
Do you think the problem arises from Parasitic Drag due to the flap airfoil or it's choosing wrong solution monitor coordinates for Cl & Cd of the airfoil system that results in negative drag of main airfoil.
Attached Images
File Type: jpg Domain-1.jpg (95.6 KB, 32 views)
File Type: jpg Domain-2.jpg (91.0 KB, 32 views)
File Type: jpg Domain-3.jpg (97.6 KB, 34 views)
File Type: jpg Force Report.jpg (48.3 KB, 31 views)

Last edited by Masoud.A1; August 26, 2014 at 04:37.
Masoud.A1 is offline   Reply With Quote

Old   March 20, 2015, 00:34
Default
  #2
New Member
 
remi
Join Date: May 2014
Location: China
Posts: 26
Rep Power: 11
remir is on a distinguished road
Hello,

have you found a solution?

I have the exact same problem, for a similar configuration. (velocity 11m/s, main element angle 3, second element angle 38 (respectively S1223, Eppler420).


As the angle of the second element is so high, maybe I should try LES instead of k-w SST low Re.
remir is offline   Reply With Quote

Old   March 21, 2015, 05:12
Default
  #3
New Member
 
Masoud Arianezhad
Join Date: Mar 2014
Posts: 4
Rep Power: 11
Masoud.A1 is on a distinguished road
Hi Remi,

Unfortunately not yet, I just accept it as it is. I hope someone finds a reasonable answer for it!
About your suggestion for using SST-k-omega, I should say that I used SST-k-omega in majority of simulations however my free stream speed was 66.67 m/s for a 240mm main element airfoil in 0 deg & 84mm flap in 35 deg $ 40 deg. I also tried Spalart-Allmaras, realizable k-epsilon but the same thing occurred.
Masoud.A1 is offline   Reply With Quote

Old   March 21, 2015, 20:52
Default
  #4
New Member
 
remi
Join Date: May 2014
Location: China
Posts: 26
Rep Power: 11
remir is on a distinguished road
Thanks for your reply.

I will investigate with LES, and try to "see" what happens!
Maybe it's as you said, a parasitic drag occuring from the second element. In that case, it would be extremely beneficial for an aircraft, as it would push it forward. I highly doubt it, though (but that would be great ).

I'll keep this post updated if I find anything.
remir is offline   Reply With Quote

Old   April 8, 2015, 23:17
Default
  #5
New Member
 
Join Date: Nov 2014
Posts: 9
Rep Power: 10
Exkalabur22 is on a distinguished road
I am also simulating a 2 element airfoil for a race car and have negative drag on the main element. However my overall drag is positive. My I don't know if this is right but my supervisor told me that it is due to a phenomenon called leading edge suction. You should be able to see that your stagnation point on the main element is underneath the airfoil (the high pressure side) and that the flow raps around the leading edge at very high speed . Apparently it is actually separating and reattaching very quickly and this high speed results in very low pressure on the leading edge which creates a suction force.

As long as your overall drag is positive I think you are ok.
Yann Scott likes this.
Exkalabur22 is offline   Reply With Quote

Reply

Tags
drag, multi element, negative, rear wing, s1223 airfoil

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
[Other] mesh airfoil NACA0012 anand_30 OpenFOAM Meshing & Mesh Conversion 13 March 7, 2022 17:22
[blockMesh] non-orthogonal faces and incorrect orientation? nennbs OpenFOAM Meshing & Mesh Conversion 7 April 17, 2013 05:42
[blockMesh] error message with modeling a cube with a hold at the center hsingtzu OpenFOAM Meshing & Mesh Conversion 2 March 14, 2012 09:56
Question about reference values --> Area (multi element wing) Zweeper FLUENT 7 March 28, 2010 11:29
multi wing element - negative coefficient of drag? Zweeper FLUENT 10 March 11, 2010 12:20


All times are GMT -4. The time now is 14:49.