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

Drag in inviscid flow

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   June 1, 2014, 11:46
Default Drag in inviscid flow
  #1
New Member
 
Hakjin lee
Join Date: Jun 2014
Posts: 4
Rep Power: 12
jinny is on a distinguished road
Dear all

- Euler simulation
- Test model :NACA0012 airfoil (symmetric airfoil)
- Mach number : 0.1 (incompressible flow)
- Angle of attack : 0 deg.

As above condition, there is no lift and drag
As you known, D'Alembert proved that for incompressible flow and inviscid potential flow - the drag force is zero on a body moving with constant velocity relative to the fluid

- Euler simulation
- Test model :NACA0012 airfoil (symmetric airfoil)
- Mach number : 0.1 (incompressible flow)
- Angle of attack : 2 deg.

However, in case of angle of attack of 2 deg. the drag force occur !
I don't know where the draq come from ?

please explain the drag force in inviscid flow (Euler simulation)

Thanks in advance
Jinny
jinny is offline   Reply With Quote

Old   June 1, 2014, 12:02
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,882
Rep Power: 73
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
this is a classical issue in an aerodynamic course, generally the topic is illustrated by the flow problem of the lift generated by a 2D cylinder.
The key is in the circuitation generated by the position of the rear stagnation point (Kutta condition says it at trailing edge). Any textbook of basic fluid/aerodynamic can help you...
just start see in...wikepedia
http://en.wikipedia.org/wiki/Lift_%28force%29
FMDenaro is online now   Reply With Quote

Old   June 1, 2014, 13:42
Default
  #3
Senior Member
 
Martin Hegedus
Join Date: Feb 2011
Posts: 500
Rep Power: 19
Martin Hegedus is on a distinguished road
Artificial/numerical dissipation and numerical error. A high order code with very little numerical dissipation should produce zero drag.
Martin Hegedus is offline   Reply With Quote

Old   June 1, 2014, 14:01
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,882
Rep Power: 73
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by Martin Hegedus View Post
Artificial/numerical dissipation and numerical error. A high order code with very little numerical dissipation should produce zero drag.
As you said, the answer is that the code "implicitly" fix the Kutta condition ...it is as a check to compute the integral of the tangential velocity along the airfoil
FMDenaro is online now   Reply With Quote

Old   June 2, 2014, 00:18
Default
  #5
New Member
 
Hakjin lee
Join Date: Jun 2014
Posts: 4
Rep Power: 12
jinny is on a distinguished road
Dear Martin Hegedus

Thnak you for your reply !

You means that the drag force generated in Euler simulation is numerical error ?
If I used high-oder scheme, such as DGM, the drag force does not occur ?
Um....Could you explain why the drag force does not occur in inviscid flow (Not potential flow)

Thanks in advance
jinny
jinny is offline   Reply With Quote

Old   June 2, 2014, 00:21
Default
  #6
New Member
 
Hakjin lee
Join Date: Jun 2014
Posts: 4
Rep Power: 12
jinny is on a distinguished road
Dear FMDenaro

Thanks, FMDerao
As you said, I have to check the surface integral routine which calculate a normal force to the aifoil surface

Thank you, have a nice day
Jinny
jinny is offline   Reply With Quote

Old   June 2, 2014, 00:59
Default
  #7
Senior Member
 
Martin Hegedus
Join Date: Feb 2011
Posts: 500
Rep Power: 19
Martin Hegedus is on a distinguished road
Jinny,

Inviscid, incompressible, steady state flow is irrotational (due to vorticity transport equation), therefore inviscid incompressible steady state Euler is potential flow. Once the bound vortex reaches steady state, vorticity is not shed into the flow field, and then drag goes to zero. However, numerical error causes the bound vortex to decay and therefore vorticity needs to be constantly shed into the field to maintain it (and the implicit kutta condition). If there is no error, then the bound vortex does not decay and eventually all the vorticity in the field is transported away.

On this page is a grid refinement study of the NACA 0012 airfoil at M=0.5 and alpha=1.25. http://www.hegedusaero.com/examples/.../Vassberg.html. You can see that drag drops as the grid is refined. For the case of FLO82-HCUSP, drag basically approaches zero. AO_CFD has a more challenging time probably because of higher artificial dissipation. I assume the same is true for OVERFLOW.
Martin Hegedus is offline   Reply With Quote

Reply

Tags
d'alembert paradox, drag force, euler simulation, inviscid flow, naca0012


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
Inviscid flow solver luca_g OpenFOAM Running, Solving & CFD 3 August 11, 2024 11:52
Drag coefficient too high at flow around a cyclinder Gunni OpenFOAM Running, Solving & CFD 17 October 31, 2019 03:18
Inviscid Turbulent flow Vino Main CFD Forum 11 May 8, 2019 11:35
Nozzle flow simulation in inviscid and viscous Cath FLUENT 0 January 28, 2007 03:16
Can 'shock waves' occur in viscous fluid flows? diaw Main CFD Forum 104 February 16, 2006 06:44


All times are GMT -4. The time now is 12:36.