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

Calculating forces on a non-closed surface

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 1, 2011, 18:18
Default Calculating forces on a non-closed surface
  #1
New Member
 
Join Date: Sep 2010
Posts: 4
Rep Power: 16
ScottN is on a distinguished road
Hello everyone,

I'm having a bit of trouble determining the correct way of calculating the net force on one surface of an object with a flow field on only one side of the surface (i.e. I'm calculating the net force on a non-closed surface that only has pressure data for one side).

I am modeling the external aerodynamics of a vehicle in an incompressible airflow at 22m/s. The upper and lower surfaces of the vehicle in the real world are separate from each other (obviously in FLUENT the car body is a closed surface) and thus I need to determine the net force on the upper surface of the car so that the design requirements of the latches holding the upper surface can be defined.

The problem I run into is this: For FLUENT to calculate the correct force on a non-closed surface that does not have a flow field on one side, it needs a correct reference pressure that it will use for the side of the surface that the flow is not modeled for (i.e. the inside of the car). I have tried using the volume averaged total pressure of the flow volume, but using this value gives me a lift force on the top half of the car of 3000N! Let me first say that this car has already been built and run at up to 36m/s and the top half has yet to come off, and I'm completely sure the current latches are not capable of restraining 3kN, let alone whatever force it would be at 36m/s. So clearly this is wrong.

I've also tried using an area-averaged value of the total pressure on the car surfaces (which is actually calculated using FLUENTs area-weighted average of static pressure on the car surface - I confirmed this by writing a udf to calculate the total pressure in the near wall cells on the surface (Ptot=Pstat+1/2*density*v^2), and it's results matched flawlessly with the FLUENTs static pressure on a surface...so does anyone know what FLUENTs Total pressure on a surface is calculating?), but the result it gives me on the lower half of the car is that it is actually producing thrust, not drag, so that obviously can't be correct either.

So, does anyone have suggestions for what reference pressure to use or know of another way to calculate accurate net forces on non-closed surfaces (like a UDF, I just don't know how to extract the viscous forces from within a UDF)?

Thanks in advance,

Scott
ScottN 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
Help! problems in calculating forces with SimpleFoam DLC OpenFOAM 3 April 26, 2022 00:43
[OpenFOAM] 3D graph of lift/drag forces (coeffs) on a surface - is it possible in paraview??? Domi ParaView 1 May 19, 2011 08:16
Recover surface from surface mesh in IcemCFD Jerry Tanner CFX 0 August 20, 2008 13:48
CFX4.3 -build analysis form Chie Min CFX 5 July 13, 2001 00:19
free convection heat transfer from a heated horizontal surface through a liquid to a thin cooled fin Kaushik FLUENT 1 May 8, 2000 07:47


All times are GMT -4. The time now is 23:25.