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-   -   Wedge Impact Forces (6DOF high velocity problem) (http://www.cfd-online.com/Forums/fluent/72472-wedge-impact-forces-6dof-high-velocity-problem.html)

der_meister February 7, 2010 01:40

Wedge Impact Forces (6DOF high velocity problem)
 
Hi everyone,

I am trying to investigate the variation in impact forces on a wedge at specific impact velocities (5 - 35m/s). I'm using the 6DOF model with dynamic meshing. I have used solver settings as specified in the FLUENT tutorial "Solving a 2D Box falling into water". (namely coupled pressure-velocity at Courant Number of 1000000, PRESTO, Geo-Reconstruct and Second Order Discretization for momentum).

I have positioned the wedge at a distance of 0.2m above the water surface. The idea behind this was to allow the solution to stabilize while it is in freefall. I calculate total force using a UDF to sum pressure values along the wedge.

1.) At 6m/s I get an approximate drag of about 70N (body weight = 143N) during the freefall. When running the simulation at 10m/s however, I get a negative drag (i.e., downward force on the body falling downward.) The pressure contours show me a huge area of negative pressure above the wedge. This is clearly unphysical. Similar results were observed for a velocity of 15m/s also. Does someone here have an idea of what I am doing wrong? I have a hunch it might have something to do with the initial pressure distribution and that starting off from the first time instant with high velocities might be throwing the solver off. (I have taken time steps as small as 1e-8). Do I need to change solver settings?

2.) My second question is on whether I should use Static Pressure or Total Pressure in calculating pressure force on a body. I know F_P returns static pressure and the value calculated from it matches the FLUENT post processed force report. Could someone tell me why static and not total pressure is used?

3.) Why is the Courant Number used in the tutorial so high?

Regards,

Deepak

bowenNZ June 14, 2010 23:32

Reply to Deepak
 
Hi Deepak,

I assume the pressure mis-reading has to do with your mesh setup and the frame of reference with respect to the wedge at initialization. The dynamic pressure must be a function of the velocity gradient at the wedge wall so a high wall velocity (or gradient if a no slip wall) may result in negative (downwards) pressure. i.e. pressure resultant would be due to more of a lifting force than a drag force. (though this should only show up in total and dynamic pressure)

Are you using a deforming mesh for the falling wedge or is the wedge stationary with the fluid approaching from below?
*Ah, I see it is a dynamic mesh. Perhaps try the later?

Perhaps using total pressure to calculate the force on the wedge will not result in the negative forces? (I would think you want to use total pressure to calculate pressure force on the body)

As for the courant number can you tell me what version this tutorial is in? I would like to have a look at the tutorial myself and then maybe I can give you a more useful answer to your questions.

Also is your wedge 2D or 1D. i.e. 1D would be just a line drawn in the mesh as a wall which fluent (I believe) creates two surfaces for. If that were the case you may have selected the wrong surface, one of which is labeled "shadow" in fluent and it's location is dependent on which direction you drew the line if you are using Gambit. But that is all probably too much info.

Cheers,
B


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