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-   -   Low Speed Flow Boundary Conditions (http://www.cfd-online.com/Forums/fluent/44098-low-speed-flow-boundary-conditions.html)

 Paul Bateman March 13, 2007 06:32

Low Speed Flow Boundary Conditions

Hi,

This is a question from a newbie CFD user regarding a problem that really has me stumped.

I have successfully used FLUENT in the past for transonic aerofoil and wing analysis using a compressible flow setup and the coupled solver. However, I'm now trying to simulate the flow around various 2D bodies at low speeds (e.g. 10 to 30 m/s at sea level) and small angles of attack. No matter what I try I seem to get x and y velocities that converge, but mass continuity gets "stuck" at about 10e-1 or 10e-2.

The flow field looks sensible, there are no "hot spots" in terms of velocity in the domain, and lift coefficient seems sensible(ish). Mass fluxes into the domain = mass fluxes out of the domain.

Mesh:

"Extended D" shaped domain with aerofoil at centre.

Boundaries are at least 50 chord's away from aerofoil. ~30000 elements.

I have tried:

1) unstructured tet mesh (with high quality elements)

Boundary conditions:

Front of D & bottom of D - set to velocity inlet at 30 m/s with velocity components to give 2 degree AOA.

Top of D and back of D - set to pressure outlet with a pressure of 101325 Pa normal to boundary.

Aerofoil - set as a wall.

Solution set up:

Segregated Solver.

Incompressible flow.

Energy equation disabled.

Operating pressure set to 101325 Pa.

I have tried both first and second order upwind discretisation.

I have tried inviscid, laminar and turbulent solutions.

I have tried changing under relaxation of momentum with no improvement.

I don't think the problem is to do with grid resolution because it won't run inviscid. Plus, if I set up the same mesh as a transonic problem with the coupled solver and pressure far-field boundaries it converges nicely. I think it might be my selection of boundary conditions, but I don't have enough experience to know better.

If anyone has any pearls of wisdom to offer I will happily try anything. Thanks in advance for any help offered.

Paul

 S Guha March 15, 2007 00:29

Re: Low Speed Flow Boundary Conditions

Plot the mass flow values vs iteration at inlet and outlet. If it is straight (i.e. they dont have any gradients) and your mass flux report shows a difference of less than 0.5 % at inlet and outlet, then there is no necessity for decresaing your residuals i suppose. if it satisfies all these crtireion (as mentioned above) with larger residuals then it might mean it is possible......

 micro11sl January 10, 2013 14:01

Precision

Are you using single precision?
Change it to double and try again...

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