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March 13, 2007, 06:32 |
Low Speed Flow Boundary Conditions
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#1 |
Guest
Posts: n/a
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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) 2) Structured quad mesh 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 |
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March 15, 2007, 00:29 |
Re: Low Speed Flow Boundary Conditions
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#2 |
Guest
Posts: n/a
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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......
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January 10, 2013, 14:01 |
Precision
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#3 |
Member
Sheng
Join Date: Jun 2011
Posts: 62
Rep Power: 15 |
Are you using single precision?
Change it to double and try again... |
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