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Fixed value pressure at the outlet and zeroGradient pressure at the inlet

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Old   February 13, 2020, 15:47
Default Fixed value pressure at the outlet and zeroGradient pressure at the inlet
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Goli
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Dear All,

I am simulating an incompressible flow past an airfoil. I have read the comments and also Versteeg book about the proper boundary conditions. All of them say that we should not use zero gradient condition for the pressure at the outlet. I was wondering that why we should put fixed value pressure at the outlet and zeroGradient pressure at the inlet?

Even if we go far from the airfoil (e.g. 30chord for the inlet and 30chord for the outlet), why should not we fix the pressure at the inlet and why should not we put zerogradient pressure at the outlet?

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Goli
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Old   February 13, 2020, 18:05
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You can. What all those remarks state actually state that you need to impose a fixed value for pressure at somewhere on your domain boundaries for incompressible flor computations. If you do not specify it, the matrix system behind the curtain is ill-conditioned.

The most intuitive combination element is setting outlet pressure to zero for incomp sims, so you assume at the very 'far' off the field of interest, the flow state reaches the ambient reference conditions, free from the effects of the upstream perturbations.

I could write out a more formal, more systematic, more math-concrete explanations. But I think you can find the answer by reviewing the literature, which is abundant for these matters, I think.
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Old   February 13, 2020, 21:39
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Dear HPE,


Thank you for your reply.

In the book "An Introduction to Computational Fluid Dynamics" by Versteeg, on page 271, it it is said:


"If the location of the outlet is selected far away from geometrical disturbances the flow eventually reaches a fully developed state where no change occurs in the flow direction. In such a region we can place an outlet surface and state that the gradients of all variables (except pressure) are zero in the flow direction."


I would be grateful if someone can help me to understand the reason of the above sentence.


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Goli
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