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Setting a pressure loss in cfx

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Old   June 10, 2021, 01:04
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Glenn Horrocks
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If your simulation is close enough to validation results then you have some confidence the simulation is working accurately. There is a small chance the result was just a fluke and if you make a change it will be miles off - running a different mesh size would confirm whether this has happened.

Your second question is an FAQ: https://www.cfd-online.com/Wiki/Ansy...gence_criteria
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Old   June 10, 2021, 11:33
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Just be careful.

Solutions are ONLY converged when the mathematical statement is satisfied, i.e. the residual of the system is ZERO.

Solutions can ONLY be unique for a given method when they no longer depend on any of the discretizations parameters, i.e. mesh size, timestep, under relaxation, etc

Achieving the above is most of the time too costly, and sometimes not worth the additional effort since the models being solved already include a certain amount of error, i.e. they only approximate the real problem. Then, the experience working with specific models comes in, and in the end, it is YOUR criteria and responsibility to decide.

However, be careful of the practice of "solution is converged because my variable of choice is no longer changing". Here is a counterexample,

Say we want to find the root of f(x) = x^2 + 1, i.e. solving the problem

f(x) = x^2 + 1 = 0

If you use an iterative method, it will lead towards a constant value of 0, i.e. exactly where the residual is at its minimum, but a real-valued solution does NOT exist. What will you do then?

Food for thought
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Last edited by Opaque; June 10, 2021 at 18:33.
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Old   June 15, 2021, 00:40
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Thank you for your reply. I got a better understanding.
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cfx, negative heat transfer, pressure drop, pump, steam generator

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