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is time in unsteady problems exactly equivalent to what occurs in nature?

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Old   February 19, 2013, 16:24
Default is time in unsteady problems exactly equivalent to what occurs in nature?
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Ehsan
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I've faced with a essential problem and doubt about what i set forth at the title.for example at simple cavity problem at 1micro second are pressure and velocity equal exactly and precisely to what experiment shows.does it takes same time to reach steady state as takes place in nature?or about other unsteady problems.
Can anyone submit a reasonable answer to this doubt?
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Old   February 19, 2013, 16:29
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Originally Posted by immortality View Post
I've faced with a essential problem and doubt about what i set forth at the title.for example at simple cavity problem at 1micro second are pressure and velocity equal exactly and precisely to what experiment shows.does it takes same time to reach steady state as takes place in nature?or about other unsteady problems.
Can anyone submit a reasonable answer to this doubt?

To be formally equivalent with an experiment, you should prescribe exactly the same initial and boundary conditions. For laminar flow you can be quite confident that you can reproduce a natural time-evolution for sufficiently fine grid.
For turbulence the problem is much more complex ...
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Old   February 19, 2013, 18:18
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then your answer seems yes or nearly yes.in turbulent flows may be very small difference to what occurs in nature but in laminar if initial and boundary conditions be defined precisely we can trace phenomena accurately at same time evolution.am i correct?
thanks.
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Old   February 19, 2013, 18:26
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To be truly time accurate one must use the compressible, rather than incompressible, set of equations to capture the pressure wave. However, for low speeds, that means there will be a big time scale difference between the rate at which the particles and pressure wave travels. I'm not sure what error is incurred by using the unsteady incompressible equations. Hopefully it's small.
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