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

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 February 19, 2013, 16:24 is time in unsteady problems exactly equivalent to what occurs in nature? #1 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,210 Rep Power: 20 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?

February 19, 2013, 16:29
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Filippo Maria Denaro
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Quote:
 Originally Posted by immortality 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 ...

 February 19, 2013, 18:18 #3 Senior Member     Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,210 Rep Power: 20 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.

 February 19, 2013, 18:26 #4 Senior Member   Martin Hegedus Join Date: Feb 2011 Posts: 488 Rep Power: 12 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. immortality likes this.

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