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-   -   is time in unsteady problems exactly equivalent to what occurs in nature? (http://www.cfd-online.com/Forums/main/113437-time-unsteady-problems-exactly-equivalent-what-occurs-nature.html)

immortality February 19, 2013 16:24

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

FMDenaro February 19, 2013 16:29

Quote:

Originally Posted by immortality (Post 408816)
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 ...

immortality February 19, 2013 18:18

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.

Martin Hegedus February 19, 2013 18:26

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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