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oscillating netMassFlux with fireFoam

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Old   February 13, 2012, 14:00
Default oscillating netMassFlux with fireFoam
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Aram Amouzandeh
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dear all,

i m currently simulating a fire in a tunnel-like geometry using fireFoam (OF 1.7.1). I added the calculation of mass flux through all boundaries taken from http://openfoamwiki.net/index.php/Sn...ting_mass_flow. the netMassFlux is calculated as the sum of fluxes of alll patch thus representing the temporal variation of density in the domain.

in case of a constant rate of heat release the netMassFlux is expected to konverge towards zero. this is the case for my simulations, though, even if small, it sometimes fluctuates. now the situation drastically changes if I consider a rate of heat release changing over time where the fluctuations increase seriously (see e.g. attached pic).

I tried a lot of different variations of BC, different solvers and tolerance settings or different Co-numbers but no change in behaviour.

does somebody have an idea what the problem could be?? I greatly appreciate your comments!!

cheers,
aram
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File Type: pdf netMassFlux.pdf (52.7 KB, 22 views)
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Old   February 14, 2012, 04:39
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Aram Amouzandeh
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hi !

I had a look at the simulation results again, specially at the pressure prgh (= p - rho g h) and realized a constant change (over time) in the sign of the pressure difference between inlet (left end) and outlet (right end) as seen in the attached pics for two consecutive time steps. the flow is expected to move from left to right.

as BCs I used:

inlet (left): fixedValue for U and T (or fixed mass flux) and buoyantPressure for prgh, calculated for p.

outlet (right): inletOutlet for U and T with inletValue 0 and 293 K, respectively, and totalPressure for prgh with p0 = 101325 Pa, calculated for p.

fuel inlet: mass flux for U, fixedValue for T and buoyantPressure for prgh, calculated for p.

as the prgh at the outlet gets higher than at the inlet, the flow even changes direction. hence, I assume either a U-prgh BC problem not allowing the pressure equation to determine a proper pressure and velocity field, or the pressure equation is not converged. I keep on digging ...

appreciate your comments!

cheers,
Aram
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File Type: jpg prgh_t100.jpg (10.0 KB, 21 views)
File Type: jpg prgh_t150.jpg (11.5 KB, 15 views)
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Old   March 21, 2012, 07:23
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Aram Amouzandeh
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hi!

realised that the observed oscillations strongly reduce when I switch off radiation. even prgh iterations reduce drastically when radiation is off.

any idea?

thanks in advance!

Aram
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