Momentum Equation and hydrostatic pressure gradient
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Dimitrios
Jun 14, 2012, 5:47:32 AM6/14/12
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why the hydrostatic pressure gradient subtracted from both sides of
the momentum equation is defined as time dependent rhon(z,t)?
RHO_0(K) is time independent.
the momentum equation is defined as time dependent rhon(z,t)?
RHO_0(K) is time independent.
Kevin
Jun 14, 2012, 8:39:41 AM6/14/12
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Inside a sealed compartment, the background density can change with
time.
time.
Dimitrios
Jun 14, 2012, 9:40:37 AM6/14/12
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how is these possible, if within a sealed compartment global mass and
volume does not change?
Or is there computationally a clear destinction between background and
local fluid density?
volume does not change?
Or is there computationally a clear destinction between background and
local fluid density?
Kevin
Jun 14, 2012, 9:46:30 AM6/14/12
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In FDS, you can blow air into a sealed compartment and the background
pressure and density will rise.
pressure and density will rise.
Dimitrios
Jun 14, 2012, 9:48:51 AM6/14/12
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ok!
and if these is not the case the hydrostatic pressure subtracted is
only a function of z?
and if these is not the case the hydrostatic pressure subtracted is
only a function of z?
Kevin
Jun 14, 2012, 9:52:31 AM6/14/12
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Yes.
Dimitrios
Jun 14, 2012, 10:05:15 AM6/14/12
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Kevin one last question,
in the momentum equation, the hydrostatic pressure gradient ist
subtracted from both sides (pressure gradient on the right, gravity
acceleration on the left side).
If the pressure gradient is taken to be also on the left side, can
this treatment of the hydrostatic pressure can be thought as a
rearrangement of the hydrostatic pressure gradient between this two
terms (pressure gradient and gravity acceleration), from gravity
acceleration to the pressure gradient?
rho*(Du/Dt) = (rho*g - rhon*g) + (-gradp + rhon*g) + ...
in the momentum equation, the hydrostatic pressure gradient ist
subtracted from both sides (pressure gradient on the right, gravity
acceleration on the left side).
If the pressure gradient is taken to be also on the left side, can
this treatment of the hydrostatic pressure can be thought as a
rearrangement of the hydrostatic pressure gradient between this two
terms (pressure gradient and gravity acceleration), from gravity
acceleration to the pressure gradient?
rho*(Du/Dt) = (rho*g - rhon*g) + (-gradp + rhon*g) + ...
Dimitrios
Jun 14, 2012, 10:13:41 AM6/14/12
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replace left by right and vice versa in the text :)
Kevin
Jun 14, 2012, 10:22:47 AM6/14/12
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I am not sure what you mean. The reason for subtracting off the
background pressure is so that we can write our equations in terms of
a relatively small pressure perturbation. This is convenient for the
numerical solution.
background pressure is so that we can write our equations in terms of
a relatively small pressure perturbation. This is convenient for the
numerical solution.
Dimitrios
Jun 14, 2012, 10:33:55 AM6/14/12
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I'm just trying to find a physical interpretation, that the
hydrostatic pressure gradient occurs twice in the equation
hydrostatic pressure gradient occurs twice in the equation
Jaime Cadena
Jun 15, 2012, 6:56:49 AM6/15/12
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If you review seccion 5.1 of the Technical Reference Guide in step 1 by subtracting Ro*g in both sides allow making a variable substitution (deltaP=Ro*g+deltaRoP2, in which P2 relates to the perturbation and changes to the pressure due to the flow field behavior - hidrodynamic pressure). Basically as Kevin said, it just allows to have a better rearranged equation that help the solution algorithm.
Jaime Cadena
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