Issue 596 in fds-smv: Unexpected air flow for a jet-fan blowing in the negative X direction

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Status: New
Owner: ----
Labels: Type-Defect Priority-Medium

New issue 596 by pi...@gofree.indigo.ie: Unexpected air flow for a jet-fan
blowing in the negative X direction
http://code.google.com/p/fds-smv/issues/detail?id=596

Application Version: 5.2.5 Serial
SVN Revision Number: 2828
Compile Date: 10 Dec 2008
Operating System: Windows XP Pro

Dear All,

I'm working on a simulation involving several jet-fans. Each jet-fan is
represented by a simplified casing plus an obstacle (with POROUS variable
set to .TRUE.) at the "discharge" end, which is supposed to give air the
required velocity.
Some of my jet fans are required to blow air in the direction opposite to
the positive X direction.
The jet of air created by the jet-fans blowing air in the negative X-
direction is strongly "tilted" (i.e. it is not symmetrical about the
longitudinal fan axis as one would expect).
I am attaching a much simplified input file which displays the problem,
together with a screen shot ilustrating the extend of the "asymmetry" in
the flow.
The problem does not occur if the airflow is positive X-direction. Also,
removing the casing solves the problem of the asymmetry.
Improving grid resolution to 10 cm does not seem to help much.

Did anyone experience similar problems? I'd most appreciate suggestions on
how to overcome this malfunction.

Kind Regards,

Piotr

Attachments:
jet_fan.fds 726 bytes
jet_fan_0738.jpg 78.3 KB

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Updates:
Status: Accepted
Owner: mcgratta

Comment #1 on issue 596 by mcgratta: Unexpected air flow for a jet-fan

blowing in the negative X direction
http://code.google.com/p/fds-smv/issues/detail?id=596

I'll take a look.

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Updates:
Status: Started
Cc: randy.mcdermott
Labels: OpSys-All Performance

Comment #2 on issue 596 by mcgratta: Unexpected air flow for a jet-fan

blowing in the negative X direction
http://code.google.com/p/fds-smv/issues/detail?id=596

A quick fix is to do the following:

&OBST XB= 9.0, 9.8, 6.0, 6.0, 2.40, 2.85, COLOR='BLUE', SAWTOOTH=.FALSE. /
Fan
casing
&OBST XB= 9.0, 9.8, 6.4, 6.4, 2.40, 2.85, COLOR='BLUE', SAWTOOTH=.FALSE. /
Fan
casing
&OBST XB= 9.0, 9.8, 6.0, 6.4, 2.40, 2.40, COLOR='BLUE', SAWTOOTH=.FALSE. /
Fan
casing
&OBST XB= 9.0, 9.8, 6.0, 6.4, 2.80, 2.85, COLOR='BLUE', SAWTOOTH=.FALSE. /
Fan
casing
&OBST XB= 9.4, 9.4, 6.0, 6.4, 2.40, 2.85, COLOR='RED', SURF_ID='BLOW1' /

The SAWTOOTH=.FALSE. tells FDS not to impose a velocity "slip" condition on
the fan
casing. The trouble with any obstruction that is zero cells thick is that
the
velocity BC is applied on one side or the other, which explains why the air
was
being blown sideways. Usually, the effect is not that dramatic, but in your
case it
was. Second, I moved the RED obstruction into the center of the fan casing.
That
helps, too.

Long term, we need to ensure consistent velocity BCs for thin obstructions.
I will
keep the case open to work on the long term solution, but hopefully, you
can do what
you need to do now.

Randy -- will the alternative differencing of the momentum equation
eliminate the
bias that is apparent in the input file above? The current differencing
scheme uses
the vorticity vector and stress tensor at cell edges with no regard to
orientation.

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Comment #3 on issue 596 by randy.mcdermott: Unexpected air flow for a

jet-fan blowing in the negative X direction
http://code.google.com/p/fds-smv/issues/detail?id=596

I doubt that using the conservative form of the momentum equations will
help. I
think part of the problem is that the obstructions are snapped to cells and
that in
the staggered scheme (conservative or not) the momentum control volume is
staggered
relative the obstruction. So, you end up with a partially blocked cell
either way.
I think that Ruddy's material point formulation will ultimately be the most
consistent fix for these kinds of problems.

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Comment #4 on issue 596 by mcgratta: Unexpected air flow for a jet-fan

blowing in the negative X direction
http://code.google.com/p/fds-smv/issues/detail?id=596

I think that you are thinking about porous boundaries. The issue in
question here is
when you have a thin obstruction and you try to apply a tangential boundary
condition to it.

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Comment #5 on issue 596 by pi...@gofree.indigo.ie: Unexpected air flow for

a jet-fan blowing in the negative X direction
http://code.google.com/p/fds-smv/issues/detail?id=596

Thanks Kevin - it does resolve the problem.
As regards the location of the "red" OBST - the reason why I've located it
at the
end of the casing was to add tangential velocity components which would
account for
the deflector plates installed on the jest-fans and the fact that some of
the jet
fans are not exactly aligned with the axis of the coordinate system.
One thing I'm not sure I understand is why the problem that you have
explained only
manifests itself when the direction of the flow is in the "negative"
direction? I
should say that I've got a similar effect for the negative Y direction as
well!

Regards,

Piotr

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Comment #6 on issue 596 by mcgratta: Unexpected air flow for a jet-fan

blowing in the negative X direction
http://code.google.com/p/fds-smv/issues/detail?id=596

The reason for the direction dependence is that the boundary conditions are
being
applied in the order -x, +x, -y, +y, -z, +z. For a thin sheet, the boundary
condition is applied at the cell edge, in which case the same BC is seen on
both
sides. This is not right -- there should be two BCs associated with each
edge. When
the sheet is not zero cells thick, this is not an issue -- the BCs are
unique.