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Hi Nitesh,<br>
<br>
It seems to me like you will need to initialize the non-jet portion
of your domain in hydrostatic equilibrium so that it doesn't just
start falling. A ballistic trajectory with an initial velocity of
15cm/s will give you only 0.1 cm rise before it peaks and starts to
fall back down. And nominally you have fluid pushing down in
addition to just gravity. Though I'm unsure if your domain is
initialized with fluid at rest or not, but regardless it will start
to fall pretty quickly because there is no pressure gradient.<br>
<br>
Flash does have a "hydrostatic" boundary condition that works like
the ones discussed in the paper I referenced, though you would have
to put in the "hole" for your jet but that might not be too hard a
modification. Putting things in hydrostatic equilibrium also should
be pretty easy since you are using an ideal gas EOS, so the profile
is analytic with dP/dz = -rho g. The necessary pressure gradient
isn't even really that big, but anything is big compared to zero.
With zero gradient the fluid on the domain will just immediately
start to free-fall.<br>
<br>
I think in your second boundary condition the material is probably
trying to support itself, since the pressure at the bottom boundary
is free to increase, but there is not enough damping for this to
ever find an equilibrium.<br>
<br>
I hope that helps,<br>
<br>
Dean<br>
<br>
<br>
<br>
On 05/03/2012 07:14 PM, Nitesh Attal wrote:
<blockquote cite="mid:4FA31F7B.8090205@uncc.edu" type="cite">
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Hello Dean,<br>
I would have a look at the paper you mentioned. It looks like my
previous discussion with Dangwook was not displayed on the user
forum and it is as follows.<br>
_____________________________________________________________________<br>
Hello Dongwook,<br>
Let me describe the problem again just to make sure we are on the
same page,<br>
The details are as follows------------------------<br>
Coordinate System 2D cylindrical (r and z)<br>
Left BC-Reflecting<br>
Right BC-Outflow <br>
Top BC-Outflow or Diode<br>
Bottom BC-User<br>
_________________________________<br>
User BC details -<br>
If r<=0.355cm then<br>
Vz = 800cm/s<br>
Vr = 0 cm/s<br>
P = 1atm<br>
T = 350K<br>
Species - N2 and O2 (AIR) (I plan to change this Later)<br>
else<br>
Vz = 15cm/s<br>
Vr = 0 cm/s<br>
P = 1atm<br>
T = 300K<br>
Species - N2 and O2 (AIR)<br>
_________________________________<br>
Domain Initialized with Air at 300K and 1atm pressure.<br>
Direction of Gravity = Z<br>
Acceleration = (-981.0)<br>
This means that the gravity is acting against the inlet flow.<br>
Domain Size - r = 0 to 15 and z = 0 to 21<br>
_________________________________<br>
Solvers - Split PPM<br>
EOS - Multigamma<br>
_________________________________<br>
<br>
I have tried the following,<br>
1) Gravity OFF = No flow reversal<br>
2) Gravity ON = Flow Reversal<br>
3) Gravity ON but only for z>0, this means in the guard cells
used to apply the lower BC experience no gravity (acceleration is
zero) = Flow reversal<br>
4) Gravity ON (Increase Co-flow) = Flow reversal<br>
<br>
From the above I concluded that there is pressure rise in the
domain when the gravity is turned on because of which flow
reversal is observed. Thus I changed the the Inlet BC as follows,<br>
_________________________________<br>
If r<=0.355cm then<br>
Vz = 800cm/s<br>
Vr = 0 cm/s<br>
P = <b>Pressure at (guard+1) cell</b><br>
T = 350K<br>
Species - N2 and O2 (AIR) (I plan to change this Later)<br>
else<br>
Vz = 15cm/s<br>
Vr = 0 cm/s<br>
P = <b>Pressure at (guard+1) cell</b><br>
T = 350K<br>
Species - N2 and O2 (AIR)<br>
_________________________________<br>
<br>
This results in NO flow reversal but the pressure in the domain
keeps on increasing with time which I dont want to simulate. I
want to simulate an atmospheric jet.<br>
Thank you for helping me out.<br>
Nitesh<br>
On 5/3/2012 6:56 PM, Dean Townsley wrote:
<blockquote cite="mid:4FA30D2D.1080902@ua.edu" type="cite">
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Nitesh,<br>
<br>
From your description it sounds like the simulation is doing
just what it is supposed to, just the gravity you are applying
is much stronger than what you might have expected.<br>
<br>
In order for the fluid to remain static when a simulation is
started, it must be initialized in hydrostatic equilibrium --
i.e. it must have a pressure/density/temperature gradient of the
appropriate size and direction. Without this the fluid will
just start to "fall" toward such an equilibrium as soon as the
simulation starts.<br>
<br>
There is some discussion of initializing fluid in hydrostatic
equilibrium in Zingale et al. (<a moz-do-not-send="true"
href="http://adsabs.harvard.edu/abs/2002ApJS..143..539Z">2002ApJS..143..539Z</a>),
though it is in the astrophysical context. This also discusses
relevant boundary conditions to support the fluid.<br>
<br>
If the gravity is too strong, it is possible for an inlet
boundary condition to be inconsistent, i.e. the imposed pressure
may not be high enough to actually support the material in the
domain against gravity. In this situation the pressure is not
consistent with sustained inflow and things will just get weird.<br>
<br>
I have implemented a top-to-bottom flow-through hydrostatic
domain, but it is pretty touchy and it doesn't sound like this
is what you are looking for. For a buoyant jet it seems like
you want the hydrostatic background to be mostly static, not
steadily flowing. Though honestly I'm unsure what you are
trying to reproduce. Maybe if you have a reference for an
example?<br>
<br>
Dean<br>
<br>
<br>
On 05/02/2012 12:43 PM, Nitesh Attal wrote:
<blockquote cite="mid:4FA1725D.2070800@uncc.edu" type="cite">Hello,
<br>
1) I use a constant acceleration of gravity (-981.0) along x
direction. The x-velocity decreases throughout the domain at
early time and as time progress the rate at which it decreases
increases. Also, pressure inside the domain increases where as
we are fixing the pressure at the inlet boundary (xl). This
causes the reverse flow, and I am wondering if it has anything
to do with the gravity implementation or its compatibility
with the inlet boundary. What should I use for the inflow
boundary? <br>
2) (i)When I changed constant of gravitational acceleration to
+981.0 along x direction. The velocity inside the domain rises
rapidly and there is no flow reversal. <br>
(ii) I would get back on that <br>
(iii) I would get back on that <br>
3) I am using FLASH4-beta but observed this in the previous
releases also(FLASH3.3 and FLASH4-alpha) <br>
On 5/2/2012 1:06 PM, <a moz-do-not-send="true"
class="moz-txt-link-abbreviated"
href="mailto:dongwook@flash.uchicago.edu">dongwook@flash.uchicago.edu</a>
wrote: <br>
<blockquote type="cite">Hi Nitesh, <br>
<br>
I am not sure if anyone has replied to your email, but if
not, I am sorry <br>
that your email has not been anwered yet. <br>
<br>
Just few questions to understand your issue: <br>
<br>
(1) To what direction do you apply your constant gravity? Is
this to the <br>
negetive x-direction, and as a result, do you see flow
reversal at the <br>
xl-boundary? <br>
<br>
The Windtunnel uses inflow boundary condition at the
xl-boundary, and this <br>
should not allow any matter to leave across the boundary.
But if your <br>
gravity is applied to the negative x-direction, it may as
well be the case <br>
that flow reversal would happen (depending on how strong
your gravity is <br>
relative to the inflow velocity), but the inflow boundary
condition at the <br>
xl-boundary doesn't seem to be a sensible thing to have. <br>
<br>
(2) In case that your gravity is NOT to the negative
x-direction but still <br>
experiencing the flow reversal, it could be useful for you
to understand <br>
your problem by trying several things: <br>
<br>
(i) change gravity directions and identify which one has
the flow reversal, <br>
(ii) change the magnetitude of gravity and identify the
problems as a <br>
function of its magnitude, <br>
(iii) use an alternative solver, for example, if you use
the split PPM <br>
then use the unsplit hydro solver, or vice versa. <br>
<br>
(3) By the way, what FLASH do you use? Do you use the most
recent release? <br>
<br>
Hope this helps. <br>
<br>
Best, <br>
Dongwook <br>
========================================= <br>
Dongwook Lee, Ph.D., Research Scientist <br>
The Flash Center for Computational Science <br>
The University of Chicago <br>
5747 S. Ellis Ave., Room 319 <br>
Chicago, IL 60637 <br>
(773) 834-6830 <br>
<br>
<blockquote type="cite">Hello All, <br>
I intend to perform a Multi-species laminar buoyant Jet
simulation. <br>
I began with the turning on Gravity in the Supplied
Windtunnel test <br>
problem. <br>
The Windtunnel test problem when ran with constant Gravity
results into <br>
complete flow reversal (towards xl-boundary). Even when
the xl-boundary <br>
is made outflow flow reversal is observed. <br>
Is this a known issue? Could some one suggest how to
overcome this? <br>
Thanks, <br>
Nitesh <br>
<br>
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