[FLASH-USERS] Flow Reversal for Windtunnel with Gravity

Thu May 3 20:14:51 EDT 2012

Hello Dean,
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.
_____________________________________________________________________
Hello Dongwook,
Let me describe the problem again just to make sure we are on the same page,
The details are as follows------------------------
Coordinate System 2D cylindrical (r and z)
Left BC-Reflecting
Right BC-Outflow
Top BC-Outflow or Diode
Bottom BC-User
_________________________________
User BC details -
If r<=0.355cm then
     Vz = 800cm/s
     Vr = 0 cm/s
     P = 1atm
     T = 350K
     Species - N2 and O2 (AIR) (I plan to change this Later)
else
     Vz = 15cm/s
     Vr = 0 cm/s
     P = 1atm
     T = 300K
     Species - N2 and O2 (AIR)
_________________________________
Domain Initialized with Air at 300K and 1atm pressure.
Direction of Gravity = Z
Acceleration = (-981.0)
This means that the gravity is acting against the inlet flow.
Domain Size - r = 0 to 15 and z = 0 to 21
_________________________________
Solvers - Split PPM
EOS - Multigamma
_________________________________

I have tried the following,
1) Gravity OFF = No flow reversal
2) Gravity ON = Flow Reversal
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
4) Gravity ON (Increase Co-flow) = Flow reversal

 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,
_________________________________
If r<=0.355cm then
     Vz = 800cm/s
     Vr = 0 cm/s
     P = *Pressure at (guard+1) cell*
     T = 350K
     Species - N2 and O2 (AIR) (I plan to change this Later)
else
     Vz = 15cm/s
     Vr = 0 cm/s
     P = *Pressure at (guard+1) cell*
     T = 350K
     Species - N2 and O2 (AIR)
_________________________________

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.
Thank you for helping me out.
Nitesh
On 5/3/2012 6:56 PM, Dean Townsley wrote:
> Nitesh,
>
> 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.
>
> 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.
>
> There is some discussion of initializing fluid in hydrostatic 
> equilibrium in Zingale et al. (2002ApJS..143..539Z 
> <http://adsabs.harvard.edu/abs/2002ApJS..143..539Z>), though it is in 
> the astrophysical context.  This also discusses relevant boundary 
> conditions to support the fluid.
>
> 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.
>
> 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?
>
> Dean
>
>
> On 05/02/2012 12:43 PM, Nitesh Attal wrote:
>> Hello,
>> 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?
>> 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.
>>      (ii) I would get back on that
>>      (iii) I would get back on that
>> 3) I am using FLASH4-beta but observed this in the previous releases 
>> also(FLASH3.3 and FLASH4-alpha)
>> On 5/2/2012 1:06 PM, dongwook at flash.uchicago.edu wrote:
>>> Hi Nitesh,
>>>
>>> I am not sure if anyone has replied to your email, but if not, I am 
>>> sorry
>>> that your email has not been anwered yet.
>>>
>>> Just few questions to understand your issue:
>>>
>>> (1) To what direction do you apply your constant gravity? Is this to 
>>> the
>>> negetive x-direction, and as a result, do you see flow reversal at the
>>> xl-boundary?
>>>
>>> The Windtunnel uses inflow boundary condition at the xl-boundary, 
>>> and this
>>> should not allow any matter to leave across the boundary. But if your
>>> gravity is applied to the negative x-direction, it may as well be 
>>> the case
>>> that flow reversal would happen (depending on how strong your 
>>> gravity is
>>> relative to the inflow velocity), but the inflow boundary condition 
>>> at the
>>> xl-boundary doesn't seem to be a sensible thing to have.
>>>
>>> (2) In case that your gravity is NOT to the negative x-direction but 
>>> still
>>> experiencing the flow reversal, it could be useful for you to 
>>> understand
>>> your problem by trying several things:
>>>
>>>    (i) change gravity directions and identify which one has the flow 
>>> reversal,
>>>    (ii) change the magnetitude of gravity and identify the problems 
>>> as a
>>> function of its magnitude,
>>>    (iii) use an alternative solver, for example, if you use the 
>>> split PPM
>>> then use the unsplit hydro solver, or vice versa.
>>>
>>> (3) By the way, what FLASH do you use? Do you use the most recent 
>>> release?
>>>
>>> Hope this helps.
>>>
>>> Best,
>>> Dongwook
>>> =========================================
>>> Dongwook Lee, Ph.D., Research Scientist
>>> The Flash Center for Computational Science
>>> The University of Chicago
>>> 5747 S. Ellis Ave., Room 319
>>> Chicago, IL 60637
>>> (773) 834-6830
>>>
>>>> Hello All,
>>>> I intend to perform a Multi-species laminar buoyant Jet simulation.
>>>> I began with the turning on Gravity in the Supplied Windtunnel test
>>>> problem.
>>>> The Windtunnel test problem when ran with constant Gravity results 
>>>> into
>>>> complete flow reversal (towards xl-boundary). Even when the 
>>>> xl-boundary
>>>> is made outflow flow reversal is observed.
>>>> Is this a known issue? Could some one suggest how to overcome this?
>>>> Thanks,
>>>> Nitesh
>>>>
>>>
>>
>>

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