[FLASH-USERS] Flow Reversal for Windtunnel with Gravity

[Dean Townsley]

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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