[FLASH-USERS] double rarefraction question

[Dean Townsley]

Mike, Bob, Greg, thanks for the comments!

I'll reply all in one place...

As Greg says, the density at the center is not spectacularly low, only 
about 0.02 (this is given in Toro so I know its right :) ).  So no 
problems with the small density limit.  And this doesn't seem 
particularly low from a numerical point of view either.  Or am I wrong 
about that?

I've attached a plot of the internal energy, kinetic energy and density 
from the reference (direct) solution.  You can see that the internal 
energy is also not particularly low compared to the kinetic, being about 
eint/ekin=0.5 at the lowest, and actually in the middle the kinetic 
energy is zero.

So I still don't understand what is going so wrong.  0.02 doesn't seem 
particlarly close to zero, but maybe it is(?).  And this doesn't seem to 
be a kinetically dominated flow.  The eintSwitch is set at 0.0001 and we 
don't go below 0.5.

Further thoughts are welcome, of course.  The next thing we might try is 
somewhat lower velocities to see when/if things start to look more 
reasonable.

Thanks again,
Dean


Weirs, V Gregory wrote:
> The large internal energy error you observe is precisely why this is a
> good verification problem. Flash (probably) conserves total energy and the
> internal energy is backed out by subtracting the kinetic energy from the
> total. In this problem all most all the energy is kinetic, so one large
> number is being subtracted from another of nearly the same size, leading
> to large errors in the difference (the internal energy.) Almost all direct
> Eulerian methods have this problem.
>
> Algorithmic remedies generally involve solving a PDE for the internal
> energy, as well as total energy, momentum and mass, and then specifying
> (generally ad hoc) conditions under which to sacrifice total energy
> conservation for internal energy conservation.
>
> For the conditions you specified from Toro (gamma=1.4 ideal gas) the
> interior is NOT a vacuum state. Toro has the conditions for the vacuum to
> be formed and the velocities are not high enough (one way to adjust the
> problem) in this case. (Setting up a case where vacuum is formed would be
> an interesting experiment in itself.) I do not think the adjusting the
> floors should matter in this case.
>
> Greg
>
> On 3/15/11 11:53 AM, "Dean Townsley" <Dean.M.Townsley at ua.edu> wrote:
>
>   
>> Hi flash users and developers,
>>
>> I am working with an undergraduate (cc'd here) on some Riemann solver
>> and verification stuff and we got something a bit strange when we tried
>> a double-rarefraction wave with flash.  So I thought I would ask around
>> about it.
>>
>> We are just doing the standard double-rarefraction case out of Toro's
>> textbook.  I've attached our flash.par for the Sod problem, but
>> basically it is:
>> sim_rhoLeft     = 1.
>> sim_rhoRight    = 1.
>> sim_pLeft               = 0.4
>> sim_pRight              = 0.4
>> sim_uLeft               = -2.
>> sim_uRight              = 2.
>>
>> Broxton has implemented the solver out of Toro to make some comparison
>> plots, and they look fine compared to the text solutions.  But I tried
>> it with flash and it did something really funky in the internal energy.
>> I've attached a plot that compares the energy I get out of flash (blue)
>> to that from the explicit riemann solver (red) at a time of 0.15.
>>
>> This is effectively with flash 3.2, but I tried it with 3.3 and I get a
>> slightly weirder structure of similar magnitude.  I attached a plot of
>> that too.  I was quite surprised to see this change from one version to
>> another.
>>
>> So my question to anyone who might know:  Is this a known issue with
>> PPM?  Perhaps I'm just unfamiliar with it.  I tried turning off the
>> steepening, just to try something, and it didn't seem to do anything.
>> Am I doing something else dumb?
>>
>> Any insight would be much appreciated!
>>
>> (hopefully the attachments will come through, otherwise I'll send links)
>>
>> Thanks,
>> Dean
>>     
>
>
>
>   
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