[FLASH-USERS] double rarefraction question

[Weirs, V Gregory]

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