Porous Wall Boundary Condition

[Gary Sharpe]

Hi all,
Matei and I wanting to simulate his porous wall detonation
experiments.
Does anyone know of or have any ideas for a boundary condition which
mimics
the "loss" effects or non-perfect reflectivity of a porous wall?
My idea was to include a non-reactive layer of complient (i.e. a gas
of some non-infinite density)
in the calculation, but Matei thinks doing it through a boundary
condition would
be preferable.
Gary

[James Quirk]

Gary,

It's not difficult to add a transpiration boundary condition,
but without knowing the details of Matei's experimental setup
it's hard to advise on what formulation the transpiration
should take.

James

[Gary Sharpe]

Hi James,
in the experiments, a cellular detonation in a channel runs over a
section of "porous wall"
- in the experiments the "wall" consists of a network of criss-
crossing wires with
a given "pore density" I believe. Its role is to partially absorb or
destroy transverse waves
which impact on it. We want a boundary condition which mimics
qualitatively this and weakens
any waves which reflect from it. If the degree of absorbtion can be
specified in the b.c.
all the better.
Cheers

[James Quirk]

Gary,

On Sun, 1 Nov 2009, Gary Sharpe wrote:

> Hi James,
> in the experiments, a cellular detonation in a channel runs over a
> section of "porous wall"
> - in the experiments the "wall" consists of a network of criss-
> crossing wires with
> a given "pore density" I believe. Its role is to partially absorb or
> destroy transverse waves
> which impact on it. We want a boundary condition which mimics
> qualitatively this and weakens
> any waves which reflect from it. If the degree of absorbtion can be
> specified in the b.c.
> all the better.

So you really want an absorbing boundary condition.
These are often used in the aero community for far-field
boundary conditions, and you'll find a slew of references
in the literature.


If you think of Roe's schem, applied in the transverse
wave directon, then all you're doing is filling in a
ghost state such that the incoming characteristic has
zero strength. Of course, such a BC may not work in
practice because of non-linearities. But it's easy
enough to try.

James

> Cheers
> Gary
>
> > It's not difficult to add a transpiration boundary condition,
> > but without knowing the details of Matei's experimental setup
> > it's hard to advise on what formulation the transpiration
> > should take.
> >
> > James
>

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[Gary Sharpe]

James,
hmmm...I'm no sure the 'far-field' bc is appropriate here
a) if you run a znd wave parallel to wall, the wave would
remain 1D so wouldn't have the desired the effect.
b) there would be no way to control the "porosity"
i.e. degree of absorbtion.
We are currently looking at specifing the normal velocity
at the wall as a function of (p-p0) and porosity - perhaps assuming
choked flow in the pores.
Gary

[James Quirk]

Gary,

On Wed, 4 Nov 2009, Gary Sharpe wrote:

> James,
> hmmm...I'm no sure the 'far-field' bc is appropriate here
> a) if you run a znd wave parallel to wall, the wave would
> remain 1D so wouldn't have the desired the effect.
> b) there would be no way to control the "porosity"
> i.e. degree of absorbtion.
> We are currently looking at specifing the normal velocity
> at the wall as a function of (p-p0) and porosity - perhaps assuming
> choked flow in the pores.
> Gary
>

I wasn't suggesting you lift a far-field bc verbatim, it was more a case
of suggesting you look at the associated characteristic analysis for some
insight as what to do.

At a practical level, the first thing you should do is to implement
your own reflecting wall bc using the template from the bdy manpage.
Then when you're convinced that is working the same as the built-in
version you can adjust the routine to allow for a transpiration
velocity based on your porous model.

James