Closed gforney closed 9 years ago
We do need to do more work on evaporation, but we would prefer to integrate the
evaporation and movement equations separately. In short, the time step for movement
is based on the droplet's speed, and the time step for evaporation would be based
(mainly) on the droplet's size.
I will mark this as "On Hold" to keep it as a suggestion when we look at this
routine again. In the meantime, if you can suggest a robust integration scheme for
the evaporation, we'd like to hear it.
Original issue reported on code.google.com by mcgratta
on 2008-03-25 12:07:13
Let's keep in mind though, that we may have to start integrating them jointly, since
they are so tightly coupled. Has anyone checked what fraction of heat and mass
balances do those droplets, that require movement sub-time stepping, actually represent?
Adding myself to Cc.
Original issue reported on code.google.com by shostikk
on 2008-03-26 12:59:45
The biggest issue with moving droplets is not the sub-time steps, but rather the
drag coefficient. Currently, we assume the droplets are completely indepedent of
each other. When the spray is dense, the drag coefficient should be reduced because
droplets are "protected" by their neighbors.
For evaporation, it is the very small droplets that cause the problems, because they
evaporate so rapidly, and again, each evaporates independently of neighbors.
Original issue reported on code.google.com by mcgratta
on 2008-03-26 13:16:36
Simo -- I have not had a chance to work on this. Has the issue arisen in your work
with water mist?
Original issue reported on code.google.com by mcgratta
on 2008-11-07 18:37:05
Kevin, before going home this friday, I just started runs of a single mist nozzle.
One with standard FDS, and one with drag coefficient reduction formula that was
published in Int. Journal of Multiphase flow. The current version is an extreme - as
if all the droplets were moving in a row. That is, I'm using a formula for the
second of the two trailing spheres. If this shows any significant effect, I must
find a way to compute how strongly I actually reduce the drag, based on local volume
fraction of liquid (which is sometimes quite grid sensitive, though).
Original issue reported on code.google.com by shostikk
on 2008-11-07 20:22:38
I started this discussion and I would like to make a clear statement: The problem for
a single isolated droplet is very simple: There are three equations to be solved for
each droplet: mass, impulse, energy. These equations should be solved simultaneously
using the same integration scheme.
Because of obstacles in the way, the simple explicit EULER is a good choice to keep
things simple. To keep mathematics straight all droplets should be integrated using
the same time step. In order to obtain reasonable results the time step should be
chosen small enough, i.e. the droplet should interact with each grid cell it is
passing through. This is what has to be done if we only look at the droplet.
But now comes the big problem: the Lagrangian droplet is interacting with the
Eulerian flow field which is modelled as turbulent Smagorinski. I have not the
slightest idea how this interaction has to be modelled adequately.
But I have modified the part.f90 such that the droplets are integrated as described
above using a subtime step of Delta L / Umax = 0.1 and I have run many tests. The
finding is that the interaction has a substantial effect on the results. Cuuting off
by FLUXMAX is not the solution.
My personal findings are:
1) the droplet spray is not modelled adequately: primary break up, secondary break
up
and reduced drag are missing
2) The droplet - flow interaction is an unresolved issue but of substantial influence
on the results
3) Final conclusion: We need specific experiments to develop reliable simulation models.
Original issue reported on code.google.com by mur@grs.de
on 2008-11-08 19:45:16
I am transferring control over this Issue to Simo Hostikka, who has been working
with the droplet routine recently. Before I leave, let me just say that NIST has
invested millions of dollars in water spray experiments over the past few decades.
I
appreciate that there are still issues, like break-up and turbulence interaction,
but I do not think that these are as important as fire suppression, both solid and
gas phase. I would prefer to put resources into suppression. We can model water
sprays reasonably well, but suppression is still in its infancy. For me, the biggest
single issue for droplets is the reduction in drag of the droplets near the nozzle.
If we can solve this problem, then I would suggest focussing on suppression issues.
Original issue reported on code.google.com by mcgratta
on 2008-11-10 20:22:20
I am closing this issue. The droplet routines have been changed significantly since
this posting. Let's start a new issue if there are still remaining problems.
Original issue reported on code.google.com by mcgratta
on 2013-02-26 15:00:15
Original issue reported on code.google.com by
mur@grs.de
on 2008-03-25 09:46:13