Computational Acoustics #2, BCs

[rih5342]

I'm trying to reduce cell count by reducing the
amount of model being meshed.

I've got CAD of an intake manifold consisting of
several runners pulling air from a common plenum.

At this time, I prefer to model just 1 runner,
by trimming the model where it connects with
the plenum.

Which of the following is a better way to close
the model?

1. draw a planar surface and use an absorbing BC?

2. draw a domed surface and use an absorbing BC?

3. draw a domed surface and use a mixed BC where
some percentage of the energy is reflected
and some is absorbed?

TIA