  We present detailed simulations of the interaction of a stream emanating from
a mass-losing secondary with the core of a massive supergiant in the slow
merger of the two stars inside a common envelope. The dynamics of the stream
can be divided into a ballistic phase, starting at the L_1 point, and a
hydrodynamical phase where the stream interacts strongly with the core.
Considering the merger of a 1 and 5Msun star with a 20Msun evolved supergiant,
we present two-dimensional hydrodynamical simulations using the PROMETHEUS code
to demonstrate how the penetration depth and post-impact conditions depend on
the initial properties of stream material (e.g. entropy, angular momentum,
stream width) and the properties of the core (e.g. density structure and
rotation rate). Using these results, we present a fitting formula for the
entropy generated in the stream--core interaction and a recipe for the
determination of the penetration depth based on a modified Bernoulli integral.
