  (shortened) We perform 3D hydrodynamic simulations of gas flowing around a
planetary core of mass \mplan=10\me embedded in a near Keplerian background
flow, using a modified shearing box approximation. We employ a nested grid
hydrodynamic code with as many as six nested grids, providing spatial
resolution on the finest grid comparable to the present day diameters of
Neptune and Uranus. We find that a strongly dynamically active flow develops
such that no static envelope can form. The activity is not sensitive to
plausible variations in the rotation curve of the underlying disk. It is
sensitive to the thermodynamic treatment of the gas, as modeled by prescribed
equations of state (either `locally isothermal' or `locally isentropic') and
the temperature of the background disk material. The activity is also sensitive
to the shape and depth of the core's gravitational potential, through its mass
and gravitational softening coefficient. The varying flow pattern gives rise to
large, irregular eruptions of matter from the region around the core which
return matter to the background flow: mass in the envelope at one time may not
be found in the envelope at any later time. The angular momentum of material in
the envelope, relative to the core, varies both in magnitude and in sign on
time scales of days to months near the core and on time scales a few years at
distances comparable to the Hill radius. We show that material entering the
dynamically active environment may suffer intense heating and cooling events
the durations of which are as short as a few hours to a few days. Peak
temperatures in these events range from $T \sim 1000$ K to as high as $T \sim
3-4000$ K, with densities $\rho\sim 10^{-9}-10^{-8}$ g/cm$^3$. These time
scales, densities and temperatures span a range consistent with those required
for chondrule formation in the nebular shock model.
