  Stream-stream collision may be an important pre-peak energy dissipation
mechanism in tidal disruption events (TDEs). We perform local three-dimensional
radiation hydrodynamic simulations in a wedge geometry including the gravity to
study stream self-crossing, with emphasis on resolving the collision and
following the subsequent outflow. We find that the collision can contribute to
pre-peak optical emissions by converting $\gtrsim5\%$ of stream kinetic energy
to radiation, yielding prompt emission of $\sim10^{42-44}\rm erg~s^{-1}$. The
radiative efficiency is sensitive to stream mass fallback rates, and strongly
depends on the downstream gas optical depth. Even for a sub-Eddington ($10\%$)
mass fallback rate, the strong radiation pressure produced in the collision can
form a local super-Eddington region near the collision site, where a fast,
aspherical outflow is launched. Higher mass fallback rate usually leads to more
optically-thick outflow and lower net radiative efficiency. For
$\dot{M}\gtrsim0.1\dot{M}_{\rm Edd}$, the estimated photosphere size of the
outflow can expand by one to two orders of magnitudes reaching $\sim10^{14}\rm
cm$. The average gas temperature at this photospheric surface is a few
$\times10^{4}$K, roughly consistent with inferred pre-peak photosphere
properties for some optical TDEs. We find that the dynamics is sensitive to
collision angle and collision radius, but the radiative efficiency or outflow
properties show more complex dependency than is often assumed in ballistic
models.
