  A new general relativistic magnetohydrodynamics (GRMHD) code ``RAISHIN'' used
to simulate jet generation by rotating and non-rotating black holes with a
geometrically thin Keplarian accretion disk finds that the jet develops a
spine-sheath structure in the rotating black hole case. Spine-sheath structure
and strong magnetic fields significantly modify the Kelvin-Helmholtz (KH)
velocity shear driven instability. The RAISHIN code has been used in its
relativistic magnetohydrodynamic (RMHD) configuration to study the effects of
strong magnetic fields and weakly relativistic sheath motion, c/2, on the KH
instability associated with a relativistic, Lorentz factor equal 2.5, jet
spine-sheath interaction. In the simulations sound speeds up to c/1.7 and
Alfven wave speeds up to 0.56 c are considered. Numerical simulation results
are compared to theoretical predictions from a new normal mode analysis of the
RMHD equations. Increased stability of a weakly magnetized system resulting
from c/2 sheath speeds and stabilization of a strongly magnetized system
resulting from c/2 sheath speeds is found.
