We present a comprehensive analysis of the morphology and dynamics of
relativistic pressure-matched axisymmetric jets. The numerical simulat
ions have been carried out with a high-resolution shock-capturing hydr
ocode based on an approximate relativistic Riemann solver derived from
the spectral decomposition of the Jacobian matrices of relativistic h
ydrodynamics. We discuss the dependence of the jet morphology on sever
al parameters, paying special attention to the relativistic effects ca
used by high Lorentz factors and large internal energies of the beam f
low. The parameter space of our analysis is spanned by the ratio of th
e beam and ambient medium rest mass density (eta), the beam Mach numbe
r (M(b)), the beam Lorentz factor (W-b), and the adiabatic index (gamm
a) of the equation of state (assuming an ideal gas). Both the ultrarel
ativistic regime (W-b greater than or equal to 20) and the hypersonic
regime (relativistic Mach number greater than 100) have been studied.
Our results show that the enhancement of the effective inertial mass o
f the beam due to relativistic effects (through the specific enthalpy
and the Lorentz factor) makes relativistic jets significantly more sta
ble than Newtonian jets. We find that relativistic jets propagate very
efficiently through the ambient medium, at speeds that agree very wel
l with those obtained from an estimate based on a one-dimensional mome
ntum balance. The propagation efficiency of a relativistic jet is an i
ncreasing function of the beam flow velocity. Relativistic jets seem t
o give rise to two different morphologies, according to the relevance
of relativistic effects. Hot beams (i.e., with internal energies compa
rable to the beam rest-mass energy) show little internal structure (as
they are almost in pressure equilibrium with their surroundings) and
relatively smooth cocoons forming lobes near the head of the jet. High
ly supersonic models, in which the kinematic relativistic effects due
to high beam flow Lorentz factors dominate, display extended cocoons t
hat are overpressured with respect to the environment. The cocoon thic
kness decreases, and its mean pressure increases with increasing beam
Lorentz factor.