  The enormous sizes and variability of emission of radio-loud AGNs arise from
the relativistic flows of plasma along two oppositely directed jets. We use the
Athena hydrodynamics code to simulate an extensive suite of 54 propagating
three-dimensional relativistic jets with wide ranges of input jet velocities
and jet-to-ambient matter density ratios. We determine which parameter sets
yield unstable jets that produce jet dominated FR I type radio galaxy
morphologies and which tend to produce stable jets with hot-spots and FR II
morphologies. Nearly all our simulations involve jets with internal pressures
matched to those of the ambient medium but we also consider over-pressured jets
and discuss differences from the standard ones. We also show that the results
are not strongly dependent on the adiabatic index of the fluid. We focus on
simulations that remain stable for extended distances (60-240) times the
initial jet radius. Scaled to the much smaller sizes probed by VLBI
observations, the fluctuations in such simulated flows yield variability in
observed emissivity on timescales from months. Adopting results for the
densities, pressures and velocities from these simulations we estimate
normalized rest frame synchrotron emissivities from individual cells in the
jets. The observed emission from each cell is strongly dependent upon its
variable Doppler boosting factor. We sum the fluxes from thousands of zones
around the primary reconfinement shock. The light curves an power-spectra, with
red-noise slopes between -2.1 and -2.5, so produced are similar to those
observed from blazars.
