Jets with relativistic flow speed are a ubiquitous feature of the
Universe, being associated with pulsars, Galactic microquasars, and
active galactic nuclei. Their collimation and stability are
extraordinary, and understanding how these flows retain their
integrity over more than nine orders of magnitude in length scale is a
major challenge. I will present the results of both numerical
relativistic hydrodynamic studies, and linear stability analyses. I
will show how each approach is, by itself, severely limited in terms
of the insight it can yield, but that taken together, these studies
provide a powerful method for exploring relativistic flows: the
linear analysis provides quantitative information on the normal modes
of the flow, and thus a solid basis for interpreting the numerical
simulations, while the simulations validate use of the linear theory
in the domain of macroscopic disturbances to the flow structure.
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