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We present results for 2-D, axisymmetric simulations of flows with Lorentz factors $\sim 5$ -- $10$, typical of values inferred for superluminal BL~Lacs and QSOs.
The simulations were performed with a numerical hydrodynamic code that admits
relativistic flow speed. We exploit the property that the relativistic Euler
equations for mass, momentum and total energy densities in the laboratory
frame have the same form as the nonrelativistic equations, to solve for
laboratory frame variables using a conventional Godunov-type scheme with
approximate Riemann solver: the HLLE method. The relativistic nature of the
flow is incorporated by performing a Lorentz transformation at every step, at
each cell center or cell boundary where pressure, sound speed or velocity are
required. Determination of the velocity in this manner is a robust algebraic
procedure within which we can ensure that $v
This work was supported by NSF grant AST 9120224 and by the Ohio
Supercomputer Center from a Cray Research Software Development Grant.