  We compare the results of two--dimensional simulations to experimental data
obtained at Los Alamos National Laboratory in order to validate the FLASH code.
FLASH is a multi--physics, block--structured adaptive mesh refinement code for
studying compressible, reactive flows in various astrophysical environments.
The experiment involves the lateral interaction between a planar Ma=1.2 shock
wave with a cylinder of gaseous sulfur hexafluoride (SF$_6$) in air. The
development of primary and secondary flow instabilities after the passage of
the shock, as observed in the experiments and numerical simulations, are
reviewed and compared. The sensitivity of numerical results to several
simulation parameters are examined. Computed and experimentally measured
velocity fields are compared. Motivation for experimental data in planes
parallel to the cylinder axis is provided by a speculative three--dimensional
simulation.

Astrophysics