  We address the challenging proposition of using real experimental parameters
in a three-dimensional numerical simulation of fast rotating Bose-Einstein
condensates. We simulate recent experiments [V. Bretin, S. Stock, Y. Seurin and
J. Dalibard, Phys. Rev. Lett. 92, 050403 (2004); S. Stock, V. Bretin, S. Stock,
F. Chevy and J. Dalibard, Europhys. Lett. 65, 594 (2004)] using an anharmonic
(quadratic-plus-quartic) confining potential to reach rotation frequencies
($\Omega$) above the trap frequency ($\omega_\perp$). Our numerical results are
obtained by propagating the 3D Gross-Pitaevskii equation in imaginary time. For
$\Omega \leq\omega_\perp$, we obtain an equilibrium vortex lattice similar (as
size and number of vortices) to experimental observations. For
$\Omega>\omega_\perp$ we observe the evolution of the vortex lattice into an
array of vortices with a central hole. Since this evolution was not visible in
experiments, we investigate the 3D structure of vortex configurations and
3D-effects on vortex contrast. Numerical data are also compared to recent
theory [D. E. Sheehy and L. Radzihovsky, Phys. Rev. A 70, 063620 (2004)]
describing vortex lattice inhomogeneities and a remarkably good agreement is
found.
