A combined finite-difference/spectral method is used to model the 3D
viscous Rayleigh-Taylor instability. Numerically calculated growth rate
spectra are presented for an initial sinusoidal perturbation of the
interface separating two fluids with amplitude 10(-3)H and 0.2H, where H
is the height of the system. At small initial amplitude, growth rate
spectra closely follow linear theory, whereas the calculation with
higher initial amplitude shows wavelength selection towards 3D
perturbations. Numerical simulations and analytical theory are used to
evaluate the applicability of previous 2D numerical models, which is
shown to depend on (1) the wavelength and amplitude of an initially 2D
sinusoidal perturbation and (2) the amplitude of background noise. It is
also shown that reverse (backward) modeling is capable of restoring the
initial geometry as long as overhangs are not developed. If overhangs
are present, the possibility of restoring the initial conditions is
largely dependent on the stage of overhang development.