  Classical novae are thermonuclear explosions that take place in the envelopes
of accreting white dwarfs in binary systems. The material piles up under
degenerate conditions, driving a thermonuclear runaway. The energy released by
the suite of nuclear processes operating at the envelope heats the material up
to peak temperatures about 100 - 400 MK. During these events, about 10-3 - 10-7
Msun, enriched in CNO and, sometimes, other intermediate-mass elements (e.g.,
Ne, Na, Mg, Al) are ejected into the interstellar medium. To account for the
gross observational properties of classical novae (in particular, the large
concentrations of metals spectroscopically inferred in the ejecta), models
require mixing between the (solar-like) material transferred from the secondary
and the outermost layers (CO- or ONe-rich) of the underlying white dwarf.
Recent multidimensional simulations have demonstrated that Kelvin-Helmholtz
instabilities can naturally produce self-enrichment of the accreted envelope
with material from the underlying white dwarf at levels that agree with
observations. However, the feasibility of this mechanism has been explored in
the framework of CO white dwarfs, while mixing with different substrates still
needs to be properly addressed. Three-dimensional simulations of mixing at the
core-envelope interface during nova outbursts have been performed with the
multidimensional code FLASH, for two types of substrates: CO- and ONe-rich. We
show that the presence of an ONe-rich substrate, as in "neon novae", yields
larger metallicity enhancements in the ejecta, compared to CO,rich substrates
(i.e., non-neon novae). A number of requirements and constraints for such 3-D
simulations (e.g., minimum resolution, size of the computational domain) are
also outlined.
