  We experimentally and numerically investigated the generation of plumes from
a local heat source (LHS) and studied the interaction of these plumes with
cellular convective motion (CCM) in a rectangular cavity filled with silicon
oil at a Prandtl number (Pr) of approximately two thousand. The LHS is
generated using a 0.2-W green laser beam. A roll-type CCM is generated by
vertically heating one side of the cavity. The CCM may lead to the formation of
an unusual spiral convective plume that resembles a vertical Archimedes spiral.
A similar plume is obtained in a direct numerical simulation. We discuss the
physical mechanism for the formation of a spiral plume and the application of
the results to mantle convection problems. We also estimate the Reynolds (Re)
and Rayleigh (Ra) numbers and apply self-similarity theory to convection in the
Earth's mantle. Spiral plumes can be used to interpret mantle tomography
results over the last decade.
