Geoscience Reference
In-Depth Information
Figure 8.13.
Photographs
of the planforms of
convection in a layer of
viscous fluid. (From White
(1988).) (a) Horizontal rolls
of rising, hotter fluid (dark
lines) and sinking, colder
fluid (light lines) are
stable over a wide range
of Rayleigh numbers.
(b) The bimodal pattern
has a primary set of
horizontal rolls with a
weaker perpendicular set
of rolls. The develops at a
higher Rayleigh number
than in (a). (c) A
hexagonal pattern with a
central rising plume and
six sinking sheets.
(a)
(b)
(c)
Rayleigh-Benard convection occurs when a tank of Newtonian viscous fluid
is uniformly heated from below and cooled from above. Initially, heat is trans-
ported by conduction, and there is no lateral variation. As heat is added from
below, the fluid on the bottom of the tank warms and becomes less dense, so a
light lower fluid underlies a denser upper fluid. Eventually, the density inversions
increase to a magnitude sufficient for a slight lateral variation to occur sponta-
neously and a convective flow starts. In plan view, the first convective cells are
