- There is a cold upper surface and hot lower surface to a
fluid layer
and gravity
points downward.
- A blob of fluid (which is usually
macroscopic scale,
and so bigger than a
fluid element (AKA fluid parcel))
absorbs heat from
lower surface.
The heat absorption can be
by heat conduction,
radiative transfer,
smaller scale convection, or some combination the
3
aforesaid heat transfer processes.
- The heat raises the blob's
temperature and causes the blob to expand
and become less dense.
- When the blob becomes less dense than its surroundings, it is subject to the
buoyancy force
which tries to make it float upward.
Buoyancy
is actually a fluid
pressure
effect.
It is familiar from playing in the pool.
The gravity on the expanded blob stays constant, but the
pressure force increases with size. So there is a net force
upward and the blob will accelerate upward.
- If the blob rises and cools sufficiently by
adiabatic expansion or
heat flow to its surroundings,
it contracts, becomes more dense, and sinks.
The fluid layer is
then STABLE with respect convection.
In practice, a fluid layer is stable/unstable
if its temperature gradient
is sufficiently unsteep/steep.
Note convection
can only happen when the macroscopic
parts of layer can move relative to each other.
Therefore convection
only happens in
fluids:
gases,
liquids,
and
solids of sufficient plasticity---e.g.,
the Earth's mantle
in plate tectonics.
- But if the blob does NOT
cool sufficiently, it can keep expanding and rising in a runaway fashion.
We assume this behavior for the rest of the explication.
- Eventually the blob reaches the cold upper surface
and transfers some heat
to the cold upper surface
by heat conduction,
radiative transfer,
smaller scale convection, or some combination the
3 aforesaid heat transfer processes.
The possibly broken-up, cooled blob loses
buoyancy and
sinks back to the hot lower surface.
- At the hot lower surface, blob reheats and starts its journey all over again.
- Convection forms a cycle of
heat transfer
from hot lower surface to cold upper surface.
The cycle though generally
chaotic in detail is
often rough approximate closed loop called a
convection cell???.
- Convection is a universally important
macroscopic scale
heat transfer process.
- Convection is qualitatively easy to understand
as we have seen.
Alas, it is a
turbulent
and often
chaotic process and requires
three-dimensional calculations for
high accuracy/precision.
The upshot is that convection is very hard to calculate
accurately/precisely even with
supercomputers and
huge amounts of CPU time.
Such calculations often need a lot of verification to be trustworthy.
Often one uses some approximate method of calculation of
convection: e.g., the
well-known
mixing-length theory.
- Dealing with
convection
is one of the difficult and uncertain parts of
astrophysics.
For example, our understanding of
stellar evolution
is we think quite good, but uncertainty about
convection is one of the weak links.
- See Convection videos
below.