Beyond the well-mixed room Natural convection
PROFESSOR: So another important source of convection in an air filled room is buoyancy due to differences in the density of the air as the temperature varies. Even relatively small variations in temperature can lead to significant flows. There's another dimensionless number, which controls the appearance and strength of such flows, which is the Rayleigh number, written Ra. And this is also a physical property of the-- it's a combination of physical properties of the fluid plus the geometry. So in this case, the relevant geometrical scale is the height, because this is a gravitational instability. So in the Rayleigh number, we have gravity. I'll just define all these-- a gravitational acceleration, which is 9.8 meters per second squared. We have-- well, if we define the change in air density relative to the initial air density that's caused by changes in temperature, if the temperature changes aren't too big, there is a linear response, which is defined...