One-dimensional non-permanent flows in internal mechanics
Fluid mechanics

Fluid circulation can be ensured by the effect of the gravity field alone if the fluid temperature is not uniform. Archimedes' forces act differentially on the fluid masses, with the warmest (least dense) being forced upwards while the coldest are forced downwards. If the fluid is contained in a loop with heating in the lower position (point A on figure 30 ) and cooling in the upper position (point B), the fluid circulates under the driving action of gravity.

The first central heating systems used this type of movement, but the instabilities created by the closing or opening of some radiators soon led to a preference for forced circulation...