Investigation of thermal convection in water columns using particle image velocimetry

Particle image velocity measurements were applied on thermally driven convection at low Rayleigh numbers. In a model experiment using a water column heated from bottom and cooled from above, the velocity field was studied at different vertical temperature gradients. In the testing facility with high aspect ratio (about 19) representing a 1-m-long column with 5 cm diameter, occurrence of free convection was verified for destabilizing temperature gradients of 0.1-2 K/m. The PIV results revealed that significant flow exists already at low vertical temperature gradients. The velocity of the stable large-scale circulations increased linearly with temperature gradient (< 1 K/m) from 8 x 10(-5) to 1 x 10(-3) m/s. At higher temperature gradients (1-2 K/m), a transition from quasi-stationary into time-dependent flow was observed, where convection cells changed position, number, and form temporarily. The motivation of this research was to gain more insight into density-driven convection in boreholes and groundwater monitoring wells.