Interferometric observations of the temperature structure in water cooled or heated from above

Controlled laboratory experiments were performed in a water tank to study energy transfer during cooling and radiant reheating of water from above. Accurate instantaneous temperature measurements were obtained using a Mach-Zehnder interferometer which did not disturb the temperature, radiation, and flow fields. The buoyancy induced flow field was visualized by employing an electro-chemical dye production technique. Cooling of initially uniform layer of water and stratification-cooling-restratification were studied. The experimental observation during cooling of an initially uniform temperature column of water indicated intermittent free convection near the surface. The phenomenon was characterized by randomly descending cooler plumes penetrating 10 to 25 cm into the warmer underlying region. The cooling rate had a decisive influence on the frequency and intensity of the descending parcels of water. For lower cooling rates the colder water usually descended in a form of sheets. Observations during cooling of initially stratified water showed that buoyancy driven convection occurred near the surface. The motion had an initial regular roll pattern, but as the cooling continued the roll pattern in the convective (mixed) layer deteriorated into motion similar to that observed during cooling of an initially uniform temperature layer of water. When the water was restratified by radiant heating, the circulation in the convective layer was suppressed, and the temperature profiles obtained were similar to those observed in a fluid during free convection between two solid parallel walls.