Showing papers on "Convective available potential energy published in 1947"

Convective Motion in Air Over the Sea

Abstract: It has long been known that, when a confined sheet of fluid is heated from below, it becomes unstable and there develops in it a systematic pattern of circulation. The classical experiments of Benard,' performed nearly 50 years ago, deal with liquids. Benard found that, if a thin layer of liquid, usually free at its upper surface, is heated uniformly at the lower surface, there sets in a regime of polygonal convection cclls as soon as a certain critical temperature gradient is reached. The walls of these cells are vertical, and the movement of the liquid is upward in the center and downward a t the periphery. At first, the pattern is somewhat irregular, the polygons formed by ti horizontal section through the cells being of different sizes and the number of sides varying from 4 to 7. After a period of time, however, ranging from one or. two seconds to many minutes, depending on the viscosity of the liquid, a much more regular condition is established, in which the polygons approach regular hexagons. The relative dimensions of the cells vary somewhat with the conditions of the experiment, but the ratio of the length of the side of a hexagon to the cell height, i e . , the thickness of the layer, is in the neighborhood of 2. Estimates of the velocities involved were made on the basis of observations of minute suspended particles. These showed that the horizontal components of the motion are along radii from the centers of the hexagons, and that the downward vertical motion is a maximum along the edges common to three adjacent cells, represented in horizontal section by the vertices of the hexagons. When the liquid is subjected to shear, as a result of horizontal motion, the vertical cells are replaced by horizontal strips, or double rolls, with axes parallel to the direction of shear.