. Compendium of meteorology. Meteorology. 1260 LABORATORY INVESTIGATIONS is filled with polygonal convection cells, as described earlier. 2. When the top plate is moved at a rate not exceed- ing a certain low limiting value mi, the original convec- tion cells are distorted into a horseshoe pattern, as shown in Fig. 9. 3. When the top plate is moved at a rate exceeding Ml, but not exceeding another limiting value M2, the chamber is filled with transverse rolls. 4. When the top plate is moved at a rate exceeding Ms, the chamber is filled with longitudinal rolls. For a given depth of chamber, Ui
. Compendium of meteorology. Meteorology. 1260 LABORATORY INVESTIGATIONS is filled with polygonal convection cells, as described earlier. 2. When the top plate is moved at a rate not exceed- ing a certain low limiting value mi, the original convec- tion cells are distorted into a horseshoe pattern, as shown in Fig. 9. 3. When the top plate is moved at a rate exceeding Ml, but not exceeding another limiting value M2, the chamber is filled with transverse rolls. 4. When the top plate is moved at a rate exceeding Ms, the chamber is filled with longitudinal rolls. For a given depth of chamber, Ui and M2 will both increase as the difference of temperature between top and bottom of the chamber increases. It is not possible to state the values of Ui and u^ for different depths of chamber and for different temperature gradients. Only one value, M2 for depth 10 mm and temperature differ- ence 40C, is known, ih then being cm sec~', ap- proximately. Depth of Chamber Less Than 7 mm. In the absence of shear the chamber is filled with structures such as are shown in Fig. 5. When a large shearing motion is imposed, the chamber is filled with spindlelike struc- tures, and never shows the long continuous rolls which occur in deeper chambers. As the rate of shear is de- creased the spindlelike structures become shorter and more irregular, but at no stage is the chamber filled with transverse rolls. Some idea of the completely different structures obtained in different depths of fluid may be gathered from Fig. 11, which was taken in a. Fig. 11.—Change of cell pattern with depth. The picture represents a chamber of depth 8 mm at the top and 6 mm at the bottom of the picture. The temperature difference between top and bottom of the chamber was about 42C, and the rate of motion of top plate was 7 cm sec"'. chamber in which the depth varied from 6 mm at one side to 8 mm at the opposite side, with a temperature difference of 42C. The photograph in Fig. 11 was taken shortly after
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