On the Transition to Turbulent Convection 



rod in order to keep the Illuminated region in focus. Simultaneously, 

 the back of the camera rolls on an inclined plane since the camera 

 is free to rotate about an axis through its lens. Thus , different 

 regions of the fluid produce images on different parts of the film. 

 In this way, one obtains a picture of the flow pattern as if one were 

 viewing from above. 



For each steadily maintained external condition, the steadi- 

 ness or non- steadiness of the resulting flow was to be determined. 

 This was found to be too difficult by simply observing moving tracers 

 through the fluid since there were gentle time dependencies with 

 time scales of the order of several minutes to several hours. In 

 order to have a record of the flow at an earlier time against which 

 to compare the flow at a later time, the following photographic 

 technique was devised. The apparatus used is shown schematically 

 in Fig. 3. Two narrow overlapping beams of light illuminate 

 cilumiinum flake tracers along a line in the x-direction, say, through 

 the fluid. The beam remained fixed in space throughout the obser- 

 vation time. The camera was free to rotate about an axis through 

 its lens. With the camera aperature open, a synchronous motor 

 drew a wedge under the back of the camera at a rate determined by 

 the time scale of the time dependence of the flow. Thus, the photo- 

 graph displays an (x,t) representation of the flow, where t is the 

 tinne coordinate. At t = 0, the camera recorded alternating bright 

 and dark regions, corresponding to the cellular structure, as a 

 narrow strip of image across the film. When the flow was steady, 

 the cell boundaries remained fixed in time, thus producing straight 

 lines parallel to the t-axis on the photograph. With the beam near 

 the top (or bottom) of the convecting layer, the tracer particles 

 have an x- component of velocity which is given by the slope of the 

 trajectories in the (x,t) representation. 



I Convecting liquid 

 Light beam 



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Fig. 3, The apparatus for photographing the time evolution of flow 



293 



