A • TRANSITION FROM LAMINAR TO TURBULENT FLOW 



The exact mechanism by means of which the growing oscillations pro- 

 duce transition is not yet completely understood. It appears, however, 

 that the laminar instability is not a factor except when all sources of 

 disturbance are made very small. 



When the transition region is investigated with a hot wire anemometer 

 capable of indicating the instantaneous velocity, it is found that, whereas, 

 from measurements of mean speed, transition appears to be a gradual 

 process, it is in fact quite sudden [1]. As the test probe is moved down- 

 stream, turbulent "bursts" appear, at first infrequently, then more fre- 

 quently and of longer duration, until finally the flow is continually and 

 completely turbulent. These observations were interpreted as a wander- 

 ing of transition back and forth about a mean position. 



In 1951 Emmons [3] suggested as a result of observations of the 

 gravity flow of a thin sheet of water over an inclined flat plate that 

 turbulence appears in more or less random fashion at localized spots 

 which grow in size as they move downstream. Mitchner [4] developed an 

 experimental method of generating local turbulent spots in air artificially 

 by passing an electrical spark through the boundary layer. 



Recently Schubauer and Klebanoff [5] made extensive studies of the 

 mechanism of transition from amplifying Tollmien-Schlichting waves. 

 They showed that turbulence did in fact originate as localized spots 

 in natural transition. They studied the growth of artificial spots by 

 Mitchner's technique. 



Since publication of the cited reference, Schubauer and Klebanoff have 

 shown that the Tollmien-Schlichting waves exhibit variations in ampli- 

 tude along a direction parallel to the leading edge of the plate, and that 

 turbulent spots appear in the regions of maximum amplitude of the wave. 



A,3. Effect of Pressure Gradient on Transition on a Flat Plate. 



The variation of pressure along the outer edge of the boundary layer has 

 a marked effect on the location of transition. Many years ago experi- 

 ments were made by Wright and Bailey [6] on the effect of pressure 

 gradient on transition in a tunnel in which the turbulence was about 

 0.2 per cent. Relatively small gradients produced large effects, changing 

 Ret from 2 X 10^ for zero pressure gradient to 0.7 X 10^ and 2.5 X 10® 

 for small positive and negative gradients. Schubauer and Skramstad [2] 

 gave the striking demonstration shown in Fig. A, 3 of the stabihzing effect 

 of a pressure which decreases in the downstream direction and the de- 

 stabilizing effect of a pressure which increases in the downstream direc- 

 tion. In those cases where transition results from the instability of laminar 

 flow at low turbulence levels, transition is hastened by a positive (adverse) 

 pressure gradient and delayed by a negative (favorable) one. The behavior 

 of the Tollmien-Schlichting oscillations in a pressure gradient has been 

 computed [7] and the above-mentioned observations are in qualitative 



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