Figure 3. 



like array manifested by the transition process as shown began only after the laminar 

 flow was disrupted and turbulent bursts were present. 



An important aspect especially in the region of finite amplitude is the ampli- 

 fication of the wave. Figure 4 shows preliminary measurements of the amplification 

 of the wave from the ribbon to transition at a frequency of 160 c.p.s. for different 

 input levels, and was measured along a line corresponding to a peak in the intensity 

 distribution. The intensity u ' is the level at x which is a reference position two inches 

 downstream from the ribbon. The distance from the surface at which the measurements 

 were made was at about 0.2 the thickness of the boundary layer, which is where the 

 maximum in the intensity distribution across the boundary layer occurs in the Tollmien- 

 Schlicting theory. At input levels of 0.2 and 0.3 percent transition did occur past 

 the neutral point, but here it occurred in an intermittent fashion with the wave some- 

 times damping and at other times increasing and causing transition. As the input level 

 was raised still further, transition occurred steadily and at positions successively farther 

 upstream. The data shown ends at about the beginning of the transition region. The 

 departure from the small disturbance rate is an exponential-like growth. The departure 

 for the four highest input levels takes place at an intensity level of about 1.5 percent. 

 As pointed out by Dr. Lin on a visit to our laboratory the other day, a level of this 

 magnitude would correspond to a fluctuating shear stress which is on the order of one- 

 tenth the viscous shear and would be a reasonable assumption for the beginning of 

 non-linear effects. However, it should be mentioned that there is no discernible effect 

 on the mean-velocity distribution. In fact there is no significant distortion of the mean- 

 velocity profile until the actual breakdown of laminar flow occurs. At any rate it is 

 felt that the significance of non-linearity needs further investigation and that the effect 

 of curvature as proposed by Gortler should not be overlooked. 



369 



