F. /?. Hama 



I am not quite prepared to discuss Dr. Corrsin's interesting and stimulating lec- 

 ture, and 1 am rather anxious to read his manuscript carefully. 



However, in connection with his Fig. 15, which was taken in a fully-developed 

 turbulent boundary layer at Dr. Corrsin's request, I would like to show one other pic- 

 ture ( Fig. 1 ) taken in a laminar boundary layer disturbed by vortices due to a trip wire. 



In the absence of the wire, the dye comes out uniformly and no filamentation 

 of the dye occurs. Once the wire is applied, the flow of the dye shows a concentration 

 as shown in Fig. 1, indicating the existence of secondary spiral motion near the bound- 

 ary surface, although the boundary layer still remains laminar or, more properly, is 

 in the stage toward transition. 



There is naturally a distinctive difference between the two cases. Whereas the 

 attached eddies are smaller and randomly distributed in turbulent flow, the wisps are 

 larger and regular in the other flow. Nevertheless, there seems to be a resemblance 

 for the two flow conditions, i.e., secondary spiral motion. 



I don't know, first, how we can explain this secondary flow formation which 

 takes place when a laminar boundary layer is disturbed by strong-two-dimensional 

 vortices. I don't know, second, whether there is any relation between the predominant 

 secondary flow in turbulent boundary layers and that in laminar layers in the transition 

 region. 



/?, W. Stewart 



We seem to get very close to the boundary. There is one phenomenon that 

 must be connected with this, which I thought I would mention. We have examined 

 very closely the bottom of some ships in dry dock. Whenever these ships were painted 

 in Hongkong, apparently they used very soft paint, and along the bottom of the hull, 

 along the direction of the mean flow — this has no relation whatsoever to the way the 

 paint brush strokes went, or the direction of the plates on the ships — but in the direction 

 of the mean flow one finds streaks of raised paint. Sometimes a single streak will be 



403 



