SECTION A 



m t ^ 



TRANSITION FROM LAMINAR TO 

 TURBULENT FLOW 



H. L. DRYDEN 



A,l. Introduction. The motions of real fluids exhibit phenomena of 

 a bewildering degree of diversity. The pressure distribution and the re- 

 sultant force and moment on a body moving through a fluid are but 

 grosser manifestations of complex motions of the fluid in the surrounding 

 space. The body is surrounded by a flow field in which the velocity and 

 pressure vary. There is no single theory adequate to describe this flow 

 field in its entirety. Progress can be made only by recognizing charac- 

 teristic phenomena in limited regions of complex flow fields that can be 

 reproduced and studied in a simpler experimental environment. 



Transition is one of these characteristic phenomena of wide occur- 

 rence. Everyone has observed it in the rising column of smoke from a 

 cigarette lying on an ash tray in a quiet room, and at the same time has 

 observed the types of flow which we designate as laminar and turbulent. 

 For some distance above the cigarette the smoke rises in smooth filaments 

 characteristic of laminar flow, only to break up into the confused swirling 

 turbulent motion at a height which is dependent on the quietness of the 

 surrounding air. If the drafts or convective air currents are strong, 

 transition occurs close to the cigarette. If the air is very quiet, the fila- 

 ment may persist to a considerable height. This breakdown of the laminar 

 flow is transition. 



Considerable progress has been made in the study of transition at low 

 subsonic speeds where the motion may be regarded as that of an incom- 

 pressible fluid of uniform temperature. While transition has also been ob- 

 served in localized regions of flow at transonic and supersonic speed, the 

 information available is less extensive. The effects of heat transfer at 

 subsonic speed are discussed in Art. 23. Art. 24, 25, and 26 deal with 

 transition at supersonic speed. Art. 2 to 23, inclusive, deal with low speed 

 subsonic flow. 



A, 2. Transition on a Flat Plate in a Stream of Constant Velocity. 



Transition is observed to occur in the boundary layer, that region of the 

 flow fleld near the surface of the body where the steepest velocity gradi- 



(3) 



