G,4 • TRANSPIRATION-COOLED BOUNDARY LAYER 



turbulent boundary layer is of much greater importance than in the lami- 

 nar because it is more often encountered in engineering problems, yet 

 little progress has been made in the development of methods for the calcu- 

 lation of turbulent boundary layer even without transpiration cooling. 

 The difficulty in this problem is that no precise knowledge of the surface 

 shear and the shearing-stress distribution across the turbulent boundary 



12 3 4 5 6 7 



Free stream Mach number Moo 



Fig. G,4f. Limiting wall temperature required for complete 

 stabilization of boundary layer. (From [23].) 



layer (essential quantities in connection with the solution of boundary 

 layer equations) is available. 



It has long been recognized that the momentum exchange in turbu- 

 lent flow is impossible when the fluid stream approaches the vicinity of 

 a solid wall where a thin laminar sublayer exists and that the transfer of 

 shearing stress must depend on viscous action. Outside of this thin sub- 

 layer only the turbulent exchange mechanism is effective while the trans- 

 fer by molecular action may be neglected. In a like manner, the rate of 

 heat transfer between a fluid stream in turbulent flow and a smooth wall 



(453 > 



