text: Premature stagnation of the boundary layer 
  means that all subsequent airflow will overrun 
  this point and the boundary layer will separate 
  from the surface. Surface flow which is aft of 
  the separation point will 
  indicate a local flow 
  direction forward toward theseparation point- 
  a flow reversal. 
  If separation occurs the posi- 
  tive pressures are not recovered and form drag 
  results. The points of separation on any aero- 
  dynamic surface may be noted by the reverse 
  flow area. Tufts of cloth or string tacked to 
  lie streamlined in an area of 
  the surface will 
  in an 
  unseparated flow but will 
  area behind the separation point. 
  
  to increase the kinetic energy of the upper sur- 
  the more 
  face boundary layer to withstand 
  severe pressure gradients common to the higher 
  lift coefficients. Extreme surface roughness 
  on full scale aircraft (due to surface damage, 
  heavy frost, etc.) causes higher skin friction 
  and greater energy loss in the boundary layer. 
  The lower energy boundary layer may cause a 
  noticeable change in C,-” and stall speed. In 
  the same sense, vortex generators applied to 
  the surfaces of a high speed airplane may allay 
  compressibility buffet to some degree. The 
  function of the vortex generators is to create a 
  strong vortex which introduces high velocity, 
  high energy air next to the surface to reduce 
  or delay the shock induced separation. These 
  examples serve as a reminder that separation is 
  the result of premature stagnation of 
  the 
  boundary layer-insufficient 
  kinetic energy in 
  the presence of an adverse pressure gradient. 
  
  lie forward 