text: The flow pattern for the cylinder in an actual 
  fluid demonstrates the effect of friction or 
  viscosity. The viscosity of air produces a thin 
  layer of retarded flow immediately adjacent 
  to the surface. The energy expended in this 
  “boundary 
  layer” can alter the pressure dis- 
  tribution and destroy the symmetry of the 
  pattern. The force unbalance caused by the 
  change in pressure distribution creates a drag 
  force which is in addition to the drag due to 
  skin friction. 
  
  The effect of the addition of circulatory flow 
  is appreciated by the change in the pressure 
  distribution on the cylinder. The increased 
  local velocity on the upper surface causes an 
  increase in upper surface suction while 
  the 
  decreased local velocity on the lower surface 
  causes a decrease in lower surface suction. As 
  a result, the cylinder with circulation will 
  produce a net lift. This mechanically induced 
  circulation-called Magnus effect-illustrates 
  the relationship between circulation and lift 
  and is important to golfers, baseball and tennis 
  players as well as pilots and aerodynamicists. 
  The curvature of the flight path of a golf ball 
  or baseball rcluites an unbalance df force 
  which is created by rotation of the ball. The 
  pitcher that can accurately control a .powerful  The streamline pattern for the symmetrical 
  airfoil of figure 1.7 again provides the basis 
  for 
  the velocity and pressure distribution. 
  At the leading edge the streamlines are widely 
  diverged in the vicinity of the positive pres- 
  sures. The maximum 
  local velocities and 
  suction (or negative) pressures exist where the 
  streamlines are the closest together, One 
  notable difference between the flow on the 
  cylinder and the airfoil 
  is that the maximum 
  velocity and minimum pressure points on the 