THRUST GENERATION DUE TO AIRFOIL FLAPPING 

   Abstract: 

   Thrust generation on a single flapping airfoil and a
   flapping/stationary airfoil combination in tandem is
   studied parametrically.  A multi-block Navier-Stokes
   solver is employed to compute unsteady flowfields.  The
   unsteady flowfield around a single flapping airfoil is
   also computed by an unsteady potential flow code and a
   Navier-Stokes/potential flow interactive solution
   method.  The numerical solutions
   predict thrust generation in flapping airfoils and a
   significant  augmentation of thrust in
   flapping/stationary airfoil combinations in tandem. The
   propulsive efficiency is found to be  a strong function
   of reduced frequency and the amplitude of the flapping
   motion.  At a flapping amplitude of 0.40 chord lengths
   and a reduced frequency of 0.10, the propulsive
   efficiency of a single NACA0012 airfoil was computed to
   be more than 70\%. For the airfoil combination in
   tandem, the propulsive efficiency was augmented more
   than 40\% at a reduced frequency of 0.75 and a flapping
   amplitude of 0.20 chord lengths when the airfoils are
   separated by about two chord lengths.