Jet characteristics of a plunging airfoil

Water tunnel tests of a NACA 0012 airfoil that was oscillated sinusoidally in plunge are described. The flowfield downstream of the airfoil was explored by dye flow visualization and single component laser Doppler velocimetry (LDV) measurements for a range of freestream speeds, frequencies, and amplitudes of oscillation. The dye visualizations show that the vortex patterns generated by the plunging airfoil change from drag-producing wake flows to thrust-producing jet flows as soon as the ratio of maximum plunge velocity to freestream speed, i.e., the nondimensional plunge velocity, exceeds approximately 0.4. The LDV measurements show that the nondimensional plunge velocity is the appropriate parameter to collapse the maximum streamwise velocity data covering a nondimensional plunge velocity range from 0.18 to 9.3. The maximum streamwise velocity at a given streamwise distance downstream starts to exceed the freestream speed as soon as the nondimensional plunge velocity exceeds 0.25, Furthermore, this maximum jet velocity has been shown to be a linear function of the nondimensional plunge velocity.