Characteristics of Dynamic Thrust on an Unsteady Airfoil in Pitching and Heaving Motions

The unsteady flow at low Reynolds number regions has been attracted significant attentions in recent years. However, detailed vortex flow structures and characteristics of dynamic forces acting on an unsteady airfoil at a low Reynolds number region have not been clarified sufficiently. In this study, the detailed vortex flow structures behind an unsteady airfoil in pitching and heaving motions at low Reynolds number regions have been measured by PIV measurement. Moreover, the dynamic thrust measurements on an airfoil have been performed by a six-axes sensor in a water tunnel. An unsteady airfoil in pitching and heaving motions forms thrust producing vortex streets with vortices accompanied with large vorticities, without disturbing flow around an airfoil, comparing to airfoils with independent pitching and heaving motions. In the wake of an unsteady airfoil in pitching and heaving motions, jet flow is generated at φ = π/3, π/2, π and 3π/2. In particular, jet velocity becomes over 2.0 at φ = π/3 and π and the values are greater than those of the heaving airfoil. For φ = π/3 and π/2, large dynamic thrusts are produced. The thrust efficiency increases drastically with increase of Strouhal numbers and reaches its maximum value at around St = 0.2, then decreases gradually. At φ = π/3, 2π/3, 5π/6, π, 7π/6 and 4π/3, significantly large thrusts are produced on an unsteady airfoil however at the same time large lifts and moments are generated therefore thrust efficiency becomes small. The maximum thrust efficiency is φ = π/2 and is approximately 0.40.