Airfoil Thickness Effects on the Thrust Generation of Plunging Airfoils

Anumerical study was conducted to investigate the effects of airfoil thickness on the thrust generation of plunging airfoils and to assess the contributions of pressure and viscous forces in flapping propulsion. A series of NACA symmetric airfoils with thickness ratio ranging from 4.0% to 20.0% of the airfoil chord length were used in the present study to undertake a same sinusoid plunging motion at a low Reynolds number of Re 1200 with the plunging Strouhal number Str 0:45 and reduced frequency k 3:5. It was found that the thickness of the airfoils would affect the evolution of the unsteady vortex structures around the plunging airfoils significantly, even though the airfoilswere set to undertake the sameplungingmotion.The different behaviors of the unsteady vortex structures shedding from the airfoils with different thickness were found to cause dramatic changes to the resultant aerodynamic force acting on the plunging airfoils. For a thick plunging airfoil with its thickness ratio greater than 9.0%, pressure force was found to play a dominant role in the thrust generation, and viscous force would be almost negligible and contribute mainly to drag production. It confirms that the traditional inviscid model of the Knoller– Betz effect (i.e., ignoring viscous effect) can be used to explainmany phenomena associatedwith flapping propulsion. A new finding of the present study is the substantial contribution of viscous force to the thrust generation for thin plunging airfoils (i.e., the thickness ratio less than 8.0%). Viscous force was found to become thrust-producing, instead of drag-producing, and it played a nonnegligible role in the thrust generation for the thin airfoils (i.e., viscous force would produce up to 20.5% of the total thrust for NACA0004 airfoil in the present study). The role change of viscous force in the thrust generation of the plunging airfoils was found to be closely related to the variations of the dynamics of the unsteady vortex structures around the plunging airfoils.