Force generation of smooth and corrugated airfoil performing unsteady motions at low Reynolds number

Abstract

Structurally, corrugated airfoil for flapping wing MAV applications is more desirable due to its increased strength. Therefore, any aerodynamic (penalty) effects due to corrugation on such airfoil have found considerable attention. The objective of the paper is to computationally examine the effects of corrugation on an elliptical airfoil performing translation and pitching motion at Re = 100 by comparing it with the performance of a smooth NACA 0012 airfoil. The forces and the flow structures generated over the airfoils during their unsteady motion are solved using Navier–Stokes equations. These unsteady motions include the airfoil accelerating from the rest position and quickly pitching up in a constant free stream. The two airfoils are considered in the study with 50 and 100 equally spaced perturbations on the upper and the lower surfaces, respectively. The surface of a standard ellipse has been modified with the regular perturbations or ‘corrugation’ of the order of 1% c on the upper and the lower surfaces. The current study reveals that during the airfoil acceleration from the rest to U and, subsequently, in a constant velocity translation, the corrugated airfoils (regular surface perturbations) have similar behavior of the force and the moment coefficients (also similar vorticity plots) in comparison with the smooth airfoil. However, it is also found that for the fast and the slow pitching motion in a free stream, the corrugated airfoil (after certain angle of attack) has slightly lower values of the lift and the moment coefficients in comparison with the smooth airfoil; which is due to the small effect of corrugation on the velocity profile. It is concluded that the unsteady effect dominates the geometric differences (smooth versus corrugated airfoil); therefore, corrugated airfoil can be utilized to provide structural strength for flapping wing MAV applications.