Effect of airfoil blade orientation on added mass and fluid damping

This study experimentally investigates the effect of orientation angle α (= 0° − 180°) on the added mass and fluid damping of two symmetrical airfoil models (NACA 0018 and NACA 0021) under still-water conditions using free-decay tests. The dynamic responses are captured using a laser Doppler vibrometer, from which natural frequencies, damping ratios, added mass, and added damping are extracted. Both added mass and fluid damping are strongly dependent on α. The added-mass ratio attains its minimum value at α = 90°, where the airfoil chord is aligned with the oscillation direction, and increases rapidly as α deviates from this orientation, reaching maximum values (45%–47% of the total effective mass) at α = 0° and 180°. A similar orientation dependence is observed for the fluid damping, which reaches its minimum at α = 90° and increases toward maximum values at α = 0° and 180°. The thicker NACA 0021 airfoil consistently produces lower added mass but higher fluid damping than the NACA 0018 airfoil. Empirical correlations are proposed to describe the dependence of added mass and added damping on both α and airfoil thickness. These findings highlight the importance of accounting for orientation-dependent added-mass and fluid-damping effects.