Unsteady aerodynamics of a plunging airfoil in transient motion

Abstract

The peak loads experienced by aircraft of all scales will typically be during gusts, turbulence or extreme manoeuvres. Understanding the aerodynamic response to these transient disturbances is therefore crucial, particularly when Leading-Edge Vortices (LEVs) occur. This fundamental study investigates the aerodynamic response to a wide range of transient plunging motions. The peak loads exhibited a strong dependence to motion amplitude yet remained relatively insensitive to motion duration. Within the parameter range tested (motion duration of T ¡ 20τ, or equivalent reduced frequency k ¡ 1, and plunge amplitude of αpl,peak≤ 30°), the peak lift did not exceed that of the quasi-static thin airfoil theory prediction, permitting its use as a safe limit for structural design. The normalized peak lift change displayed weak collapse with the timescale of the motion and instead showed better correlation with the non-dimensional plunge rate. The peak pitching moment scales well with plunge rate according to the theoretical prediction due to the added-mass component for plunge-up motions, but quickly diverges for plunge-down motions. At post-stall angles of attack, large-scale vortex shedding was observed and caused decaying oscillations in the loads long after the transient motion ends. For both a NACA 0012 and flat plate airfoil, the first vortex shedding cycle after the transient motion occurs around the subharmonic of the static shedding frequency. Subsequent shedding cycles then increase in frequency and asymptotically approach the static shedding frequency in around 15 to 20 convective times. This is the first study to experimentally quantify this behavior and is an aspect currently missing in existing reduced-order models, which could be significant for the prediction of successive transient disturbances. Finally, Reynolds number insensitivity was demonstrated for transient disturbances between 20,000 and 150,000, even for post-stall angles of attack where large-scale vortex shedding can occur.