Systematic Investigation of Thrust Production during Plunging Motion of the Airfoil

The effect of various conditions on the thrust generation of ۲-D airfoil in pure plunging motion has been investigated. These conditions include different airfoil shapes, different Reynolds numbers (Re) and reduced frequencies (K). The three different shapes used in this study are the NACA۰۰۱۴, the ellipse, and the flat plate airfoil, whereas, the three Re used in the study are ۱۰۰۰, ۱۰۰۰۰, and ۲۵۰۰۰ for the three values of K at ۲.۰, ۱.۰, and ۰.۵. For all these parametric studies, the thickness (t/c ratio) of all the airfoil has been kept as constant at ۱۴% t/c ratio. During sinusoidal plunging motion, CL and CD varies in a sinusoidal manner however CL and CD lags with the airfoil motion and the time averaged lift coefficient over one complete cycle is zero whereas the time averaged drag coefficient is negative and non-zero i.e. thrust is produced. The reason behind the thrust generation is due to the formation of the Reverse Karman Vortex Street in the wake of the airfoil.NACA۰۰۱۴ airfoil produces more negative values of the drag coefficient as compared to the ellipse and flat plate which indicates that the shape effect is important for thrust generation which is due to the pressure changes that occur close to the leading edge of the airfoil and it is more pronounced for an airfoil with large Δy variation near the leading edge , for instance NACA ۰۰۱۴. As the Re is increased, the time averaged drag coefficient becomes more negative and the thrust produced by the NACA۰۰۱۴ airfoil remains higher as compared to the other two airfoil which shows that the airfoil shape effect is dominant. As K reduces, time averaged drag coefficient (thrust) decreases and the airfoil shape effect becomes less prominent as K is decreased (or the unsteady effect decreases). It is seen that for all the cases, the CDv (drag due to viscous forces) is very small and major contribution of negative drag (thrust) comes from the pressure forces.

Keywords:

Reverse Vortex shedding , Thrust , Plunge , Airfoil , Reduced frequency , vorticity