Time-Varying Velocity Effects on Dynamic Stall

As discussed in Chapter 8, for a rotor in forward flight a blade element will en­counter a time-varying incident velocity and there will be additional unsteady aerodynamic effects to be considered. In nominally attached flow, these effects include more complicated circulatory contributions resulting from the nonuniform shed wake convection velocity, also with additional noncirculatory contributions. The problem of dynamic stall under these con­ditions is not completely understood but has been studied experimentally on 2-D oscillating airfoils by Pierce et al. (1978a, b), Maresca et al. (1981), and Favier et al. (1988). A time – varying onset velocity was obtained in the experiment of Pierce et al. by using choking of the upstream flow by means of rotating vanes and in the experiment of Maresca & Favier by means of fore-and-aft movement of the airfoil. In both experiments the AoA and free-stream velocity were varied harmonically with different relative phase angles.

Pierce et al. (1978a, b) have measured the airfoil pitching moment, with a view to under­standing the possible effects of varying free-stream velocity on the torsional aerodynamic damping at dynamic stall onset. The measurements in fully attached flow were found to be in good agreement with the “classical,” linearized unsteady thin airfoil models, as discussed in Chapter 8. In the vicinity of stall reduced aerodynamic damping was observed, although as shown previously, this is a characteristic found with steady onset flows and is related to the phasing of the dynamic stall events with respect to the forcing function (a, a, etc.).

In the other experiments by Maresca & Favier, measurements of the lift, drag, pitching moment, and chordwise pressure distribution have suggested some considerable influence of the varying free-stream velocity on the dynamic stall process. Depending on the phasing of the velocity variations with respect to the AoA, initiation of leading edge vortex shedding and the chordwise convection of this vortex appear to be different. Favier et al. (1988) measured a phase lead of the unsteady lift response for conditions with constant free- stream velocity, but these seem to be at variance both with other measurements for the same problem and also with linearized theory. In this regard, the various problems associated with the subscale Reynolds number simulation of the problem cannot be overlooked. The issue of time-varying incident flow velocity, unfortunately, has not yet been studied in detail using the various mathematical models of dynamic stall, and it would seem to be an ideal problem whose investigation is overdue. It would also seem that because of the difficulties in conducting experiments of this problem, it would form a good challenge for the various first – principles based CFD approaches to dynamic stall modeling currently under development (see Section 14.10.1)