Experimental Determination of Flow Fields behind a Pitching Cascade

Abstract

Unsteady flow fields behind an isolated airfoil and a cascade oscillating in pitch about their mid-chords are determined from a computer aided measurement. Behavior of shedding vortices is investigated and validity of the Kutta condition for a pitching airfoil is examined. Then, the response of a boundary layer on an airfoil under both stalled and unstalled conditions is studied by the wake analysis. The Kutta condition holds quite strictly for an airfoil pitching at reduced frequencies up to 0.45. Shedding vortices are conveyed downstream on a wake center in the form of a quite small-sized vortex or a vortex sheet. As long as a flow separation on an airfoil does not occur influence of pitching motion on a boundary layer is very small. When a separation occurs, however, the boundary layer is affected severely by pitching motion and a dynamic stall appears remarkably with an increase in a reduced frequency. An unsteady response of a boundary layer under a dynamic stall condition can be represented relatively well by the dead lag model.