Flow patterns in the wake of an oscillating aerofoil

Abstract

Part I of the report gives an account of experiments made with the National Physical Laboratory smoke generator to obtain smoke patterns in the wake of an aerofoil performing a rolling oscillation in a wind stream. Calculations are given in Part II relating to the smoke patterns produced by a nozzle in uniform motion relative to (a) an infinite row of equally spaced two-dimensional discrete vortices of alternate sign and (b) an infinite two dimensional vortex sheet with sinusoidal distribution of strength. Comparison with certain of the smoke patterns discussed in Part I suggests that the wake vorticity can be treated very closely as a system of discrete vortices, and this is supported by the consideration that the elements of a continuous vortex sheet would in general be subject to normal induced velocities, which would tend to break the sheet up. The tendency for this to occur would be greater for the higher values of ω, wince the variation of vorticity with distance along the wake would be greater. The induced velocities for a uniform infinite vortex sheet would be zero. On the assumption that the theoretical infinite vortex sheet in the wake 0f an oscillating aerofoil breaks up into discrete vortices, the size of the vortex cores is calculated by satisfying the energy equation relating the two systems.