Abstract:
Photographs have been taken of the flow round a 10 per cent thick RAE 104 aerofoil performing pitching oscillations at low values of the frequency parameter (ω < 0.1) in subsonic and supersonic air streams. Apart from a difference of phase, the general flow patterns appeared to be similar to those observed in steady motion, the pattern for a particular instantaneous incidence of the oscillation resembling that for steady motion at a different incidence. It is suggested that, for the range of frequency parameter covered, the observed phase lag of the flow pattern corresponds to the lag in the circulation. The phase lag of the flow pattern (as indicated in most cases by the positions of the regions where transition from laminar to turbulent flow occurs in the boundary layer) has been determined for various conditions of frequency, amplitude and Mach number, and for two positions of the axis of oscillation. For one axis position direct measurements of the pitching-moment derivatives had been made previously, and the phase lag of the flow pattern found in the present experiment is approximately the same as that in the pitching moment. At the supersonic speed (M = 1.6) used in the present tests any phase difference that was present was too small to be detected in a photograph.
