Control of flow around an aerofoil at low Reynolds numbers using periodic surface morphing

Abstract

This thesis combines experimental and computational methods to investigate the low Reynolds number flow (Re = 50,000) around a NACA 4415 aerofoil, and its control using periodic surface motion. A physical model was fabricated and tested in a closed-loop wind tunnel and a good comparison between the experiments and computations was achieved.

Time-resolved measurements of the surface reveal that the peak-to-peak displacement is a function of both the amplitude and frequency of the input voltage signal but the addition of aerodynamic forces does not cause significant changes in surface behaviour. The vibration mode shape exhibits a single peak and is uniform in the spanwise direction at frequencies below 80Hz, above which a change in the vibration mode occurs.

The flow around the actuated aerofoil was compared with the baseline (i.e. unactuated) flow. The latter exhibits a large separation region and, as a result, produces relatively high drag and low lift forces. By analysing the experimental and computational data, the large separation zone was found to be the result of laminar separation without reattachment. Transition to turbulence does occur but too close to the trailing edge, and far from the wall, for sufficient pressure recovery to take place for reattachment.

When actuated at 70 Hz, the frequency spectra in the vicinity of the trailing edge and near-wake was found to be dominated by the actuation frequency. Sharp peaks suggest the production of Large Coherent Structures at this frequency. In agreement with the experiments, the computations revealed that the vortex shedding from the shear layer was `locked-on' to the surface motion and spanwise coherent vortices were produced during each actuation cycle. The increased momentum entrainment associated with them enabled a large suppression of the separated region, which was seen in both the experiments and computations. The result was a simultaneous increase in Lift and decrease in Drag and therefore a large increase in the L/D ratio.