Flow structure and coherence in the wake of a lifting wing with multiscale sinusoidal cut-in trailing edge serrations

Abstract

This thesis presents an experimental investigation into the wake of a lifting wing with multiscale cut-in trailing edge serrations. More specifically, it looks into how the wake is affected by the serration patterns, both in terms of mean properties and dynamic and spectral information. The flow is investigated by means of particle image velocimetry and hot-wire anemometry to combine the dynamic and spatial strengths of the two experimental techniques. The response of a wing fitted with such serrations and subjected to unsteady inflows is studied by means of lift measurement and hot-wire anemometry in the wake. Cut-in trailing edges are found to generate a strong secondary flow from below the peaks towards the troughs. Such a flow pattern increases the velocity deficit behind the peaks while re-energising the wake behind the troughs, giving rise to span-wise inhomogeneity of the velocity deficit and wake width which persists far downstream. The introduction of cut-in serrations exhibits bluntness that in turn generates vortex shedding that locks into a bent cell structure. The use of multiscale patterns weakens the cell structures which translates into a reduction of vortex shedding intensity as well as a reduction of span-wise correlation at both vortex shedding and low frequencies. The use of an alternative sharp approach offers a compromise that gives less coherence reduction on the edges of the wake but avoids the introduction of a clear vortex shedding signal. Under unsteady inflow conditions, the 2-iteration multiscale pattern provide the strongest overall reduc- tion of coherence. Statistical analysis of lift fluctuations under unsteady and intermittent inflow conditions reveals that the single tone sinusoidal pattern has the strongest reduction of the distribution’s tails. Finally, the multiscale patterns are found to smooth the time-evolution of the lift fluctuations when the wing is submitted to a sinusoidal variation of the flow incidence.