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The forced reattachment of flow to an inclined flat surface, simulating a simple flap, was investigated experimentally. The transition from a separated to an attached state of the flow was initiated by an abrupt change in the frequency and the amplitude of periodic perturbations emanating from a slot at the flap shoulder. The excitation parameters determined the total duration of the reattachment process. Minimum reattachment time occurred at an optimal excitation frequency of $F_{\hbox{\scriptsize{\it opt}}}^{+} \approx 1.5$, which was independent of amplitude and flap inclination. The control over the process was achieved by enhancing large spanwise vortices in the flow. Spatial amplification of consecutive vortices induces mean transport of fluid away from the flap surface which causes the main stream to reattach. The time scales of the excitation are at least an order of magnitude smaller than the typical reattachment times.

The controlled separation of flow from an inclined straight flap at high inclination angles was investigated experimentally. The separation process was initiated by an abrupt change in the excitation emanating from a slot at the flap shoulder. A complete cessation of the actuation resulted in formation of a large vortex above the flap akin to the familiar ‘dynamic stall vortex’ (DSV) seen over oscillating airfoils in pitch. The DSV temporarily increased the aerodynamic load over the flap before it dropped to its low separated value. The duration of this overload decreased as the flap inclination increased. The use of periodic excitation during separation slowed down the rate of separation and changed its character depending on the amplitude and the frequency used. Forcing separation by switching the excitation to a high frequency ($3\,{<}\,F^{+}\,{<}\,8$) reduced or even eliminated the increase in flap loading that is associated with the DSV. A switch to low frequencies ($F^{+}\,{<}\,1$) extended the duration of separation and increased the transient overload during the initial stage of the process.

Following the high penetration of synchronous generators (SGs) in the power network, optimal overcurrent coordination improvement under faulty conditions has become a crucial problem. To reduce the overcurrent relay operating time, a new overcurrent relay curve is proposed in this study. Then, the overcurrent coordination problem is overcome by using a robust combinatorial optimisation method. Additionally, SG sizing and location is performed to verify the merits of both the proposed relay curve and the applied optimisation algorithm. The proposed relay curve performance is compared with other non-standard relays characteristic available in the literature for a standard microgrid. Then, the proposed relay curve is applied to both the 8-bus transmission and the 33 kV distribution portion of the 30-bus IEEE standard power test systems. Then, the SG transient stability for different fault locations is analysed. Finally, an accurate comparison between the proposed relay curve and standard/non-standard curves available in the literature is provided by applying the same optimisation method and network topology. The simulation results confirm the superiority of the proposed relay curve.

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