Abstract

The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite the parameters influencing the development of the LEV topology must be fully understood and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities using water and air as working media by making use of a common baseline case. The motivation behind this comparison is that with two facilities the overall parameter range can be significantly expanded. This comparison includes an overview of the respective parameter ranges, control of the airfoil kinematics and careful scrutiny of how post-processing procedures of velocity data from time-resolved particle image velocimetry (PIV) influence the integral properties and topological features used to characterise the LEV development. Second, and based on results coming from both facilities, the appearance of secondary structures and their effect on LEV detachment over an extended parameter range is studied. A Lagrangian flow field analysis based on finite-time Lyapunov Exponent (FTLE) ridges allows precise identification of secondary structures and reveals that their emergence is closely correlated to a vortex Reynolds number threshold computed from the LEV circulation. This threshold is used to model the temporal onset of secondary structures. Further analysis indicates that the emergence of secondary structures causes the LEV to stop accumulating circulation if the shear layer angle at the leading edge of the flat plate has ceased to increase. This information is of particular importance for advanced flow control applications, since efforts to strengthen and/or prolong LEV growth rely on precise knowledge about where and when to apply flow control measures. Graphical abstract

A new flow control strategy based on leading-edge vortex (LEV) manipulation is proposed to improve the aerodynamic performance of a plunging airfoil. It has been found that the low pressure region produced by the LEV contributes to the high lift during dynamic stall, while the growth of the secondary vortex would weaken the LEV and result in a decrease in lift. Accordingly, the vortex control hypothesis is that we change the evolution of the secondary vortex and LEV, thus achieving a higher lift coefficient with a longer duration. The suction actuator is placed at different positions on the upper surface of the airfoil to test the control hypothesis. When the suction actuator is near the leading edge, the LEV detaches from the shear layer earlier and it can only enhance the lift slightly while not delay stall time. When the suction actuator is near the middle region, it could inhibit the growth of the secondary vortex and, thus, reduce its strength greatly. Therefore, the LEV circulation could continue to increase. As a result, the suction control could increase the lift coefficient and also prolong the high-lift duration. When the suction actuator is near the trailing edge, an increase in lift could also be achieved by an increase in the negative pressure over the upper surface as well as the LEV circulation. Thus, we present and validate the lift enhancement strategy for an unsteady airfoil based on vortex control.

Moiré patterns in physics are interference fringes produced when a periodic template is stacked on another similar one with different displacement and twist angles. The phonon in two-dimensional (2D) material affected by moiré patterns in the lattice shows various novel physical phenomena, such as frequency shift, different linewidth, and mediation to the superconductivity. This review gives a brief overview of phonons in 2D moiré superlattice. First, we introduce the theory of the moiré phonon modes based on a continuum approach using the elastic theory and discuss the effect of the moiré pattern on phonons in 2D materials such as graphene and MoS2. Then, we discuss the electron–phonon coupling (EPC) modulated by moiré patterns, which can be detected by the spectroscopy methods. Furthermore, the phonon-mediated unconventional superconductivity in 2D moiré superlattice is introduced. The theory of phonon-mediated superconductivity in moiré superlattice sets up a general framework, which promises to predict the response of superconductivity to various perturbations, such as disorder, magnetic field, and electric displacement field.

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