The control of dynamic stall over a periodically pitching NACA 0015 airfoil using alternating current (AC) and nanosecond (NS) dielectric barrier discharge (DBD) plasma actuators is investigated by means of numerical simulation. This study employs a two-dimensional unsteady Reynolds averaged Navier–Stokes (URANS) approach to resolve the flow control process. The pulsed AC and NS plasma discharges are modeled by an empirical body force model and a sophisticated self-similar plasma formulation, respectively. Our study concentrates on the resolution of detailed control process of dynamic stall under DBD plasma forcing at two Reynolds numbers (Re) as well as on comparison of AC and NS plasma actuations in terms of control mechanism and authority. It appears that the dynamic stall without control at both moderate Reynolds number of Re = 2.5 × 10⁵ and high Reynolds number of Re = 7.5 × 10⁵ can be categorized into the so-called trailing edge stall. The trailing edge stall initiates with flow reversal near the trailing edge. Regarding the dynamic stall control, it is found that the jet flow produced by AC DBD or residual heat of NS DBD is responsible for inducing large-scale spanwise vortices, which, in turn, dominate the flow control. For the moderate Re flow, AC and NS plasma actuators have comparable performance and both achieve good control authority. However, for the dynamic stall control at high Re, the NS DBD achieves surprising success in enhancing lift of the airfoil and reducing aerodynamic hysteresis, whereas AC DBD nearly has no effect on the flow. It is found that the superiority of the NS plasma actuator over other control means is due to the thermal convection characteristic peculiar to NS plasma discharge. This characteristic makes the NS DBD plasma actuator more flexible in extending its influence region and acquiring a better control effect.

Leading edge flow separation control over a stalled National Advisory Committee for Aeronautics 0015 airfoil using a fluidic oscillator (FO) is investigated by means of numerical simulation possibly for the first time to elucidate the flow control mechanism and evaluate control authority. The flow is assumed to be two-dimensional and fully turbulent and resolved using unsteady Reynolds-averaged Navier–Stokes calculations with the elaborate Reynolds stress turbulence model employed. Our simulation is proved to be able to successfully resolve the basic characteristics of a FO operating in quiescent air, which include both the qualitative prominent flow structures and quantitative jet oscillation frequency. It is seen that the driving force behind the self-induced and self-sustaining oscillation of jet flow inside the oscillator is Coanda effect induced alternating development of a recirculation bubble on either side of the mixing chamber walls. We provide a comprehensive analysis of the flow control procedure over an airfoil at Reynolds number of Re = 4.8 × 10⁵ and an elucidation of the flow control mechanism. It is found that the most prominent flow feature resulting from the interplay between an oscillating jet and external crossflow over an airfoil is the production of spanwise vortices. The strong entrainment effect of the induced spanwise vortices is the dominant mechanism leading to the mitigation of flow separation. Periodic jet oscillation generates a series of downstream moving vortices over an airfoil surface and results in a greatly delayed flow separation. The recovery of a strong suction pressure peak near the leading edge and significant lift enhancement and drag reduction reflects the improvement of an aerodynamic performance of the airfoil under control. Also observed is the phenomenon of local flow frequency lock-in to forcing frequency near the leading edge region. Moreover, the mass supply rate at the inlet of the oscillator is found to have an appreciable effect on the flow control authority. A higher mass flow usually leads to a better flow control performance.

The flow separation control over an NACA 0015 airfoil using continuous alternating current (AC) dielectric barrier discharge (DBD) plasma actuator is investigated experimentally and numerically. This work is intended to report some observations made from our experiment, to which little attention is paid in the previous studies, but which is thought to be important to the understanding of control of complex flow separation with AC DBD. To this end, the response of separated flow to AC plasma actuation is visualized through the time-resolved particle image velocimetry (PIV) measurement, whereas numerical simulation is carried out to complement the experiment. The flow control process at chord-based Reynolds number (Re) of 3.31 × 10⁵ is investigated. It is found that the response of external flow to plasma forcing is delayed for up to tens of milliseconds and the delay time increases with angle of attack increasing. Also observed is that at the intermediate angle of attack near stall, the forced flow features a well re-organized flow pattern. However, for airfoil at high post-stall angle of attack, the already well suppressed flow field can recover to the massively separated flow state and then reattach to airfoil surface with the flow pattern fluctuating between the two states in an irregular manner. This is contrary to one’s first thought that the forced flow at any angles of attack will become well organized and regular, and reflects the complexity of flow separation control.

The design of a thermal cavity receiver and the arrangement of the fluid flow layout within it are critical in the construction of solar parabolic dish collectors, involving the prediction of the thermal–fluid physical field of the receiver and optimization design. However, the thermal–fluid analysis coupled with a heat loss model of the receiver is a non-linear and computationally intensive solving process that incurs high computational costs in the optimization procedure. To address this, we implement a net-based thermal–fluid model that incorporates heat loss analysis to describe the receiver’s flow and heat transfer processes, reducing computational costs. The physical field results of the net-based thermal–fluid model are compared with those of the numerical simulation, enabling us to verify the accuracy of the established thermal–fluid model. Additionally, based on the developed thermal–fluid model, a topology optimization method that employs a genetic algorithm (GA) is developed to design the cavity receiver and its built-in net-based flow channels. Using the established optimization method, single-objective and multi-objective optimization experiments are conducted under inhomogeneous heat flux conditions, with objectives including maximizing temperature uniformity and thermal efficiency, as well as minimizing the pressure drop. The results reveal varying topological characteristics for different optimization objectives. In comparison with the reference design (spiral channel) under the same conditions, the multi-objective optimization results exhibit superior comprehensive performance.

The purpose of this study is to investigate deep dynamic stall over a pitching National Advisory Committee for Aeronautics 0015 airfoil and its control with an alternating current dielectric barrier discharge (AC-DBD) plasma actuator by means of large eddy simulation. The airfoil oscillates at a reduced frequency of k = 0.08 with the angle of attack (AoA) varying between 5° and 25°. The chord-based Reynolds number is Re = 7.6 × 10⁴. Rich fluid dynamics and physics involved in the dynamic stall are resolved by large eddy simulation. It is found that the dynamic stall under consideration initiates with the onset of a leading edge vortex (LEV). The deep dynamic stall is characterized and dominated by four successive LEVs at high angles of attack. The control of dynamic stall with AC-DBD plasma actuation is investigated in depth. At low AoAs, the plasma forcing can delay the laminar-to-turbulent transition of the attached flow over the suction surface of the airfoil. Although the formation of the first LEV is delayed by the plasma actuation, the first three LEVs cannot be effectively suppressed in high AoAs beyond 20°. The impact of the AC-DBD on the flow becomes increasingly pronounced in the downstroke phase, and at the intermediate angles of attack, the flow separation can be effectively controlled. Substantial gain from the flow control is obtained. The area of the lift hysteresis loop and drag force are appreciably reduced, and the nose-down pitch moment is alleviated notably. Finally, the influence of forcing frequency on flow control is examined.