Active Flow Control Mechanism Research Articles

Long-lasting ceramic composites for surface dielectric barrier discharge plasma actuators

The developed research presents a novel experimental study of the cost-effective MgO-Al2O3, MgO-CaZrO3 perovskite, and thermally stable YSZ ceramic composites for DBD plasma actuators in aerospace applications. This study focuses on the implementation of ceramic DBD plasma actuators for aerodynamic flow control and ice creation mitigation. For this purpose, electrical power consumption analysis, induced flow velocities assessment, and mechanical and thermal characterization were performed. MgO-Al2O3 presented higher induced velocities than its zirconia-based counterparts of up to 3.3 m/s, and lower heat dissipation, achieving a ceiling temperature of 46 ºC, being thereby the best-suited candidate for active flow control mechanisms. In contrast, YSZ had very high-power consumption translated into a maximum surface temperature of 155.4 ºC, establishing itself for ice mitigation. This extensive research evinces that the strategic combination of the developed ceramics' thermomechanical, thermoelectric, and electromechanical properties allows them to be a promising breakthrough material for DBD plasma actuators.

Numerical Investigation of Plasma Actuator Effects on Flow Control Over a Three-Dimensional Airfoil With a Sinusoidal Leading Edge

Abstract The impetus of the present bio-inspired work is to investigate the impact of simultaneously using wavy leading-edge (WLE) airfoils in combination with curved multidielectric barrier discharge (DBD) plasma actuators as hybrid passive and active flow control mechanisms, respectively. A precise distinction of the produced frequency and noise signals, altogether with the acoustic effect of using WLE and DBD plasma actuators, is herein analyzed with precision. Two specific DBD plasma actuators are designed to actuate at x/C = 3% and x/C = 30% on a NACA 634-021 airfoil with sinusoidal WLE that bears a wavelength of 25% and an amplitude of 5% of the mean chord length and straight-leading-edge (SLE). A large eddy simulation (LES) turbulence model was used. This includes the dynamic control of unsteady flow separation, the three-dimensional vortical structure and induced trains of vortices, the aerodynamic forces, the velocity variation, and also the spanwise flow. The momentum transfer between the main flow and boundary layer was improved by the DBDs-induced vortices train and formed streamwise counter-rotating pair-of-vortices over the tubercle. Also, both the continuous wavelet transform (CWT) and fast Fourier transform (FFT) methods were used to investigate the induced plasma flow spectral content for the WLE and SLE geometries. We witnessed an optimized flow control, by using DBD plasma actuators with the WLE airfoil, that resulted in less massive flow separation, faster turbulent transition, and a robust earlier flow reattachment. This modification was beneficial in increasing the efficiency and decreasing the noise for low Reynolds number operational conditions.

Control of Separated Flow Using Actuated Compliant Membrane Wings

Membrane wings exhibit several aerodynamic advantages at low Reynolds numbers, passively adapting to flow conditions and delaying stall to significantly higher angles of attack relative to rigid wings. Rigid wings, on the other hand, often rely on active flow control mechanisms to achieve high angles of attack, injecting momentum to induce vortex roll-up in the shear layer. Active flow control for membrane wings is limited by the flexible nature of the wing surface. However, it is possible to achieve active flow control with a membrane wing by using a dielectric elastomer actuator as the membrane material. In this work, the performance of a sinusoidally actuated membrane wing is characterized for a range of actuation frequencies, freestream velocities, and angles of attack. Lift measurements show lift enhancement of up to 20%. Time-resolved membrane kinematics are used to calculate the coefficient of added momentum of the actuator, and the effect on the flow field is shown using phase-averaged particle image velocimetry measurements. Dynamic mode decomposition is used to show vortical structures being shed from the leading edge in phase with actuation, and comparisons are made with synthetic jet literature to show the similarities to separated flow control techniques used on rigid wings.

Propulsive Efficiency of Ridge/Inlet Configuration

Controlling and directing the boundary layer on the surfaces of a flight vehicle are two of the most demanding challenges in advanced aerodynamic designs. The design of highly integrated and submerged inlets with a large offset between the entrance and compressor face is particularly challenging because of the need for controlling or reducing the adverse effects of the boundary layer on propulsive efficiency. S-duct diffusers are used widely in flight vehicles when the compressor face needs to be hidden, and their performance is generally sensitive to the quality of ingested boundary layer from the fuselage. Passive or active flow control mechanisms are needed to prevent flow separations at the bends. In this paper, a new method is presented for optimal inlet/body integration based on a pair of ridges ahead of the inlet and its effects on the performance of a semicircular S-duct inlet integrated on a flat surface using CFD. In this design, the ridge changes an inefficient inlet concept to one with acceptable performance. The new method of integration is practicable for top-mounted inlet configurations where the use of diverters and other mechanisms results in higher amounts of drag, weight, and complexity.

Active Flow Control for Low-Pressure Turbines

Recent applications of active flow control to the low-pressure turbine section of a gas turbine are reviewed and evaluated. Unique aspects of the low-pressure turbine flow environment such as freestream turbulence, secondary flows, and unsteady wakes create challenges for flow control that must be considered during design and implementation. Applications of four different active flow control mechanisms to two challenging flow phenomena (separation and secondary flows) are described in detail. Although significant work is yet required before a viable active flow control implementation can be fielded, several mechanisms show considerable promise.

Accurate Discrete Adjoint Approach for Optimal Active Separation Control

This paper presents the development of an accurate and robust discrete adjoint approach for the optimization of active flow control mechanism. The adjoint solver is developed by applying algorithmic differentiation techniques to an incompressible unsteady Reynolds-averaged Navier–Stokes solver. Coupled with a quasi-Newton gradient-search optimization algorithm, its applicability is demonstrated by finding the optimal synthetic jet actuation on the suction side of the NACA 4412 airfoil at a Reynolds number and angle of incidence of 20 deg. Numerical results have shown that the actuation sensitivities based on the unsteady discrete adjoint solver are in excellent agreement with the values obtained from the tangent-linear solver and finite differences. Compared to the stalled baseline flow, optimal actuated flow significantly increased the time-averaged lift by 54%, and the stall is delayed from 16 to 22 deg. A detailed analysis of sensitivities and the effect of optimal actuation on the separation is presented.

Numerical analysis of synthetic jet flows using a diagonally implicit harmonic balance method with preconditioning

Abstract Synthetic jet flow is one of the active flow control mechanisms used to suppress flow instabilities including separation and wake vortices. It induces turbulent flows by ejecting or suctioning periodic jet streams into freestream air. A single piezoelectric actuator is often used to generate a periodic synthetic jet flow into quiescent air. The main purpose of the current study is to develop an efficient diagonally implicit harmonic balance method based on the nonlinear frequency domain method. This method is implemented in a compressible RANS flow solver with a preconditioning treatment to improve the performance of the numerical analysis. The transpiration wall boundary condition for periodic velocity of jet flows is used to avoid mesh deformation associated with the movement of the jet actuator diaphragm, where the periodic velocity profile is modeled from existing experimental data. With varying numbers of harmonics of up to fifteen the results are compared with the experimental data as well as those of the time-accurate computations. Reasonable agreement with the experimental data is obtained at reduced computational costs.

Flow control for the vertical axis wind turbine by means of flapping flexible foils

An active flow control mechanism is proposed to improve the efficiency of the energy extraction for the vertical axis wind turbine. The proposed system consists of a vertical axis wind turbine with flexible blades. The conception is inspired from the vortex control mechanism utilized by the aero-/aqua animals to improve their performance via the flexion of their fins. The viscous non-stationary flow around the turbine is simulated using the ANSYS-FLUENT 15 software. The complex flapping motion is reproduced using a dynamic mesh technique and a user-defined function. The results show that, with this strategy of control, the turbine generates a higher moment coefficient due to the increase in the peaks of lift force caused by a better difference in the pressure between the two sides of the blade due to the flexure motion. The turbine power coefficient can reach 38 % enhancement for the optimal flow control conditions.

Active flow control for reduction of fluctuating aerodynamic forces of a blunt trailing edge profiled body

Vortex shedding in the wake of two-dimensional bluff bodies is usually accompanied by three dimensional instabilities. These instabilities result in streamwise and vertical vorticity components which occur at a certain spanwise wavelength. The spanwise wavelength of the instabilities ( λ Z ) depends on several parameters, including profile geometry and Reynolds number. The objective of the present work is to study the three dimensional wake instabilities for a blunt trailing edge profiled body, comprised of an elliptical leading edge and a rectangular trailing edge, and to manipulate these instabilities to control the aerodynamic forces. Results of numerical simulations of flow around the body at Re( d) = 400, 600, and 1000, as well as planar Laser Induced Fluorescence (LIF) flow visualizations at Re( d) = 600 and 1000 are analyzed to determine the wake vorticity structure and λ Z . Based on the findings of these analyses, an active flow control mechanism for attenuation of the fluctuating aerodynamic forces on the body is proposed. The flow control mechanism is comprised of a series of trailing edge injection ports distributed across the span, with a spacing equal to λ Z . Injection of a secondary flow leads to amplification of the three dimensional instabilities and disorganization of the von Kármán vortex street. Numerical simulations indicate that the flow control mechanism can attenuate the fluctuating aerodynamic forces at lower Reynolds numbers (Re( d) = 400 and 600) where λ Z is constant in time. However, the control mechanism loses its effectiveness at Re( d) = 1000, due to the temporal variations of λ Z .

PASSIVE AND ACTIVE FLOW CONTROL BY SWIMMING FISHES AND MAMMALS

What mechanisms of flow control do animals use to enhance hydrodynamic performance? Animals are capable of manipulating flow around the body and appendages both passively and actively. Passive mechanisms rely on structural and morphological components of the body (i.e., humpback whale tubercles, riblets). Active flow control mechanisms use appendage or body musculature to directly generate wake flow structures or stiffen fins against external hydrodynamic loads. Fish can actively control fin curvature, displacement, and area. The vortex wake shed by the tail differs between eel-like fishes and fishes with a discrete narrowing of the body in front of the tail, and three-dimensional effects may play a major role in determining wake structure in most fishes.