Flow Control Actuators Research Articles

Recent advancements in flow control using plasma actuators and plasma vortex generators

AbstractFlow‐control techniques have attracted significant attention in many scientific areas due to their ability to improve the effectiveness and regulate the flow of aerodynamic devices. This study explores the latest developments in flow‐control techniques, specifically concentrating on the cutting‐edge technologies of plasma vortex generators (PVGs) and actuators. By taking advantage of the ionization of gases or air, plasma actuators have become a viable method for modifying an object's aerodynamic properties without needing physical moving parts. These actuators create localized plasma discharges that interact with the surrounding flow to provide accurate separation control, boundary‐layer dynamics, and aerodynamic forces on aircraft wings, wind turbine blades, and other surfaces. PVG, which produce controlled vortical structures, offer a novel way to manipulate airflow with plasma actuators. These generators create swirling motions through plasma discharges that can be used in various technical applications, such as automotive, marine, and aviation, to modify boundary layers, reduce drag, and improve lift characteristics. This study offers an overview of recent work, focusing on the theoretical underpinnings, experimental validations, and practical applications of plasma‐based flow‐control technologies. Advances in plasma‐generating techniques, computational modeling approaches, and experimental configurations to optimize and comprehend the intricate fluid–structure interactions are covered in the debate. Moreover, the study delves into incorporating plasma‐based flow management into cars, renewable energy systems, and next‐generation aerospace designs, highlighting the possibility of increased agility, decreased emissions, and efficiency. It also discusses the difficulties and potential paths for developing these technologies further for use in business and industry, highlighting the necessity of dependable, scalable, and durable solutions. Finally, this study summarizes the most recent advancements in vortex generators and plasma actuators for flow control. It demonstrates how they have the power to revolutionize fluid dynamics and aerodynamics in a variety of engineering fields.

An experimental investigation of the unsteady counter-flow generated by a DBD plasma actuator for flow control

An experimental investigation of the unsteady counter-flow generated by a DBD plasma actuator for flow control

Magnetic polyurethane based composites as contactless valves in microfluidic applications

Magnetoactive polymer composites have garnered significant attention for their potential use in diverse applications, owing to their rapid and reversible response to external magnetic fields. By incorporating magnetic nanoparticles (MNPs) into an elastomeric matrix, these composites exhibit unique properties under static or alternating magnetic fields. In this context, thermo-polyurethane-based magnetic active composites are promising materials for developing microfluidic system components such as valves and peristaltic pumps. In the current study, we investigated the utilization of cobalt ferrite (CoFe2O4) magnetic nanoparticles in conjunction with a non-toxic synthesis method for polyurethane. It was explored the impact, on the overall success of the process, of cobalt ferrite nanoparticles incorporation at various stages of the thermo-polyurethane (TPU) synthesis reaction. Finally, the effects of different amounts of MNPs on the physicochemical properties of the resulting composites and their behavior as actuators under the influence of a magnetic field, was investigated. Our studies reveal that the actuator response of the composites increases proportionally with the percentage of MNPs present.Finally, the performance of a TPU/7.5% (V/V) CoFe2O4 composite strip as a flow control actuator within a microfluidic system was evaluated. This actuator responds to magnetic fields by bending, resulting in a 10% reduction in flow rate of microfluidic system. Reversing the magnetic field restores the flow rate to its initial value. Our cyclic tests illustrate the actuator's capacity to locally and temporarily modulate the microfluidic system's resistance. When combined with tailored TPU elasticity, these materials show significant potential for the fabrication of microfluidic valves and pumps.

Sweeping JET actuator for the control of flow separation on a hump airfoil

Flow control can effectively improve lift and delay flow separation. Sweeping jet (SWJ) actuators have attracted increasing interest for as active flow control actuators. In this paper, the characteristics of the SWJ actuator were studied through large-eddy simulation and experiments. It was discovered that the oscillation frequency of the SWJ actuator is directly proportional to the mass flow rate, and its working principle was determined. Subsequently, SWJ actuators with [Formula: see text], 1.9%, and 6.6% were applied to flow control on the hump airfoil. It was observed that with an increase in momentum coefficient, the SWJ actuator was able to push the shear layer toward the hump airfoil surface, leading to a reduction in the size of the separation vortex. At [Formula: see text], it was achieved that the reattachment point of the separation vortex shifted upstream from [Formula: see text] to [Formula: see text], and the separation vortex size essentially disappeared. During the investigation of the flow control mechanism of the SWJ actuator, it was found that the SWJ actuator periodically generates a separation vortex V1, which has entrainment effects, resulting in the formation of an external jet on the airfoil surface, effectively inhibiting flow separation.

Dielectric barrier discharge actuators: Momentum injection into co-flow and counter-flow freestream

In a quiescent environment, dielectric barrier discharge (DBD) plasma actuators generate a wall jet through the interaction of ionized and neutral molecules in an electric field. In external flow, the coupling between electrohydrodynamic (EHD), turbulence, inertial, and viscous effects in the flow boundary layer is more complex and requires additional investigation. We experimentally study momentum injection by DBD actuators into the free stream flow with Re = 35,000 and 75,000 in co-flow and counter-flow scenarios over a range of VAC = 12 kV–19.5 kV peak-to-peak at a frequency of 2 kHz. In co-flow, the momentum injection leads to boundary layer thinning and fluid entrainment from the freestream into the DBD forcing region, while in the counter-flow configuration, flow separation can occur. A separation bubble is observed at Re = 35,000 for the tested condition. The momentum difference in the counter-flow configuration is six times greater than the EHD jet momentum in a quiescent environment. Both co-flow and counter-flow momentum injections show diminishing effects with higher Re. We show that the resulting flow pattern is not a superposition of the EHD jet and the free stream but is determined by the coupling and competition of inertial, viscous, and Coulombic effects between the EHD-driven forcing and the external flow. The proposed non-dimensional momentum ratio (M*) of EHD jet momentum to momentum in the external flow boundary layer can be used to predict the onset of separation; however, additional experimental and numerical studies are required to generalize this concept to other flow scenarios. The velocity profiles and momentum measurements presented here can be used to validate numerical models and inform the design of DBD actuators for active flow control.

Flow Control Over a Finite Wall-Mounted Square Cylinder by Using Multiple Plasma Actuators

Abstract The present study aims to investigate the effectiveness of plasma actuators in controlling the flow around a finite wall-mounted square cylinder (FWMSC) with a longitudinal aspect ratio of 4. The test is conducted in a small-scale closed return-type wind tunnel. The Reynolds number of the experiments, Red, is 500 based on the width of the bluff body and the freestream velocity. The plasma actuators are installed on the top surface and the rear surface of the square cylinder. The induced flow velocities of the plasma actuators are modulated by adjusting the operating voltage and frequency of the high-voltage generator. In this work, particle image velocimetry (PIV) is used to obtain the velocity fields. Furthermore, force calculations are conducted to investigate the effect of using plasma actuators with different driving voltages on the drag force. Our results show that the plasma actuators can successfully suppress flow separation and reduce the turbulent kinetic energy (TKE) in the wake. A correlation between the drag coefficient and the operating voltage of the power generator is also revealed, and the mean drag coefficient is found to decrease with increasing imposing voltage. The plasma actuators can enhance the momentum exchange and the interactive behavior between the shear layer and the flow separation region, resulting in flow reattachment at the free end and shrinkage of the recirculation zone in the near-wake region of the bluff body. Overall, the present study demonstrates the practical effectiveness of using plasma actuators for flow control around FWMSC.

Response of a Turbulent Boundary Layer to an Imposed Synthetic Large-Scale Structure

The dynamic response of a zero-pressure gradient turbulent boundary layer (TBL) to an active flow control actuator was experimentally investigated. The experimental TBL had a sufficiently low momentum thickness Reynolds number, such that there were no energetic organized large-scale structures in the outer region, as evidenced from measurements of the premultiplied wavenumber–frequency spectra. The periodically pulsed plasma actuator, placed inside the outer region of TBL, introduced a synthetic large-scale structure, and the boundary-layer response to this synthetic structure downstream of the actuator at select wall-normal and streamwise locations was investigated. The turbulence amplitude modulating effect of the synthetic large-scale structure, within the inner and log-linear regions of the boundary layer, was isolated and analyzed using a phase-locked analysis. The dynamic interaction of the synthetic large-scale structure and smaller-scale turbulent motions was quantified using a modulation coefficient, and a strong positive correlation within the inner and log regions of the boundary layer was measured. The streamwise development of the synthetic large-scale structure and its modulating effect on the near-wall turbulence across several streamwise locations are described. Profiles of the phase speed at these streamwise locations were extracted and were found to be constant within the log region, further confirming a strong coupling between the near-wall fluctuations in turbulence intensity and the synthetic large-scale motions introduced in the outer region. Overall, the turbulence amplitude modulation effect induced by the synthetic large-scale structure was found to be dynamically similar to the large-scale modulation measured in canonical TBLs at higher Reynolds numbers.

Numerical study on the periodic control of supersonic compression corner flow using a nanosecond pulsed plasma actuator

This study investigates the effectiveness of a pulsed nanosecond dielectric barrier discharge (NSDBD) plasma actuator for flow control over a supersonic compression corner through numerical simulations. The effects of varying applied voltages, repetitive frequencies, and activated locations of the plasma actuator are examined under large-scale laminar flow separation conditions around a compression corner. The unit Reynolds number and Mach number are 7.8 × 106 m−1 and 4, respectively. The results indicate that the discharge induces a pressure rise and leads to misalignment between the pressure gradient and density gradient in the residual heat region. Because of the interaction between the supersonic freestream and the actuation-induced shock/compression flow, convection, compressibility of the fluid element, and baroclinicity of the residual heat region collectively lead to the formation of an induced spanwise vortex, which in turn enables momentum migration. The induced vortex disrupts the initial flow structures and entrains high-energy fluid from the main flow into the boundary layer, promoting momentum mixing between the main flow and the separated flow, which increases the energy of the boundary layer to resist the adverse pressure gradient. The time-averaged flow structures imply that it is possible to totally eliminate the flow separation near the supersonic compression corner. For aerodynamics on the surface, the normal force produces a pitching moment that can be potentially utilized to control the body's orientation and trajectory. Additionally, total drag on the surface can be reduced by 5 %. This suggests that choosing the most appropriate position based on its local fluid characteristics can strongly increase the control effectiveness.

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.

Experimental characterization of a modified high-speed plasma synthetic jet actuator with oblique-slot exit

In the field of flow control research, oblique jets are known to offer several advantages over vertical jets. To gain a comprehensive insight into the flow field characteristics of a plasma synthetic jet actuator with an oblique-slot exit, the related experiments are conducted. The experiment employed high-speed schlieren imaging techniques and electrical parameter measurements to acquire the flow field characteristics and discharge properties of the oblique-slot actuator, followed by a comparative analysis with a vertical circular orifice actuator. The oblique-slot plasma synthetic jet exhibits a wall-attaching effect and asymmetric flow characteristics, which differ from those of the vertical circular orifice actuator. The actuator generates a wall jet with an initial velocity of 389.5 ± 15.08 m/s, effectively propelling the fluid within the boundary layer. The Mach number of the precursor shock wave in the direction of the jet reaches 1.59, but decreases to just 1.02 in the opposite direction. Over a period in the range of 10–70 μs, the Froude number of the plasma jet decreases from 1841 to 238. The dominant role of the inertial force gradually weakens, while the influence of buoyancy increases, causing the jet boundary to move upward. The oblique-slot jet configuration represents a typical planar jet, exhibiting superior flow control uniformity compared with the vertical circular orifice jet. The results indicate that the high-speed oblique-slot plasma synthetic jet actuator designed in this study possesses distinct advantages over vertical circular orifice actuators for high-speed fluid flow control.

Bayesian Optimization of Wall-Normal Blowing and Suction-Based Flow Control of a NACA 4412 Wing Profile

Active flow-control techniques have shown promise for achieving high levels of drag reduction. However, these techniques are often complex and involve multiple tunable parameters, making it challenging to optimize their efficiency. Here, we present a Bayesian optimization (BO) approach based on Gaussian process regression to optimize a wall-normal blowing and suction control scheme for a NACA 4412 wing profile at two angles of attack: 5 and 11 ∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document}, corresponding to cruise and high-lift scenarios, respectively. An automated framework is developed by linking the BO code to the CFD solver OpenFOAM. RANS simulations (validated against high-fidelity LES and experimental data) are used in order to evaluate the different flow cases. BO is shown to provide rapid convergence towards a global maximum, even when the complexity of the response function is increased by introducing a model for the cost of the flow control actuation. The importance of considering the actuation cost is highlighted: while some cases yield a net drag reduction (NDR), they may result in an overall power increase. Furthermore, optimizing for NDR or net power reduction (NPR) can lead to significantly different actuation strategies. Finally, by considering losses and efficiencies representative of real-world applications, still a significant NPR is achieved in the 11∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document} case, while net power reduction is only marginally positive in the 5∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^\\circ$$\\end{document} case.

Innovative Control Effectors for Maneuvering of Air Vehicles

Flow control is a rapidly developing key technological area which is subject to extensive research as it offers the potential to enhance aircraft aerodynamic performance, control, and maneuverability. The current paper details the methods and the results of a research which envisions assisting the newly formed NATO Task Group STO-AVT-239, with assessing the ability of modern active flow control actuators to improve the performance of Unmanned Combat Air Vehicles (UCAV). The project was supported by the ACE Undergraduate Research Topics Program at IIT and investigated the feasibility of the Coanda effect in achieving Circulation Control (CC) in order to produce rolling motion in cruise. Understanding of the performance of the SACCON airframe design and the complex vortex dominated flow field around was achieved through research papers and previous work of NATO Task Groups. It was discovered that a mid-span vortex influences and disturbs the flow above the control surfaces diminishing their effectiveness. A simple freeware computer code was used to predict the behavior of the airframe and the results were compared to experiment. The predictive abilities of the code were assessed to be good given the limitations of the software. Along with the airframe, CC was researched inorder to predict its maneuver control capabilities.The experimental data and the computationalpredictions were then used in combination witha semi-empirical aileron control model to predictthe aileron effectiveness and compare it tothe CC results. In order to be able to apply forresearch funding from appropriate institutions,a windtunnel experiment was designed andplanned to be conducted to validate the developedmodel.

Experimental investigation of turbulent boundary layer dynamics via active manipulation of large-scale structures

The dynamic response of a zero pressure gradient turbulent boundary layer (TBL) to an active flow control actuator was experimentally studied using continous laser particle imaging velocimetry (PIV). In previous experiments using a single hot-wire, it was shown that the synthetic large-scale structure (LSS) introduced by the plasma-based actuator, located in the outer region of TBL, had a strong modulating and reorganizing effect on the near-wall turbulence. In the study reported here, an actuator, optimized for the experimental TBL, was placed at the upper boundary of the log-linear region of the TBL to produce a spanwise uniform, periodic synthetic LSS in order to study the response of the TBL to these large-scale perturbations. Planar PIV over a narrow streamwise region was used to measure the time-resolved, two-dimensional velocity downstream of the actuator at a series of streamwise point locations. Using PIV, the modulating effect of the synthetic LSS on the near-wall turbulence is described in more detail. The results are discussed and compared with previous hot-wire measurements and numerical simulations of the actuated TBL.

Compressor and Valve Control Performance Implications on Active Flow Control Aircraft

This study presents the feasibility of implementing commercial-off-the-shelf reciprocating piston compressors for a developed active flow control (AFC) actuation framework to quantify the aerospace relevant performance ramifications of control architecture and compressor operational choices. Three architectures are studied: supply valve metering, exit area metering, and a combined approach. All concepts are studied under varying compressor operation schedules. The analysis framework in this study integrates internal pneumatic actuation and discharge dynamics, an experimentally calibrated compressor pressure and thermal dynamics model, three feedback control architectures, and flight dynamics models. The framework is implemented in simulation to provide a user-friendly tool for linking AFC architecture choices to achievable flight trajectories. Actuator performance is evaluated using actuation time, output, compressor duty cycle, and specific energy consumption. Aircraft tracking performance is evaluated as usable time and slalom centerline deviation. The analysis indicates that a supply-volume-based metering approach is comparatively inefficient concerning an exit-area-based metering, resulting in high flight tracking error. Exit area metering provides the best efficiency and run time with some structural drawbacks, while the combined approach provides the best flight tracking performance at the expense of additional complexity. These results inform the choice of onboard AFC hardware choices.

Experimental Observations of Transient Flows in Separation Control Using a Plasma Actuator

This paper presents the experimental results of separation and reattachment transient flow processes over a NACA0015 airfoil wing when using a plasma actuator for flow control. In addition, it addresses the flow behavior in the transient processes when the flow control device is activated or deactivated, providing insights for future feedback-based active flow control. This approach offers the benefit of enhanced aerodynamic capabilities. The experiments were conducted at a Reynolds number of 66,000 and an angle of attack of 13 degrees for leading-edge separation without control. The plasma actuator was installed on the leading edge of the wing, with a voltage of 8 kV, base frequency of 30 kHz, and burst frequencies ranging from 100 Hz to 600 Hz. Particle image velocimetry was employed for the flow field velocity measurements, and surface pressure data were obtained using eight piezoelectric pressure sensors. The first proper orthogonal decomposition mode of the transient flow velocity field is the focus of this paper and the flow behavior is quantitatively discussed. The results reveal details about the flow separation and reattachment transient processes such as their flow structures and their evolution over time. It is concluded that the time asymmetry between the separation and reattachment transient processes could be leveraged for further improvements to the efficiency of actuators.

Development of a Background-Oriented Schlieren (BOS) System for Thermal Characterization of Flow Induced by Plasma Actuators

Cold climate regions have great potential for wind power generation. The available wind energy in these regions is about 10% higher than in other regions due to higher wind speeds and increased air density. However, these regions usually have favorable icing conditions that lead to ice accumulation on the wind turbine blades, which in turn increases the weight of the blades and disrupts local airflow, resulting in a reduction in wind turbine performance. Considering this problem, plasma actuators have been proposed as devices for simultaneous flow control and deicing. These devices transfer momentum to the local airflow, improving the aerodynamic performances of the turbine blades while producing significant thermal effects that can be used to prevent ice formation. Considering the potential application of plasma actuators for simultaneous flow control and deicing, it is very important to investigate the thermal effects induced by these devices. However, due to the significant electromagnetic interference generated by the operation of these devices, there is a lack of experimental techniques that can be used to analyze them. In the current work, a background-oriented Schlieren system was developed and is presented as a new experimental technique for the thermal characterization of the plasma-induced flow. For the first time, the induced flow temperatures are characterized for plasma actuators with different dielectric materials and different dielectric thicknesses. The results demonstrate that, due to the plasma discharge, the temperature of the plasma-induced flow increases with the increase of the applied voltage and may achieve temperatures five times higher than the room temperature, which proves the potential of plasma actuators for deicing applications. The results are presented and discussed with respect to the potential application of plasma actuators for simultaneous flow control and deicing of wind turbine blades.

Modal analysis of compressor cascades with sweeping jet actuator and pulsed jet for active flow control based on large Eddy simulation

This study utilized large eddy simulation (LES) to explore the mitigation effects of endwall pulsed jet (PJ) and suction-surface sweeping jet actuator (SJA) controls on corner separation. Two modal analysis methods, proper orthogonal decomposition (POD) and dynamic modal decomposition (DMD), were applied to extract the main structures in the flow field and to analyze the mechanism of the effect of an unsteady jet on flow separation. First, LES was first performed in a linear cascade to capture the pulsating information in the flow field. The spectral analysis shows the dominance of the shedding vortex dynamic feature in the measured position of the original scheme. Furthermore, the dominant flow pattern in the flow field was obtained using POD. Both unsteady jet schemes made the energy distribution in the decomposed reconstructed flow field of the POD more dispersed than that in the origin. However, POD loses the actual physical meaning of the eigenvalues and corresponding modes of that order when finding the optimal orthogonal basis. Therefore, this study used DMD to retain information reflecting the time-evolution process. The magnitude of each order of energy corresponded to the characteristic frequency of the flow field. The highest energy modal frequency was consistent with the jet frequency for both jet schemes. The results of the modal analysis showed that the PJ and its induced small-scale flow structures dominate most of the higher energy modes. These small-scale flow structures inhibit the development of transverse secondary flows at the endwall. In comparison, the dominant flow-field structure using the SJA is a large-scale streamwise vortex. The sweeping jet accelerates the corner low-energy fluid over a broader area by the induced streamwise vortex itself, thus controlling the corner separation.

Design of an Acoustic Synthetic Jet Actuator for Flow Control

Synthetic jet technology is widely adopted in active flow control. An actuator with an oscillating diaphragm is a commonly used excitation device for synthetic jet generation. However, it has a disadvantage wherein the volume at the cross-section of the cavity varies unevenly when the diaphragm vibrates, which makes it difficult to use multiple jets corresponding to one diaphragm. In this paper, an acoustic synthetic jet actuator that can generate multiple jets with one diaphragm was designed. The diaphragm vibrated in a cylindrical cavity, transferring air to another constant-volume square cavity through pipes. The square cavity was covered with a multiple-orifice plate for the expulsion and suction of the ambient air. Through this means, the implementation of multiple jets corresponding to one diaphragm was achieved. The multiple jets are called distributed synthetic jets in this paper. Governing parameters that determined the performance of the distributed synthetic jets were given by theoretical derivation. It was found that, under specific geometry conditions, the governing parameters were mainly the frequency and voltage of the input signal to the actuator. Then, the velocity characteristics of the distributed synthetic jets were measured by using a constant-temperature anemometer and the parameter space was determined. The results showed that it was practicable to apply the acoustic actuator to turbulent boundary layer flow control.

Smart hybrid VIV control of a linearly sprung cylinder using an internal semi-active NES-based vibration absorber coupled with two active rotating wake-control rods

Extensive two dimensional CFD simulations were carried out to investigate the hybrid active/semi-active cross-flow vortex induced vibration (VIV) suppression of a flexibly-mounted circular cylinder. The cylinder is equipped with two smart actively rotating control rods that are symmetrically oriented 120° apart in the near-wake region in real-time collaboration with an internally-fitted semi-active NES-based vibration absorber in the low Reynolds number flow regime (Re=100) over a typical range of lock-in reduced flow velocities (3≤U∗≤10). The control rods function as flow control actuators by enforcing the desired momentum injection into the near-wake region, while the semi-active structural control is realized by switching the nonlinear absorber force between some minimum and maximum values consistent with certain control logics. Selected multi-input-multi-output (MIMO) feedback control laws are introduced into the coupled transient dynamic simulation framework through a manually hooked user defined function (UDF) subroutine. The modified active rotating rod control system equipped with passive NES is found to be fairly successful in overall mitigation of the time response amplitudes (i.e., up to 56% reduction in the cylinder displacements, 78% reduction in the lift amplitudes, and 8% reduction in the drag coefficients as compared to the hybrid passive system). Similarly, notable reductions are obtained when a modified continuous feedback control strategy is directly applied in the NES-based semi-active vibration absorber (i.e., up to 55% in the displacements, and up to 11% in the drag coefficients). Moreover, the superior performance of proposed smart hybrid active/semi-active VIV control system in comparison to the hybrid passive system is demonstrated (i.e., up to 68% reduction in the displacements, 61% in the lift amplitudes, and 12% in the drag coefficient). Lastly, the key novelties, advantages, practical limitations, and trade-offs of the proposed numerical model are briefly pointed out in regard to the real-world marine/offshore applications and forthcoming investigations.

Current state and future trends in boundary layer control on lifting surfaces

Successful flow control may bring numerous benefits, such as flow stabilization, flow reattachment, separation delay, drag reduction, lift increase, aerodynamic performance improvement, energy efficiency increase, shock delay or weakening, noise reduction, etc. For these purposes, many different flow control devices, which can be classified as passive, semi-active and active, have been designed and tested. This review paper aims to highlight the most promising and commonly employed boundary layer control methods as well as outline their potential in specific applications in aerospace and energy engineering. Referenced studies, performed on various geometries (flat plates, channels, airfoils, wings, blades, cylinders), are primarily numerical or experimental. Although enhanced aerodynamic performance is achieved in many cases, further research is required to draw general conclusions. This paper aims to demonstrate that, in the future, we may expect further developments of flow control actuators, as well as their increased application.