Active Flow Control Technique Research Articles

Performance enhancement of an axial flow compressor using active flow control technique

Performance enhancement of an axial flow compressor using active flow control technique

Multiple boundary layer suction slots technique for performance improvement of vertical-axis wind turbines: Conceptual design and parametric analysis

Vertical-axis wind turbines (VAWTs) are gaining attention for urban and offshore applications. However, their development is hindered by suboptimal power performance, primarily attributable to the complex aerodynamic characteristics of the blades. Flow control techniques are expected to regulate the flow on the blade surface and improve blade aerodynamics. In the present study, an effective active flow control technique, multiple boundary layer suction slots (MBLSS), is designed for VAWTs performance improvement. The impact of MBLSS on the aerodynamic performance of VAWTs is examined using high-fidelity computational fluid dynamics simulations. The response surface methodology is employed to identify the relatively optimal configuration of MBLSS. Three key parameters are considered, i.e., number of slots (n), distance between slots (d), and slot length (l), which vary from 2 to 4, 0.025c to 0.125c, and 0.025c to 0.075c, respectively. The results show that MBLSS positively affects the power performance and aerodynamics of VAWTs. Parameter n has the most significant effect on VAWT power performance and the importance of d and l is determined by tip speed ratios (TSRs). Tight and loose slot arrangements are recommended for high and low TSRs, respectively. The relatively optimal configuration (n = 2, d = 0.025c, l = 0.05c) results in a remarkable 31.02% increase in the average net power output of the studied TSRs. The flow control mechanism of MBLSS for VAWT blade boundary layer flow has also been further complemented. MBLSS can prevent the bursting of laminar separation bubbles and avoid the formation of dynamic stall vortices. This increases the blade lift-to-drag ratio and mitigates aerodynamic load fluctuations. The wake profiles of VAWTs with MBLSS are also investigated. This study would add value to the application of active flow control techniques for VAWTs.

Multiple boundary layer suction slots technique for performance improvement of vertical-axis wind turbines: Conceptual design and parametric analysis

Vertical-axis wind turbines (VAWTs) are gaining attention for urban and offshore applications. However, their development is hindered by suboptimal power performance, primarily attributable to the complex aerodynamic characteristics of the blades. Flow control techniques are expected to regulate the flow on the blade surface and improve blade aerodynamics. In the present study, an effective active flow control technique, multiple boundary layer suction slots (MBLSS), is designed for VAWTs performance improvement. The impact of MBLSS on the aerodynamic performance of VAWTs is examined using high-fidelity computational fluid dynamics simulations. The response surface methodology is employed to identify the relatively optimal configuration of MBLSS. Three key parameters are considered, i.e., number of slots (n), distance between slots (d), and slot length (l), which vary from 2 to 4, 0.025c to 0.125c, and 0.025c to 0.075c, respectively. The results show that MBLSS positively affects the power performance and aerodynamics of VAWTs. Parameter n has the most significant effect on VAWT power performance and the importance of d and l is determined by tip speed ratios (TSRs). Tight and loose slot arrangements are recommended for high and low TSRs, respectively. The relatively optimal configuration (n = 2, d = 0.025c, l = 0.05c) results in a remarkable 31.02% increase in the average net power output of the studied TSRs. The flow control mechanism of MBLSS for VAWT blade boundary layer flow has also been further complemented. MBLSS can prevent the bursting of laminar separation bubbles and avoid the formation of dynamic stall vortices. This increases the blade lift-to-drag ratio and mitigates aerodynamic load fluctuations. The wake profiles of VAWTs with MBLSS are also investigated. This study would add value to the application of active flow control techniques for VAWTs.

Experimental Study on Pulsed Suction under Different Momentum Coefficients in Variable Compressor Cascade with Annular Gaps

Variable Stator Vanes (VSVs) with penny cavities are crucial in the compressors of variable cycle engines, where complex interactions between annular and radial gaps can adversely affect aerodynamic performance. Unsteady active flow control techniques have significant potential for reducing secondary flow losses in compressors. This research investigates the internal flow characteristics of a variable compressor cascade with penny cavities using low-speed wind tunnel experiments and flow visualization techniques. This study is the first to experimentally explore the relationship between loss reduction and momentum coefficient using pulsed suction (PS) compared to steady continuous suction (SCS), evaluating the effects of various excitation frequencies. The findings show that at a momentum coefficient of 0.11 and an excitation frequency of 100 Hz, PS achieves optimal aerodynamic performance, reducing total pressure losses by 11.59%, marking a 92.59% improvement over SCS. As the momentum coefficient increases, the effectiveness of PS control significantly improves, whereas the changes in SCS effectiveness are less pronounced. The control effects of PS under varying excitation frequencies are complex and influenced by multiple factors. At higher momentum coefficients, the loss reduction effectiveness of PS is strongly correlated with excitation frequency, while at lower coefficients, performance changes due to frequency variations are minimal. This study provides valuable guidance for the design of modern multistage variable compressors.

Optimization of unsteady jet control flow method for aerodynamic drag reduction of heavy truck model

The aerodynamic drag reduction of ground vehicles is crucial for meeting the increasingly strict emission standards. In this context, a potential approach to achieve this objective is to use active flow control techniques. Many active flow control techniques have been applied for ground vehicles, and the unsteady jet device has been proven to have better control energy efficiency than some other flow actuators. Therefore, this study focuses on applying this unsteady jet technique to a realistic truck model. The main parameters of the control flow device, including jet amplitude, jet angle, and control frequency were optimized to reduce the aerodynamic drag of the truck model and achieve the maximum control energy efficiency. Numerical studies were performed based on the CFD method with the k-ω SST turbulence model. A 25-level optimal Latin Hypercube design experimental study was conducted. The optimization method based on the Kriging response surface model and genetic algorithm was chosen to determine the optimal solution. The results show that the maximum energy efficiency is 9.174 % using the unsteady jet devices with the optimal parameters on the small-scale truck model.

Reduction of rotorcraft BVI using synthetic jets; computational studies using LES

In rotorcraft, the blade-vortex interaction (BVI) has been recognized as one of the main sources of noise and vibrations, affecting its aerodynamics performance and stability. Active and passive flow control techniques, for the reduction of BVI, have been investigated. The passive flow control techniques have proved to be inefficient due to the complex aerodynamics of rotorcraft. Thus, active flow control techniques has become of particular interest. In the present research we propose a novel approach, for the reduction of rotorcraft’s noise and vibrations due to BVI. The main idea of this approach is to inject air at the leading-edge of the blade to alter the characteristics (strength and core size) of the incoming vortex and thus, minimize the noise and vibrations associated with BVI. Computational studies are carried out using the large-eddy simulation (LES) with the dynamic Smagorisky sub-grid scale model. The computations are carried out for a flow of Reynolds number R e = 1.3 × 10 6 , based on the chord of NACA0012 airfoil and free-stream velocity. The studies show that injecting air at the leading edge of the blade, the influence of blade-vortex interaction on the aerodynamic coefficients is significantly reduced. Higher reduction of BVI effect, on the aerodynamic coefficients, was achieved with the increase of the speed of the synthetic jet. The reduction of the BVI, also, leads to the reduction of the vibrations.

Intelligent active flow control of long-span bridge deck using deep reinforcement learning integrated transfer learning

Aerodynamic forces of the Great Belt Bridge are mitigated using deep reinforcement learning (DRL) based active flow control (AFC) techniques at a Reynolds number of 5×105 in this study. The flow control method involves placing a suction slit at the bottom of the trailing edge of the bridge. The DRL agent as a controller is able to optimize the velocity of the suction to reduce the fluctuating coefficients of bending moment, drag, and lift by 99.1%, 73.7%, and 95.8% respectively. To reduce the high computational demands associated with DRL-based AFC training, this study integrates transfer learning technique into DRL, which calls transfer learning based deep reinforcement learning (TL-DRL) method. Specifically, the transfer learning method based on a DRL pre-trained model trained with a coarse mesh scheme is implemented. Results indicate that with TL-DRL method, the training cost can be reduced by 53%, while achieving the same control strategy and control effect as the DRL training from scratch. This study shows that the TL-DRL based flow control method is highly effective in reducing aerodynamic forces on long-span bridges. Furthermore, TL-DRL based flow control approach can adapt flexibly to different flow field environments, ultimately enhancing energy utilization efficiency.

Pneumatic Microballoons for Active Control of the Vibration-Induced Flow.

Vibration-induced flow (VIF), in which a mean flow is induced around a microstructure by applying periodic vibrations, is increasingly used as an active flow-control technique at the microscale. In this study, we have developed a microdevice that actively controls the VIF patterns using elastic membrane protrusions (microballoons) actuated by pneumatic pressure. This device enables on-demand spatial and temporal fluid manipulation using a single device that cannot be achieved using a conventional fixed-structure arrangement. We successfully demonstrated that the device achieved displacements of up to 38 µm using the device within a pressure range of 0 to 30 kPa, indicating the suitability of the device for microfluidic applications. Using this active microballoon array, we demonstrated that the device can actively manipulate the flow field and induce swirling flows. Furthermore, we achieved selective actuation of the microballoon using this system. By applying air pressure from a multi-input channel system through a connection tube, the microballoons corresponding to each air channel can be selectively actuated. This enabled precise control of the flow field and periodic switching of the flow patterns using a single chip. In summary, the proposed microdevice provides active control of VIF patterns and has potential applications in advanced microfluidics, such as fluid mixing and particle manipulation.

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.

NUMERICAL STUDY OF THE EFFECT OF TAPER ANGLES AND TAPER LENGTH ON THE REDUCTION OF AERODYNAMIC DRAG

Aerodynamic drag is still one of the main challenges facing the automotive industry. The design engineers are trying to develop novel techniques that could help to control the flow and optimize the shape of a vehicle to make it able to increase the base pressure drag at the wake region. The pressure drag is one of the main contributors to the aerodynamic drag. In our investigation, we have dealt with a benchmark car model (Windsor geometry) and visualized its effect on the characteristic parameters of the flow. In order to maximize the reduction of aerodynamic drag of vehicles, a numerical study has been undertaken based upon the CFD analysis with an active flow control technique including tapering of the rear lower and upper body along the length and height of the Windsor geometry. Taper angles from 12° to 20° in 4° steps have been implemented on the upper and lower part of the body while a taper length has been implemented along with the height of the body sides for the two cases. In comparison to other studies, we have come to sufficient results that gave us a significant reduction of aerodynamic drag for all the cases that we have tested. Thus, our study showed that the aerodynamic drag reductions are maximized, which indicates the efficiency of the technique we have applied.

Vortex shedding characteristics and hydrodynamic forces of stationary and elastically mounted side-by-side cylinders fitted with small diameter control rods

Vortex shedding behind bluff bodies can induce noise and vibration problems when it engages in a feedback cycle of oscillations with an acoustic or structural mode. Therefore, several passive and active flow control techniques have been developed to suppress vortex shedding behind a single bluff body. However, less attention was given to the case of multiple bluff bodies in cross-flow. Therefore, in this paper, a numerical investigation is performed to study the effect of adding small diameter control rods in the vicinity of two side-by-side cylinders on the vortex shedding characteristics, the hydrodynamic fluid forces, as well as the suppression of vortex-induced vibration. The simulations are conducted at a Reynolds number of Re = 200 and the side-by-side cylinders have a spacing ratio of T∗=2.5. The number of control rods and their angular positions around the main cylinders are varied. Fitting the side-by-side cylinders with control rods has resulted in three different flow regimes compared to the base case with no control rods. These flow regimes depend on the angular positions of the control rods and they are classified as; bistable flow, stable biased flow, and merged wake flow regimes. The stable biased flow regime has resulted in the maximum reduction of the fluctuating lift coefficient of the side-by-side cylinders, especially when the control rods are placed at θ=135° from the flow stagnation point. Increasing the number of control rods around the cylinders has further reduced the fluctuating lift coefficient and produced a more symmetric wake at the rod angles which produce the stable biased flow, and the merged wake flow regimes. The vortex-induced vibration of the side-by-side cylinders is investigated when the control rods are placed at the angular position that results in a stable biased flow regime. It is revealed that fitting the cylinders with control rods at an angular position of θ=135° has resulted in reducing the vibration amplitude over the whole range of the simulated reduced flow velocities, which makes this technique a viable one to control the vortex shedding and the vortex-induced vibration of side-by-side cylinders.

Mechanisms of Plasma Actuators Controlling High-Aspect-Ratio Rectangular Jet Width for Automobile Air Conditioning Systems

High-aspect-ratio (HAR) rectangular jets have attracted attention in automobile air conditioning (A/C) systems and turbulent jet applications owing to their excellent air delivery and mixing and attractive interior design. Active flow control (AFC) of rectangular jets using plasma actuators (PAs) has proven to be a promising technique because the actuator is simple, has low energy consumption, and can create flow features without interference. This research aims to understand the interaction between PAs and flow from a HAR rectangular nozzle using hot-wire anemometry, particle image velocimetry, and theoretical studies. Understanding how PAs affect the flow is beneficial for designing air vents to fit automobile A/C systems and various engineering applications by recreating the flow features with other AFC techniques and actuators. The combination of periodic excitation and vectoring effects transfers the flow’s mean energy to organized structures—known as spanwise vortexes—as large as 6 mm. The interaction between these coherent structures and the dissipative environment compresses the vortexes, resulting in the flow converging on the spanwise–streamwise (X–Z) plane and diverging on the transverse–streamwise (X–Y) plane. HAR rectangular jet flow features controlled by PAs can be predicted for specific cases by calculating the Strouhal number based on PA operating parameters.

Improvement of Helicopter Operations on Frigates Using the Coanda Effect

The flow surrounding a frigate is characterized by high-velocity gradients and flow detachment behind the superstructure of the ship. Therefore, the helicopter pilot’s workload during the recovery maneuvers above the flight deck can increase. Different techniques of flow control have been tested for reducing the flow detachment and pilot workload. In this paper, a combined numerical optimization and experimental tests in a wind tunnel are performed to analyze the implementation of the Coanda effect at the back part of the hangar as an active flow control technique to reduce the flow detachment above the flight deck. The numerical optimization is conducted by testing different velocities and geometries of the Coanda hangars tested. The best results show a complete elimination of the flow detachment above the flight deck and an up to 2.85 times reduction of the low-speed area with respect to the base case. The geometries selected from the numerical results were installed in a scaled simple frigate shape 2 (SFS2) and tested in a wind tunnel with particle image velocimetry (PIV). The experimental results also show a drastic reduction of the flow detachment above the flight deck combined with reduced levels of turbulence intensity at the height where the helicopter rotor must operate during the recovery maneuver.

Optimisation and Analysis of Streamwise-Varying Wall-Normal Blowing in a Turbulent Boundary Layer

Skin-friction drag is a major engineering concern, with wide-ranging consequences across many industries. Active flow-control techniques targeted at minimising skin friction have the potential to significantly enhance aerodynamic efficiency, reduce operating costs, and assist in meeting emission targets. However, they are difficult to design and optimise. Furthermore, any performance benefits must be balanced against the input power required to drive the control. Bayesian optimisation is a technique that is ideally suited to problems with a moderate number of input dimensions and where the objective function is expensive to evaluate, such as with high-fidelity computational fluid dynamics simulations. In light of this, this work investigates the potential of low-intensity wall-normal blowing as a skin-friction drag reduction strategy for turbulent boundary layers by combining a high-order flow solver (Incompact3d) with a Bayesian optimisation framework. The optimisation campaign focuses on streamwise-varying wall-normal blowing, parameterised by a cubic spline. The inputs to be optimised are the amplitudes of the spline control points, whereas the objective function is the net-energy saving (NES), which accounts for both the skin-friction drag reduction and the input power required to drive the control (with the input power estimated from real-world data). The results of the optimisation campaign are mixed, with significant drag reduction reported but no improvement over the canonical case in terms of NES. Selected cases are chosen for further analysis and the drag reduction mechanisms and flow physics are highlighted. The results demonstrate that low-intensity wall-normal blowing is an effective strategy for skin-friction drag reduction and that Bayesian optimisation is an effective tool for optimising such strategies. Furthermore, the results show that even a minor improvement in the blowing efficiency of the device used in the present work will lead to meaningful NES.

Control mechanism of vortex-generator jet on turbulent separation in a highly loaded compressor cascade

The active flow control technique of vortex-generator jet (VGJ) was used to control the turbulent separation of a highly loaded compressor cascade at a high incidence. VGJ schemes on suction surface with different jet locations, skewed angles, pitch angles were numerically performed and the control mechanism of VGJ parameters on the flow field of the cascade was revealed. For avoiding the effect of endwall boundary layer, both endwall surfaces of the cascade were set to translational periodicity boundary (TPB). Results show that VGJ significantly eliminates the turbulent separation and improves the performance of cascade. The overall loss coefficient of cascade is reduced by 52.3% at most. The optimal VGJ location in this study is not separation location but 7% axial chord downstream of it, which is probably due to the ultra-high loading of the cascade. The spanwise location and intensity of jet vortices are mainly affected by skewed angle, and will increase with the skewed angle. The VGJ case with a higher pitch angle features better control effect on suction surface but increases the pitchwise scale of the low-velocity region. In addition, as pitch angle increases, the mixing loss between mainstream and jet flow increases, which brings a negative effect on cascade performance. In order to enhance the momentum transporting between mainstream flow and separation flow, counter vortex-generator jet (CVGJ) schemes were designed and studied. Though CVGJ strengthens the jet vortices near the trailing edge and achieves minimum loss near TPB sections at 150% axial chord section, the interaction of jet vortices with opposite rotating direction brings extra loss, which leads to the control effect of CVGJ is weaker than that of VGJ.

Thermal contraction of the boundary layer from progressive cooling and its possible use as active flow control technique

In this work, consideration is given to thermal contraction of the boundary layer from the progressive cooling of the airfoil and its possible technological use as an active flow control technique. Here, it is thought that if the airfoil is progressively cooled from the leading-edge toward the trailing-edge the increase in density will translate into a progressive contraction of the boundary layer which eventually could be used as technique for either re-attachment or delay detachment of the boundary layer. Utilizing the von Karman momentum integral an analytical expression for the reduction of the thickness of the boundary layer by thermal contraction was derived which predicts contractions around 10 percent for temperatures drops around 50° K. Experimental measurements made on a heated flat plate with controlled thermal gradient agree with the theoretical prediction within a ± 10 per cent. The possibility of prompt contraction of the boundary layer by condensation (of the air or its moisture content) if the temperature drops below the subcooling temperature, is an open question which should be addressed in future research.

Combined passive and active flow control for fixed-wing micro air vehicles

This study presents the design, implementation, and assessment of a combined passive and active flow control technique with the aim of increasing the aerodynamic performance of fixed-wing Micro Air Vehicles (MAVs). Power consumption restrictions in MAVs support the choice of passive flow control solutions such as the use of a modified (tubercled) wing leading edge. This strategy successfully allows to delay and mitigate aerodynamic stall but detrimental effects are found at pre-stall operating conditions. In order to retrieve the lift-generation capabilities of the baseline wing at pre-stall, a subsidiary active flow control method making use of air blowing was designed and installed in the modified wing. Guidance to the selection of optimum settings was provided by experimental and computational analyses. The resulting hybrid flow control system demonstrated its effectiveness, thus producing generalized lift enhancements irrespectively of the attitude of the wing.

Winglets effect on the aerodynamics of aircraft wing; computational studies

Aircraft drag reduction has been of interest to aerospace industry for long time. Therefore, in the past decade different flow control techniques have been used such as active and passive flow control techniques. Winglet design has been explored in the past both experimentally and computationally. Experimental studies pose significant challenges due to the wing-span and costly equipment and facilities. Therefore, this study concerns the computational studies of winglet design using the large-eddy simulation approach. The results show that the winglet design reduces the flow separation and drag force, while the pressure and lift force increases.

A Numerical Analysis of Active Flow Control Techniques for Aerodynamic Drag Reduction in the Square-Back Ahmed Model

Aerodynamic drag reduction is required with new stringent constraints on pollutant emissions and fuel efficiency of ground vehicles. In this context, active flow control is a promising approach to achieve this target. This study focuses on applying different flow control strategies on the square-back Ahmed model to reduce aerodynamic drag. A steady blowing jet, a synthetic jet and an unsteady jet are located at the back edges of the model as flow control devices. A numerical study based on the 3D WMLES simulation was performed to evaluate the drag reduction capabilities of these devices at different operating conditions. The results showed that a maximum aerodynamic drag reduction of 26.51% was achieved when using a steady blowing jet, and 17.27% with an unsteady jet. In contrast, the effect of the synthetic jet on the aerodynamic drag of the model is solely at high control frequencies.

Geometric Optimization of Blunt Bodies with Aerodisk and Opposing Jet for Wave Drag and Heat Reduction

The use of passive, active, or hybrid flow control techniques is often investigated to reduce the acoustic signature, wave drag, and aerodynamic heating associated with the supersonic flow regime. This research explores passive and hybrid flow control techniques to achieve an optimal reduction in wave drag and aerodynamic heating on a blunt body using an aerodisk. While passive techniques use one or two aerospikes, hybrid techniques employ opposing jets and aerospikes. Numerical analysis is performed using Reynolds-Averaged Navier–Stokes (RANS) equations to analyze the bodies’ flow field. The statistical technique, Design of Experiments (DOE), is combined with Response Surface Method (RSM) to find the optimal configuration for four cases by generating design space. Two cases were considered for the optimization: single aerospike with and without opposing jet and double aerospike with and without opposing jet. Variables used for the design of the aerodisks were spike length and diameter, while the response variables were wave drag and normalized heat flux. The current study has established an optimum relationship between spike length and aerospike diameter located in front of the main blunt body for both single and double aerospikes. The study’s results suggest that a double aerodisk configuration is more beneficial for reducing drag and heat flux at supersonic speed than a single aerodisk. By incorporating an opposing jet at a pressure ratio of 0.8 from the frontal aerodisk to the spiked blunt body, it can reduce drag and heat flux by 86% and 95%, respectively. Finally, numerical verification is performed for statistically optimized designs.