Active Flow Control Research Articles

Deep reinforcement learning for active flow control in a turbulent separation bubble

The control efficacy of deep reinforcement learning (DRL) compared with classical periodic forcing is numerically assessed for a turbulent separation bubble (TSB). We show that a control strategy learned on a coarse grid works on a fine grid as long as the coarse grid captures main flow features. This allows to significantly reduce the computational cost of DRL training in a turbulent-flow environment. On the fine grid, the periodic control is able to reduce the TSB area by 6.8%, while the DRL-based control achieves 9.0% reduction. Furthermore, the DRL agent provides a smoother control strategy while conserving momentum instantaneously. The physical analysis of the DRL control strategy reveals the production of large-scale counter-rotating vortices by adjacent actuator pairs. It is shown that the DRL agent acts on a wide range of frequencies to sustain these vortices in time. Last, we also introduce our computational fluid dynamics and DRL open-source framework suited for the next generation of exascale computing machines.

Active flow control for bluff body under high Reynolds number turbulent flow conditions using deep reinforcement learning

This study employs deep reinforcement learning for active flow control in a turbulent flow field of high Reynolds numbers at Re = 274 000. That is, an agent is trained to obtain a control strategy that can reduce the drag of a cylinder while also minimizing the oscillations of the lift. Probes are placed only around the surface of the cylinder, and a proximal policy optimization (PPO) agent controls nine zero-net mass flux jets on the downstream side of the cylinder. The trained PPO agent effectively reduces drag by 29% and decreases lift oscillations by 18% of amplitude, with the control effect demonstrating good repeatability. Control tests of this agent within the Reynolds number range of Re = 260 000 to 288 000 show that the agent's control strategy possesses a certain degree of robustness, with very similar drag reduction effects under different Reynolds numbers. Analysis using power spectral energy reveals that the agent learns specific flow frequencies in the flow field and effectively suppresses low-frequency, large-scale structures. Graphically visualizing the policy, combined with pressure, vorticity, and turbulent kinetic energy contours, reveals the mechanism by which jets achieve drag reduction by influencing reattachment vortices. This study successfully implements robust active flow control in realistically significant high Reynolds number turbulent flows, minimizing time costs (using two-dimensional geometrical models and turbulence models) and maximally considering the feasibility of future experimental implementation.

Lift and stall behavior of a co-flow jet airfoil: a detailed analysis

This study investigates the aerodynamic performance of a co-flow jet (CFJ) airfoil with the objective of enhancing lift and reducing drag through active flow control. National Advisory Committee for Aeronautics 0018 airfoil was employed for the computational fluid dynamics (CFD) study. This work examines the effect of slot placement, coefficient of jet momentum, and CFJ metrics namely injection velocity, injection mass flow rate, and injection height on the variation of aerodynamic coefficients. Additionally, the tradeoff between aerodynamic coefficients and associated power consumption was investigated. The study indicates that positioning injection slots near regions of minimum static pressure on the upper surface of the leading edge increases the coefficient of lift and mitigates power consumption. A maximum improvement in lift coefficient up to 30% was achieved, while the stall margin increased by 25%. The findings also indicate that CFJ configurations with higher jet momentum coefficients exhibit an improvement in lift-to-drag ratios by approximately 40% with a 15% reduction in drag in comparison with those of conventional airfoil. This research underlines the transformative potential of CFJ technology in optimizing aerodynamic performance with minimal energy impact, suggesting promising applications for enhancing aircraft efficiency and sustainability.

Implementation Of Flow Control Methods For Design Of Connecting Duct Of Annular Sector Cascade Tunnel

Abstract The present study discusses the effect of various flow control methods implemented to develop an Intermediate Turbine Duct (ITD), providing a continuous flow path between the contraction duct of the wind tunnel and the annular sector cascade (ASC) test section to deliver the required flow. The S-shaped diverging passage of the ITD with continuously increasing mean radius imposes a major challenge to achieving the required inlet flow angle with periodicity at the entry of the ASC. The numerical study was performed in ANSYS CFX® to analyze the effect of active and passive flow control methods on the ITD exit flow delivered to the ASC test section. It was found that Inlet Guide Vanes (IGVs), incorporated within the ITD as a passive flow control method, direct the flow through ITD, and approximately required inlet flow condition with reasonable periodicity was achieved at the entry of the ASC test section. However, the opening slot on the casing endwall, an active flow control method, shows the adverse effect on the flow angle distribution at the entry of the ASC test section. The change in axial spacing between IGVs and the test vane cascade shows a negligible effect on the cascade entry flow field. The experimental flow field measurement performed using a five-hole pressure probe showed that IGVs successfully directed the flow through the ITD, precisely meeting the requirements at the test section. The surface oil flow visualization confirms the formation of various flow features captured in the computational study.

Improvement of Aerodynamic Performance of Bilaterally Symmetrical Airfoil by Co-Flow Jet and Adaptive Morphing Technology

For a special bilaterally symmetric airfoil (BSA), this paper designs an active flow control scheme based on the Co-Flow Jet (CFJ) and adaptive morphing technology, and establishes a numerical simulation method which is suitable for simulating aerodynamic characteristics. The accuracy and effectiveness of the numerical method has been verified through benchmark cases. This study investigates the effects of jet intensity, suction slot position and angle, and deflection angles of the leading and TE flap on the aerodynamic performance parameters and flow field structure of the bilaterally symmetric airfoil. The results show that the adaptive morphing technology can significantly improve the equivalent lift coefficient and equivalent lift-to-drag ratio of the bilaterally symmetric airfoil, without obviously increasing the CFJ power consumption coefficient. Selecting an appropriate CFJ intensity can achieve a relatively high equivalent lift-to-drag ratio with a low compressor power requirement. Moving the suction slot rearward can increase the lift coefficient, and placing it on the trailing edge (TE) flap can more efficiently delay flow separation, reduce power consumption, and increase the equivalent lift-to-drag ratio. The suction slot angle has little effect on the lift coefficient, but a larger suction slot angle can enhance the equivalent lift-to-drag ratio. Increasing the TE flap deflection angle enhances both the lift coefficient and drag coefficient, as well as the power consumption coefficient at high angles of attack. But it has little effect on the maximum equivalent lift-to-drag ratio. Increasing the leading edge flap deflection angle can improve the maximum equivalent lift-to-drag ratio while increasing the angle of attack corresponding to it. Overall, choosing a CFJ and adaptive morphing parameters by considering different factors can enhance the aerodynamic performance of the bilaterally symmetric airfoil.

Near-Field Aeroacoustics of Spanwise Forcing on a Transonic Wing: A DNS Study

The transonic airflow around a supercritical wing with a shock wave is described via direct numerical simulations. Flow control for turbulent drag reduction is applied via streamwise traveling waves of spanwise velocity applied on a finite portion of the suction side. The near-field modifications caused by the forcing are studied via the analysis of the wake profile downstream of the trailing edge. Moreover, for the first time, the effects of spanwise forcing on aeroacoustic noise are considered to establish whether active flow control for drag reduction could possibly increase noise. By extracting the acoustic signals on a circumference placed in the near-field around the wing and by studying them in terms of sound intensity and frequency content, it is found that noise intensity is not significantly increased by spanwise forcing and that frequency content is only minimally altered. Furthermore, if the angle of attack is reduced to take into account the increased lift and the reduced drag made possible by the control action, changes in the noise characteristics become negligible.

An active torque model for regulating tuna finlets

Finlets, characterized by their series of small triangular fins, represent a notable adaptation observed in tuna and other scombrid fish renowned for their remarkable high-speed swimming abilities. This study focuses on elucidating the control mechanism underlying the pitching motion of finlets by establishing and numerically analyzing both passive and active finlet models through coupled simulations. While the passive model demonstrates qualitative similarities with experimental data, discernible disparities highlight the necessity for active control mechanisms to fully understand and replicate the intricate dynamics of finlet motion. An active torque model is introduced, derived from the correlation between the necessary torques and the observed experimental pitching motions. The results of this active model demonstrate good alignment with the prescribed model, confirming its effectiveness in reproducing responses similar to those of the muscles attached to the finlet root. These findings provide quantitative validation for the active control mechanism that regulates tuna finlets, thus elucidating the underlying mechanics of the fluid. Moreover, they offer valuable insights for advancing research in active flow control, not only in tuna but also in other biomimetic propulsion systems.

Time-resolved deep reinforcement learning for control of the flow past an airfoil

The current work proposes a method for the active control of flow over a National Advisory Committee of Aeronautics 0012 airfoil under turbulent condition based on time-resolved deep reinforcement learning (DRL). To leverage the coherent structures in the temporal evolution of the flow field, we integrate the long short-term memory (LSTM) network with the proximal policy optimization (PPO) method. Based on this LSTM-PPO method, the model obtained an improved strategy for controlling the mass flow rates of the three jets located on the upper surface of the airfoil to control the flow and increase the lift-to-drag ratio. The LSTM-PPO method is first compared with the traditional PPO method at Re = 2500, achieving a remarkable 160.9% enhancement of the lift-to-drag ratio. Then, the trained LSTM-PPO model is tested under several operation conditions, manifesting its adaptability. Dynamic mode decomposition is also used to study the change in the dynamics with and without the active flow control (AFC) based on the LSTM-PPO method. This study highlights the potential of recurrent neural networks to cooperate with DRL algorithms, paving the way for real-world applications of AFC.

Flow Control over a Generic Tailless Chined Forebody Delta Wing

Active flow control via finite-span synthetic jet (SJ) actuators was used to affect the aerodynamic loads on a half model of a chined forebody delta wing. Three different SJ orientations were explored, employing surface-normal SJs, horizontal SJs, or SJs angled 45 deg away from the leading edge relative to the normal direction. Six cases were explored, one with each jet individually actuated and one with both jets actuated together, each with and without pulse-modulation waveform having a modulation frequency corresponding to the helical mode frequency. In all cases, the synthetic jets were activated with Cb =1.667 (Cμ =7.15*10−5 per jet). Aerodynamic load measurements were conducted to explore the effects of flow control, along with detailed flowfield measurements using SPIV to shed light on the reasons for these effects. It was found that the surface-normal SJs had a much larger effect on the aerodynamic coefficients than the other two SJ orientations. This resulted in an increase in nose-down pitching moment, increased lift and increased drag, as well as reduced wandering of the chine and wing vortices. In addition, pulse modulation made the actuation much more energy-efficient as the modulation caused the helical mode to lock onto the actuation frequency.

Transition of counter-rotating plasma-induced vortices into a jet

Two low-frequency-modulated plasma actuators are symmetrically positioned on either side of a thin flat plate to continuously generate pairs of counter-rotating vortices, with velocity fields captured using time-resolved particle image velocimetry (PIV). Convolutional neural networks with a U-Net architecture are adopted to generate the phase-averaged velocity field of plasma-induced vortices from a single PIV snapshot to eliminate the requirement of multiple measurements. The influence of the number of input features and samples on the model accuracy is examined. The model-predicted results match well with the measurements, accurately restoring the vortex dynamics and capturing the strength variation. The proposed model is then utilized to reconstruct the phase-averaged results of the starting vortex for continuous plasma jets, revealing that the evolution of plasma-induced vortices and their transition into a jet are characterized by four distinct stages: formation, boosting, distortion, and jetting, all governed by the vortex convection velocity. The vortices exhibit a marked increase in circulation after a delayed development, moving linearly downstream while diverging from the centerline. Consequently, the vortex-induced effect significantly enhances the centerline velocity. The vortices are subsequently stretched and distorted from a circular into a chain-like shape due to the large velocity gradient between the vortex pair, leading to the vortex breakdown and transition into a jet, accompanied by a collapse in the velocity magnitude. Insights into vortex-to-jet transition inform the optimal placement of plasma actuators, thereby enhancing control efficiency in active flow control applications.

Transformer-based in-context policy learning for efficient active flow control across various airfoils

Transformer-based in-context policy learning for efficient active flow control across various airfoils

Wind Turbine Enhancement via Active Flow Control Implementation

The present research enhances the efficiency of an airfoil section from the DTU-10MW Horizontal Axis Wind Turbine (HAWT) via Active Flow Control (AFC) implementation and when using synthetic jets (SJ). The flow around two airfoil sections cut along the wind turbine blade and for a wind speed of 10 m/s is initially simulated using the CFD-2D-RANS-Kω-SST turbulence model, from where the time-averaged boundary layer separation point and the associated vortex shedding frequency are obtained. On a second stage of the paper, and considering one of the two airfoil sections, the boundary layer separation point previously determined is used to locate the SJ groove as well as the groove width; the three remaining AFC parameters, momentum coefficient, jet inclination angle, and jet pulsating frequency, are parametrically optimized. Thanks to the energy assessment presented in the final part of the paper, the study shows that a considerable power increase of the airfoil section can be obtained when attaching the former separated boundary layer. The extension of the optimization process to the rest of the blade sections where the boundary layer is separated would lead to an efficiency increase of the HAWT. The Reynolds numbers associated to the respective airfoil sections analyzed in the present manuscript are Re = 14.088×106 and Re = 14.877×106, the characteristic length being the corresponding chord length for each airfoil.

Advances in Flow Control Methods for Pump-Stall Suppression: Passive and Active Approaches

This article provides a comprehensive review of key approaches to suppressing stall flow in pumps, offering insights to enhance pump performance and reliability. It begins by outlining the formation mechanisms and characteristics of stalls, followed by an in-depth analysis of various stall types. The discussion highlights passive and active flow control methods, emphasizing their roles in suppressing stall phenomena. Passive flow-control strategies, including surface roughness, grooves, obstacles, fixed guide vanes, and vortex generators, are examined with a focus on their mechanisms and effectiveness in suppressing stall. Similarly, active flow-control techniques, such as jets and adjustable guide vanes, are explored for their capacity to regulate the flow field and suppress stall. The novelty of this review lies in its exploration of the effectiveness of passive and active flow-control methods in suppressing pump stall, with a focus on their mechanisms of action and the underlying principles of stall formation. The findings reveal that appropriate flow-control measures can mitigate laminar flow separation and reduce performance losses associated with stall. However, careful attention must be given to the optimal arrangement of control devices. Finally, the article highlights the limitations of current implementations of combined active and passive flow-control methods while offering insights into the future potential of advanced flow-control technologies in regard to suppressing stall.

Synthetic Jet Actuators for Active Flow Control: A Review

A synthetic jet actuator (SJA) is a fluidic device often consisting of a vibrating diaphragm that alters the volume of a cavity to produce a synthesized jet through an orifice. The cyclic ingestion and expulsion of the working fluid leads to a zero-net mass-flux and the transfer of linear momentum to the working fluid over an actuation cycle, leaving a train of vortex structures propagating away from the orifice. SJAs are a promising technology for flow control applications due to their unique features, such as no external fluid supply or ducting requirements, short response time, low weight, and compactness. Hence, they have been the focus of many research studies over the past few decades. Despite these advantages, implementing an effective control scheme using SJAs is quite challenging due to the large parameter space involving several geometrical and operational variables. This article aims to explain the working mechanism of SJAs and provide a comprehensive review of the effects of SJA design parameters in quiescent conditions and cross-flow.

Design optimization and active control of unsteady flow in large-scale annular linear induction pumps

This study addresses the issue of unstable flow in large-scale annular linear induction pumps (ALIPs), with a focus on optimizing their design and enhancing performance. Utilizing the ALIP model developed by Toshiba Corporation as a reference, the design process employs the equivalent circuit method to improve the hydraulic performance of high-flow ALIP systems. A comparison of various hydraulic and excitation structure parameters facilitated the identification of an optimal design scheme. A numerical simulation of the ALIP’s internal magneto-fluid coupling field was then conducted, based on magnetohydrodynamic (MHD) theory. The simulation results were validated against experimental data, confirming the model’s accuracy. Further simulations under various operational conditions were performed to analyze the distribution and magnitude of the axial Lorentz force (FL) and the axial pressure gradient across different flow rates and currents. The analysis indicated that the unstable flow primarily results from inverse pressure gradients, which are caused by the uneven distribution of these forces. To mitigate this issue, the study proposes the addition of a regulating coil winding to the inner stator. This addition significantly reduces the uneven distribution of magnetic fields and pressure gradients. These coils generate a compensating magnetic field that enhances FL within the electromagnetic section, thereby improving the axial force on the magnetic fluid. The results demonstrate that this active regulation method markedly reduces unsteady flow phenomena, stabilizes fluid movement, and offers a novel design strategy for large-scale ALIP systems. The ratio of the area of the regulating coils to that of the driving coils is only 0.33, which minimally increases the pump dimension. Additionally, the energy conversion rate of different regulation currents between the inner and outer regulation coils was compared. It was found that variations in the regulation current alter the total efficiency of the ALIP by no more than 1%, indicating that the control coil winding consumes minimal energy and that the stability of the magnetic fluid can be effectively controlled, making this approach feasible for engineering applications.

Optimal DSSC’s deployment in power system using PSO

Owing to the high cost of installation and operation, distributed flexible AC transmission system (FACTS) technology gives opportunity to provide cost-effective solution in power system operation and control. A distributed static series compensator (DSSC) is a series FACTS device and it is placed equidistantly placed on the existing line to vary the line current. Active power can be controlled in the line with the help of DSSC. This paper presents DSSC for active power flow control to enhance system loadability index and to minimize reactive power generation by generators. Since DSSC is of low power. a small value of reactance is emulated in the line. Large number of DSSC’s are distributed along the transmission line at regular intervals to realize considerable change in the current. A particle swarm optimization is implemented to determine DSSC’s emulated reactance optimally. In this condition, all the lines flowing power in their limits with increased loading condition. Maximum system loadability index is evaluated by employing optimal number of DSSC’s on the line. A multiobjective problem is formulated. One objective function is formed such that no line would become overloaded even when loading is increased. Other objective is formulated to minimize reactive power generation by generators. A compromise solution is investigated for optimally connected of DSSC devices to achieve both the objective functions. IEEE 14 bus system is taken for MATLAB simulation of DSSC compensated system.

Mitigation of train-tunnel aerodynamics through active airflow control at front and rear noses: Impact of slit area

As railway transportation advances toward higher speeds, traditional passive measures may struggle to meet the stringent aerodynamic criteria in tunnels, necessitating the exploration of novel active flow control techniques. This study employs three-dimensional, compressible, unsteady Reynolds-averaged Navier–Stokes simulations to investigate the aerodynamic effects of the suction and blowing slit area (S) positioned on the front and rear noses of the train. The results indicate that suction and blowing activation is particularly effective in alleviating pressure on the narrower side of the tunnel. Specifically, with a 4 m2 slit, the original 4.8% pressure difference between symmetrical points on the train body is fully eliminated. The influence of suction and blowing on the positive pressures is confined to the front and rear noses where the slits are located. Notably, only suction at the front nose mitigates pressure gradients, while blowing at the rear is unrelated. The peak-to-peak pressure (ΔP) on both the train surface and tunnel wall exhibits a linear decline, with reductions of 17.4% and 16.6%, respectively, as S increases from 0 to 4 m2. Similarly, the slipstreams on both sides of the tunnel decrease linearly with increasing slit area: with u/Umax = −0.008S + 0.24 for the near side, and u/Umin = 0.014S − 0.265 for the far side. Additionally, expanding the slit area further boosts the stability and safety of the train during tunnel exit by reducing lateral forces and rolling moments, while also decreasing overall drag, thereby partially compensating for the energy input. Although the maximum lift on the head car increases with slit area, the lift on the tail car initially rises and then decreases, helping to mitigate instability upon tunnel exit. Overall, the hybrid suction and blowing technique offers promising potential for enhancing the tunnel aerodynamics in the future.

Active control of wake-induced vibration using deep reinforcement learning

Wake-induced vibration (WIV) is a typical type of flow-induced vibration. Effectively controlling such vibration is of significant value in engineering fields. In this study, we focus on the feasibility, effectiveness, and efficiency of the deep reinforcement learning (DRL)-guided active flow control for WIV control. Here an elastically mounted circular cylinder is interfered by the wake of an upstream equal-size cylinder at Reynolds number 100. With different center-to-center in-line distances, the unwanted vibration is noted to be more complicated than the vortex-induced vibration, which is then controlled by the rotary control with sensory motor cues as feedback signals. The control strategy is established by the DRL and is trained in the numerical environment built upon the lattice Boltzmann solver. For the tandem configuration, the DRL learns effective control strategies that can control the vibration amplitude by 99.7%, 99.2%, and 95.7%, for the cases with nondimensionalized gap length of 2, 6, and 8, respectively. Both time-averaged flow fields and vortex dynamics are discussed, revealing that the DRL-guided control learns different control strategies for different gap spacing. With the successfully learned strategy in tandem configuration, the WIV in staggered configuration is further explored based on the transfer learning. The vibration amplitudes of all cases in the staggered configuration are mitigated by more than 97%. To conclude, this study confirms that the DRL is effective in situations involving strong wake interference. It is anticipated that the DRL can provide a general solution for controlling flow-induced vibration.

Model-Free Closed-Loop Control of Flow Past a Bluff Body: Methods, Applications, and Emerging Trends

Flow past one or multiple bluff bodies is almost ubiquitous in nature and industrial applications, and its rich underlying physics has made it one of the most typical problems in fluid mechanics and related disciplines. The search for ways to control such problems has attracted extensive attention from both the scientific and engineering fields, as this could potentially bring about benefits such as reduced drag, mitigated noise, suppressed vibration, and enhanced heat transfer. Flow control can be generally categorized into passive and active approaches, depending on whether there is an external energy input to the flow system. Active control is further divided into open-loop approaches and closed-loop approaches, depending on whether the controller depends on feedback signals extracted from the flow system. Unlike in many other applications of passive flow control and open-loop active flow control, theoretically advantageous closed-loop controls are quite rare in this area, due to the complicated features of flow systems. In this article, we review the recent progress in and future perspectives of flow past a single or multiple bluff bodies using model-free closed-loop control so as to outline the state-of-the-art research, determine the physical rationale, and point to some future research directions in this field.

Genetically-based active flow control of a circular cylinder wake via synthetic jets

Genetically-based active flow control of a circular cylinder wake via synthetic jets