Steady Actuation Research Articles

Thermal effect on the flow induced by a single-dielectric-barrier-discharge plasma actuator under steady actuation

The thermal effect of a single-dielectric-barrier-discharge plasma actuator under steady actuation is numerically investigated. A new actuator model is proposed and validated using experimental data. A discrete Galerkin method based on high-order flux reconstruction schemes is employed to solve the flow governing equations and the actuator model equations on unstructured quadrilateral grids. By comparing the induced heated and cold flow fields of the actuator with and without a plasma thermal source, its thermal effect is revealed. The actuator generates a thermal wall jet with rich vorticity, forming a monopolar starting vortex with a high-temperature and low-density core. Over time, the starting vortex becomes unstable and transforms into a dipole. Actuator heating enhances jet velocity and width, as well as vortex stability, while slowing down vorticity generation. The relative change in density and temperature fields due to actuator heating is four orders of magnitude greater than that without actuator heating. Additionally, the actuator heating causes the background thermodynamic fields to increase approximately linearly with time. Two stages in the actuator's thermal effect are distinguished due to time accumulation. Initially, the actuator heating minimally affects the monopolar starting vortex motion, and the temperature and density fields are treated as passive variables driven by the velocity field. During this stage, the momentum and thermal effects of the actuator can be studied separately. However, after the starting vortex becomes unstable, the actuator heating significantly impacts its motion and morphology, and these two effects are coupled with each other.

Corner Separation Control Using Sweeping Jet Actuator in a Compressor Cascade

Abstract In the pursuit of advancing active flow control (AFC) technology, a more promising nonsteady actuator known as sweeping jet actuator (SJA) has been equipped on the endwall to manage the corner separation in a compressor cascade, while the steady jet actuators with holes were used for comparison. Experimental investigations have been conducted in a low-speed wind tunnel with an inflow Mach number of 0.23. Five-hole probe measurements and the oil flow visualizations were carried out to demonstrate the performance and physics of steady and unsteady blowing on controlling the corner separation. The transient data at the measurement plane were also obtained using the dynamic pressure sensors to get insight into the unsteady characteristics. Variation of jet momentum of both actuators and the excitation frequency of the SJA allows determination of favorable control parameters. Using the SJA with only 0.13% of the cascade mass flow, the total pressure loss which takes the additional energy input into account is reduced by 14.2%, while the steady jet achieves a reduction of 8.4%. The results highlight the superior characteristics of the SJA in controlling the corner separation.

Closed-loop plasma flow control of a turbulent cylinder wake flow using machine learning at Reynolds number of 28 000

Machine learning is increasingly used for active flow control. In this experimental study, alternating-current dielectric barrier discharge plasma actuators are deployed for the closed-loop intelligent control of the flow around a cylinder at a Reynolds number of 28 000 based on the velocity feedback from two hot-wire sensors placed in the wake. Variations in the cylinder drag are monitored by a load cell, and the temporal response of the wake flow field is visualized by a high-speed particle image velocimetry system working at 1 kHz. The high-speed control law is operated using a field programmable gate array optimized by genetic programing (GP). The results show that the peak drag reduction achieved by machine learning is of similar magnitude to that of conventional steady actuation (∼15%), while the power saving ratio is 35% higher than with conventional techniques because of the reduced power consumption. Analysis of the best GP control laws shows that the intensity of plasma actuation should be kept at a medium level to maximize the power-saving ratio. When compared with the baseline uncontrolled flow, the best controlled cases constrain the meandering motion of the cylinder wake, resulting in a narrow stabilized velocity deficit zone in the time-averaged sense. According to the results of proper orthogonal decomposition and dynamic mode decomposition, Karman vortex shedding is promoted under the best GP control.

Aerodynamic Performance and Leakage Flow in Turbine Cascades With Sweeping Jet Actuators

Abstract In recent years, unsteady flow-control technology has been considered as more promising for reducing secondary flow loss in turbines. Sweeping jet actuators (SJAs), in particular, have steadily been used in turbomachinery to reduce loss due to their wide sweeping range. However, most of the preliminary studies were focused mainly on computational fluid dynamics calculations, with very few on wind tunnel experiments to verify the flow-control effectiveness. To fill this gap, an innovative attempt to investigate experimentally the influences of SJAs on the aerodynamic performance and leakage flow is presented in a high-pressure turbine cascade. Comparing SJAs and hole-type steady jet actuators (HSJAs), the influence on the tip leakage flow and the loss characteristics at various incidences and injection frequencies are discussed in detail. The results indicate that the SJAs effectively reduce the magnitude and extension of the leakage vortex and the passage vortex, and weaken the interactions between them. The loss at the cascade outlet drops by up to 10.3% when the jet flow of an SJA equals 0.2% of the inlet flow, which is significantly better than that with an HSJA (6.3%).

Stabilization of a multi-frequency open cavity flow with gradient-enriched machine learning control

We stabilize an open cavity flow experiment to 1 % of its original fluctuation level. For the first time, a multi-modal feedback control is automatically learned for this configuration. The key enabler is automatic in situ optimization of control laws with machine learning augmented by a gradient descent algorithm, named gradient-enriched machine learning control (Cornejo Maceda et al., J. Fluid Mech., vol. 917, 2021, A42, gMLC). The physical interpretation of the feedback mechanism is assisted by a novel cluster-based control law visualization for the flow dynamics and corresponding actuation commands. Starting points of the control experiment are two unforced open cavity benchmark configurations: a narrow-bandwidth regime with a single dominant frequency and a mode-switching regime where two frequencies compete. The flow is forced by a dielectric barrier discharge actuator located at the leading edge and is monitored by a downstream hot-wire sensor over the trailing edge. The feedback law is optimized with respect to the monitored fluctuation level. As reference, the self-oscillations of the mixing layer are mitigated with steady actuation. Then, a feedback controller is optimized with gMLC. As expected, feedback control outperforms steady actuation by achieving a better amplitude reduction with approximately 1 % of the actuation energy required for similarly effective steady forcing. Intriguingly, optimized laws learned for one regime perform well for the other untested regime as well. The proposed control strategy can be expected to be applicable for many other shear flow experiments.

Flow induced by a pair of plasma actuators on a circular cylinder in still air under duty-cycle actuation

A pair of surface mounted dielectric-barrier-discharge (DBD) plasma actuators is placed diametrically on a circular cylinder model. The flow field generated by the actuators in steady and duty-cycle actuation modes is measured by particle image velocimetry. Salient features of the flow and vortices are discovered for the duty-cycle actuation modes with frequency ranging from 5 to 1000 Hz and duty-cycle ratio from 0.01 to 0.99. The results are compared with those for continuous steady actuation. For a given duty-cycle ratio, a discrete vortex is generated by each of the DBD actuator in an actuation cycle for low frequencies. Such vortices are created alternately from each side of the cylinder. They interact and move laterally in response to the on-off signals of the actuators. As the frequency increases, the distance between the vortices generated from successive duty-cycles decreases and the lateral motion of the vortices becomes smaller so that the vortices are packed together as a vortex train along a narrow path on each side of the cylinder. As the frequency is further increased, the vortices in the vortex trains lose their individual characteristics and the vortex trains become steady jets. Regardless of the actuation frequency, the direction of the average total momentum of the air is found to be continuously dependent on the duty-cycle ratio. The magnitude of the average total momentum depends on the frequency. An optimum frequency exists for maximum average momentum, which may exceed that in the continuous steady actuation mode.

Effects of combined sweeping jet actuator and winglet tip on aerodynamic performance in a turbine cascade

Compared with steady injection, the unsteady flow control techniques have received more attention in recent years and are considered to be more hopeful for reducing secondary flow losses in turbine stages. In particular, the sweeping jet actuator (SJA) is gradually being applied to reduce losses in turbomachinery due to its larger sweeping range. The objective of this study is to investigate the effect of combined flow control of SJA and the winglet tip structure on the aerodynamic performance and tip leakage flow of the turbine cascade through experimental and numerical methods. The effect of blade tip geometry on aerodynamic performance enhancement is discussed in detail by comparing flat and groove tips. The effect of injection flow on aerodynamic performance and secondary flow of turbine cascade with different blade tip structures is discussed by comparing SJA and hole-style steady jet actuator (HSJA). Compared with the flat tip without injection case, the combination of the winglet tip and SJA results in a maximum reduction of 20.1% in the total pressure loss at the cascade outlet, which is significantly better than the combination of the winglet tip and HSJA (16.2%). The results of the validated CFD calculations are also reported to help analyze the flow field and secondary flow loss mechanisms.

Numerical Simulation of Fully Coupled Flow-Field and Operational Limitation Envelopes of Helicopter-Ship Combinations

Landing a helicopter to the ship flight deck is most demanding even for the most experienced pilots and modeling and simulation of the ship-helicopter dynamic interface is a substantially challenging technical problem. In this paper, a coupling numerical method was developed to simulate the fully coupled ship-helicopter flow-field under complete wind-over-deck conditions. The steady actuator disk model based on the momentum source approach and the resolved blade method based on the moving overset mesh method were employed to model the rotor. Two different ship-helicopter combinations were studied. The helicopter flight mechanics model was established and then the influences of coupled airwake on the helicopter were analyzed. Finally, based on the derived rejection criterion of safe landing and the developed numerical method, the flight envelopes for these two ship-helicopter combinations were predicted. The steady actuator disk model was found to be effective in the study of helicopter operations in the shipboard environment. The calculated flight envelopes indicate that an appropriate wind direction angle is beneficial to increasing the allowable maximum wind speed and the operating boundary is affected by the rotation direction of the main rotor.

Parallel-in-time adjoint-based optimization – application to unsteady incompressible flows

Gradient-based optimization algorithms, where gradient information is extracted using adjoint equations, are efficient but can quickly slow down when applied to unsteady and nonlinear flow problems. This is mainly due to the sequential nature of the algorithm, where the primal problem is first integrated forward in time, providing the initial condition for the adjoint problem, which is then integrated backward. In order to address the sequential nature of this optimization procedure parallel-in-time algorithms can be employed. However, the characteristics of the governing equations of interest in this work, and in particular, the divergence-free constraint (incompressibility effect) as well as the nonlinearity and the unsteadiness of the flow, make direct application of existing parallel-in-time algorithms less than straightforward. In this work, we introduce a parallel-in-time procedure, applied to the integration of the adjoint problem, which addresses all the existing constraints and allows quick access to local gradients. The performance of the proposed algorithm is assessed for both steady and unsteady actuation; in both cases it readily outperforms the sequential algorithm.

Numerical simulation of dynamic stall flow control using a multi-dielectric barrier discharge plasma actuation strategy

To alleviate the deterioration in wind turbine performance caused by dynamic stall, the flow control of a pitching NACA0012 airfoil is investigated through numerical simulation of an alternating current dielectric barrier discharge (AC-DBD) plasma actuator at a Reynolds number Re = 135 000. To avoid the harmonic oscillations of aerodynamic force caused by unsteady DBD actuation, this work focuses on improving the control potential for steady actuation. The control mechanisms of actuators at various positions are investigated using five groups of actuators mounted at 0%, 3%, 10%, 45%, and 80% chord lengths c above the upper surface of an airfoil. The actuator at 80%c performs more efficiently in terms of lift enhancement in the initial upstroke and the final downstroke. The actuator at 0%c suppresses the growth of the leading-edge vortex and maintains the suction of the dynamic stall vortex (DSV). After the shedding of the DSV, it suppresses the secondary separation to delay the onset of dynamic stall. At the flow reattachment stage, the actuators at 3%c and 10%c accelerate the boundary layer reattachment by momentum injection. From these results, a multi-DBD control strategy is proposed. The scheme selects the optimal actuator in operation at a certain stage of dynamic stall and takes advantage of actuators at different positions to enhance the average and maximum aerodynamic force, delay the onset of dynamic stall, accelerate flow reattachment, and avoid excessive energy consumption.

Electrohydrodynamic performance improvement using different actuation modes in a channel: A CFD simulation study

• Effect of unsteady EHD actuation modes is numerically investigated. • Performance of the unsteady actuation is compared with the steady actuation. • Using unsteady EHD actuation significantly decreases energy consumption. • Using unsteady EHD actuation leads to reducing the friction factor. • The EHD efficiency increases by reducing the time duration of the actuation. In the present work, the effect of periodic Electrohydrodynamic (EHD) actuation modes on enhancing heat transfer and cutting down energy consumption in a channel is numerically investigated. For this purpose, a simple duty-cycled voltage is applied to the wire. Moreover, the performance of the periodic actuation is compared with the steady actuation. A descriptive EHD efficiency is used to evaluate the effect of the periodic EHD actuation on heat transfer enhancement and electric power consumption. The results show that reducing the time duration of the actuation in the periodic mode leads to reducing the friction factor, heat transfer efficiency, and energy consumption. The results also show that using the periodic actuation for Reynolds numbers of 500 and 1000 compared to the steady actuation case increases the performance evaluation criterion and reduces energy consumption by 70% and 50%, respectively. The results indicate that compared to the steady EHD actuation, the energy consumption of periodic EHD with the time duration of the actuation of 0.1 s is reduced by 90%, while the heat transfer efficiency decreases by 32.05% at Re = 500. Consequently, the EHD efficiency increases by reducing the time duration of the actuation and the Reynolds number. Furthermore, the results indicate that replacing one wire in steady actuation with two wires in periodic actuation improves heat transfer or decreases friction factor without changing the energy consumption.

A comparative study on post-stall flow separation control mechanism of steady and unsteady plasma actuators

In order to improve the aerodynamic performance of the wing at post-stall conditions, the experimental comparative investigations on the flow separation control over an ONERA 212 airfoil using steady and unsteady plasma actuators are carried out at Reynolds number of 3.1 × 105. The duty cycle ratio is fixed at 80%, and the non-dimensional unsteady frequency F+ is varied from 0.04 to 1. The lift coefficients are increased by 39.6% and 66%, respectively, after steady and unsteady operations (F+ = 0.08) at an angle of attack of 18°, which indicates that the unsteady actuation is more efficient than steady operation. Meanwhile, the study provides new insight into understanding the post-stall separation flow controlling mechanism. First, different from the general view that the injection of momentum is the controlling mechanism of steady operation, flow control using the steady actuation experiences four stages, namely, flow separation, promoting the instability of the separated shear layer to produce large-scale spanwise vortices, flow re-attachment, and the continuous generation of small-scale vortices in the separated shear layer. Second, flow control with the unsteady operation consists of several quasi-periodic flow processes. Each quasi-cycle is composed of three stages, namely, flow separation, promoting the separation of shear layer instability to produce large-scale spanwise vortices, and flow re-attachment. The off-time of the plasma actuator plays an important role in realizing the control effect of the unsteady actuation, and an effective strategy to promote the control effect of the unsteady operation is proposed based on the propagation time of the induced spanwise vortex.

Binary Repetitive Model Predictive Active Flow Control Applied to an Annular Compressor Stator Cascade With Periodic Disturbances

Abstract Novel pressure gain combustion concepts invoke periodic flow disturbances in a gas turbine's last compressor stator row. This contribution presents studies of mitigation efforts on the effects of these periodic disturbances on an annular compressor stator rig. The passages were equipped with pneumatic active flow control (AFC) influencing the stator blade's suction side, and a rotating throttling disk downstream of the passages inducing periodic disturbances. For steady blowing, it is shown that with increasing actuation amplitudes cμ, the extension of a hub corner vortex deteriorating the suction side flow can be reduced, resulting in an increased static pressure rise coefficient Cp of a passage. The effects of the induced periodic disturbances could not be addressed intrinsically, by using steady blowing actuation, Considering a corrected total pressure loss coefficient ζ*, which includes the actuation effort, the stator row's efficiency decreases with higher cμ due to the increasing costs of the actuation mass flow. Therefore, a closed-loop approach is presented to address the effects of the disturbances more specifically, thus lowering the actuation cost, i.e., mass flow. For this, a repetitive model predictive control (RMPC) was applied, taking advantage of the periodic nature of the induced disturbances. The presented RMPC formulation is restricted to a binary control domain to account for the used solenoid valves' switching character. An efficient implementation of the optimization within the RMPC is presented, which ensures real-time capability. As a result, Cp increases in a similar magnitude but with a lower actuation mass flow of up to 66%, resulting in a much lower ζ* for similar values of cμ.

Drag reduction of circular cylinder using linear and sawtooth plasma actuators

The flow around a single circular cylinder modified by the Dielectric Barrier Discharge (DBD) plasma actuator is investigated experimentally at a Reynolds number Re = 5.25 × 103. The main purpose of the present study is to establish a comprehensive understanding of the effects of linear and sawtooth DBD plasma actuators (under steady and unsteady actuation modes, with different applied voltage Va, normalized modulation frequency F+, and duty cycle DC) on the time-averaged drag coefficient CD and flow characteristics of the cylinder. Extensive measurements are performed in the near wake of the cylinder using a load cell, hotwire anemometer, flow visualization, and particle image velocimetry techniques. The maximum drag reductions are 58% and 22.8% under the steady and unsteady actuation modes of the linear actuators, respectively, when a voltage of Va = 11 kV is applied. On the other hand, the maximum drag reductions achieved by the steady and unsteady actuation modes of the sawtooth actuators are 51.8% and 53.0%, respectively. The corresponding power efficiencies are 1.6%, 0.9%, 1.4%, and 2.1%, respectively. A detailed examination of the near wake dynamics unveils that the mechanisms behind the substantial drag reductions are quite different for the two different actuators, which account for the difference in drag reduction and efficiency.

A high-performing sawtooth plasma actuator with multi-electrodes

The present study for the first time develops a high performing sawtooth plasma actuator (PA) that may produce large flow velocity u and streamwise vorticity ωx′ by means of increasing the number of electrodes. This improved performance is crucial for improving the post-stall airfoil aerodynamics. It is found that the high performing sawtooth PA with three encapsulated electrodes powered by +Vdc (hereafter called the optimum sawtooth PA) produces the maximum ωx′ and u of 650 s−1 and 6.83 m/s, respectively. The plasma-induced flow is linked to the acceleration of the negatively charged species in the streamwise direction caused by the +Vdc-powered encapsulated electrodes and is 26.5% larger than that produced by a standard sawtooth PA (with two sawtooth electrodes) under the same power consumption. At a chord Reynolds number Re=1.0×105, the optimum sawtooth PA under steady mode actuation postpones the stall angle-of-attack α of a NACA 4412 airfoil by 4° and increases the maximum lift coefficient CL by 8.1%; the former is appreciably larger than that (0°) achieved by the standard sawtooth PA. Furthermore, under the optimum burst mode actuation (non-dimensional burst frequency F+=1.4 and duty cycle DC=7%), the stall α and the maximum CL increase by 5° and 19.6%, respectively, along with a 93% power saving. The physical mechanisms behind the impressive performance are discussed.

Extremum Seeking Control Applied to Airfoil Trailing-Edge Noise Suppression

Extremum seeking control (ESC) and its slope seeking generalization are applied in a high-fidelity flow simulation framework for reduction of acoustic noise generated by a NACA0012 airfoil. Two Reynolds numbers are studied for which different noise generation mechanisms are excited. For a low Reynolds number flow, the scattering of vortex shedding at the airfoil trailing edge produces tonal noise, while for a moderate Reynolds number case, boundary-layer instabilities scatter at the trailing edge, leading to noise emission at multiple tones superimposed on a broadband hump. Different control setups are investigated, and they are configured to either find an optimal steady actuator intensity or an optimal position for a blowing/suction device. Implementation details are discussed regarding the control modules and design of digital filters. Analyses of physical phenomena as well as of relevant behavior of the actuated plant are conducted to understand how the extremum seeking loop leverages the flow physics to control noise. Depending on the flow configuration studied and the control setup, results demonstrate that the ESC can provide considerable airfoil noise reductions.

Experimental study on dynamic stall control based on AC-DBD actuation

To explore AC-DBD’s ability in controlling dynamic stall, a practical SC-1095 airfoil of a helicopter was selected, and systematic wind tunnel experiments were carried out through direct aerodynamic measurements. The effectiveness of dynamic stall control under steady and unsteady actuation is verified. The influence of parameters such as constant actuation voltage, pulsed actuation voltage, pulsed actuation frequency and duty ratio on dynamic stall control effect is studied under the flow condition of k = 0.15 above the airfoil, and the corresponding control mechanism is discussed. Steady actuation can effectively reduce the hysteresis loop area of dynamic lift, and control the peak drag and moment coefficient. For unsteady actuation, there is an optimal duty ratio DC = 50%, which has the best effect in improving the lift and drag characteristics, and there is a threshold of pulsed actuation voltage in dynamic stall control. The optimal dimensionless frequency will not be found; different F + have different control advantages in different aerodynamic coefficients of different pitching stages. Unsteady actuation has obvious control advantages in improving the lift-drag characteristics and hysteresis, while steady actuation can better control the large nose-down moment.

Analysis of Separation Control Authority of DBD Plasma Actuator using Hot-Film Sensor Array

The boundary-layer control authority of a DBD plasma actuator using surface mounted hot-film sensors is evaluated. Wind tunnel experiments on a wind-turbine blade section were established at a Reynolds number of 0.27 × 106. Aerodynamic performance of the wind-turbine blade section for both plasma-ON and plasma-OFF modes are evaluated using measurements made by both surface pressure and wake survey behind the model. Two distinct boundary-layer states are recognized. A state which occurs at the onset and in proximity of the deep stall, which is affected by the low-frequency instabilities of the separated flow. In this case, the steady actuation of plasma imparts local momentum on the nearby flow, eliminating the instabilities, hence, reattaching the detached flow. The other state happens beyond the static stall angle of attack of the airfoil where the flow over the suction side of the airfoil is fully separated and coexistence of both the leading edge and the trailing edge shear-layer instabilities and natural trailing edge vortex shedding is the underlying mechanism. In this case, although the plasma actuator eliminates the instabilities, to some extent, but the corresponding momentum injection is not efficient to stabilize and reattach the flow.

Armor-Based Stable Force Pneumatic Artificial Muscles for Steady Actuation Properties.

In this article, a novel actuator called armor-based stable force pneumatic artificial muscle (AS-PAM) is presented. AS-PAM has a sealed chamber made of polygonal parts and film, which helps the actuator to be lightweight (∼100 g) and achieve a large contraction ratio (>60%). It has an armor and a constraint to guide its motion, which keeps its force output [6.25 N/(cm2·bar)] stable over its operating range (<10% deviation). An analytical model is presented to predict and control the behavior of the actuator, and various experiments were conducted to show the validity of the model. Afterward, a gripper using the actuators is presented to illustrate how it can be used in real applications. With its characteristics, the actuator shows interesting behaviors that cannot be found in other soft pneumatic actuators, and it would allow AS-PAM to expand the range of applications in which soft robots cooperate with humans.

Dual-gradient PNIPAM-based hydrogel capable of rapid response and tunable actuation

Thermoresponsive hydrogels have been received myriad attention and applied in numerous fields of soft robots, artificial muscles, and multifunctional sensors. However, developing a facial and low-cost method to obtain a thermoresponsive hydrogel with distinguished, multidirectional actuating performance and suitable mechanical properties remains a big challenge. Herein, by the introduction of the inorganic particle (IP), a thermoresponsive, dual-gradient (IP and crosslinking density) hydrogel with multidirectional actuating behaviors and improved mechanical properties were prepared by ultraviolet (UV)-induced copolymerization of the N-isopropylacrylamide (NIPAM) monomer and dispersed IP. The high thermal conductivity of IP endowed the prepared hydrogel with relatively rapid heat transmission and actuating deformation, and the deformation degree and direction could also be adjusted by changing the content of IP. Notably, although hydrogels containing a low content of IP undergo one-way deformation under a single thermal (45 ℃) stimulus, those containing a higher content of IP can exhibit bi-and even tri-directional stable actuations under the same and continuous thermal stimulus. Therefore, inspired by the adjustable and steady multidirectional actuations under a single thermal stimulus, the IP-containing hydrogels were suitable for use as the hydrogel-based gripper and jack to grab/release and lift/lower objects, respectively. Our work provides a facial and practical method for the design of anisotropic thermoresponsive hydrogels, paving the way for the further development of hydrogel-based soft robots and artificial muscles.