Numerical study of the flow structures in flat plate and the wall-mounted hump induced by the unsteady DBD plasma

In this work,the dielectric-barrier-discharge plasma actuator was employed to study the flow structures induced by the plasma actuator over a flat plate and a wall-mounted hump.A phenomenological dielectric-barrier-discharge plasma model which regarded the plasma effect as the body force was implemented into the Navier–Stokes equations solved by the method of large eddy simulations.The results show that a series of vortex pairs,which indicated dipole formation and periodicity distribution were generated in the boundary layer when the plasma was applied to the flow over a flat plane.They would enhance the energy exchanged between the near wall region and the free stream.Besides,their spatial trajectories are deeply affected by the actuation strength.When the actuator was engaged in the flow over a wall-mounted hump,the vortex pairs were also produced,which was able to delay flow separation as well as to promote flow reattachment and reduce the generation of a vortex,achieving the goal of reducing dissipation and decreasing flow resistance.     

Experimental Study of the Unsteady Actuation Effect on Induced Flow Characteristics in DBD Plasma Actuators

The main aim of this paper is to investigate unsteady actuation effects on the operation of dielectric barrier discharge(DBD) plasma actuators and to study induced flow characteristics of steady and unsteady actuators in quiescent air.The parameters affecting the operation of unsteady plasma actuators were experimentally measured and compared with the ones for steady actuators.The effects of excitation frequency and duty cycle on the induced flow pattern properties were studied by means of hot-wire anemometers,and the smoke visualization method was also used.It was observed that the current and the mean induced velocity linearly increase with increasing duty cycle while they are not sensitive to excitation frequency.Furthermore,with increasing excitation frequency,the magnitude of vortices shedding from the actuator decreases while their frequency increases.Nevertheless,when the excitation frequency grows beyond a certain level,the induced flow downstream of the actuator behaves as a steady flow.However,the results for steady actuators show that by increasing the applied voltage and carrier frequency,the velocity of the induced flow first increases and then decreases with actuator saturation and the onset of the emission of streaky glow discharge.     

Experimental investigation of dynamic stall flow control using a microsecond-pulsed plasma actuator

To alleviate the performance deterioration caused by dynamic stall of a wind turbine airfoil,the flow control by a microsecond-pulsed dielectric barrier discharge(MP-DBD) actuator on the dynamic stall of a periodically pitching NACA0012 airfoil was investigated experimentally.Unsteady pressure measurements with high temporal accuracy were employed in this study,and the unsteady characteristics of the boundary layer were investigated by wavelet packet analysis and the moving root mean square meth...     

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.     

Investigation of the interaction between NS-DBD plasma-induced vortexes and separated flow over a swept wing

The effect of nanosecond pulsed dielectric barrier discharge(NS-DBD) plasma flow separation control is closely related to the actuation frequency,because it involves the interaction between plasma-induced vortexes and separated flow.In order to study the mechanism of NS-DBD plasma flow separation control over a swept wing,especially the influence of the actuation frequency,at first,experimental studies of the actuation frequencies at 100 Hz are conducted to validate the numerical simulation meth...     

A Study of Variation Patterns of Shock Wave Control by Different Plasma Aerodynamic Actuations

Demonstrative experiments on the variation patterns of the position, angle, and intensity of shock wave are presented. Different means of aerodynamic actuation, such as variations of the distance between discharge channels, the number of discharge channels, the DC discharge voltage, the angle of ramp, and the application of magnetic field, in a supersonic flow of M=2.2 are employed. Results of both the schlieren and pressure test indicated that when the plasma aerodynamic actuation is applied, the starting point of the shock wave was shifted 1 to 8mm upstream on average, the shock wave angle was reduced 4% to 8% on average, and the shock wave intensity was decreased by 8% to 26%. The local plasma aerodynamic actuation could generate an extrusive plasma layer with high temperature and pressure. This plasma layer caused an upstream-shift of the separating point of the boundary layer, which changed the structure of the original shock wave. Moreover, in a simulation study, the plasma aerodynamic actuation was simplified as a thermal source term added to the Navier–Stokes equations, after all, the results obtained showed consistency with the experimental results.     

Large eddy simulation of unsteady flow in gas-liquid separator applied in thorium molten salt reactor

Axial gas-liquid separators have been adopted in fission gas removal systems for the development of thorium molten salt reactors. In our previous study, we observed an unsteady flow phenomenon in which the flow pattern is directly dependent on the backpressure in a gas-liquid separator; however, the underlying flow mechanism is still unknown. In order to move a step further in clarifying how the flow pattern evolves with a variation in backpressure, a large eddy simulation(LES) was adopted to study the flow field evolution. In the simulation, an artificial boundary was applied at the separator outlet under the assumption that the backpressure increases linearly. The numerical results indicate that the unsteady flow feature is captured by the LES approach, and the flow transition is mainly due to the axial velocity profile redistribution induced by the backpressure variation. With the increase in backpressure,the axial velocity near the downstream orifice transits from negative to positive. This change in the axial velocity sign forces the unstable spiral vortex to become a stable rectilinear vortex.     

Electrical and aerodynamic characteristics of sliding discharge based on a microsecond pulsed plasma supply

Plasma flow control technology has broad prospects for application. Compared with conventional dielectric barrier discharge plasma actuators (DBD-PA), the sliding discharge plasma actuator (SD-PA) has the advantages of a large discharge area and a deflectable induced jet. To achieve the basic performance requirements of light weight, low cost, and high reliability required for UAV (Unmanned Aerial Vehicle) plasma flight experiments, this work designed a microsecond pulse plasma supply that can be used for sliding discharge plasma actuators. In this study, the topology of the primary circuit of the microsecond pulse supply is determined, the waveform of the output terminal of the microsecond pulse plasma supply is detected using the Simulink simulation platform, and the design of the actuation voltage, the pulse frequency modulation function and the construction of the hardware circuit are achieved. Using electrical diagnosis and flow field analysis, the actuation characteristics and flow characteristics of sliding discharge plasma under microsecond pulse actuation are studied, the optimal electrical actuation parameters and flow field characteristics are described.     

Experimental study on plasma actuation characteristics of nanosecond pulsed dielectric barrier discharge

Combining high-speed schlieren technology and infrared imaging technology, related research has been carried out on the influence of parameters such as actuation voltage, repetition frequency, and electrode size of an actuator on the discharge characteristics, induced flow field characteristics, and thermal characteristics of nanosecond pulsed dielectric barrier discharge. The results show that increasing the value of the actuation voltage can significantly increase the actuation intensity, and the plasma discharge area is significantly extended. Increasing the repetition frequency can increase the number of discharges per unit time. Both will cause more energy input and induce more changes in the flow field. The effect of temperature rise is more significant. The width of the covered electrode will affect the potential distribution during the discharge process, which in turn will affect the extension process of the plasma discharge filament. Under the same actuation intensity, the wider the covered electrode, the larger range the induced flow field and temperature rise is. Preliminary experimental analyses of high-frequency actuation characteristics, temperature field characteristics, flow field characteristics and actuation parameter settings provide support for the parameter selection and partial mechanism analysis of plasma anti-icing.     

Reduction of turbulent boundary layer drag through dielectric-barrier-discharge plasma actuation based on the Spalding formula

Abstract It is a very difficult task to develop a method of reducing turbulent boundary layer drag. However, in recent years, plasma flow control technology has demonstrated huge potential in friction drag reduction. To further investigate this issue, a smooth plate model was designed as a testing object arranged with a bidirectional dielectric-barrier-discharge (DBD) plasma actuator. In addition, measurement of skin friction drag was achieved by applying hot wire anemometry to obtain the velocity distribution of the turbulent boundary layer. A method of quantifying the friction drag effect was adopted based on the Spalding formula fitted with the experiment data. When plasma actuation was conducted, a velocity defect occurred at the two measuring positions, compared with the no plasma control condition; this means that the DBD plasma actuation could reduce the drag successfully in the downstream of the actuator. Moreover, drag reduction caused by backward actuation was slightly more efficient than that caused by forward actuation. With an increasing distance from plasma actuation, the drag-reduction effect could become weaker. Experimental results also show that the improvement of drag-reduction efficiency using a DBD plasma actuator can achieve about 8.78% in the local region of the experimental flat model.     

Turbulent drag reduction by spanwise slot blowing pulsed plasma actuation

This work studies the turbulent drag reduction (TDR) effect of a flat plate model using a spanwise slot blowing pulsed plasma actuator (SBP-PA). Wind tunnel experiments are carried out under a Reynolds number of 1.445 × 104. Using a hot-wire anemometer and an electrical data acquisition system, the influences of millisecond pulsed plasma actuation with different burst frequencies and duty cycles on the microscale coherent structures near the wall of the turbulent boundary layer (TBL) are studied. The experimental results show that the SBP-PA can effectively reduce the frictional drag of the TBL. When the duty cycle exceeds 30%, the TDR rate is greater than 11%, and the optimal drag reduction rate of 13.69% is obtained at a duty cycle of 50%. Furthermore, optimizing the electrical parameters reveals that increasing the burst frequency significantly reduces the velocity distribution in the logarithmic region of the TBL. When the normalized burst frequency reaches f+ = 2πfpd/U∞ = 7.196, the optimal TDR effectiveness is 16.97%, indicating a resonance phenomenon between the pulsed plasma actuation and the microscale coherent structures near the wall. Therefore, reasonably selecting the electrical parameters of the plasma actuator is expected to significantly improve the TDR effect.     

Study on Free Surface and Channel Flow Induced by Low-Temperature Plasma via Lattice Boltzmann Method

Active boundary layer flow control and boundary layer manipulation in the channel flow that was based on low temperature plasma were studied by means of a lattice Boltzmann method.Two plasma actuators were placed in a row to obtain the influence rule of their separation distance on the velocity profile at three locations and maximum velocity in the flow field.Two plasma actuators were placed symmetrically inside a channel to examine the effect of channel height and voltage on the velocity profile and flow rate.It was found that the channel height controls the distribution of flow velocity,which affected the flow rate and its direction.Increasing plasma voltage had a negative effect on the flow rate due to the generation of a larger and stronger flow vortex.     

Large Eddy Simulation of the Effects of Plasma Actuation Strength on Film Cooling Efficiency

In this article, numerical investigation of the effects of different plasma actuation strengths on the film cooling flow characteristics has been conducted using large eddy simulation(LES). For this numerical research, the plasma actuator is placed downstream of the trailing edge of the film cooling hole and a phenomenological model is employed to provide the electric field generated by it, resulting in the body forces. Our results show that as the plasma actuation strength grows larger, under the downward effect of the plasma actuation, the jet trajectory near the cooling hole stays closer to the wall and the recirculation region observably reduces in size. Meanwhile, the momentum injection effect of the plasma actuation also actively alters the distributions of the velocity components downstream of the cooling hole. Consequently, the influence of the plasma actuation strength on the Reynolds stress downstream of the cooling hole is remarkable. Furthermore, the plasma actuation weakens the strength of the kidney shaped vortex and prevents the jet from lifting off the wall. Therefore, with the increase of the strength of the plasma actuation, the coolant core stays closer to the wall and tends to split into two distinct regions. So the centerline film cooling efficiency is enhanced, and it is increased by 55% at most when the plasma actuation strength is 10.     

Numerical Simulation of Stall Flow Control Using a DBD Plasma Actuator in Pulse Mode

A numerical simulation method is employed to investigate the effects of the unsteady plasma body force over the stalled NACA 0015 airfoil at low Reynolds number flow conditions.The plasma body force created by a dielectric barrier discharge actuator is modeled with a phenomenological method for plasma simulation coupled with the compressible Navier-Stokes equations.The governing equations are solved using an efficient implicit finitevolume method.The responses of the separated flow field to the effects of an unsteady body force in various interpulses and duty cycles as well as different locations and magnitudes are studied.It is shown that the duty cycle and inter-pulse are key parameters for flow separation control.Additionally,it is concluded that the body force is able to attach the flow and can affect boundary layer grow that Mach number 0.1 and Reynolds number of 45000.     

Optimum Duty Cycle of Unsteady Plasma Aerodynamic Actuation for NACA0015 Airfoil Stall Separation Control

Unsteady dielectric barrier discharge(DBD) plasma aerodynamic actuation technology is employed to suppress airfoil stall separation and the technical parameters are explored with wind tunnel experiments on an NACA0015 airfoil by measuring the surface pressure distribution of the airfoil.The performance of the DBD aerodynamic actuation for airfoil stall separation suppression is evaluated under DBD voltages from 2000 V to 4000 V and the duty cycles varied in the range of 0.1 to 1.0.It is found that higher lift coefficients and lower threshold voltages are achieved under the unsteady DBD aerodynamic actuation with the duty cycles less than 0.5as compared to that of the steady plasma actuation at the same free-stream speeds and attack angles,indicating a better flow control performance.By comparing the lift coefficients and the threshold voltages,an optimum duty cycle is determined as 0.25 by which the maximum lift coefficient and the minimum threshold voltage are obtained at the same free-stream speed and attack angle.The non-uniform DBD discharge with stronger discharge in the positive half cycle due to electrons deposition on the dielectric slabs and the suppression of opposite momentum transfer due to the intermittent discharge with cutoff of the negative half cycle are responsible for the observed optimum duty cycle.     

Analysis of flow separation control using nanosecond-pulse discharge plasma actuators on a flying wing

Abstract Dielectric barrier discharge (DBD) plasma is one of most promising flow control method for its several advantages. The present work investigates the control authority of nanosecond pulse DBD plasma actuators on a flying wing model’s aerodynamic characteristics. The aerodynamic forces and moments are studied by means of experiment and numerical simulation. The numerical simulation results are in good agreement with experiment results. Both results indicate that the NS-DBD plasma actuators have negligible effect on aerodynamic forces and moment at the angles of attack smaller than 16°. However, significant changes can be achieved with actuation when the model’s angle of attack is larger than 16° where the flow separation occurs. The spatial flow field structure results from numerical simulation suggest that the volumetric heat produced by NS-DBD plasma actuator changes the local temperature and density and induces several vortex structures, which strengthen the mixing of the shear layer with the main flow and delay separation or even reattach the separated flow.     

Rotor performance enhancement by alternating current dielectric barrier discharge plasma actuation

An experimental system was established to explore the plasma flow control effect for helicopter rotors in hover mode.With the plasma actuator applied at the leading edge of the rotor blades,alternating current dielectric barrier discharge(AC-DBD) plasma actuation was generated by a sinusoidal AC high-voltage generator.By direct force measurement,the influence of actuation parameters on the aerodynamic performance of the rotor was investigated at a tip Reynolds number of 1.7 × 10⁵.AC-D...     

Experimental Investigation on Aerodynamic Control of a Wing with Distributed Plasma Actuators

Experimental investigation of active flow control on the aerodynamic performance of a flying wing is conducted.Subsonic wind tunnel tests are performed using a model of a 35°swept flying wing with an nanosecond dielectric barrier discharge(NS-DBD) plasma actuator,which is installed symmetrically on the wing leading edge.The lift and drag coefficient,lift-todrag ratio and pitching moment coefficient are tested by a six-component force balance for a range of angles of attack.The results indicate that a 44.5%increase in the lift coefficient,a 34.2%decrease in the drag coefficient and a 22.4%increase in the maximum lift-to-drag ratio can be achieved as compared with the baseline case.The effects of several actuation parameters are also investigated,and the results show that control efficiency demonstrates a strong dependence on actuation location and frequency.Furthermore,we highlight the use of distributed plasma actuators at the leading edge to enhance the aerodynamic performance,giving insight into the different mechanism of separation control and vortex control,which shows tremendous potential in practical flow control for a broad range of angles of attack.     

Investigation of the DBD Plasma Effect on Flat Plate Flow

Particle image velocimetry experiments and simulations were conducted in this study to clarify the influence of the DBD plasma actuator on the flow over a flat plate. The result shows that the actuator not only effectively leads to a local rise in near-wall velocity, but also efficiently causes a decrease in the displacement thickness of the boundary layer. Actuator-induced vorticity is generated to intensify the energy exchange between the main flow and the boundary layer, and dynamic energy is thus added directly to the low-energy fluid by the actuator. Although the increase in fluid velocity also brings a rise in dynamic energy loss, the energy added by the actuator can cover this to provide growth in the energy of the boundary layer. The plasma actuator presents a better performance when the free-stream velocity is lower.     

ADS无窗靶件自由界面模拟实验和数值研究

冷却剂自由界面形态的形成和控制是加速器驱动的次临界系统(ADS)无窗靶件设计的关键技术之一。采用水介质对无窗靶件模型的自由界面特征进行了实验和数值研究。实验中采用激光诱导荧光的示踪方法实现了流场的可视化,得到Re=30000～50000范围内的自由界面和可视化流场。在高Re工况下,流场中出现大尺度的非稳定涡结构,随着Re的降低,流场中涡结构的紊乱程度增加。分别采用大涡模型（LES）和两方程动能-特征耗散率模型（kω-SST）对无窗靶件实验工况进行了数值分析,计算结果表明,LES能较好地模拟实验中所得的流场现象和界面特征。