Yongdong Cui

Numerical simulation of nanosecond pulsed dielectric barrier discharge actuator in a quiescent flow

We present a numerical study of nanosecond pulsed dielectric barrier discharge (DBD) actuator operating in quiescent air at atmospheric condition. Our study concentrates on plasma discharge induced fluid dynamics and on exploration of parametric space of interest for voltage pulse in an attempt to shed some light into elucidation of the mechanisms whereby the generated shock wave propagates through and affects the external flow. Specifically, a one-dimensional, self-similar, local ionization kinetic model recently developed to predict key parameters of nanosecond pulsed plasma discharge is coupled with the compressible Navier-Stokes equations possibly for the first time. Within the considered range of parameters of the plasma model which is justified for the modeling of surface nanosecond pulsed discharge at atmospheric pressure, our coupled method is able to provide satisfactory prediction of the shock structure generated by the actuator for comparison with experiment, not only in the qualitative shock wave shape but also in quantitative shock front displacement. We provide a comprehensive analysis of the gas heating, shock wave initiation and evolution processes. For example, the characteristic time of the rapid localized heating responsible for shock wave generation, which is yet to be quantified experimentally, is found to be ∼350 ns. We conduct a parametric investigation by varying the peak voltage from 10 kV to 50 kV and rise time from 5 ns to 150 ns. The pressure wave whose behavior is found to be dominated by input voltage amplitude, introduces highly transient, localized disturbance to the quiescent air. In addition, the vortex induced by the shock passage is relatively weak. The interplay of the induced flows by a few successive plasma discharges operating at continuous mode does not appear to be significant, especially at low voltage amplitude.

Quenching of vortex breakdown oscillations via harmonic modulation

Vortex breakdown is a phenomenon inherent to many practical problems, such as leading-edge vortices on aircraft, atmospheric tornadoes, and flame-holders in combustion devices. The breakdown of these vortices is associated with the stagnation of the axial velocity on the vortex axis and the development of a near-axis recirculation zone. For large enough Reynolds number, the breakdown can be time-dependent. The unsteadiness can have serious consequences in some applications, such as tail-buffeting in aircraft flying at high angles of attack. There has been much interest in controlling the vortex breakdown phenomenon, but most efforts have focused on either shifting the threshold for the onset of steady breakdown or altering the spatial location of the recirculation zone. There has been much less attention paid to the problem of controlling unsteady vortex breakdown. Here we present results from a combined experimental and numerical investigation of vortex breakdown in an enclosed cylinder in which low-amplitude modulations of the rotating endwall that sets up the vortex are used as an open-loop control. As expected, for very low amplitudes of the modulation, variation of the modulation frequency reveals typical resonance tongues and frequency locking, so that the open-loop control allows us to drive the unsteady vortex breakdown to a prescribed periodicity within the resonance regions. For modulation amplitudes above a critical level that depends on the modulation frequency (but still very low), the result is a periodic state synchronous with the forcing frequency over an extensive range of forcing frequencies. Of particular interest is the spatial form of this forced periodic state: for modulation frequencies less than about twice the natural frequency of the unsteady breakdown, the oscillations of the near-axis recirculation zone are amplified, whereas for modulation frequencies larger than about twice the natural frequency the oscillations of the recirculation zone are quenched, and the near-axis flow is driven to the steady axisymmetric state. Movies are available with the online version of the paper.

Study of Shock and Induced Flow Dynamics by Nanosecond Dielectric-Barrier-Discharge Plasma Actuators

The shock wave behavior generated from a single shot of pulsed nanosecond dielectric-barrier-discharge plasma actuator with varying pulse voltages in quiescent air was studied by experiments and numerical simulations. The experiments included using the schlieren technique, a fast response pressure transducer, and a two-velocity-component particle image velocimetry system to measure the propagation of the shock wave, the shock overpressure, and the shock induced flow, respectively. For the numerical simulation, a simple “phenomenological approach” was employed by modeling the plasma region over the encapsulated electrode as a jump-heated and pressurized gas layer. The present investigation revealed that the behaviors of the shock wave generated by the nanosecond pulsed plasma were fundamentally a microblast wave, and their speed and strength were found to increase with higher input voltages. The blast wave occured about 1 to 3  μs after the discharge of the nanosecond pulse, which was dependent on the inpu...

Study of shock and induced flow dynamics by pulsed nanosecond DBD plasma actuators

The shock wave behaviour generated from a single shot of nanosecond DBD plasma actuator with varying pulse voltages in quiescent air was studied by experiments and numerical simulations. The experiments includes Schlieren technique, a fast response pressure transducer and a two-velocity-component PIV system to measure the propagation of the shockwave, the shock overpressure and the shock induced flow, respectively. For the numerical simulation, a simple “phenomenological approach” is employed by modelling the plasma region over the covered electrode as a jump-heated and pressurized gas layer. The present investigation reveals that the behaviours of the shock wave generated by the nanosecond pulsed plasma is fundamentally a micro blast wave and its speed and strength is found to be increased with higher input voltages. The blast wave occurs in about 1 to 4 μs after the discharge of the nanosecond pulse, which is dependent on the input voltages, and decays quickly from supersonic to sonic level within about 5μs (2-3mm from the actuator surface). The shock induced burst perturbations (overpressure and induced velocity) is found to be restricted to a very narrow region (about 1mm) behind the shock front and last only for a few microseconds. While a fairly weak induced vortex flow is observed in a relative long time period after the discharge of the plasma. These results imply that the pulsed plasma actuators have stronger local effects in time and spatial domain.

Experimental and numerical investigation of the competition between axisymmetric time-periodic modes in an enclosed swirling flow

Time-periodic vortex flows in an enclosed circular cylinder driven by the rotation of one endwall are investigated experimentally and numerically. This work is motivated partly by the linear stability analysis of Gelfgat et al. [J. Fluid Mech. 438, 363 (2001)], which showed the existence of an axisymmetric double Hopf bifurcation, and the purpose of the experiment is to see if the nonlinear dynamics associated with this double Hopf bifurcation can be captured under laboratory conditions. A glycerin/water mixture was used in a cylinder with variable height-to-radius ratios between Γ=1.67 and 1.81, and Reynolds numbers between Re=2600 and 2800 (i.e., in the neighborhood of the double Hopf). Hot-film measurements provide, for the first time, experimental evidence of the existence of an axisymmetric double Hopf bifurcation, involving the competition between two stable coexisting axisymmetric limit cycles with periods (nondimensionalized by the rotation rate of the endwall) of approximately 31 and 22. The dyna...

An electronically tunable duct silencer using dielectric elastomer actuators

A duct silencer with tunable acoustic characteristics is presented in this paper. Dielectric elastomer, a smart material with lightweight, high elastic energy density and large deformation under high direct current/alternating current voltages, was used to fabricate this duct silencer. The acoustic performances and tunable mechanisms of this duct silencer were experimentally investigated. It was found that all the resonance peaks of this duct silencer could be adjusted using external control signals without any additional mechanical part. The physics of the tunable mechanism is further discussed based on the electro-mechanical interactions using finite element analysis. The present promising results also provide insight into the appropriateness of the duct silencer for possible use as next generation acoustic treatment device to replace the traditional acoustic treatment.

Control of Vortex Breakdown over a Delta Wing Using Forebody Slot Blowing

In this paper we study the effectiveness of forebody slot blowing to control vortex breakdown over a generic delta wing-body configuration. The motivation is to exploit the benefits of both slot blowing and canards to control vortex breakdown over a delta wing. Parameters investigated include slot length, slot width, single and double-sided blowing, and the Reynolds number. Time-averaged flow images show that Reynolds number has little effect on vortex breakdown location, at least for the range of conditions studied here, and a single-sided blowing has favorable effects on the blowing side and unfavorable effects on the nonblowing side. It is postulated that when fluid is discharged from the slot at the forebody in the spanwise direction, it produces a vortex sheet that interacts with the freestream and is rolled up to form a trailing vortex further downstream. The rolled-up vortex sheet produces a downwash effect on the blowing side and a sideslip effect on the opposite side. The downwash modifies the flowfield around the leading edge of the wing, causing a delay in vortex breakdown. To compensate for the unfavorable effects of blowing on the opposite side, a much higher blowing momentum (more than two times the single-sided slot case) is needed to delay vortex breakdown on both sides of the wing when double-sided blowing is employed. Our results also show that for a given Reynolds number and angle of attack, increasing the blowing momentum leads to a substantial delay in the vortex breakdown position. In addition, for the same blowing momentum coefficient, varying the slot width has little effect on the breakdown location, whereas increasing the slot length has favorable effect, particularly at lower angles of attack.

On the generation of a spiral-type vortex breakdown in an enclosed cylindrical container

Earlier experimental studies [see H. U. Vogel, “Experimentelle Ergebnisse uber die Laminare Stromung in einem zylindrischen Gehause mit darin rotierender Scheibe,” Max-Planck-Inst. Bericht 6 (1968) and M. P. Escudier, “Observations of the flow produced in cylindrical container by a rotating endwall,” Exp. Fluids 2, 189 (1984)] showed that vortex breakdown in an enclosed cylindrical container with one rotating endwall could exhibit either one, two, or three recirculating bubbles depending on the combination of Reynolds number Re and aspect ratio H∕R, at least for H∕R⩽3.5. However, a recent numerical study by Serre and Bontoux [“Vortex breakdown in a three-dimensional swirling flow,” J. Fluid Mech. 459, 347 (2002)] at H∕R=4.0 showed that under some conditions, an S-shape vortex structure follows by a spiral-type vortex breakdown could also be produced. This finding is most interesting because, to the best of our knowledge, a spiral-type vortex breakdown in an enclosed cylindrical container has not been prod...

Harmonically forced enclosed swirling flow

The response of steady state vortex flows in an enclosed circular cylinder driven by the harmonic modulation of the rotating end wall is investigated experimentally and numerically. Three dynamic regimes have been identified, with a continuous variation in forcing frequency between them. For very low forcing frequency, the synchronous flow approaches quasistatic adjustment, and for very large forcing frequencies the oscillations in the synchronous flow are localized in the boundary layers on the various cylinder walls. These localized wall oscillations drive the synchronous flow in the cylinder interior to the underlying axisymmetric steady basic state. The third regime occurs for forcing frequencies in the range of the most dangerous axisymmetric Hopf eigenfrequencies, with the 1:1 resonances leading to greatly enhanced oscillation amplitudes localized in the axis region where the flow manifests vortex breakdown recirculation zones.

Acoustic characteristics of a dielectric elastomer absorber.

The present paper is devoted to study the acoustic characteristics of a dielectric elastomer (DE) absorber, which has a wide variety of potential applications as a novel actuator technology. DE, a lightweight and high elastic energy density smart material, can produce a large deformation under high DC/AC voltages. These excellent characteristics can be used to improve the present typical noise control systems. The performance of using this new soft-controlled-material is experimentally investigated. It is found that the voltage on the DE could tune the resonance frequencies of DE absorber thus it could absorb broadband noise. The results also provide insight into the appropriateness of the absorber for possible use as an active noise control system for replacing the traditional acoustic treatment.