Zhenbo Lu

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.

Time-domain In Situ Characterization of Acoustic Liners in a Flow Duct

A novel method for measuring liner characteristics in situ in a flow duct is proposed with the immediate aim of obtaining the impedance boundary condition for use in time-domain acoustics. This method as an integrated system in the present demonstration employs an accurate computational aeroacoustics model for prediction of the transient response of a liner to a plane-wave source; an acoustic impulse generator capable of producing sharp, repeatable, millisecond-short sound pulses of desired waveforms; a physical setup of a partially lined duct in a wind tunnel; and an optimization method for parametric identification of the embedded liner. Based on the assumption that impedance can be characterized by a sum of damped harmonic oscillators, the characterizing parameters of the liner are inferred by minimizing the differences between the model-predicted and the in situ measured pressure responses at strategically and conveniently placed sensor locations and are immediate applicable as causal broadband time-domain impedance-equivalent boundary conditions for the computational aeroacoustic model. The use of acoustic impulses of narrow space―time extents affords this method an easy resolve of the influence of neighboring but irrelevant boundaries such as the duct exit. The reliability, effectiveness, and potential benefits of this method are studied and demonstrated. This method is expected to provide broadband inferences and insights into the mean flow effect on liners in a wide range of practical applications with the present setup. It is believed that the feasibility demonstrated here opens a new dimension for theoretical, experimental, and numerical studies in the time domain.

Electrically tunable and broader-band sound absorption by using micro-perforated dielectric elastomer actuator

Most membrane-type acoustic absorbers for low-frequency sound attenuation are applicable only at a fixed resonant frequency for a narrow bandwidth. Tuning of the acoustic absorption spectrum is desired to match the varying noises. This letter presents a micro-perforated dielectric elastomer actuator (MPDEA) to make a broader-band acoustic absorber electrically tunable. Voltage activation of the MPDEA reduces the membrane tension and hole size and thus enables the active shifting of the acoustic absorption spectrum. Such a membrane tuning method does not require discrete mechanical parts as for the cavity tuning method. Also presented are the analytical models to predict the voltage-induced hole size change and acoustic characteristics of MPDEA.

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.

Active control of flow-induced acoustic resonance through surface perturbation

DOI: 10.2514/1.J051748Asurface perturbation technique was applied to control the flow-induced acoustic resonance inside downstreamcavities.Thetechniquemadeuseofpiezoceramicactuatorsembeddedonthesurfaceofanupstreamtestmodelinacrossflow to generate a controllable motion to alter vortex formation, as well as the subsequent acoustic resonance.Experimentswereconductedusingvariousconfigurations.Itwasobservedthattheflow-inducedacousticresonancecould be effectively reduced after applying a surface perturbation technique. This was caused not only by animpairmentofthevortex-shedding strengthbut alsobyashiftin thesheddingfrequencyresulting fromthecontrolaction. The vortex-strength abatement mechanism was discussed, and the estimation of the frequency-shiftphenomenon as well as its effect on the impairment of acoustic resonance was experimentally assessed.

Closed-loop control of flow-induced sound in a flow duct with downstream resonant cavities

A closed-loop-controlled surface perturbation technique was developed for controlling the flow-induced sound in a flow duct and acoustic resonance inside downstream cavities. The surface perturbation was created by piezo-ceramic THUNDER (THin layer composite UNimorph Driver and sEnsoR) actuators embedded underneath the surface of a test model with a semi-circular leading edge. A modified closed-loop control scheme based on the down-sampling theory was proposed and implemented due to the practical vibration characteristic limitation of THUNDER actuators. The optimally tuned control achieved a sound pressure reduction of 17.5 dB in the duct and 22.6 dB inside the cavity at the vortex shedding frequency, respectively. Changes brought up by the control in both flow and acoustic fields were analyzed in terms of the spectrum phase shift of the flow field over the upper surface of the test model, and a shift in the vortex shedding frequency. The physical mechanism behind the control was investigated in the view of developing an optimal control strategy.

A novel duct silencer using dielectric elastomer absorbers

A novel duct silencer was developed using dielectric elastomer absorbers (DEAs). Dielectric elastomer, a lightweight, high elastic energy density and large deformation under high DC/AC voltages smart material, was used to fabricate this new generation actuator. The acoustic performances of this duct silencer were experimentally investigated in a transmission loss (TL) measurement system using two-load method. It was found that the resonance peaks of this new duct silencer could be controlled by applying various DC voltages, a maximum resonance shift of 59.5Hz for the resonance peaks was achieved which indicated that this duct silencer could be adjusted to absorb broadband range noise without any addition mechanical part. Furthermore, the resonance shift and multiple resonances mechanisms using DEAs were proposed and discussed in the present paper which was aiming to achieve broadband noise reduction. The present results also provide insight into the appropriateness of the absorber for possible use as new acoustic treatment to replace the traditional acoustic treatment.

Reduced-order modeling and flutter suppression control of an experimental wing

This paper presents a numerical and experimental study of the reduced-order modeling (ROM) and flutter suppression control of a wind-tunnel wing model. The modeling work is computation based and performed in modal coordinates of the wing structure. A nonlinear ROM of aerodynamic and structural responses is obtained via computational aero-elasticity simulation, finite-element analysis and system identification. Then, casting the linearized model in linear fractional transform, a fixed-order robust controller is obtained that achieves flutter suppression over uncertain air speed.

Impedance Boundary Condition for Truncated Open Spaces

The propagation of waves governed by hyperbolic systems at a surface is in general reflective. The numerical solution of many problems in mechanics necessitates the truncation of an open space to a finite domain. The reflectivity of the boundary of a truncated domain can be effectively characterized by a few parameters and implemented as a time-domain impedance boundary condition. It is shown that the concept of a time-domain impedance boundary condition affords a simple and effective treatment of waves, allowing them to exit the surfaces of a truncated domain as a causal, local, space–time continuation rather than a global boundary confinement. We presentanddiscusstheopen-spaceimpedancesofradiativeandconvective fields,theirmodeling,analyticstructures, implementation, and solution effectiveness as an open-space impedance boundary condition. We further propose a generalmethodologyfornumericaldefinitionofopen-spaceimpedanceanditsapplicationwithinexistingschemesto reduce truncated domains and computational resources.

Ink-jet printing of transparent and stretchable electrodes for dielectric elastomer actuator

Dielectric elastomer actuators have recently been used to drive loudspeakers and acoustic absorbers. So far, these acoustic devices are opaque due to use of metallic or carbongrease compliant electrodes. A transparent device of acoustic-absorber is desirable for large-areal installation to glass window or roof. There were reports of transparent compliant electrode based on ionic hydrogel, which however does not last long when water evaporates. This paper investigates the use of transparent conductive polymer and its printing to make a transparent acoustic absorber. We formulated the aqueous ink with improved ink’s wettability to the elastomeric substrate. In addition, we optimized the droplet spacing to form a continuous electrode coating. The ink-jet printing enables the hassle-free patterning of transparent compliant electrodes to make a micro-perforated dielectric elastomer actuator. Testing shows that this transparent membrane DEA can produce a maximum voltage-induced radial expansion of 20%; whereas, a transparent perforated-membrane DEA can size down the holes by 15%, for tuning the acoustic resonant frequency.