The acoustic properties of a compound micro-perforated panel (MPP) absorber array are investigated. The absorber array consists of three parallel-arranged MPP absorbers with different cavity depths. ...A finite element procedure is used to simulate its acoustic behaviors under normal incidence. Experimental studies are carried out to verify the numerical simulations. Due to different reactance matching conditions in the absorber array, strong local resonance occurs and the corresponding local resonance absorption dominates. Compared with single MPP absorber, the absorber array requires lower acoustic resistance for good absorption performance, and the resonance frequencies shift due to inter-resonator interactions. The different acoustic resistance requirement is explained by considering the reduced effective perforation rate of the MPP in the absorber array. The performance of the absorber array varies with the sizes and spatial arrangement of the component absorbers. When the distance between component absorbers is larger than a quarter-wavelength, the above-mentioned parallel absorption mechanism diminishes. In the experimental study, the normal incidence absorption coefficients of a prototype MPP absorber array are tested. The measured results compare well with the numerical predictions. The experimental study also shows that although other absorption mechanisms may exist, dissipation by the MPP is dominant in the MPP absorber array.
The emerging Internet of Things (IoT) applications, such as smart manufacturing and smart home, lead to a huge demand on the provisioning of low-cost and high-accuracy positioning and navigation ...solutions. Inertial measurement unit (IMU) can provide an accurate inertial navigation solution in a short time but its positioning error increases fast with time due to the cumulative error of accelerometer measurement. On the other hand, ultrawideband (UWB) positioning and navigation accuracy will be affected by the actual environment and may lead to uncertain jumps even under line-of-sight (LOS) conditions. Therefore, it is hard to use a standalone positioning and navigation system to achieve high accuracy in indoor environments. In this article, we propose an integrated indoor positioning system (IPS) combining IMU and UWB through the extended Kalman filter (EKF) and unscented Kalman filter (UKF) to improve the robustness and accuracy. We also discuss the relationship between the geometric distribution of the base stations (BSs) and the dilution of precision (DOP) to reasonably deploy the BSs. The simulation results show that the prior information provided by IMU can significantly suppress the observation error of UWB. It is also shown that the integrated positioning and navigation accuracy of IPS significantly improves that of the least squares (LSs) algorithm, which only depends on UWB measurements. Moreover, the proposed algorithm has high computational efficiency and can realize real-time computation on general embedded devices. In addition, two random motion approximation model algorithms are proposed and evaluated in the real environment. The experimental results show that the two algorithms can achieve certain robustness and continuous tracking ability in the actual IPS.
► A porous superhydrophobic PVDF coating was prepared on a wind turbine blade. ► The present method of preparing superhydrophobic coating is novel and simple. ► The superhydrophobic PVDF coating ...showed excellent anti-icing.
A novel and simple method was developed to prepare a porous superhydrophobic polyvinylidene fluoride (PVDF) coating on a wind turbine blade. The water contact angle and sliding angle of the superhydrophobic PVDF coating were 156±1.9° and 2°, respectively. The superhydrophobic PVDF coating showed excellent anti-icing property. The present work will provide a new way to fabricate anti-icing coating on wind turbine blades and other outdoor equipments.
In the present study, we surveyed the performance of closed-type centrifugal pump with a single blade and its fluctuations through an experiment and a CFD analysis, and comparatively verified the ...blade outlet flow, with the objective of elucidating the internal flow structure and loss generation mechanism of the pump. Furthermore, we quantitatively evaluated each hydraulic loss on the pump and examined the relationship with the vortex structure in an impeller. The result showed that fluctuations in the impeller loss were notable at any flow rate with the maximum value at a blade phase angle of 133° when the best efficiency point flow rate. It suggests an impact of the vortex generated on the shroud side near the end of the blade outlet winding. While the impeller loss indicates similar behavior to the best efficiency point flow rate at a high flow rate, but its fluctuation was extremely large, and we found that such fluctuations are the behavior at a low flow rate was opposite to other flow rates.
Extensive studies have been carried out on the acoustic performance of a micro-perforated panel (MPP) absorber which typically consists of a flat MPP fitted in front of a rigid backing wall. The ...present study concerns its acoustic properties when the conventionally flat perforated panel is replaced with a corrugated one that finds many applications in architecture. A three-dimensional finite element model is used to simulate the acoustic performance of a such corrugated MPP absorber at normal incidence, oblique incidence and diffuse field. Results show that the absorption performance of corrugated MPPA can be very different from conventional flat MPPA when the acoustic wave length is smaller than the corrugation depth. At low frequencies, the corrugated MPP performs similarly as a flat one of the same bulk perforation ratio. The absorption levels of the corrugated MPPA are considerably enhanced at the dips of absorption curves, which is favorable for reverberation control or broadband random noise reduction in large spaces and buildings. Further modal analysis shows that multiple modes of the corrugated configuration are excited at the peak and dip frequencies, and the acoustic response by the non-resonating modes give rise to the performance improvement. Absorption coefficients are also measured experimentally using an impedance tube. The measured normal incidence absorption coefficients compare well with the numerical predictions, which validates the numerical model and, in part, the numerical findings.
Abstract
Wave frequency is a critical parameter for applications ranging from human hearing, acoustic non-reciprocity, medical imaging to quantum of energy in matter. Frequency alteration holds the ...promise of breaking limits imposed by the physics laws such as Rayleigh’s criterion and Planck–Einstein relation. We introduce a linear mechanism to convert the wave frequency to any value at will by creating a digitally pre-defined, time-varying material property. The device is based on an electromagnetic diaphragm with a MOSFET-controlled shunt circuit. The measured ratio of acoustic impedance modulation is up to 45, much higher than nonlinearity-based techniques. A significant portion of the incoming source frequency is scattered to sidebands. We demonstrate the conversion of audible sounds to infrasound and ultrasound, respectively, and a monochromatic tone to white noise by a randomized MOSFET time sequence, raising the prospect of applications such as super-resolution imaging, deep sub-wavelength energy flow control, and encrypted underwater communication.
Hollow and heterostructured architectures are recognized as an effective approach to improve photocatalytic performance. In this work, ternary TiO
2
/CdTe/BiOI with hollow structure was constructed ...via a step-by-step method. In addition, the effect of TiO
2
structural regulation and the energy band alignment of BiOI and CdTe quantum dots (CdTe QDs) with TiO
2
in TiO
2
/CdTe/BiOI on photocatalytic dye removal were also studied. The results reveal that the TiO
2
/CdTe/BiOI heterostructures with hollow substrates exhibit much higher photocatalytic activities than pure TiO
2
, P25, TiO
2
/CdTe, and TiO
2
/BiOI and ternary TiO
2
/CdTe/BiOI with solid substrates. For TiO
2
(H)/CdTe/BiOI, several synergistic factors may be responsible for the remarkable visible-light photodegradation performance, such as strong visible-light absorption by BiOI and larger specific surface area.
A new duct noise control device is introduced based on the mechanism of human middle ear which functions as a compact, broadband impedance transformer between the air motion in the outer ear and the ...liquid motion in the inner ear. The system consists of two rigid endplates, simulating the tympanic membrane and the stapes footplate, and they are connected by a single rigid rod, simulating the overall action of the ossicular chain. These three pieces are placed in a side-branch cavity, and the whole device is called an ossicular silencer. A specific configuration is investigated numerically with a two-dimensional finite element model. Results show that broadband noise attenuation can be achieved in the very low frequency regime. Typically, two or more resonance peaks are found and the transmission loss between two neighbouring peaks is maintained at a high level. The cavity length is found to be the most crucial parameter that determines the effective frequency range of the ossicular silencer. The total cavity volume, which is a major controlling factor in most existing noise control devices, becomes less influential. The fluid medium in the enclosed cavity mainly acts like an added mass, while its stiffness effect is negligible. Simplified plane wave analysis is also conducted to reveal the mechanisms of the system resonances. The first resonance is identified as of the mass-spring system with mass contributions from both fluid and the plates, while the second one is of the Herschel–Quincke (HQ) tube resonance.
► A duct noise cancellation device is introduced based on the middle ear mechanism. ► Broadband noise attenuation can be achieved in the very low frequency regime. ► Mass-spring resonance and HQ tube resonance account for the resonance mechanism. ► The volume of the side-branch cavity is not a controlling design parameter.
In this study, a 3D printing technology, selective laser sintering (SLS), and infiltration techniques were used to prepare a complex three-dimensional metamaterial absorber with high performance. The ...raw carbonyl iron (CI) powders were coated by polyamide12 (PA12), where the dissolution-precipitation method was used to improve their uniformity. The coated powders were then printed by SLS. The influence of laser power on the relative density of the composites was studied, where 18W was thought as the optimal laser power. And the SLS samples were infiltrated by thermosetting epoxy resin (EP, E−44) to increase their relative densities and strength. Mechanical tests showed that the infiltrated composites have an ultimate tensile strength of 41.98 MPa, almost 15 times higher than those without infiltration. The final metamaterial absorber (MMA) exhibited the reflection loss below −10 dB in 2.7–18 GHz. The above results indicate that the fabricated processes proposed in this paper have great potential for the preparation of three-dimensional metamaterial absorbers.
•The metamaterial absorber has broadband and strong absorption characteristic.•The metamaterial absorber is manufactured by selective laser sintering and infiltraion processes, instead of traditional preparation method.•The metamaterial absorber has high strength, almost 15 times higher than those without infiltration.
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