Focus control of wide-angle metalens based on digitally encoded metasurface

Based on the principle of super-symmetric lens with quadratic phase gradient transformation, combined with the principle of digital coding of metasurface, we propose a wide-angle coded metalens for focusing control in two-dimensional space. This metalens achieves focus shift in the x-direction by changing the oblique incidence angle of the incident wave,and focus control in the y-direction by combining with the convolution principle of the digitally coded metasurface to achieve flexible control of light focusing in the two-dimensional plane. The metasurface unit is mainly composed of threelayer of metal structure and two layers of medium, and the transmission phase is obtained by changing the middle layer of metal structure, which in turn obtains the required phase distribution of the metalens. The design of the metalens realizes the function of the lens with a large viewing angle at the x-polarized incidence, and realizes two-dimensional focus control. Experimentally, we prepared the designed coding metalens and tested the focus control function of the wide-angle coding metalens. The experimental results are in good agreement with the design results.

Optimization of the End Effect of Hilbert-Huang transform(HHT)

In fault diagnosis of rotating machinery, Hil- bert-Huang transform （HHT） is often used to extract the fault characteristic signal and analyze decomposition results in time-frequency domain. However, end effect occurs in HHT, which leads to a series of problems such as modal aliasing and false IMF （Intrinsic Mode Func- tion）. To counter such problems in HHT, a new method is put forward to process signal by combining the general- ized regression neural network （GRNN） with the bound- ary local characteristic-scale continuation （BLCC）. Firstly, the improved EMD （Empirical Mode Decompo- sition） method is used to inhibit the end effect problem that appeared in conventional EMD. Secondly, the gen- erated IMF components are used in HHT. Simulation and measurement experiment for the cases of time domain, frequency domain and related parameters of Hilbert- Huang spectrum show that the method described here can restrain the end effect compared with the results obtained through mirror continuation, as the absolute percentage of the maximum mean of the beginning end point offset and the terminal point offset are reduced from 30.113% and 27.603% to 0.510% and 6.039% respectively, thus reducing the modal aliasing, and eliminating the false IMF components of HHT. The proposed method caneffectively inhibit end effect, reduce modal aliasing and false IMF components, and show the real structure of signal components accuratelX.

A Numerical Investigation on the Characteristics of the Radial Force in a Cycloid Gerotor Pump

In order to improve the performances of a cycloid gerotor pump,the variations of the radial force induced by different rotating speeds and outlet pressures are analyzed numerically.Using the numerical simulations as a basis,an improved oil inlet and outlet groove structure is proposed.The results show that the radial force decreases with the decrease of the outlet pressure and of the rotor speed.Compared with the original model,the large-end oil inlet line and pressure line of the new oil groove are claw-shaped.This configuration can effectively weaken the pressure changes inside the gerotor pump and reduce accordingly the radial force on the inner rotor.

Simulation of flow through porous anode in MFC at higher power density

Microbial fuel cell （MFC） is a novel environmental friendly energy device which has received great attention due to its technology for producing electricity directly fi-om organic or inorganic matter by using bacteria as catalyst. To date, many experiments have been carried out to achieve the maximum power output with advective flow through porous anode to the cathode in the MFC. However, the precise mechanical mechanism of flow through anode and the quantified relationship between electrode spacing and MFC performance are not yet clearly understood. It hasbeen found experimentally that the power output can be increased apparently at certain electrode spacing configuration. Based on these available experimental data, this paper investigates the effect of spacing between electrodes, the Darcy number of porous anode and the Reynolds number on the power production performance of MFC by using lattice Boltzmann method. The numerical simulation results present that the distance between electrodes significantly influences the flow velocity and residence time of the organic matter attached to the anode in the MFC. Moreover, it is found that the Darey number of porous anode and the Reynolds number can regulate the output efficiency of MFC. These results perform better understanding of the complex phenomena of MFC and will be helpful to optimize MFC design.

Flexible tuning of multifocal holographic imaging based on electronically controlled metasurfaces

Programmable hyper-coded holography has the advantage of being programmable as well as being flexibly modifiable.Digitally coded metamaterials with excellent electromagnetic modulation capability and the ability to control the phase to modulate the spatial radiation field through external excitation in the form of switching can be used to realize low-cost digital arrays.We design a 1-bit encoded programmable metasurface,which is electrically connected to control the PIN diode in the switching state and to switch the condition of each metasurface cell between“0”and“1.”Using the designed programmable metasurface,we can randomly encode the cell structure to realize single-focus focusing,multi-focusing,and simple holographic letter imaging.Based on the nonlinear holographic model,we employ the Gerchberg-Saxton improvement algorithm to modulate the energy distribution at the focus by adjusting the phase distribution.Importantly,we introduce the Fourier convolution principle to regulate the holographic imaging focus flexibly.

Three-dimensional color particle image velocimetry based on a cross-correlation and optical flow method

Rainbow particle image velocimetry(PIV)can restore the three-dimensional velocity field of particles with a single camera;however,it requires a relatively long time to complete the reconstruction.This paper proposes a hybrid algorithm that combines the fast Fourier transform(FFT)based co-correlation algorithm and the Horn–Schunck(HS)optical flow pyramid iterative algorithm to increase the reconstruction speed.The Rankine vortex simulation experiment was performed,in which the particle velocity field was reconstructed using the proposed algorithm and the rainbow PIV method.The average endpoint error and average angular error of the proposed algorithm were roughly the same as those of the rainbow PIV algorithm;nevertheless,the reconstruction time was 20%shorter.Furthermore,the effect of velocity magnitude and particle density on the reconstruction results was analyzed.In the end,the performance of the proposed algorithm was verified using real experimental single-vortex and double-vortex datasets,from which a similar particle velocity field was obtained compared with the rainbow PIV algorithm.The results show that the reconstruction speed of the proposed hybrid algorithm is approximately 25%faster than that of the rainbow PIV algorithm.

Improvement of terahertz beam modulation efficiency for baseless all-dielectric coded gratings

Optical metasurfaces are two-dimensional ultrathin devices based on single-layer or multilayer arrays of subwave-length nanostructures. They can achieve precise control of phase, amplitude, and polarization on the subwave-length scale. In this paper, a substrate-free all-silicon coded grating is designed, which can realize the phase control of the outgoing beam after the y-polarized plane wave is vertically incident on the metasurface at 0.1 THz.Through a single-layer silicon nanoarray structure, a low-reflection anomalous transmission metasurface is realized, and a variety of different beam deflectors are designed based on these encoded gratings. We propose a coded grating addition principle, which adds and subtracts two traditional coded grating sequences to obtain a new coded grating sequence. The encoded supergrating can flexibly control the scattering angle, and the designed substrate-free all-silicon encoded grating can achieve a deflection angle of 48.59°. In order to verify the principle of coded grating addition, we experimented with cascade operation of two coded sequence gratings to obtain the flexible control of the terahertz beam of the composite supergrating. The principle of grating addition provides a new degree of freedom for the flexible regulation of the terahertz wavefront. At the same time, this method can be extended to the optical band or microwave band, opening up new ways for electromagnetic wave manipulation and beam scanning.

Investigation of Self-Priming Process of a Centrifugal Pump with Double Blades

The gas-liquid two-phase flow patterns of a centrifugal pump during the self-priming process were investigated numerically and experimentally.The Euler-Euler multiphase model and SST k-ω turbulence model were applied for simulating the self-priming process.Meanwhile,the changes of motor speed and self-priming height were considered in the simulation.The overall transient two-phase flow features and water level distributions were mapped.Results showed that the self-priming process was divided into three stages.The liquid level in inlet-pipe rose in oscillation during self-priming process.The variations of water level during self-priming process of numerical simulation and test result agreed well.The inlet-pipe(Ver)was filled at 22 s and 24 s respectively numerically and experimentally.The bubble cloud circulated in the volute during middle stage of self-priming process,and breakup into smaller bubbles by shear force and tongue,and then discharged into chamber.The bubbles in the outlet-pipe mainly included bubbly flow and slug flow at the last stage of self-priming process,which is morphologically consistent with the test results.Also,during the self-priming process,the reflux liquid was pressed by blades and fully mixed with gas;that is the way to realizing the function of self-priming.

Shape Memory of Elastic Capsules Under the Effect of Spontaneous Shape

Red blood cells can recover their resting shape after having been deformed by shear flow.Their rims are always formed by the same part of the membranes,and the cells are said to have shapememory.Modeled as two-dimensional elastic capsules,their recoverymotion and shapememory is studied,mainly focused on the effect of the spontaneous shape.The fluid-structure interaction ismodeled using immersed boundary method.Based on the simulations,the resting shapes of capsules are obtained and the area ratio of spontaneous shape is found to play an important role.After remove of shear flow,all capsules can recover their resting shapes,while only capsules with noncircular spontaneous shapes present shape memory.As the spontaneous shape approaches a circle but still noncircular,the capsule spends more time on recovery process.We consider how these capsules deform depending on the membrane bending energy,and find that the relaxation speed is positive correlated to the range of values of dimensionless bending energy.These results may help to identify different spontaneous shapes for capsules especially RBCs through future experiments.

Broadband cross-circular polarization carpet cloaking based on a phase change material metasurface in the mid-infrared region

In view of the fact that most invisibility devices focus on linear polarization cloaking and that the characteristics of mid-infrared cloaking are rarely studied,we propose a cross-circularly polarized invisibility carpet cloaking device in the mid-infrared band.Based on the Pancharatnam–Berry phase principle,the unit cells with the cross-circular polarization gradient phase were carefully designed and constructed into a metasurface.In order to achieve tunable cross-circular polarization carpet cloaks,a phase change material is introduced into the design of the unit structure.When the phase change material is in amorphous and crystalline states,the proposed metasurface unit cells can achieve highefficiency cross-polarization conversion,and reflection intensity can be tuned.According to the phase compensation principle of carpet cloaking,we construct a metasurface cloaking device with a phase gradient using the designed unit structure.From the near-and far-field distributions,the cross-circular polarization cloaking property is confirmed in the broadband wavelength range of 9.3–11.4μm.The proposed cloaking device can effectively resist detection of cross-circular polarization.