Wave Beam Research Articles

Fast Multigrid Algorithm for Non-Linear Simulation of Intact and Damaged Ship Motions in Waves

This paper proposes a fast multigrid algorithm to simulate the non-linear motion of ships in both intact and damaged conditions. The simulations of ship motions in waves are known to require much time to calculate due to the strong non-linear interactions between ship and waves. To improve the calculation efficiency while retaining the accuracy, a prediction-correction strategy was designed to accelerate the simulation through three sets of locally refined meshes. The flow field was first estimated in a coarse mesh and then mapped to a locally refine mesh for further higher-fidelity corrections. A partitioned radial basis function (PRBF) method is proposed to interpolate and reconstruct the flow field for the refined mesh. A new two-phase flow solver was developed with a fast multigrid algorithm based on the Reynolds-averaged Navier–Stokes equations (RANSE). The new solver was applied to study the non-linear behavior of a damaged ship in beam waves and the effect of damaged compartments on ship rolling motion. Validation against the solution with the original method of single set meshes and experimental data indicates that the proposed algorithm yields satisfactory results while saving 30–40% of the computational time.

Traveling and Standing Flexural Waves in the Micro-Beam Based on the Fraction-Order Nonlocal Strain Gradient Theory

Abstract The traveling and standing flexural waves in the microbeam are studied based on the fraction-order nonlocal strain gradient elasticity in the present paper. First, the Hamilton’s variational principle is used to derive the governing equations and the boundary conditions with consideration of both the nonlocal effects and the strain gradient effects. The fraction-order derivative instead of the integer-order derivative is introduced to make the constitutive model more flexible while the integer-order constitutive model can be recovered as a special case. Then, the Euler–Bernoulli beam and the Timoshenko beam are both considered, and the corresponding formulations are derived. Two problems are investigated: (1) the dispersion of traveling flexural waves and the attenuation of the standing waves in the infinite beam and (2) the natural frequency of finite beam. The numerical examples are provided, and the effects of the nonlocal and the strain gradient effects are discussed. The influences of the fraction-order parameters on the wave motion and vibration behavior are mainly studied. It is found that the strain gradient effects and the nonlocal effect have opposite influences on the wave motion and vibration behavior. The fraction order also has evident influence on the wave motion and vibration behavior and thus can refine the prediction of the model.

Tunable transmission of a normal incidence through a double-slab structure

In this work, a double-slab structure is proposed, in which one slab is made of near-zero-real-part-of-impedance metamaterial (NZRPIM), and the other slab is made of ordinary transparent material. We shall demonstrate that, when a beam of electromagnetic wave is normally incident into the double-slab structure, the transmission may be enhanced significantly due to existence of a cross term induced by coherent superposition between forward and backward waves arising in the NZRPIM slab, thus the corresponding transmission peak may be strongly affected by the other slab and fluid arising between the two slabs, which is instructive significance for the design and application of high sensitivity refractive index sensor based on electromagnetic wave intensity detection. Our work may provide a new possible way to design and manufacture sensors by using NZRPIM.

Electromagnetic beam scattering of the anisotropic medium cylinder

Based on the conservation of electromagnetic energy, a theoretical model, which can equate the electromagnetic anisotropic medium to be the electromagnetic isotropic medium, is established. The correctness of this proposed model is verified, and method of using the model is presented. In case of the condition that the wavelength of electromagnetic wave being similar to the geometrical parameters of the cylinder, the equivalent permittivity of the cylinder of gypsum crystal in the elliptical Gaussian beam varies with its geometrical parameters, beam parameters and beam propagation direction are studied. The results show that the equivalent permittivity fluctuated changes with the increase of the radius of the cylinder, and the change of the equivalent permittivity is very small with the increase of the length of the cylinder, and the electromagnetic mechanisms of the phenomena are presented. The analytical expression of the scattered field of an anisotropic cylinder being irradiated by an electromagnetic wave beam with a general direction is studied, the correctness of the results obtained from this analytical expression are tested by the results from the references. It is found that the radius of the cylinder and the angle between the electromagnetic wave propagation direction and the symmetry axis of the cylinder have a great effect on the scattering, and the length of the cylinder, the distance between the beam and cylinder and the beam waist width and so forth have little effects on scattering.

Near-zero-index-featured multi-band highly directional radiator with large Purcell factors

Near-zero-index medium could be used to produce perfect planar wave beams, allowing for the design of compact antennas, but implementation is difficult due to the bandwidth and loss limitations. This work proposes a new method for achieving highly directional radiation by introducing a near-zero-index-featured resonator based on optical conformal mapping. The conceptual device performs the same functions as resonators filled with near-zero-index media but can be realized using dielectrics with refractive index n ≥ 1. Multi-band highly directional radiation with large Purcell factor is numerically demonstrated by the proposed high-Q resonator, which may pave the way for efficient production of high gain directive antennas in radar systems.

Orbital maneuver technology of temporary networked satellites for earth exploration

It is very difficult to achieve effective coverage of hot-spot target in a short time period by earth perturbation for orbital maneuver, because the coverage gap of the temporary multi-satellites network in certain period of time is too large. In this paper, it is assumed that wave beam type of the satellite-payload sensor is a simple conic, the beam center of the satellite payload under the maximum lateral pendulum is calculated based on the precise ephemeris, and according to the projection ellipse of the payload wave beam on the ground, the coverage ability of the satellite payload on the ground target direction is obtained. Then the satellite is designed to encounter the ground target in the nodal period between the satellite passing the ground target and the latitude circle. The orbital maneuver control variables are calculated from the change of nodal period, and the optimal orbital maneuver control variables are selected from a specified time period. At last, the orbit maneuver in covering the target in a specified time period is achieved and the maximum revisit interval is greatly shortened.

Using Phase Jumping Method to Enhance the Beam–Wave Interaction Efficiency in Terahertz Folded-Waveguide Traveling-Wave Tube

The folded-waveguide (FWG) traveling-wave tube (TWT) is a promising high-power terahertz (THz) amplifier. However, it is challenging to develop THz FWG-TWT with high beam–wave interaction efficiency. In this work, a phase jumping technology, which originates from free-electron lasers, is proposed to boost the beam–wave interaction efficiency of THz FWG-TWT. A modified slow wave structure acts as the phase shifter to adjust the phase gap between electron beam and electromagnetic wave. By jumping the phase gap to an appropriate value, the bunching center of electron beam keeps for a long time in the deceleration region. Thus, more energy is transferred to electromagnetic wave from bunching electron beam. To verify such proposition, a THz FWG-TWT example utilizing the phase jumping is presented in this article. Compared with conventional one, its efficiency within the operation bandwidth from 212 to 224 GHz is improved by 60%–140%.

Using heat maps in teaching acoustics: Interaction of incident plane wave or incident beam with a solid plane structure.

Using heat maps provides frame and relevance to topics covered in physical acoustics courses as students can more easily visualize complex ideas and situations. Students can also form a much better understanding of the physical phenomena involved and build more advanced critical thinking when heat maps are used. It is the aim of the paper to provide ways for students to interact with heat maps by using two examples: first, the interaction of a plane wave (not only monochromatic) with an interface separating a fluid and a solid medium and, second, the interaction of an acoustic beam with such an interface (including a solid layer). These heat maps permit both to describe non-specular effects for the reflected acoustic beam due to the generation of a modal wave and, in the case of anisotropic media, highlight the difference between the direction predicted by Snell-Descartes law and that predicted by energy flux.

Focusing of inertial waves by a vertically annular forcing

A theory of inertial wave focusing generated by a vertically oscillating slender torus immersed in a uniformly rotating fluid is presented. The analytical solution of the velocity field shows that, under an axisymmetric annular forcing in inviscid rotating fluid, the wave rays form a double cone symmetric about the plane on which the torus is located. At the two vertices of the double cone, the waves are in a shock-like manner focus causing localized surges of energy. After focusing, the waves continue their propagation and form a new inverted cone with the same cone angle, such that both cones are symmetric about the focal point. These results are in good agreement with the experimental and numerical study by Duran-Matute et al. [Phys. Rev. E 87, 041001(R) (2013)]. When friction effects occur, the wave pattern changes substantially and the wave away from the focal point is significantly attenuated so that the symmetry about the focal point is broken. As a consequence, the wave beam widens and the focusing effect becomes weaker with increasing Ekman number (Ek), which indicates the ratio of the viscous force to the Coriolis force. Furthermore, for the same Ek, the focusing effect tends to disappear when the forcing frequency is close to zero or twice the angular velocity of rotating flow. For forcing frequency close to the angular velocity of a rotating fluid, the amplitude of the vertical velocity at the focal point reaches its maximum, which corresponds to a wave propagation angle of 60 degrees.

Group delay time of fermions in graphene through tilted potential barrier

The group delay time of Dirac fermions subjected to a tilting barrier potential along the $ x $-axis is investigated in graphene. We start by finding the eigenspinor solution of the Dirac equation and then relating it to incident, reflected, and transmitted beam waves. This relationship allows us to compute the group delay time in transmission and reflection by obtaining the corresponding phase shifts. We discovered that the barrier width, incident energy, and incident angle can all be used to modify the group delay time, and that the particles travel through the barrier at the Fermi velocity $ v_F $. Our findings also show that the transmission group delay might be controlled, and that gate voltage control could be useful in graphene-based tilting barriers.

Experimental Investigation of a Shape-Optimized Staggered Double-Vane Slow-Wave Structure for Terahertz Traveling-Wave Tubes

Enormous concerns focus on the high-power and wide bandwidth traveling-wave tube (TWT) for its outstanding performance. In this article, a shape-optimized staggered double-vane slow-wave structure (SWS) for terahertz (THz) sheet beam TWT is proposed. The shape-optimized staggered double-vane SWS takes the advantages of higher interaction impedance, lower transmission loss, and lower phase velocity than conventional staggered double-vane SWSs. The shape-optimized staggered double-vane SWS with 85 periods is designed, fabricated, and cold tested. The measured transmission loss of the shape-optimized staggered double-vane SWS is less than 0.79 dB/cm in the frequency range of 0.211–0.26 THz, which is in good agreement with the simulation results. Furthermore, the beam–wave interaction analysis of a sheet beam TWT with this SWS is given. The output power is predicted to be >100 W with 3-dB bandwidth of >50 GHz. The results show that the shape-optimized staggered double-vane SWS is a very promising scheme to construct a high-power and wideband THz TWT for future applications such as communication and imaging.

Time-Domain Simulation of Helical Gyro-TWTs With Coupled Modes Method and 3-D Particle Beam

A new self-consistent time-domain model for the simulation of gyrotron traveling-wave tubes with a helically corrugated interaction space (helical gyro-TWTs) is presented. The new model links classical methods using the approach of slowly varying variables together with an expansion of the electromagnetic field in eigenmodes and advanced full-wave particle-in-cell (PIC) solvers. The aim is to significantly reduce the required calculation time compared to full-wave PIC solvers, while less strict assumptions are introduced as in the classical approaches of slowly varying variables. For the first time, the classical theory of coupled circular waveguide modes for the description of the operating electromagnetic eigenmode in the helical interaction space is combined with a 3-D PIC representation of the electron beam. This allows the simulation of the beam–wave interaction over a broad bandwidth and at arbitrary harmonics of the cyclotron frequency. In addition, arbitrary electron beams (with spreads, offsets of the guiding center from the symmetry axis, and so on) can be investigated. The new approach is compared with the full-wave 3-D PIC code CST Microwave Studio. A good agreement of the simulation results is achieved, while the computing time is significantly reduced.

Amplification of THz waves by beam-wave interaction in self-assembled helical slow-wave structures with single and double chirality

We investigate the interaction between an electron beam and a THz guided electromagnetic wave in a helical slow-wave structure formed by self-assembly of a conductive ribbon. We have previously shown the controlled fabrication of this slow-wave structure and its potential to form the basis for widely deployable millimeter-through-THz traveling-wave tube amplifiers. The process allows the fabrication of helical slow-wave structures with single and double chirality. Here, we use three-dimensional simulations to perform a comparative analysis of beam–wave interaction in self-assembled gold helices with single and double chirality. First, the structures are modeled without the electron beam (cold helices) to calculate the distribution of the electric field generated by the high-frequency wave. We perform simulations of cold helices by using Computer Simulation Technology Microwave Studio. Second, we evaluate the interaction between an electron beam and the THz travelingwave by using a particle in cell simulator in Computer Simulation Technology Particle Studio. Simulation studies show that a switch in chirality in the middle of self-assembled helices generates a reflected wave that boosts beam–wave interaction. We demonstrate that this efficient energy exchange will potentially provide high gain in THz traveling-wave tube amplifiers based on self-assembled helices.

Chiral trabeated metabeam for low-frequency multimode wave mitigation via dual-bandgap mechanism

An elastic wave in a physical beam naturally possesses many wave modes, such as flexural, longitudinal, and torsional. Therefore, suppression of all possible vibration modes has been rarely achieved in beam-shaped periodic systems, especially at low frequencies. Here we present a low-frequency complete bandgap mechanism by overlapping the flexural bandgap with the longitudinal-torsional bandgap. To strengthen the general framework, we enforce an extra degree of freedom (rotational and torsional-spring) on the spring-mass system for the flexural and coupled (longitudinal-torsional) modes. The low rotational stiffness provides a low flexural bandgap, whereas the torsional stiffness yields a coupled-mode bandgap. To meet these prerequisites in physical modeling, a chiral trabeated metabeam is conceived, which allows all wave modes to be suppressed by a complete bandgap. Apart from single-mode mitigation, our work provides a route to implementing a low-frequency complete bandgap in a periodic fashion, potentially enabling the use of chirality in elastic structures.

Twisted electron impact single ionization coincidence cross-sections for noble gas atoms

We present the angular profiles of the triple differential cross-section (TDCS) for the (e, 2e) process on the noble gas atoms, namely He (1s), Ne (2s and 2p), and Ar (3p), for the plane wave and the twisted electron impact. We develop the theoretical formalism in the first Born approximation (FBA). The present study compares the TDCS for different values of orbital angular momentum number m and opening angles θ p of the twisted electron beam with that of the plane wave beam in co-planar asymmetric geometry. In addition, we also investigate the TDCS for macroscopic targets to explore the influence of the opening angle θ p of the twisted electron beam on the TDCS. Our results show that the peaks in binary and recoil regions shift from the momentum transfer direction. The results also show that for larger opening angles, the peaks for p-type orbitals split into double-peak structures, which are not observed in the plane wave results for the given kinematics. The angular profiles for averaged cross-section show the dependence of TDCS on the opening angles, especially for the larger opening angles, which are significantly different from the plane wave TDCS.

Analysis of wander and spreading of an optical beam by using the oceanic turbulence optical power spectrum

Variance of beam displacement and short-term and long-term spreading of a Gaussian beam propagating in the presence of underwater turbulence are examined by using the oceanic turbulence optical power spectrum (OTOPS). Analytical expressions for both beam wander displacement variance and beam spreading are presented. Results show that the underwater turbulent channel causes deflection from the on-axis mean irradiance and brings significant wander and spreading effects to the propagating Gaussian beam wave. The variations of beam wander and short- and long-term spreading are obtained depending on the underwater medium parameters such as the average temperature, average salinity concentration, temperature-salinity gradient ratio, and temperature and energy dissipation rates. In particular, the real values of the average temperature and salinity concentration of turbulent water are used to obtain the results. In addition, the effects of propagation distance, Gaussian beam source size, and wavelength are shown. The results demonstrate that the underwater turbulent channel brings displacements in the centroid and spreading of the optical beam.

CFD database method for roll response of damaged ship during quasi-steady flooding in beam waves

A database method is proposed to predict the roll response of a damaged ship upon quasi-steady flooding in regular beam waves. To construct the hydrodynamic database, the hydrodynamic loads acting on the damaged ship are decomposed into wave excitation moment and motion-induced moment, which are calculated based on computational fluid dynamics (CFD). The wave excitation moments for different wave frequencies are formulated using the high-order trigonometric function. The ship hydrodynamic coefficients (i.e., added moment of inertia and damping coefficient) for different roll frequencies and amplitudes are regressed from the motion-induced moment and then used as samples for database construction by cubic spline interpolation. The Runge-Kutta method is employed to solve the ship's roll motion equation, in which the hydrodynamic coefficients are updated according to the roll amplitude in real time. To test the performance of the present method, the roll motion responses of a damaged frigate DTMB-5415 are computed. The results obtained by the database method are in good agreement with the CFD results and experimental data. The characteristics of motion-induced moment, roll hydrodynamic coefficients and wave excitation moment are analyzed. In addition, the influence of various factors (e.g., sensitivity of mesh and time step, order of regression method, interval of sample points and type of load decomposition model) on the computation accuracy are also discussed. Compared to directly performing CFD simulation of damaged ship motion in waves, the proposed database method can predict the ship's roll response with high efficiency and acceptable accuracy.

Performance of a free-space optical communication system employing receive diversity techniques in anisotropic atmospheric non-Kolmogorov turbulence

In this paper, bit error rate (BER) performance of a free-space optical communication (FSOC) system operating in anisotropic non-Kolmogorov weak turbulence is investigated together with the spatial diversity techniques. The spatial diversity techniques are implemented as maximum ratio combining (MRC), equal gain combining (EGC), and selection combining (SC) and applied to the receiver. The propagating beam is the Gaussian beam wave, and the modulation scheme is binary phase-shift keying (BPSK). Results are obtained for various parameters such as the anisotropy factor, non-Kolmogorov power law exponent, photodetector responsivity, equivalent load resistor, electronic bandwidth, Gaussian beam radius, wavelength, propagation distance, and turbulence structure constant. It is found that the spatial diversity technique used at the receiver causes significant improvement in the performance of an FSOC system under the conditions of anisotropic non-Kolmogorov atmospheric turbulence. It is also observed that BER performance improves as the atmospheric turbulence becomes more anisotropic. Among the spatial diversity techniques, SC is inferior to EGC and EGC is inferior to MRC in terms of BER performance.

Smart acoustic 3D cell construct assembly with high-resolution

Precise and flexible three-dimensional (3D) cell construct assembly using external forces or fields can produce micro-scale cellular architectures with intercellular connections, which is an important prerequisite to reproducing the structures and functions of biological systems. Currently, it is also a substantial challenge in the bioengineering field. Here, we propose a smart acoustic 3D cell assembly strategy that utilizes a 3D printed module and hydrogel sheets. Digitally controlled six wave beams offer a high degree of freedom (including wave vector combination, frequency, phase, and amplitude) that enables versatile biomimetic micro cellular patterns in hydrogel sheets. Further, replaceable frames can be used to fix the acoustic-built micro-scale cellular structures in these sheets, enabling user-defined hierarchical or heterogeneous constructs through layer-by-layer assembly. This strategy can be employed to construct vasculature with different diameters and lengths, composed of human umbilical vein endothelial cells and smooth muscle cells. These constructs can also induce controllable vascular network formation. Overall, the findings of this work extend the capabilities of acoustic cell assembly into 3D space, offering advantages including innovative, flexible, and precise patterning, and displaying great potential for the manufacture of various artificial tissue structures that duplicate in vivo functions.

Probability density of irradiance for electromagnetic waves propagating through turbulent magnetized plasma

SummaryNumerous physical mediums can be modelled as turbulent magnetized plasma, for example, the solar corona, and the ionosphere. Scintillation effects on the intensity of electromagnetic beam wave propagating through magnetized plasma necessitate the introduction of a statistical distribution valid over a large range of scintillation conditions, and incorporating the plasma physical properties. Under some simplifying assumptions, such a theoretical model can be derived based on the Rytov solution, the external magnetic field variation, and the space permeating plasma electron density variation. The resultant wave intensity distribution is compared with the Lognormal, Rician, and the Nakagami distributions for different scintillations conditions. The work presented in this paper is applicable to the analysis and design of optical wireless communication links through the ionosphere and deep space plasma.