Study of numerical simulation on dual-frequency IP method with FEM

Using the finite element method and Cole-Cole model for dual-frequency IP method to research numerical simulation, the authors introduced the fundamental principle of the dual-frequency IP method and the boundary value problem and variational equations, then replaced the complex resistivity of the model with the Cole-Cole model's parameters under ignoring the EM effect. Through solving the last linear equations, electric potentials of all the model's points were obtained. With changing model's parameters, the authors got different curves of the Fs and phases. According to the results of the simulation, the algorithm is proved to be correct and adaptable.

The numerical simulation on electromagnetic wave attenuation characteristics of coal face in time-frequency domain

This paper took the abnormal geological objects with high or low resistivity in the coal face as the background to establish the physical model. 2D forward numerical simulation for electromagnetic wave equation was implemented by the finite-difference scheme. According to the simulative results, the attenuation-absorption coefficient were calculated respectively based on field intensity and frequency shift parameter. Research result indicates, when coal-bed contains high electric resistivity geological abnormal object or low electric resistivity geological abnormal object, absorption attenuation function researched by frequency shift parameter of electromagnetic wave signal is more sensitive than by electromagnetic field intensity parameter.

Numerical study on characteristics of radio-frequency discharge at atmospheric pressure in argon with small admixtures of oxygen

In this paper, a 1D fluid model is developed to study the characteristics of a discharge in argon with small admixtures of oxygen at atmospheric pressure. This model consists of a series of equations, including continuity equations for electrons, positive ions, negative ions and neutral particles, the energy equation, and the Poisson equation for electric potential. Special attention has been paid to the electron energy dissipation and the mechanisms of electron heating, while the admixture of oxygen is in the range of 0.1%-0.6%. It is found that when the oxygen-to-argon ratio grows, the discharge is obviously divided into three stages： electron growth, electron reduction and the electron remaining unchanged. Furthermore, the cycle-averaged electric field, electron temperature, electron Ohmic heating, electron collisionless heating, electron energy dissipation and the net electron production are also studied in detail, and when the oxygen-to- argon ratio is relatively larger （R = 0.6%）, double value peaks of electron Ohmic heating appear in the sheath. According to the results of the numerical simulation, various oxygen-to-argon ratios result in different amounts of electron energy dissipation and electron heating.

Two-Dimensional Simulation of a Dual Frequency Sheath Near an Electrode with a Cylindrical Hole

The characteristics of a collisional dual frequency （DF） sheath near an electrode with a cylindrical hole are studied by utilizing a two-dimensional model which includes time-dependent fluid equations coupled with the Poisson equation and an equivalent-circuit model, The effects of the gas pressure on the two-dimensional profiles of the potential, electric field, ion fluid velocity in a DF sheath are investigated. The simulation results show that the cylindrical hole on the electrode has a significant influence on the DF sheath structure, i.e., the sheath profile tends to wrap around the contour of the hole feature. Moreover, it is shown that the structure of the DF sheath is different from that of a single frequency （SF） sheath because the profile of the DF sheath is modulated by the combination of the high and low frequency sources. In addition the characteristics of the DF sheath are obviously affected by the collisional effects in the DF sheath.

Numerical simulation and temporal characterization of dual-pumped microringresonator-based optical frequency combs

Dual-pumped microring-resonator-based optical frequency combs(OFCs) and their temporal characteristics are numerically investigated and experimentally explored. The calculation results obtained by solving the driven and damped nonlinear Schr?dinger equation indicate that an ultralow coupled pump power is required to excite the primary comb modes through a non-degenerate four-wave-mixing(FWM) process and, when the pump power is boosted, both the comb mode intensities and spectral bandwidths increase. At low pump powers, the field intensity profile exhibits a cosine variation manner with frequency equal to the separation of the two pumps, while a roll Turing pattern is formed resulting from the increased comb mode intensities and spectral bandwidths at high pump powers. Meanwhile, we found that the power difference between the two pump fields can be transferred to the newly generated comb modes, which are located on both sides of the pump modes, through a cascaded FWM process. Experimentally, the dual-pumped OFCs were realized by coupling two self-oscillating pump fields into a microring resonator. The numerically calculated comb spectrum is verified by generating an OFC with 2.0 THz mode spacing over 160 nm bandwidth. In addition, the formation of a roll Turing pattern at high pump powers is inferred from the measured autocorrelation trace of a 10 free spectral range(FSR) OFC. The experimental observations accord well with the numerical predictions. Due to their large and tunable mode spacing, robustness,and flexibility, the proposed dual-pumped OFCs could find potential applications in a wide range of fields,including arbitrary optical waveform generation, high-capacity optical communications, and signal-processing systems.

Numerical Simulation of Electromagnetic Field and Temperature Field in Medium-Frequency Induction Furnace Melting Process

A mathematical model for describing the melting process in the medium-frequency induction furnace was developed.Finite difference method was applied to deal with coupling electromagnetic field and temperature field in the melting process.The magnetic induction,temperature distribution and the phase interface moving characteristic during melting of the furnace burden were calculated.The effects of the direct current and inductive heating frequency on the process were analyzed.The simulation results show that:In the direction of burden radius,magnetic induction decreases from the outside of the burden to the center.Solid/liquid interface moves gradually from the outside of the burden to the center.The movement speed increases when the burden begins to melt.In the direction of the burden height,the distribution of eddy current in the surface is accord with the edge effect of the coil.Solid/liquid interface moves gradually from the center to the two sides.The direct current has a greater effect on the electromagnetic field and temperature field than frequency.

Crack detection using a frequency response function in offshore platforms

Structural cracks can change the frequency response function (FRF) of an offshore platform. Thus, FRF shifts can be used to detect cracks. When a crack at a specific location and magnitude occurs in an offshore structure, changes in the FRF can be measured. In this way, shifts in FRF can be used to detect cracks. An experimental model was constructed to verify the FRF method. The relationship between FRF and cracks was found to be non-linear. The effect of multiple cracks on FRF was analyzed, and the shift due to multiple cracks was found to be much more than the summation of FRF shifts due to each of the cracks. Then the effects of noise and changes in the mass of the jacket on FRF were evaluated. The results show that significant damage to a beam can be detected by dramatic changes in the FRF, even when 10% random noise exists. FRF can also be used to approximately locate the breakage, but it can neither be efficiently used to predict the location of breakage nor the existence of small hairline cracks. The FRF shift caused by a 7% mass change is much less than the FRF shift caused by the breakage of any beam, but is larger than that caused by any early cracks.

Gas Breakdown of Radio Frequency Glow Discharges in Helium at near Atmospheric Pressure

A one-dimensional self-consistent fluid model was developed for radio frequency glow discharge in helium at near atmospheric pressure, and was employed to study the gas breakdown characteristics in terms of breakdown voltage. The effective secondary electron emission coefficient and the effective electric field for ions were demonstrated to be important for determining the breakdown voltage of radio frequency glow discharge at near atmospheric pressure. The constant of A was estimated to be 64：t=4 cm-lTorr-1, which was proportional to the first Townsend coefficient and could be employed to evaluate the gas breakdown voltage. The reduction in the breakdown voltage of radio frequency glow discharge with excitation frequency was studied and attributed to the electron trapping effect in the discharge gap.

Effect of a preload force on anchor system frequency

The interrelationship between preload forces and natural frequencies of anchors was obtained from the structure of an anchor and its mechanical characteristics. We established a numerical model for the dynamic analysis of a bolt support system taking into consideration the working surroundings of the anchor. The natural frequency distribution of the system under various preload forces of the anchor was analyzed with ANSYS. Our results show that each order of the system frequency varied with an increase in preload forces. A single order frequency decreased with an increase in the preload force. A preload force affected low-order frequencies more than high-order frequencies. We obtained a functional relationship by fitting preload forces and fundamental frequencies, which was in agreement with our theretical considerations. This study provides theoretical support for the detection of preload forces.

Investigations of high-frequency induction hardening process for piston rod of shock absorber

The microhardness of piston rods treated with different induction hardening processes was tested. The experimental results reveal that the depth of the hardened zone is proportional to the ratio of the moving speed of the piston rod to the output power of the induction generator. This result is proved correct through the Finite Element Method (FEM) simulation of the thermal field of induction heating. From tensile and impact tests, an optimized high frequency induction hardening process for piston rods has been obtained, where the output power was 82%×80 kW and the moving speed of workpiece was 5364 mm/min. The piston rods, treated by the optimized high frequency induction hardening process, show the best comprehensive mechanical performance.

A note for the mechanism of high-frequency oscillation instability resulted from absorbing boundary conditions

In this paper the explanation of the mechanism of high-frequency oscillation instability resulted from absorbing boundary conditions is further improved. And we analytically prove the proposition that for one dimensional discrete model of elastic wave motion, the module of reflection factor will be greater than 1 in high frequency band when artificial wave velocity is greater than 1.5 times the ratio of discrete space step to discrete time step. Based on the proof, the frequency band in which instability occurs is discussed in detail, showing such high-frequency waves are meaningless for the numerical simulation of wave motion.

Formation of Well-Mixed Warm Water Column in Central Bohai Sea During Summer: Role of High-Frequency Atmospheric Forcing

The influence of high-frequency atmospheric forcing on the formation of a well-mixed summer warm water column in the central Bohai Sea is investigated comparing model simulations driven by daily surface forcing and those using monthly forcing data. In the absence of high-frequency atmospheric forcing, numerical simulations have repeatedly failed to reproduce this vertically uniform column of warm water measured over the past 35 years. However, high-frequency surface forcing is found to strongly influence the structure and distribution of the well-mixed warm water column, and simulations are in good agreement with observations. Results show that high frequency forcing enhances vertical mixing over the central bank, intensifies downward heat transport, and homogenizes the water column to form the Bohai central warm column. Evidence presented shows that high frequency forcing plays a dominant role in the formation of the well-mixed warm water column in summer, even without the effects of tidal and surface wave mixing. The present study thus provides a practical and rational way of further improving the performance of oceanic simulations in the Bohai Sea and can be used to adjust parameterization schemes of ocean models.

Numerical Study of the Forming Process of High Frequency Welded Pipe

The roll forming process is applied to the manufacturing of high frequency welded (HFW) pipes,section steels,etc. In this paper,the roll forming process of the HFW pipe is simulated with the finite element method (FEM). A user-defined material routine of the commercial finite element code ABAQUS/Explicit is developed,and the mixed hardening constitution model is realized through the user-defined material routine. Based on the mixed hardening constitutive equation,the numerical simulation of roll forming process of HFW pipe is performed. The evolutions of equivalent stress and strain are analyzed,and the calculated results are also compared between different hardening models. The results show that the different material hardening models have some important effects on the variation of equivalent stress and strain of strip steel during the simulation of the roll forming process.

Numerical simulations of the impact of wavefront phase distortions of pump on the beam quality of OPA

A numerical model of optical parametric amplification （OPA） is introduced to investigate the impact of wavefront phase distortion of pump on the beam quality of signal. Numerical results show that the unidentical walk-off directions of the pump and the idler waves are the main factors leading to the transfer of wavefront phase distortions of the pump to the signal, and by reducing the angle between the two directions, the beam quality factor （M2） can be greatly decreased and hence the good beam quality of the signal can be maintained.

The influence of various frequency chorus waves on electron dynamics in radiation belts

Non-adiabatic behaviour induced by the chorus-electron interaction is an important contributor to the radiation belt dynamics,and largely relies on wave frequency distribution.During the geomagnetic storm on August 2,2016,upper-band,lower-band and extremely low frequency(ELF)chorus waves were simultaneously observed within one orbit period of the Van Allen Probe B.Numerical simulations are performed to investigate the electron evolution by the observed chorus with different frequencies.The results show that various frequency chorus waves have different effects on electron dynamics.For chorus in the range f≈0.3 fce–0.7 fce,energy diffusion is the dominant process in electron evolutions.For chorus in the range 0.1 fce≤f≤0.25 fce,the pitch angle diffusion tends to be comparable to energy diffusion for Ek>0.5 MeV.For ELF chorus below 0.1 fce,the pitch angle diffusion rate is much above the energy diffusion rate,leading to potential scattering losses.

The nature frequency identification of tunnel lining based on the microtremor method

Many tunnels all over the world have been in service for several decades,which require effective inspection methods to assess their health conditions.Microtremor,as a type of ambient vibration originating from natural or artificial oscillations without specific sources,has attracted more and more attentions in the recent study of the microtremor dynamic properties of concrete structures.In this study,the microtremors of the tunnel lining were simulated numerically based on the Distinct Element Method(DEM).The Power Spectra Density(PSD)of signals obtained from numerical simulations were calculated and the nature frequencies were identified using the peak-picking method.The influences of the rock-concrete joint,the rock type and the concrete type on the nature frequencies were also evaluated.The results of a comprehensive numerical analysis show that the nature frequencies lower than 100 Hz can be identified.As the bonding condition becomes worse,the nature frequencies decrease.The nature frequencies change proportionally with the normal stiffness of the rock-concrete joint.As the concrete grade decreases,the third mode of frequency also decreases gradually while the variation of the first two modes of frequencies can hardly be identified.Additionally,the field microtremor measurements of tunnel lining were also carried out to verify the numerical results.

Parametric instability of a liquid metal sessile drop under the action of low-frequency alternating magnetic fields

A 2-D mathematical model is developed in order to simulate a parametric electromagnetic instability oscillation process of a liquid metal droplet under the action of low frequency magnetic field. The Arbitrary Lagrangian-Eulerian （ALE） method and weak form constraint boundary condition are introduced in this model for implementation of the surface tension and electromagnetic force on liquid droplet free surface. The results of the numerical calculations indicate the appearance of various regimes of oscillation. It is found that according to the magnetic field frequency various types of oscillation modes may be found. The oscillation is originated from an instability phenomenon. The stability diagram of liquid metal droplet in the parameter space of magnetic frequency and magnetic flux density is determined numerically. The diagram is very similar to that found in the so-called parametric instability.

The effect of actuation frequency on the plasma synthetic jet

The flow induced by plasma synthetic jet actuator was simulated through solving the Reynolds-averaged Navier-Stokes equations augmented by body force phenomenological plasma model.The effect of actuation frequency on the plasma synthetic jet was examined by case study.The numerical results present that with the actuation frequency increasing,the stream-wise distance of the adjacent vortex pairs induced by the actuator decreases monotonically,which is the same as the situation of the velocity fluctuations field caused by the vortex pairs.When the actuation frequency is 60 Hz,the vortex pairs formed during the adjacent actuation periods merge together quickly,and the flow structure in the downstream region is more close to that of the steady case.The actuation frequency has no visible influence on the time-averaged flow field of plasma synthetic jet.However,when the actuation frequency is relatively low(f<40 Hz),the momentum flux close to the actuator increases with the actuation frequency increasing,which is contrary to the situation in the far field from the wall.

Domain decomposition based on the spectral element method for frequency-domain computational elastodynamics

We propose a domain decomposition method based on the spectral element method(DDM-SEM)for elastic wave computation in frequency domain.It combines the high accuracy of the spectral element method and the high degree of parallelism of a domain decomposition technique,which makes this method suitable for accurate and efficient simulations of large scale problems in elastodynamics.In the DDM-SEM,the original large-scale problem is divided into a number of well designed subdomains.We use the spectral element method independently for each subdomain,and the neighboring subdomains are connected by a frequency-domain version of Riemann transmission condition(RTC)for elastic waves.For the proposed method,we can employ the non-conforming meshes and different interpolation orders in different subdomains to maximize the efficiency.By separating the internal and boundary unknowns of each subdomain,an efficient and naturally parallelizable block LDU direct solver is developed to solve the final system matrix.Numerical experiments verify its accuracy and efficiency,and show that the proposed DDM-SEM can be a promising numerical tool for accurately and effectively solving large and multi-scale problems of elastic waves.It is potentially valuable for the frequency domain seismic inversion where multiple source illuminations are required.