Numerical simulation of electromagnetic acoustic transducers in time domain

The present paper is aimed at introducing a new algorithm for image encryption using chaotic tent maps and the desired key image. This algorithm consists of two parts, the first of which working in the frequency domain and the second, in the time domain. In the frequency domain, a desired key image is used and a random number is generated, using the chaotic tent map, in order to change the phase of the plain image. This change in the frequency domain causes changes in the pixels value and shuffles the pixels location in the time domain. Finally, in the time domain, a pseudo random image is produced using a chaotic tent map, to be combined to the image generated through the first step, and thus the final encrypted image is created. A computer simulation is also utilized to evaluate the proposed algorithm and to compare its results to images encrypted by other methods. The criteria for these comparisons are chi square test of histogram, correlation coefficients of pixels, NPCR (number of pixel change rate), UACI (unified average changing intensity), MSE (mean square error) and MAE (mean absolute error), key space, and sensitivity to initial condition. These comparisons revealed that the proposed chaotic image encryption method shows a higher performance and is of more security.

Local defect correction techniques : analysis and application to combustion

Reduction of computing time for seismic applications based on the Helmholtz equation by Graphics Processing Units

Accurate prediction of loads on mechanical couplings is crucial in assessment of loads on coupled structures, and in reliable prediction of the motions of coupled bodies. This thesis presents a numerical articulation method, which is capable of modeling various mechanical couplings. The proposed numerical method relies on contact elements. The first objective of the work is to prove the eligibility of contact elements in representation of mechanical couplings, and to highlight the shortcomings and positive aspects of contact elements for this representation. The second objective of the thesis is to study the influence of different hydrodynamic, kinematic, and coupling models on the motion behavior of articulated bodies and coupling forces. To accomplish these objectives extensive and systematic studies on a twofold pushing convoy are conducted. The convoy bodies are interconnected with hinge joints. The hinges are modeled by means of kinematic constraints, and with contact elements. Frequency and time domain boundary element methods with linearized motions of equation, as well as a Reynolds-averaged Navier-Stokes (RANS) code coupled with a nonlinear kinematic solver are used to compute the body motion responses and coupling forces. The simulation results are compared to model basin measurements. It is shown that the contact element model can simulate mechanical couplings efficiently and it provides a suitable method to idealize free and suppressed modes at articulation locations. Furthermore, it is shown, that RANS in association with the nonlinear equation of motion and kinematically constrained joints, as well as boundary element methods in adherence to the contact element model provide more accurate prediction of articulation forces, and motion responses of coupled bodies. However, frequency and time domain boundary element methods with kinematically constrained joints still deliver agreeable and reliable results. Moreover, a case study is presented, in which a river-sea pushing convoy is applied in time domain numerical simulations. The convoy bodies are interconnected with prismatic joints and are free to heave and pitch relative to each other. Prismatic joints are modeled by means of contact elements. The case study demonstrates a successful application of contact elements in simulation of complexly articulated multibody systems. Furthermore, the findings obtained in this work contribute sustainably to the numerical approaches in assessment of hydrodynamic and articulation loads on mechanically coupled floating bodies.

Last years, Wavelet Packet Modulation (WPM) or Wavelet Packet Transform based Orthogonal Frequency Division Multiplexing (WPT-OFDM) have been introduced to wired and wireless communication fields as efficient Multicarrier Modulation (MCM) techniques. The wavelets have interesting features such as flexibility, compatibility and localization in both time and frequency domains with no need to use rectangular window function. As a result, the transmitted signal is naturally less sensitive to inter-symbol and inter-carrier interferences (ISI and ICI). Also, it is possible to implement OFDM modulation without adding cyclic prefix (CP) and it is enough only to use time domain or overlap frequency domain equalization (TEQ or overlap FEQ) in order to shorten the effective channel impulse response length with the purpose of avoiding the ISI and decreasing the ICI. In this paper, we compare BER performance for FFT-OFDM and WPT-OFDM in the presence of two types of channels defined by ETSI (i.e. Rayleigh P1 and Rician F1 channels). In our simulation Haar, Daubechies6, Symlet5 and Coiflet5 wavelets and overlap frequency domain equalization (overlap FEQ) are used for WPT-OFDM in contrast with FFT-OFDM which uses FEQ equalization. Simulation results show that the performance of OFDM will be improved by using WPT transform and due to no need of CP, the power/bandwidth efficiency of OFDM modulation will be improved as well.

The prototypical signal processing pipeline can be divided into four blocks. Representation of the signal in a basis suitable for processing. Enhancement of the meaningful part of the signal and noise reduction. Estimation of important statistical properties of the signal. Adaptive processing to track and adapt to changes in the signal statistics. This thesis revisits each of these blocks and proposes new algorithms, borrowing ideas from information theory, theoretical computer science, or communications. First, we revisit the Walsh-Hadamard transform (WHT) for the case of a signal sparse in the transformed domain, namely that has only K ≀ N non-zero coefficients. We show that an efficient algorithm exists that can compute these coefficients in O(K log2(K) log2(N/K)) and using only O(K log2(N/K)) samples. This algorithm relies on a fast hashing procedure that computes small linear combinations of transformed domain coefficients. A bipartite graph is formed with linear combinations on one side, and non-zero coefficients on the other. A peeling decoder is then used to recover the non-zero coefficients one by one. A detailed analysis of the algorithm based on error correcting codes over the binary erasure channel is given. The second chapter is about beamforming. Inspired by the rake receiver from wireless communications, we recognize that echoes in a room are an important source of extra signal diversity. We extend several classic beamforming algorithms to take advantage of echoes and also propose new optimal formulations. We explore formulations both in time and frequency domains. We show theoretically and in numerical simulations that the signal-to-interference-and-noise ratio increases proportionally to the number of echoes used. Finally, beyond objective measures, we show that echoes also directly improve speech intelligibility as measured by the perceptual evaluation of speech quality (PESQ) metric. Next, we attack the problem of direction of arrival of acoustic sources, to which we apply a robust finite rate of innovation reconstruction framework. FRIDA — the resulting algorithm — exploits wideband information coherently, works at very low signal-to-noise ratio, and can resolve very close sources. The algorithm can use either raw microphone signals or their cross- correlations. While the former lets us work with correlated sources, the latter creates a quadratic number of measurements that allows to locate many sources with few microphones. Thorough experiments on simulated and recorded data shows that FRIDA compares favorably with the state-of-the-art. We continue by revisiting the classic recursive least squares (RLS) adaptive filter with ideas borrowed from recent results on sketching least squares problems. The exact update of RLS is replaced by a few steps of conjugate gradient descent. We propose then two different precondi- tioners, obtained by sketching the data, to accelerate the convergence of the gradient descent. Experiments on artificial as well as natural signals show that the proposed algorithm has a performance very close to that of RLS at a lower computational burden. The fifth and final chapter is dedicated to the software and hardware tools developed for this thesis. We describe the pyroomacoustics Python package that contains routines for the evaluation of audio processing algorithms and reference implementations of popular algorithms. We then give an overview of the microphone arrays developed.

The ring cyclotron of the Paul Scherrer Institute (PSI) accelerates an intense proton beam from 72MeV up to 590MeV. This happens in four cavities of very high quality factor, oscillating in the fundamental mode. The beam can excite parasitic oscillation modes (HOMs), because of its time structure. Measurements showed that their field can leak out into the vacuum chamber. Until now, there is no tool available to predict the potentially harmful effect of these HOMs onto the beam operation of the cyclotron. It is foreseeable that these effects might play a role if even higher beam currents have to be accelerated. This dissertation therefore deals with the numerical analysis and measurement of beam-cavity interactions. First calculations for a single cavity, interacting with a proton bunch were performed with MAFIA's eigenmode- (E3), time domain- (T3) and particle-in-cell (TS3) solvers. However, the structured grid and the limited computing performance of MAFIA make realistic simulations impossible. A simplified computation method is developed in this dissertation since a self-consistent simulation is impossible on today's computers: The parallel eigensolver Omega3P of the Stanford Linear Accelerator Center (SLAC) allowed us to calculate eigenmodes of the entire ring cyclotron for the first time ever. The rf fields are expanded onto a superposition of these modes and the excitation is calculated in frequency domain. Trajectories of the particles in the static magnetic field, superposed with the space charge fields and the beam excited HOMs, are then simulated. However, the quantitative accuracy of this model is still limited. On the one hand, because of the simplification in the geometry of the simulated rf structure, which otherwise would lead to a problem size going beyond the available computing resources. On the other hand, because it is not yet possible to simulate strongly absorbing boundaries more accurately. The simulation results confirm that up to proton beam currents of 2mA, corresponding to the routinely accelerated beam intensities, only a small deformation of the charge distribution appears. This thesis leads to a new simulation tool for further studies of intensity increases in high power cyclotrons.

In previous work, a complete dynamic inflow model for flow above the rotor disk has been developed; and numerical results have been presented in both the frequency domain and the time domain. In this paper, we extend the inflow model to all three components of inflow below the disk. The inflow model, which is expressed in terms of a finite number of states, is derived in a mathematically rigorous way. The essence of the extension is that if one computes the co-states of the inflow equations (along with the normal states), then one can find the velocity in the hemisphere below the plane of the rotor disk (including the velocity within the wake) with accuracy equal to that of the flow above the rotor plane. The derivation is for the case of general skew angle. Numerical comparisons with exact solutions for the z-component of flow in axial flow––given for some special cases––illustrate the effectiveness of the new model. The simulations also illustrate that the model is valid for either the frequency domain or the time domain. Nomenclature n m a : real part for cosine induced inflow expansion coefficients n m b : imaginary part for cosine induced inflow expansion coefficients [ ] D :

Gyrotrons are microwave tubes capable of providing mega-watt power at millimetric wavelengths. The microwave power is produced by the conversion of the kinetic energy of an electron beam to electromagnetic wave energy. Simulations of the beam-wave interaction in the gyrotron cavity are essential for gyrotron design, as well as theoretical and experimental studies. In the usual gyrotron operation the spectrum of the generated radiation is concentrated around the nominal frequency. For this reason, the usual simulations consider only a narrow-band output spectrum (e.g. several GHz bandwidth comparing with the working frequency in the range of 100-200 GHz). As a result, the typical existing codes use a single-frequency radiation boundary condition for the generated electromagnetic field in the cavity. This condition is matched only at one frequency. However, there are two important aspects, which motivate an advanced formulation and implementation of the cavity boundary condition. Firstly, the occurrence of broadband effects (which may be several tens of GHz) in some cases, like dynamic after-cavity-interaction or modulation side-bands, requires a broadband boundary condition. Secondly, there are reflections from inside and outside of the gyrotron, which can only be considered in the simulation through a boundary condition with user-defined, frequency-dependent reflections. This master thesis proposes an improved formulation of the broadband boundary condition in the self-consistent, beam-wave interaction code Euridice. In this new formulation, two physical variables — the wave impedance and the axial wavenumber are expanded in polynomial series in the frequency domain. Because the beam-wave interaction process is simulated transiently in the time domain, the boundary condition should be also expressed in the time domain. This involves a non-trivial inverse Fourier transform, for which two solutions are proposed, tested and validated. It has been shown that, through the newly developed formulation, the existing matched boundary condition (that should yield zero-reflection in ideal case) can be improved by 15 dB even with a first-order polynomial series. Moreover, a user-defined, frequency-dependent complex reflection coefficient can be introduced. This was not possible with the previously existing boundary condition in Euridice.

A numerical study of magnetohydrodynamic flows and heat transfer past a circular cylinder in a duct under a strong magnetic field parallel to the cylinder axis is presented. In this configuration, the flow is quasi-two-dimensional and the modified Navier–Stokes equations are solved in a two–dimensional domain. The numerical simulations have been performed over a range of parameters including the Reynolds number 50 ≤ Re ≤ 3000, modified Hartmann number 50 ≤ Ha⋆ . 500, blockage ratio 0.1 ≤ β ≤ 0.5, offset ratio 0.25 ≤ γ ≤ 1, velocity amplitude 0 ≤ A ≤ 3 and forcing frequency 0 ≤ Ste ≤ 10. The primary aim of this study is to understand the fundamental mechanism that governs transition to unsteady flow in this system and exploit this for further improvements in heat transfer for MHD cooling duct flows. With this aim in mind, a spectral-element method is employed to compute the MHD flow and heat transfer past a confined circular cylinder in a rectangular duct. Meshes have been constructed to deal with the significant number of geometric flow parameters combinations. Thorough validation and grid resolution studies have been performed to ensure adequate domain sizes, and spatial and temporal resolutions to accurately resolve all flow and thermal features for the reported flow variable ranges. Studies show that the reported flow parameters are converged to better than 0.3% in terms of spatial and temporal accuracy and 1% with respect to the domain size. For the optimal growth studies, the dependence of energy growth on upstream domain length is also considered through the calculation of the energy growth over a fixed time span. This verifies that the effect of truncating the upstream length from 32d to 8d causes an error of less than 3% in the growth rate prediction. The critical Reynolds number Rec for the transition from steady to periodic flow is determined as a function of Ha⋆ and β, and this is found to increase with increasing Ha⋆ and β. In addition, the variation of the wake recirculation length in the steady flow regime is determined as a function of Reynolds number, Hartman number and blockage ratio, and a universal expression is proposed. The characteristics of heat transfer depend strongly on the proximity of the cylinder to the heated wall. For small blockage ratios, it increases significantly as the gap ratio decreases from 1 to 0.25. However, there is a substantial drop in heat transfer for high blockage ratio. Downstream cross-stream mixing induced by the cylinder wake is found to increase the heat transfer augmentation by more than a factor of two in some cases. The maximum gain in heat transfer generated by placing the cylinder in the channel near to the wall with that at the centerline was obtained for the lowest blockage ratio β = 0.1, as the cylinder is further approached the heated wall. For all β, a very significant transient energy growth is found in the subcritical regime below the onset of vortex shedding. This suggests a potential for the design of an actuation mechanism to invoke vortex shedding and then enhance heat transfer in these ducts. The energy amplification of the disturbances is found to decrease significantly with increasing Hartmann number and the peak growth shifts towards smaller times while it increases significantly with increasing blockage ratio. The structure of the optimal initial disturbance is found to be consistent across the all blockage ratios being tested. In line with similar problems, it convects along the separating region being amplified to the peak growth state downstream of the recirculation bubble. The maximum time for maximum energy growth τmax is found to increase significantly as recirculation length increases which demonstrates the amplifying nature of the separated shear layers in the wake. The critical Reynolds number for the onset of positive growth at different Hartmann numbers and blockage ratios is determined. It is found that it increases rapidly with increasing Hartmann number and blockage ratio. For all β, the peak energy amplification grows exponentially with Re from low Hartmann numbers. Direct numerical simulation studies in which the inflow is perturbed by random white noise confirms the predictions arising from the transient growth analysis: that is, the perturbation excites and feeds energy into the global mode. A considerable increase in heat transfer from the heated channel wall occurs from rotational oscillation of the cylinder, with a maximum enhancement of approximately 22% observed at higher amplitude over that for steady flow. The oscillation frequency range for effective enhancement is widened in both directions, while the frequency at which the peak of the Nusselt number occurs is shifted slightly to a lower frequency as A is increased. It is found that as the amplitude was reduced, the forcing frequency approaches the global frequency mode.

Centrifugal pumps are required to sustain a stable operation of the system they support under all operating conditions. Minor modifications of the surfaces defining the pump's water passage can influence the tendency to unstable system operation significantly. The action of such modifications on the flow are yet not fully understood, leading to costly trial and error approaches in the solution of instability problems. The part-load flow in centrifugal pumps is inherently time-dependent due to the interaction of the rotating impeller with the stationary diffuser (Rotor-Stator Interaction, RSI). Furthermore, adverse pressure gradients in the pump diffuser may cause flow separation, potentially inducing symmetry-breaking non-uniformities, either spatially stationary or rotating and either steady or intermittent. Rotating stall, characterized by the presence of distinct cells of flow separation on the circumference, rotating at a fraction of the impeller revolution rate, has been observed in thermal and hydraulic turbomachines. Due to its complexity, the part-load flow in radial centrifugal pumps is a major challenge for numerical flow simulation methods. The present study investigates the part-load flow in radial centrifugal pumps and pump-turbines by experimental and numerical methods, the latter using a finite volume discretization of the Reynolds-averaged Navier-Stokes (RANS) equation. Physical phenomena of part load flow are evidenced based on three case studies, and the ability of numerical simulation methods to reproduce part-load flow in radial centrifugal pumps qualitatively and quantitatively is assessed. A numerical study of the flow in a high specific speed radial pump-turbine using steady approaches and the hypothesis of angular periodicity between neighboring blade channels evidences the relation of sudden flow topology changes with an increase of viscous losses, impacting on the energy-discharge characteristic, and thus increasing the risk of unstable operation. When the flow rate drops below a critical threshold, the straight through-flow with flow separation zones attached to the guide vanes changes to an asymmetrical flow. Energy is drawn off the mean flow and dissipated in a large vortex-like structure. Besides flow separation in some diffuser channels, time-dependent numerical simulations of the flow in a double suction pump evidence a flow rate imbalance between both impeller sides interacting with asymmetric flow separation in the diffuser. Viscous losses increase substantially as this imbalance occurs, the resulting segment of positive slope in the energy-discharge characteristic is found for a flow rate sensibly different from measurements. Different modes of rotating stall are identified by transient pressure measurements in a low-specific-speed pump-turbine, showing 3 to 5 zones of separated flow, rotating at 0.016 to 0.028 times impeller rotation rate, depending on discharge. For operating conditions where stall with 4 cells is most pronounced, velocity is measured by Laser-Doppler methods at locations of interest. The velocity field is reconstructed with respect to the passage of stall cells by definition of a stall phase obtained from simultaneous transient pressure measurements. Time-dependent numerical simulation predicting rotating stall with 4 cells shows velocity fields that are in reasonable agreement with the measured velocity fields, but occurring at a sensibly higher flow rate than found from experiments. In consideration of the quantitative shortcomings of the numerical simulation, a novel modelling approach is proposed: Replacing the costly 3-dimensional simulation of the major part of the impeller channels by a 1-dimensional model allows a significant economy in computational resources, allowing an improved modeling for the remainder of the domain at constant computational cost. The model is validated with the challenging cases of rotating stall and impeller side flow rate imbalance. The satisfying coherence of the results with the simulation including the entire impeller channels qualifies this approach for numerous turbomachinery applications. It could also provide improved, time-dependent boundary conditions for draft tube vortex rope simulations at reasonable computational cost. Parameter studies modifying deliberately some quantities of mean flow and turbulence at the modeled boundary surfaces can be implemented in the framework of the method.


 
 
 This paper documents the investigation of fuel injector fault detection methods for a seven liter diesel engine. This effort was conducted for the Tank Automotive Research Development Engineering Center (TARDEC) Condition Based Maintenance (CBM) team. The task was to develop algorithms capable of real-time detection of injector misfire events. The purpose of this task was to enable TARDEC’s Engine Control Management (ECM) research and development efforts to evaluate the technical feasibility of integrating automated on-board condition monitoring algorithms with future ECM monitoring and control operations.
 
 

In this paper, a problem in engineering area which the output variables are not corresponding to input variables is presented. To improve it, a kind of method to the identification and modeling of a common linear state system with delay factor are studied. The domain of this system with time-delay factor is transformed from the time-domain to the frequency-domain firstly, and then the subspace identification model with the hiding delay factor is constructed by using the data of frequency domain response. The coefficient matrix of the constructed model is identified by using the principal component analysis. And the engineering system can be modeled by knowing the state matrices in time domain which can be extracted from the coefficient matrices and using the least squares method from the frequency domain. On this basis, the time-delay factor of original system is split from input matrix by a kind of separated method. At last, the method proposed is used to identify the temperature system model of ceramic shuttle kiln. Simulation results show that the proposed method is effective and feasible.

The modelling of microelectromechanical systems provides a very challenging task in microsystems engineering. This field of research is inherently multiphysics of nature, since different physical phenomena are tightly intertwined at microscale. Typically, up to four different physical domains are usually considered in the analysis of microsystems: mechanical, electrical, thermal and fluidic. For each of these separate domains, well-established modelling and analysis techniques are available. However, one of the main challenges in the field of microsystems engineering is to connect models for the behavior of the device in each of these domains to equivalent lumped or reduced-order models without making unacceptably inaccurate assumptions and simplifications and to couple these domains correctly and efficiently. Such a so-called multiphysics modelling framework is very important for simulation of microdevices, since fast and accurate computational prototyping may greatly shorten the design cycle and thus the time-to-market of new products. This research will focus on a specific class of microsystems: microelectromechanical resonators. MEMS resonators provide a promising alternative for quartz crystals in time reference oscillators, due to their small size and on-chip integrability. However, because of their small size, they have to be driven into nonlinear regimes in order to store enough energy for obtaining an acceptable signal-to-noise ratio in the oscillator. Since these resonators are to be used as a frequency reference in the oscillator circuits, their steady-state (nonlinear) dynamic vibration behaviour is of special interest. A heuristic modelling approach is investigated for two different MEMS resonators, a clamped-clamped beam resonator and a dog-bone resonator. For the clamped-clamped beam resonator, the simulations with the proposed model shows a good agreement with experimental results, but the model is limited in its predictive capabilities. For the dogbone resonator, the proposed heuristic modelling approach does not lead to a match between simulations and experiments. Shortcomings of the heuristic modelling approach serve as a motivation for a first-principles based approach. The main objective of this research is to derive a multiphysics modelling framework for MEMS resonators that is based on first-principles formulations. The framework is intended for fast and accurate simulation of the steady-state nonlinear dynamic behaviour of MEMS resonators. Moreover, the proposed approach is validated by means of experiments. Although the multiphysics modelling framework is proposed for MEMS resonators, it is not restricted to this application field within microsystems engineering. Other fields, such as (resonant) sensors, switches and variable capacitors, allow for a similar modelling approach. In the proposed framework, themechanical, electrical and thermal domains are included. Since the resonators considered are operated in vacuum, the fluidic domain (squeeze film damping) is not included. Starting from a first-principles description, founded on partial differential equations (PDEs), characteristic nonlinear effects from each of the included domains are incorporated. Both flexural and bulk resonators can be considered. Next, Galerkin discretization of the coupled PDEs takes place, to construct reduced-order models while retaining the nonlinear effects. The multiphysics model consists of the combined reduced-order models from the different domains. Designated numerical tools are used to solve for the steady-state nonlinear dynamic behaviour of the combined model. The proposed semi-analytical (i.e. analytical-numerical) multiphysics modeling framework is illustrated for a full case study of an electrostatically actuated single-crystal silicon clamped-clamped beam MEMS resonator. By means of the modelling framework, multiphysics models of varying complexity have been derived for this resonator, including effects like electrostatic actuation, fringing fields, shear deformation, rotary inertia, thermoelastic damping and nonlinear material behaviour. The first-principles based approach allows for addressing the relevance of individual effects in a straightforward way, such that the models can be used as a (pre-)design tool for dynamic response analysis. The method can be considered complementary to conventional finite element simulations. The multiphysics model for the clamped-clamped beam resonator is validated by means of experiments. A good match between the simulations and experiments is obtained, thereby giving confidence in the proposed modelling framework. Finally, next to themodelling approach for MEMS resonators, a technique for using these nonlinear resonators in an oscillator circuit setting is presented. This approach, called phase feedback, allows for operation of the resonator in its nonlinear regime. The closedloop technique enables control of both the frequency of oscillation and the output power of the signal. Additionally, optimal operation points for oscillator circuits incorporating a nonlinear resonator can be defined.

This paper reviewed a few output-only system identification algorithms and identified the shortcomings of those popular blind source separation methods. To address the issues such as less sensors than the targeted modal modes (under-determinate problem), repeated natural frequencies as well as systems with complex mode shapes, this paper proposed a complex wavelet modified second order blind identification method (CWMSOBI) by transforming the time domain problem into time-frequency domain. The wavelet coefficients with different dominant frequencies can be used to address the under-determinate problem, while complex mode shapes are addressed by introducing the complex wavelet transformation. Numerical simulations with both high and low signal-to-noise ratios validate that CWMSOBI can overcome the above-mentioned issues while obtaining more accurate identified results than other blind identification methods.