H. Landes

Finite element simulation of nonlinear wave propagation in thermoviscous fluids including dissipation

A recently developed finite element method (FEM) for the numerical simulation of nonlinear sound wave propagation in thermoviscous fluids is presented. Based on the nonlinear wave equation as derived by Kuznetsov, typical effects associated with nonlinear acoustics, such as generation of higher harmonics and dissipation resulting from the propagation of a finite amplitude wave through a thermoviscous medium, are covered. An efficient time-stepping algorithm based on a modification of the standard Newmark method is used for solving the nonlinear semidiscrete equation system. The method is verified by comparison with the well-known Fubini and Fay solutions for plane wave problems, where good agreement is found. As a practical application, a high intensity focused ultrasound (HIFU) source is considered. Impedance simulations of the piezoelectric transducer and the complete HIFU source loaded with air and water are performed and compared with measured data. Measurements of radiated low and high amplitude pressure pulses are compared with corresponding simulation results. The obtained good agreement demonstrates validity and applicability of the nonlinear FEM.

Finite element modeling of the pulse-echo behavior of ultrasound transducers

A new method for the numerical computation of the pulse-echo behavior of ultrasound transducers is presented. In this numerical scheme, the given physical problem is split into two finite element models for which three subsequential computer runs are performed. In a first finite element model, the transducer with its fluid environment is set up and, in a second finite element mesh an obstacle reflecting the outgoing ultrasound wave is modelled. The wave propagation in the fluid area between the first and the second mesh is computed via Helmholtz integral. The reported numerical scheme is applicable to any type of piezoelectric transducer. In this paper, the pulse-echo voltages due to reflecting elastic obstacles with different geometries are presented for ultrasound antennas which are of practical interest in respect to medical imaging. The obtained simulation results are in good agreement with experimental data. The new method allows the precise computer simulation of complex piezoelectric transducers on a highend PC

Computer-aided design of clinical magnetic resonance imaging scanners by coupled magnetomechanical-acoustic modeling

This paper reports on an efficient finite-element scheme for computer-aided design of a clinical magnetic resonance imaging (MRI) scanner. The modeling scheme allows the full three-dimensional calculation of the electromagnetic, mechanical, and acoustic fields, including their mutual couplings. The validity of the computer simulations has been verified by appropriate measurements. Applications include the optimization of the gradient coil and the superconducting magnet with respect to eddy-current losses in the cryostat and the emitted noise.

Modelling of acoustic antennas with a combined finite-element-boundary-element-method

A technique for the computer modeling of fluid loaded electromechanical transducers is introduced. This method is based on the coupling of piezoelectric and acoustic finite elements with acoustic boundary elements. The new technique is superior to the pure finite element method, particularly with respect to the necessary computer resources, whenever the fluid region to be modeled is large. The restriction of the boundary element method to continuous wave applications is overcome by Fourier analysis of the exciting pulses and by performing a computer run for each spectral line. An annular array antenna as used in acoustical imaging is analyzed. With the new method, precise computer simulation of the acoustic field of that antenna could be performed on a general-purpose workstation.<<ETX>>

A coupled finite-element, boundary-integral method for simulating ultrasonic flowmeters

Todays most popular technology of ultrasonic flow measurement is based on the transit-time principle. In this paper, a numerical simulation technique applicable to the analysis of transit-time flowmeters is presented. A flowmeter represents a large simulation problem that also requires computation of acoustic fields in moving media. For this purpose, a novel boundary integral method, the Helmholtz integral-ray tracing method (HIRM), is derived and validated. HIRM is applicable to acoustic radiation problems in arbitrary mean flows at low Mach numbers and significantly reduces the memory demands in comparison with the finite-element method (FEM). It relies on an approximate free-space Greens function which makes use of the ray tracing technique. For simulation of practical acoustic devices, a hybrid simulation scheme consisting of FEM and HIRM is proposed. The coupling of FEM and HIRM is facilitated by means of absorbing boundaries in combination with a new, reflection-free, acoustic-source formulation. Using the coupled FEM-HIRM scheme, a full three-dimensional (3-D) simulation of a complete transit-time flowmeter is performed for the first time. The obtained simulation results are in good agreement with measurements both at zero flow and under flow conditions

3D simulation of controlled micromachined capacitive ultrasound transducers

A finite element simulation scheme which allows the efficient calculation of electrostatic transducers immersed in an acoustic fluid is presented. This calculation scheme has been applied in investigations of the dynamical behavior of surface micromachined capacitive ultrasound arrays. In order to improve dynamics and efficiency of such transducers a controller has been designed and tested. Therewith, ring down time and acoustic crosstalk in the array are reduced.

Calculation of acoustic streaming velocity and radiation force based on finite element simulations of nonlinear wave propagation

An extension of a recently developed simulation scheme for the calculation of nonlinear wave propagation in thermoviscous fluid media for the numerical calculation of acoustic streaming is presented. Based on the nonlinear wave equation derived by Kuznetsov, the nonlinear pressure field is calculated by means of a finite element formulation. The generated force density and mean streaming velocity are obtained by time averaging of the nonlinear pressure and velocity distributions. Therewith, nonlinear distortions of the finite amplitude wave are taken into account. The numerical calculation scheme is verified by means of a plane wave problem and several parameter studies are presented.

Computer optimization of electromagnetic acoustic transducers

Electromagnetic acoustic transducers (EMATs) are well known devices that are commercially available. Their advantage is that they do not require a coupling medium and/or contact to the part being inspected; the disadvantage is their low transduction efficiency. In order to optimize their efficiency, the EMATs are best customized to their particular application with special attention to the material properties and geometry of the part to be inspected. Here we report-apparently for the first time-on numerical simulation of electromagnetic acoustic transduction for both the transmitting and receiving case. The emphasis in this paper is on guided wave generation and reception in thin plates. We have adapted the finite element/boundary element program entitled CAPA to computer simulate the electromagnetic acoustic transduction process.

Finite element simulation of acoustic wave propagation within flowing media

A finite element method for acoustic wave propagation is established, taking into account the influence of a time-independent inhomogeneous flow on the acoustic waves. The simulation scheme is verified for homogeneous flow and validated with geometrical acoustics for inhomogeneous flow profiles. As an application, the wave propagation within ultrasound flow meters for liquids is computed for different flow profiles.

Magnetomechanical field computations of a clinical magnetic resonance imaging (MRI) scanner

In this paper, an efficient magnetomechanical calculation scheme based on the finite element method is presented. This scheme is used for the precise forecast of the dynamical behavior of a clinical magnetic resonance imaging scanner. The validity of the computer simulations has been verified by means of appropriate measurements. Application examples include the optimization of the superconducting magnet regarding the eddy currents and vibrations in its cryostat.