Tim Hetkämper

Bio: Tim Hetkämper is an academic researcher from University of Paderborn. The author has contributed to research in topics: Ultrasonic sensor & Fourier transform. The author has an hindex of 1, co-authored 2 publications receiving 1 citations.

Estimation of acoustic wave non-linearity in ultrasonic measurement systems

Abstract: Most measurement methods based on ultrasound, such as sound velocity, absorption or flow measurement systems, require that the acoustic wave propagation is linear. In many cases, linear wave propagation is assumed due to small signal amplitudes or verified, for example, by analysing the received signal spectra for the generation of harmonic frequency components. In this contribution, we present an approach to quantify occurrence of non-linear effects of acoustic wave propagation in ultrasonic measurement systems based on the evaluation of the acoustic Reynolds number. One parameter required for the determination of the acoustic Reynolds number is the particle velocity of the acoustic wave, which is not trivially obtained in most measurement systems. We thus present a model-based approach to estimate the particle velocity of an acoustic wave by identifying a Mason model from electrical impedance measurements of a given transducer. The Mason model is then used to determine the transducer's velocity output for a given electrical signal, allowing for an evaluation of the acoustic Reynolds number for different target media.

Phase-preserving methods to visualise ultrasonic fields with schlieren imaging

Abstract: Abstract Schlieren imaging allows visualising local density modulations in optically transparent media, which enables to analyse ultrasonic wave propagation. In typical measurement setups, spatial filtering is applied. Commonly, phase information of the ultrasonic wave is lost, which e.g. prevents accurate tomographic reconstruction. In this work two methods are presented to preserve phase information. The first method relies on the combination of two different spatial filters, while the second method does not require any spatial filtering. The fractional Fourier transform is applied to explain and simulate the effects occurring if the position of one of the optical lenses in a schlieren measurement setup is varied. Exemplary images of ultrasonic waves are shown to demonstrate the application of both methods.

Identification of temperature-dependent model parameters of ultrasonic piezo-composite transducers

Abstract: When performing measurements, the effects of the measurement system itself on the measured data generally must be eliminated. Consequently, those effects, i.e. the system’s dynamic behavior, need to be known. For the piezo-composite transducers in an ultrasonic transmission line, a model based approach is used to describe their dynamic behavior and take into account its dependence on the environment temperature and the acoustic impedance of the target medium. Temperature-dependent model parameters are presented, which are obtained by performing a multiplepart identification process on the transducer model, based on electrical impedance measurements [1]. The identification process uses an inverse approach for optimizing a subset of the model parameters. Additionally, algorithmic differentiation methods are used to determine accurate derivatives. In a final optimization step, impedance measurements taken at different temperatures are used to determine the temperature dependencies of the model parameters. These can then be used to assess the plausibility of the identification results. Additionally, the parameters can be expressed as polynomials in the temperature to take different operating conditions into account.

Bulk viscosity of gaseous argon from molecular dynamics simulations.

Abstract: The bulk viscosity of dilute argon gas is calculated using molecular dynamics simulations in the temperature range 150-500 K and is found to be proportional to density squared in the investigated range of densities 0.001-1 kg m^{-3}. A comparison of the results obtained using Lennard-Jones and Tang-Toennies models of pair interaction potential reveals that the value of the bulk viscosity coefficient is sensitive to the choice of the pair interaction model. The inclusion of the Axilrod-Teller-Muto three-body interaction in the model does not noticeably affect the values of the bulk viscosity in dilute states, contrary to the previously investigated case of dense fluids.