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![shows the spectral centroid of the transmitted signal with respect to the distance from the transducer. Very close to the transducer, no clear tendency is visible. However, at distances greater than 30 mm the expected linear trend emerges. Using the approximation for the near field length [3], = − 4 ,](profile/Leander-Claes/publication/317304119/figure/fig2/AS:614277220151297@1523466510200/shows-the-spectral-centroid-of-the-transmitted-signal-with-respect-to-the-distance-from.png)
shows the spectral centroid of the transmitted signal with respect to the distance from the transducer. Very close to the transducer, no clear tendency is visible. However, at distances greater than 30 mm the expected linear trend emerges. Using the approximation for the near field length [3], = − 4 ,
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Next to the sound velocity, the absorption coefficient of acoustic waves in fluids is an important thermodynamic property. Although measurements of acoustic absorption have been conducted in the past, most of them negate the influence of parasitic dissipative effects or rely on either analytical or empirical compensation. A typical method for measu...
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Citations
... These investigations, however, suffer from missing, incomplete, or unreliable bulk viscosity data and were consequently restricted to predominantly qualitative results. 20 A subclass of compressible fluid dynamics problems are acoustic waves. A linearisation of the fluid-mechanical equations for a given pressure and density gives rise to the so-called acoustic approximation, which can predict the propagation and absorption of acoustic waves [8]. ...
... While raw moments of a signal's amplitude spectrum can be determined nu-105 merically, the centre frequency's derivative needs to be estimated. As an almost linear decay of the centre frequency is expected [20], finite differences of the centre frequency of the signals at z 1 and z 2 are applied. For an application to measurement signals, z can be chosen arbitrarily. ...
A measurement procedure using a modified two-chamber pulse-echo experimental setup is presented, enabling acoustic absorption and bulk viscosity (volume viscosity) measurements in liquids up to high temperature and pressure. Acoustic absorption measurements are particularly challenging, since other dissipative effects, such as diffraction at the acoustic source and at acoustic reflectors, are typically superimposed to the measurement effect. Acoustic field simulations are performed, allowing to investigate acoustic wave propagation qualitatively. The absorption coefficient is determined by evaluating the signal spectrum’s raw moments and applying a method to identify and correct systematic measurement deviations. Measurement uncertainties are estimated by a Monte Carlo method. In order to validate the present measurement procedure, the acoustic absorption in liquid methanol, n-hexane, n-octane, and n-decane is determined experimentally and compared to literature data. The measurement results for methanol are additionally validated by comparison to bulk viscosity data sampled with molecular dynamics simulation.
... Wie es von Fink [Fin83] für Humangewebe und von Quan [Qua97] für Gestein gezeigt wird, soll an dieser Stelle der Zusammenhang zwischen akustischer Absorption in Fluiden und dem spektralen Schwerpunkt, bzw. der Schwerpunkt-oder Mittenkreisfrequenz ω s (z) eines Signals, gezeigt werden [Cla17a]. Diese ist über die gewöhnlichen Momente des Signalspektrums mit der in Gleichung 4.45 beschriebenen Kurzschreibweise wie folgt bestimmt: ...
The prerequisite for a complete description of fluid dynamic and acoustic processes is that all properties of the fluid are known.
While fluid parameters such as the speed of sound or the shear viscosity are known for many liquids over a wide range of thermodynamic states, only limited measurement data exist for the bulk viscosity.
In this thesis, a measurement method for the selective determination of the bulk viscosity of liquids, based on the absorption of ultrasonic waves, is developed and implemented.
The focus is on the simulation-driven design of algorithms for processing the measurement signals as well as the analysis and further development of a measurement set-up based on the pulse-echo method.
In addition to absorption in the fluid, there are other effects (for example diffraction or incomplete reflection) that weaken or otherwise influence the acoustic signal.
Therefore, the development of procedures to separate these effects from acoustic absorption is another focus of this work.
The bulk viscosity is determined from the measured acoustic absorption for different fluids in different thermodynamic states.
An uncertainty analysis of the measured quantities concludes this thesis.
... where is the propagation distance of the acoustic wave. Similar to approaches that apply the change in centre frequency of the acoustic signal to determine absorption [3,4,5] , the -th raw moment of the spectrum ( , ) is considered: ...
... While the resulting numerical values (figure 2) are still superimposed by the influence of the measurement setup, such as diffraction, they show the numerical stability of the approach, as similar thermodynamic states of the fluid result in similar (classical) acoustic loss meas = 2 2,meas ⋅ 0 3 . Especially in comparison to the evaluation of the change in centre frequency discussed before [5] , the presented method yields stable results even though the properties of the transducer, and thus those of the transmitted signal, change with temperature. The estimation method for multi parameter absorption models is evaluated by applying it to virtual measurement results generated by performing a finite element simulation of a circular waveguide with a radius of 1 mm using CFS++ [7] . ...
Of all fluid and solid properties, quantities that describe losses are among the most challenging to quantify. In part, this is due to superimposed dissipative mechanisms, such as diffraction effects from spatially limited sources. Inherent to all these phenomena, however, is a specific frequency dependence. The nature of the frequency dependence varies, resulting from the respective absorption mechanism. Pure fluids, for example, exhibit absorption of acoustic waves with quadratic frequency dependence[1]. In solids, there are several absorption models that can be applied, each having different characteristics with respect to frequency. Other dissipative effects, such as diffraction, also show frequency dependence. In an approach using the raw moments of the signals from acoustic transmission measurements, a method to quantify absorption and dissipation phenomena with arbitrary frequency dependence is presented. The described method is applied to different absorption measurement problems. To verify that accurate results can be achieved under ideal conditions, the method is applied to signals generated using acoustic field simulation with different absorption models. To show its numerical stability, it is used qualitatively to evaluate the absorption of a fluid at different thermodynamic states.
