Doktorgradsoppgaver i akustikk

Ultrasonic Transit-Time Flowmeters (UTTF) and their modeling are on the main focus in this dissertation. UTTF can be categorized into clamp-on and inline devices depending on the needs of applications for flow measurement. Simulations, studies, and experimental verification of inline gas devices with high demands of accuracy are performed in the present work. It is demonstrated that with the use of simulations, it is conceivable to accelerate innovation, as well as to continuously improve measurement accuracy and technology for the fast-growing market of UTTF.

In the present thesis, a multiphysics, hybrid numerical method is proposed i.e., a combination of a Finite Element Method (FEM) and a Finite Volume Method (FVM), for the purpose of 3D simulations and investigation of physical phenomena that affect the behavior of UTTF. The developed method, ’Simulations of Piezoelectricity, Acoustics, Coupled with CFD’ (SimPAC2), is used as a design tool of UTTF, as well as for the improvement of understanding the operation of UTTF. For the simulation, the UTTF is split into parts and the respective, more appropriate method is used for each part. More specifically, FEM is utilized for the simulation of piezoelectricity and structural acoustics in the solid parts i.e., the transducers and, if desired, partially the meter-body of the flowmeter. FEM is also used for the simulation of wave propagation in a part of the moving fluid medium. Acoustics and computational fluid dynamics (CFD) are considered in the moving fluid medium, as well as their interaction with each other with the use of FVM, which is traditionally more appropriate for CFD and large simulations that need to be highly parallelized. The hybrid SimPAC2 method requires complex interfaces between the FEM and FVM method, which are created in the course of the present work.

A comparison of SimPAC2 results with pure CFD, FEM and measurements is carried out. A chain verification takes place, starting from a simulation of a simple geometry of piezoelectric elements in air in zero and uniform flow. Complexity is added with the simulation of a diametrical single-path flowmeter equipped with either piezoelectric elements or real transducers. Finally, a real industrial flowmeter with two chordal paths is simulated and measured in a flow rig, with the agreement of the results satisfying the set criteria. The simulations allowed for the systematic study and quantification of complex, much-anticipated effects in UTTF, such as 3D cavity effects, the position of flush, recessed, and protruded transducers, as well as the flow effect around the transducers and in the meter-body.

The performed 3D multiphysics simulations capture interactions between ultrasonic waves and flow in the 3D geometry that are, by definition, not possible to be captured by 2D simulations. Before SimPAC2, the performance of systematic 3D multiphysics simulations was computationally expensive or impossible to perform. Thus, the simulation of a full 3D geometry of an UTTF is achieved from input voltage on the transmitter to output voltage on the receiver. It is demonstrated that SimPAC2 can be further used as a tool for the design and optimization of UTTF, the reduction of the development cycle and the improvement of accuracy and linearity.

Due to the potential for using methane hydrates as an energy source, localizing, monitoring and describing hydrate deposit areas are of interest. This has previously been attempted by using acoustic methods and thus information on the relation between the hydrate saturation, SH, and acoustic properties, such as cP, cS (compressional and shear wave velocity, respectively) and αP (compressional wave attenuation coeffiscient) is needed. The overall aim of this PhD thesis is to measure and discuss cP , cS and the change in αP (∆αP ) in ten Bentheim sandstone specimen having different initial water saturations, Sw0 as a function of SH, during hydrate growth.

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Non-destructive stress measurements of structures are increasingly valued by the industry. Ultrasonic methods have the advantage that acoustic waves propagate with ease through materials, making it possible to probe the interior of structures. Classic elastic models predict constant longitudinal and shear sound velocities in a material. However, by including higher order elastic moduli, the acoustoelastic theory indicate that the sound velocities are affected by the current stress state of the material. Experiments on steel plates have confirmed the dependency of speed of sound with stress in the material. This presents a potential method to estimate pipeline wall stress using ultrasound. Pipeline wall thickness can be measured using resonant ultrasonic signals. The resonant frequencies are linked to the wall thickness via the sound velocity. Thus, equivalently the sound velocities can be estimated when the wall thickness is known. This study has investigated the possibilty of detecting changes in material properties at very high stress in steel utilising an existing acoustic nondestructive testing (NDT) technique called Acoustic Resonance Technology (ART). ART is an ultrasonic technique based on transient acoustic reflections in layer and plates. The technique utilises mainly a pulse-echo method of normally incident longitudinal acoustic signal (pressure waves), recording the longitudinal resonant frequencies across the thickness of a layer. However, it can also be set up to record shear resonant frequencies across the layer thickness by utilising the effect of mode conversion of a slightly off-normal incident pressure wave. Laboratory experiments utilising ART have been used to measure the change in both longitudinal and shear sound velocities across rectangular steel test specimens subjected to uniaxial tensional loads. In addtion the acoustoelastic theory have been implemented and used to simulate the change in sound velocities for some steel types reported in the literature for comparison of experimental results and theory. The theoretical investigation and experimental measurements on steel bars performed in this work have shown that the ART methodology is capable of detecting very small changes, in the order of 0.1%, of the sound velocities for test specimens subjected to high levels of stress. In addition, by comparing longitudinal and shear resonance frequencies, ART is capable of measuring an effect of both longitudinal and shear sound velocity changes independent of the thickness of the specimen. This might be highly valuable for potential in-line inspections along several km of pipelines where the wall thickness may vary on a scale approximately two orders of magnitude larger than the measured variation in stress induced sound velocity change.

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The subject of this thesis is to investigate methods for measuring liquid density by acoustic means, and to investigate one or more promising methods experimentally. The liquids used in this work covers distilled water and various oil qualities (as pure phases). This work was performed at the Christian Michelsen Research AS, (CMR Instrumentation) with financial support from the Norwegian Research Council (NFR) through the 4 year Strategic Institute Program “Ultrasonic technology for improved exploitation of petroleum resources”, in the period of 2003–2006.

Density (mass per unit volume) is a material property of utmost importance in many fields, such as in the process industry in general and also for fiscal use. Applications are within such diverse fields as flow measurement for converting volume flow to mass flow, basic research, fluid characterization, biomedical diagnostics (particularly measurement of bone density), process control in the industry, fluid monitoring in the petroleum industry and not the least in quality control in the food and beverage industry.

Industrial density measurements of liquids are traditionally performed by non-acoustic measurement methods. However, the use of ultrasound for industrial applications is increasing, mainly due to its non-intrusiveness and its rapid response. However, drawbacks include the invasive measuring principle along with a dependence upon deposits and air bubbles. If an acoustic density meter could be brought to industrial use, operational savings could result as the same technology was used throughout for the measurement of several parameters, such as sound speed and volumetric flow rate.

The primary objective of this doctoral work is to establish a scientific basis for developing a high precision sound velocity cell for natural gases under high pressures. A candidate measurement method for such a cell is investigated theoretically and experimentally, alongside alternative candidate methods.

In order to use an ultrasonic flowmeter (USM) as a mass and energy flowmeter, documentation and traceability to national and international standards is needed for the uncertainty of the sound velocity measurement made by the USM, obtained by an independent and accurate method. This uncertainty is not known today, and no reference measurement methods are available to establish it. It has been shown that the uncertainty of the sound velocity measurement may be a significant contributor to the gross calorific value and density measurement uncertainty. There is thus need for a high-precision sound velocity measurement cell with known uncertainty, which can be used together with the USM in a laboratory measurement setup (for the same natural gas sample, at the same pressure, temperature, etc.), to check the accuracy of the sound velocity measured by the USM. To minimize uncertainty contributions due to possible dispersion effects in the gas, the sound velocity cell should operate in the frequency band of current USMs, 100 kHz-200 kHz. Tentative technical specifications have been outlined for the sound velocity cell in a feasibility study; the measurement uncertainty for the sound velocity should be within ±(0.05-0.1) m/s → 100 ppm-200 ppm (at a 95% level of confidence), over the pressure range 0-250 bar(g), and the temperature range 0 °C-60 °C. The cell should preferably not involve any moving parts.

Such a measurement cell may also have other useful applications. One is the sound velocity correction in vibrating element densitometers, which are extensively used in oil and gas industry. Another application may be related to improving the natural gas equation of state, virial coeffients etc.

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Finite-amplitude (nonlinear) sound propagation effects in the sound beams of fisheries research echo sounders are investigated experimentally and by numerical simulations. Echo sounders with 120 kHz and 200 kHz operating frequencies are considered. An attempt is made to quantify the excess attenuation due to nonlinear effects in fresh water and seawater. Consequences of such attenuation for fisheries research applications are discussed. Results for the second harmonic frequency component generated through nonlinear distortion are also presented.

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