| 1. |
- Aitomäki, Yvonne, et al.
(författare)
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Estimating suspended fibre material properties by modelling ultrasound attenuation
- 2006
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Ingår i: Mathematical Modeling of Wave Phenomena. - Melville, NY : American Institute of Physics (AIP). - 0735403252 ; , s. 250-259
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Konferensbidrag (refereegranskat)abstract
- An analytical model for use in the inverse problem of estimating material properties of suspended fibres from ultrasonic attenuation has been developed. The ultrasound attenuation is derived theoretically from the energy losses arising when a plane wave is scattered and absorbed off an infinitely long, isotropic, viscoelastic cylinder. By neglecting thermal considerations and assuming low viscosity in the suspending fluid, we can make additional assumptions that provide us with a tractable set of equations that can be solved analytically. The model can then be to used in inverse methods of estimating material properties. We verify the model with experimentally obtained values of attenuation for saturated Nylon fibres. The experimental results from Nylon fibres show local peaks in the attenuation which are thought to be due to the resonant absorption at the eigenfrequencies of the fibres. The results of the experiments show that the model is sufficiently sensitive to detect differences in different types of Nylon. Applications for suspended fibre characterization can be found in the paper manufacturing industry.
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| 2. |
- Aitomäki, Yvonne, et al.
(författare)
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Material property estimates from ultrasound attenuation in fibre suspensions
- 2009
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Ingår i: Ultrasonics. - : Elsevier BV. - 0041-624X .- 1874-9968. ; 49:4-5, s. 432-437
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Tidskriftsartikel (refereegranskat)abstract
- An investigation of a new method for measuring fibre material properties from ultrasonic attenuation in a dilute suspension of synthetic fibres of uniform geometry is presented. The method is based on inversely solving an ultrasound scattering and absorption model of suspended fibres in water for the material properties of the fibres. Experimental results were obtained from three suspensions of nylon 66 fibres each with different fibre diameters. A forward solution to the model with reference material values is compared to experimental data to verify the model's behaviour. Estimates of the shear and Young's modulus, the compressional wave velocity, Poisson's ratio and loss tangent from nylon 66 fibres are compared to data available from other sources. Experimental data confirms that the model successfully predicts that the resonance features in the frequency response of the attenuation are a function of diameter. Consistent estimated values for the compressional wave velocity and the Poisson's ratio were found to be difficult to obtain but in combination gave values of shear modulus within previously reported values and with low sensitivity to noise. Young's modulus was underestimated by 54% but was consistent and had low sensitivity to noise. The underestimation is believed to be caused by the assumption of isotropic material used in the model. Additional tests on isotropic fibre would confirm this. Further analysis of the model sensitivity and the reasons for the resonance features are required.
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| 3. |
- Aitomäki, Yvonne, et al.
(författare)
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Sounding Out Paper Pulp : Ultrasound Spectroscopy of Dilute Viscoelastic Fibre Suspensions Acoustics and Ultrasonics
- 2006
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A model of attenuation of ultrasound in fibre suspensions is compared to a model of backscattering pressure from submersed cylinders subjected to a sound wave. This analysis is carried out in the region where the wavelength is of the same order as that of the diameter of the fibre. In addition we assume the cylinder scatterer to have no intrinsic attenuation and the longitudinal axis of the scatterer is assumed to be perpendicular to the direction of propagation of the incident wave. Peaks in the frequency response of both the backscattering pressure, expressed in the form of a form function, and the attenuation are shown to correspond. Similarities between the models are discussed. Since the peaks in the form function are due to resonance of the cylinder, we infer that the peaks in the attenuation are also due to resonance. The exact nature of the waves causing the resonance are still unclear however the first resonance peaks are related to the shear wave and hence the shear modulus of the material. The aim is to use the attenuation model for solving the inverse problem of calculating paper pulp material properties from attenuation measurements. The implications of these findings for paper pulp property estimation is that the supporting fluid could, if possible, be matched to density of that of pulp fibres and that the estimation of material properties should be improved by selecting a frequency range that in the region of the first resonance peaks.
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| 4. |
- Löfqvist, Torbjörn, et al.
(författare)
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Ultrasonic methods in determining elastic material properties of fibres in suspension
- 2008
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Ingår i: 2007 IEEE Ultrasonics Symposium proceedings. - Piscataway, NJ : IEEE Communications Society. - 9781424413843 ; , s. 46-49
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Konferensbidrag (refereegranskat)abstract
- The presented study concerns the application of the pulse-echo ultrasound measurement technique in determining the elastic properties of fibres suspended in water. The two kinds of fibre materials are investigated, nylon 6/6 fibres and softwood pulp fibres. The fibre mass fraction was 0.5% for nylon and ranges from 0% up to 1% for softwood pulp. The ultrasonic measurements are performed in the frequency range of 2-11 MHz. It is shown that the velocity dispersion of the ultrasound is small for each suspension sample. In obtaining the fibres longitudinal Young's modulus two methods are used, one based on phase velocity and one based on acoustic attenuation. It is found that both methods gives reasonable estimates of the longitudinal Young's modulus for nylon 6/6. For pulp fibres the determined Young's modulus is overestimated in comparison with earlier findings.
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| 5. |
- Niemi, Jan, et al.
(författare)
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Investigation of the photoacoustic signal dependence on laser power
- 2008
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Ingår i: Advanced Laser Technologies 2007. - : SPIE - The International Society for Optics and Photonics. ; , s. 70220C-
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Konferensbidrag (refereegranskat)abstract
- In this study we have focused on exploring the photoacoustic signal generated by laser induced dielectric breakdown process in pure water, under normal conditions. In this case the dielectric breakdown will lead to a formation of a shock wave. We investigated the relation between pulse energy and amplitude, group velocity and power spectrum of the shock wave. Also, the threshold for dielectric breakdown is estimated. A pulsed, high power Nd:YAG laser with λ = 532 nm and a pulse duration of 12 ns was used. The laser pulse energy ranges from 0.1 mJ to 7.4 mJ. Only photoacoustic signals generated from dielectric breakdown was considered. We found that the amplitude and the average group velocity of the shock wave correlates to the laser pulse energy. The frequency contents of the photoacoustic signal changes due to both non-linear behaviour and dissipative effects. We estimated the dielectric breakdown threshold to be 0.44 × 1011Wcm-2.
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| 6. |
- Niemi, Jan, et al.
(författare)
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On a new sensing strategy using a combination of ultrasonic and photoacoustic techniques
- 2006
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Ingår i: Proceedings. - Piscataway, NJ : IEEE Communications Society. - 1424402018 ; , s. 1797-1800
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Konferensbidrag (refereegranskat)abstract
- The process industry is today aiming for more advanced process control strategies. These strategies rely on quick and accurate sensing of process variables. Considering processes where particles are suspended in a fluid, e.g. paper and pulp industry, there is a need for development of a new or improved in-line sensor technique determining different properties of the particles. In this study we present a sensing strategy where a combination of acoustical and optical measurement techniques is used. For this purpose have we designed and built a measurement cell which utilises the ultrasonic signal generated from an ultrasonic transducer as well as the ultrasonic, or photoacoustic, signal generated using a pulsed laser. The photoacoustic method is using the same transducer as in the transducer based method above. In the transducer based method, the well known pulse-echo technique is used. The pulse is generated by an ultrasonic transducer, travels through the suspension and reflects at a steel reflector. In the photoacoustic method, the ultrasonic transducer receives the ultrasound that is generated when a pulse of laser light is scattered and absorbed in the suspension. The laser beam enters the cell through fused silica windows. The laser beam is crossing the cell orthogonally to the ultrasonic transducer. The photoacoustic signal contains two echoes. The first echo is the acoustic signal which is generated when photons are absorbed by the medium. The second echo is the signal that has been travelling through the suspension and reflected at the steel reflector. The two measurement techniques were tested experimentally in the cell using aqueous suspensions of Nylon 66 fibres. The samples were mixed from fibres of three different diameters 17, 51 and 55 µ m with the length of 1.2mm for the 17µm and 1.5 mm for the 51 and 55µm. The fibres were suspended in distilled and degassed water to mass fractions of 0.12% and 0.25%. Distilled and degassed water was also used as a reference medium when determining the calibration constants for the cell. The photoacoustic signal is generated using a double-pulsed laser having pulse energy of 10 mJ and the laser beam is focused at the centre of the cell. The suspension was pumped around in a closed loop using a peristaltic pump and the temperature of the suspension was monitored during the measurement cycle. The result shows that the signal generated using the transducer based method scales linearly with mass fraction for each type of fibre. The results also show that the photoacoustic signal is sensitive to the number density of scatters in the suspension, i.e. the amplitude of the signal decreases with decreasing fibre diameters. These preliminary results indicate that a combination of these two techniques might be used in resolving the proportion of different particles with different diameter in a suspension.
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| 7. |
- Niemi, Jan, et al.
(författare)
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Ultrasonic measurements and modelling of attenuation and phase velocity in pulp suspensions
- 2005
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Ingår i: 2005 IEEE Ultrasonics Symposium. - Piscataway, NJ : IEEE Communications Society. - 0780393821 ; , s. 775-779
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Konferensbidrag (refereegranskat)abstract
- In the manufacturing process of paper the mass fraction and material properties of the fibres in the pulp suspension are important for the quality of the finished product. This study presents two different methods of pulp characterisation. The first is based on phase velocity, which we use to investigate the composition of the pulp. Here a method is presented where the optimal number of circular shifts within the sampling window of the signal is determined which gives, in a weakly dispersive medium, a continuous phase spectrum and minimizes the likelihood of discontinuities within the bandwidth. Hence, the ambiguity in phase unwrapping is avoided. The results from phase velocity measurements show that the phase velocity weakly increases with increasing amount of fines in the suspension. The dispersion is caused by the fibres and it correlates with fibre mass fraction. The second method is based on attenuation and is used to characterise the wood fibres. The results of the attenuation experiments show that it is possible to inversely calculate wood fibre properties by fitting the model to the experimental data, if the fibre diameter distribution is known. However, the accuracy of these calculation is difficult to determined and more work in this area is required.
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| 8. |
- Törmänen, Matti, et al.
(författare)
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Pulp consistency determined by a combination of optical and acoustical measurement techniques
- 2006
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Ingår i: Measurement science and technology. - : IOP Publishing. - 0957-0233 .- 1361-6501. ; 17:4, s. 695-702
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Tidskriftsartikel (refereegranskat)abstract
- In this study, methods based on ultrasonic attenuation and optical time-of-flight measurements are used simultaneously in determining both the fibres and fines mass fractions, respectively, of a cellulose pulp fibre suspension. The optical measurements are done by a laser radar and the acoustical measurements are based on ultrasonic attenuation measurements in a pulse-echo set-up. Two kinds of long-fibre fractions are studied, thermo-mechanical pulp and chemical softwood pulp. Fibre and fines mass fraction ranges are 0.25–1.0% and 0–0.75%, respectively. The results show that the fibres are the predominant source for absorption and scattering of ultrasonic waves and are thus mainly contributing to the attenuation of ultrasound in the pulp. It is also found that the fines are the predominant source for optical scattering and fines are thus mainly contributing to the propagation delay of the light pulse in the laser radar set-up. By combining the ultrasonic attenuation and the optical time-of-flight measurements, it is shown that the mass fraction of fines and the mass fraction of fibres in a pulp sample could be determined, respectively.
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