Acoustics emissions in a Hele-Shaw cell due to aerofracturing
Semih Turkaya (Institut de Physique du Globe de Strasbourg, France) will present us his PhD work done with his coauthors Renaud Toussaint, Fredrik Kvalheim Eriksen, Megan Zecevic, Guillaume Daniel, Eirik G. Flekkøy, and Knut Jørgen Måløy.
Semih Turkaya(1), Renaud Toussaint(1), Fredrik Kvalheim Eriksen(1) (2), Megan Zecevic(3), Guillaume Daniel(3), Eirik G. Flekkøy(2), Knut Jørgen Måløy(2)
(1) Institut de Physique du Globe de Strasbourg, CNRS, Université de Strasbourg, 5 rue Descartes, 67084 Strasbourg Cedex, France
(2) Department of Physics, University of Oslo, P. 0. Box 1048, 0316 Oslo, Norway
(3) Magnitude, Route de Marseille 04220 Sainte Tulle, France
In this work, analogue models are developed (similar to the previous work of Johnsen) in a linear geometry, with confinement and at lower porosity to study the instabilities developing during fast motion of fluid in dense porous materials: fracturing, fingering, channelling… We study these complex fluid/solid mechanical systems using two imaging techniques: fast optical imaging and high frequency resolution of acoustic emissions using accelerometers and piezoelectrical sensors. Additionally, we develop physical models rendering for the fluid mechanics (similar to the work of Niebling[2,3]) in the channels and the propagation of microseismic waves around the fracture. We then confront a numerical resolution of this physical system with the observed experimental system.
The experimental setup consists in a rectangular Hele-Shaw cell with three closed boundaries and one semi-permeable boundary which enables the flow of the fluid but not the solid particles. During the experiments, the fluid is injected into the system with a constant injection pressure from the point opposite to the semi-permeable boundary. At the large enough injection pressures, the fluid also displaces grains and creates channels, fractures towards the semi-permeable boundary.
In the analysis phase, power spectrum of different timewindows (5 ms) obtained from the recorded signal are calculated. Then, the evolution of power spectrum is compared with the optical recordings. Power spectrum initially follows a power law trend and when the channel network is developed, stick-slip events generating peaks with a characteristic frequency can be seen. These peaks are strongly influenced by the size and branching of the channels, compaction of the medium, vibration of air in the pores and the fundamental frequency of the plate. Using direct simulations of acoustic emissions due to the air vibration in developing fractal cavities the evolution in the power spectrum is investigated.
Locations of the impulsive acoustic events recorded during the experiments are estimated using different methods which are arrival time based localization, time reversal localization and energy based localization. A new way of estimating the location using the signal energy is proposed. The strong and weak points of these methods are investigated in the case of different source size and material.
 Ø. Johnsen, R. Toussaint, K. J. Måløy, and E. G. Flekkøy, “Pattern formation during air injection into granular materials confined in a circular hele-shaw cell,” Physical Review E - Statistical,Nonlinear, and Soft Matter Physics, vol. 74, no. 1, 2006.
 Niebling MJ, Toussaint R, Flekkøy EG, Maløy KJ. “Dynamic aerofracture of dense granular packings.” Physical Review E - Statistical, Nonlinear, and Soft Matter Physics, vol. 86, no. 1, (2012).
 Niebling MJ, Toussaint R, Flekkøy EG, Maløy KJ. “Numerical studies of aerofractures in porous media.” Revista Cubana de Fisica vol. 29 (2012) 1E66–1E70.
 Farin M, Mangeney A, Toussaint R, De Rosny J, Shapiro N, “Etude expérimentale de l’emission acoustique par de petits impacteurs sur un solide élastique” CFA Proceedings, 2014