The turbulence between geomechanics and vibroacoustics: what is common point?
Résumé
This presentation covers more than 10 years of fruitful collaboration between Irphe, LMA and Irstea where our colleague and friend F. Anselmet played a central role. The presentation is organized in three parts. The first part deals with erosion of a cohesive soil by turbulent flow (F. Mercier PhD thesis). Work in progress concerning the acoustic emission of a turbulent flow in a soil is presented in a second part (C. Jeanniot PhD thesis). The last part is devoted to the interaction of a turbulent flow with acoustic sound wave inside a corrugated cylinder (G. Galeron PhD thesis) known as singing riser. Dikes and levees are prone to failure through two erosion processes [1]: i) internal erosion, induced by concentrated seepage that takes off and transports soil particles through the embankment and its foundation, ii) surface erosion induced by overflowing. Erosion of a cohesive soil by turbulent flow is studied with Computational Fluid Dynamics (CFD) numerical modelling, based on the adaptive remeshing of the water/soil interface to ensure the accurate description of the mechanical phenomena occurring near the soil/water interface [2,3,4]. The erosion law governing the interface motion is based on two erosion parameters: the critical shear stress and the erosion coefficient. The model is validated by comparison with closed-form solution of simple problems. The numerical results are then compared to laboratory test results: the Hole Erosion Test, which reproduces the mechanism of tangential erosion of a soil by turbulent flow in a circular pipe, and the Jet Erosion Test which reproduces the erosion of soil by a circular impinging turbulent jet. The shear stress at the wall appears to be well-described by the k- model while the pressure is better-described by the k- model. One means to monitor internal erosion is passive Acoustic Emission (AE) monitoring. This monitoring involves acoustic transducers to passively listen for acoustic energy released from internal sources. We study the potential of this approach at the laboratory, on the Hole Erosion Test [1]. Turbulent pipe flow in the hole generates acoustic waves that propagate in the soil sample. Acoustic sensors (accelerometers) installed on the equipment allow to measure these signals. The analysis sheds light on acoustic frequencies characteristic of the internal flow, between 0-10 kHz. The vibratory responses are analyzed from the acoustic signal using Fourier transforms and Lorenzian functions. The correlation of both frequency and intensity with flow velocity and hole diameter is studied [5]. Using a simplified model of the sound field generated inside the test sample by a wall turbulent pressure field inside a cylindrical hole, it is easy to determine, at least theoretically, the characteristics of the flow (convection velocity, diameter of the inner pipe, power spectral density of the flow) that play that contribute to the acoustic response of the test sample. The last part of this presentation is focused on a phenomenon often encountered in flow carrying pipes, since flow instabilities caused by geometric features may generate acoustic signals and thereafter interact with these signals in such a way that powerful pure tones are produced. A modern example is found in the so-called "singing risers", or the gas pipes connecting gas production platforms to the transport network. But the flow generated resonance in a fully corrugated circular pipe may be silenced by the addition of relatively low frequency flow oscillations induced by an acoustic generator.
Domaines
Acoustique [physics.class-ph]
Origine : Fichiers produits par l'(les) auteur(s)
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