Instabilities in highly deformable systems: unique opportunities for control of flow and vibrations
Applied Mechanics, Statics and Dynamics
Final Report Abstract
Acoustic waves in fluid-filled structures range from earthquakes to prenatal ultrasound. Recently, an exciting new class of soft structures is emerging that can can undergo very large deformations. Smart design allows to control the collapse of internal cavities and to tune the propagation paths of sound waves or acoustic resonances. These structures can be manufactured by 3D printing or cast from polymers. The first part of this work identified peculiarities of these compliant fluid-filled structures in comparison to geological reservoirs. The latter are described by traditional theories that have been developed for comparably rigid structures. The designed structures however show a strong interaction between the soft solid frame and the filling fluid, which is an opportunity and a challenge at the same time. On one hand, the solid-fluid coupling is beneficial for nondestructive testing as information can be gathered from fluid-bound and solid-bound signals. More precisely, the slow solid-dominated wave was observed to be susceptible with respect to compaction, while the fluid-dominated wave was strongly delayed by the solid. On the other hand, challenges arise for control of damping. Noise cancellation or acoustic cloaking, for instance, are often designed for a specific structure neglecting the various environmental fluids that appear in applications. Therefore, this work classified the acoustic behavior of various material combinations, highlighting limits in terms of stiffness ratios and density ratios. The second part of this work utilized buckling instabilities to achieve low-frequency damping materials. If a beam buckles between two fingertips, the surrounding air moves almost unnoticed. If a lot of beams buckle in a very viscous liquid such as oil or honey, the liquid movement is perceivable: the overall motion will be damped because the viscous fluid must be displaced. Such damping was observed in compliant structures with instabilities and explained by local volume changes of connected pores. Without instabilities, all pores deform equally and do not exchange fluid. With instabilities, some pores closed while other ones opened. This results in fluid displacement that acts like a damper. Tuning the geometry and the material properties allowed to achieve high damping at seismic frequencies that otherwise require large masses or distances to be damped out efficiently.
Publications
- “Oscillatory fluid flow in deformable tubes: Implications for pore-scale hydromechanics from comparing experimental observations with theoretical predictions.” J. Acoust. Soc. Am., 140(6), 4378–4395, 2016
P. Kurzeja, H. Steeb, M.A. Strutz, and J. Renner
(See online at https://doi.org/10.1121/1.4971365) - “The criterion of subscale sufficiency and its application to the relationship between capillary pressure, saturation and interfacial areas.” Proc. R. Soc. A, 472, 2016
P. Kurzeja
(See online at https://doi.org/10.1098/rspa.2015.0869)
