No abstract

In civil, mechanical, and aerospace engineering, structural dynamics is commonly understood to be a discipline concerned with the analysis and characterization of the vibratory response of structures. Key elements of the response are the amplitude, phase, and damping ratio, which are quantities that vary with the excitation frequency. In this paper, we extend the discipline of structural dynamics to the realm of materials engineering by intrinsically building localized substructures within, or attached to, the material domain itself−which is viewed as an extended medium without defined external boundaries. Our system is essentially a locally resonant elastic metamaterial, except here it is viewed from the perspective of unique dissipation characteristics rather than subwavelength effective properties or band gaps, as widely done in the literature. We provide a theory, validated by experiments, for substructurally synthesizing the dissipation under the conditions of free-wave motion, i.e., waves not constrained to a prescribed driving frequency. We use an extended elastic beam with attached pillars as an example of a metamaterial. When compared to an identical infinite beam with no attached substructures, we show that within certain frequency ranges the metamaterial exhibits either enhanced or reduced dissipation−which we refer to as positive and negative metadamping, respectively. These regimes are rigorously identified and characterized using the metamaterial's band structure and wavenumber-dependent dissipation diagram. This theory impacts applications that require a combination of high stiffness and high damping or, conversely, applications that benefit from a reduction in loss without the need to change the backbone constituent material.

Plane wave propagation in an elastic lattice material follows regular patterns as dictated by the nature of the lattice symmetry and the mechanical configuration of the unit cell. A unique feature pertains to the loss of elastodynamic isotropy at frequencies where the wavelength is on the order of the lattice spacing or shorter. Anisotropy may also be realized at lower frequencies with the inclusion of local resonators, especially when designed to exhibit directionally non-uniform connectivity and/or cross-sectional geometry. In this paper, we consider free and driven waves within a plate-like lattice−with and without local resonators−and examine the effects of damping on the isofrequency dispersion curves. We also examine, for free waves, the effects of damping on the frequency-dependent anisotropy of dissipation. Furthermore, we investigate the possibility of engineering the dissipation anisotropy by tuning the directional properties of the prescribed damping. The results demonstrate that uniformly applied damping tends to reduce the intensity of anisotropy in the isofrequency dispersion curves. On the other hand, lattice crystals and metamaterials are shown to provide an excellent platform for direction-dependent dissipation engineering which may be realized by simple changes in the spatial distribution of the damping elements.

Summary Optimization of hydrocarbon recovery requires information on the space and time behavior of the saturation of various fluids present in the reservoir. This is particularly true for oil fields under secondary recovery such as waterflooding, where an even reservoir sweep or zones of bypassed oil can be assessed by a proper description of the waterfront advance. Recently, permanent downhole electrodes have been deployed successfully in oil wells. This technology allows the time variation of the electrode potentials to be interpreted in terms of changes in saturation within the formation. However, the depth of investigation of such measurements is limited. Time-lapse pressure transient is an independent source of information with a greater depth of investigation and, therefore, it provides an adequate complement to the permanent resistivity array measurement. In this paper, we propose to use pressure buildup from repeated shut-in in association with the electrical measurements. After recalling analytical analogies in both types of measurements, we propose a quick-look method for interpreting the time-lapse pressure transients. We then compare the physical and practical advantages of each type of measurement and the domain of application of the two measurements with respect to fluid and reservoir properties. Finally, we propose an example showing the benefit obtained by coupling the two techniques.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractOptimization of hydrocarbon recovery requires information on the space and time behavior of the saturation of the various fluids present in the reservoir. This is particularly true for oilfields under secondary recovery such as water flooding, where an even reservoir sweep or zones of by-passed oil can be assessed by a proper description of the water front advance. Permanent downhole electrodes have recently successfully been deployed in oil wells. This technology allows the time variation of the electrode potentials to be interpreted in terms of changes in saturation within the formation. In practice, the depth of investigation of such measurements is however limited. Time-lapse pressure transient is an independent source of information with a greater depth of investigation, therefore provides an adequate complement to the permanent resistivity array measurement. In this paper, we propose to use pressure build-up from shut-in in association to the electrical measurements. After recalling analytical analogies in both types of measurements, we propose a quick-look method for interpreting the time-lapse pressure transients. We then compare the physical and practical advantages of each type of measurement and the domain of application of the two measurements with respect to fluid and reservoir properties. Finally we propose an example showing the benefit obtained by coupling the two techniques.