Experimental Study on Impedance-Based Damage Detection Considering Temperature Effect

The impedance-based damage detection technique has the potential for health monitoring of different types of structures. However, it is necessary to consider the temperature effect on the impedance signal in applying this technique to actual structures. In this study, an effective impedance-based damage detection method that compensates for the temperature effect was developed. Experimental tests on a steel frame structure connected with high tensile bolts were performed. Moreover, the temperature effect on the impedance damage index was compensated for detecting damage caused by bolt looseness; that is, the relationship between the impedance damage index and the temperature was established through long-term measurements. Based on this relationship, damage detection was performed by compensating for the temperature effect. Because the damage index after the bolt loosening reflects the effects of temperature and damage, it is difficult to evaluate the damage by monitoring only the damage index. However, after compensating for the temperature effect, it was observed that the damage could be estimated precisely. The damage was effectively monitored after measuring the impedance signal and temperature over a specific period for the initial healthy structural state, analyzing the correlation between the impedance damage index and temperature, and setting an appropriate warning criterion based on the correlation.

Extremely Low Effective Impedance in Stratified Graphene-Dielectric Metamaterials

The periodic reflections in frequency were observed in a stack of graphene layers and reported as a series of mini photonic bandgaps owing to the multiple interference by the graphene layers. In this research, the effective medium approach was employed to obtain the effective refractive index and Bloch impedance for understanding the wave propagation characteristic therein. Specifically, the pure real effective refractive index without attenuation as well as an extremely low Bloch impedance were found at the frequencies exhibiting periodic reflections. Some numerical examples were demonstrated to show that the series bandgap-like reflections in fact are attributed to considerable impedance mismatch caused by this ultra low Bloch impedance.

A Non-Compact Effective Impedance Model for Can-To-Can Acoustic Communication: Analysis and Optimization of Damping Mechanisms

Recently, various studies have focused on modeling the acoustic communication between adjacent cans in can-annular systems. In this study, a coupling model is presented that, in contrast to previous models, includes the effect of density fluctuations, mean flow, and dissipative effects at the connection gaps. By assuming plane acoustic waves inside each can and exploiting the discrete rotational symmetry of the can-annular system, the acoustic can-to-can interaction can be represented by an effective Bloch-type impedance. A single can modeled with the effective impedance at the downstream end emulates the acoustic response of the entire can-annular arrangement. We then propose the idea of installing a liner just upstream of the first turbine stage to damp azimuthal instabilities and discuss in detail the effect that the impedance of the liner has on the effective reflection coefficient for different Bloch wavenumbers. In the low-frequency limit, we derive an analytical condition for achieving maximum damping at a specific Bloch-number. The damping of azimuthal modes depends on the porosity of the liner, mean flow parameters and the Bloch-structure of the mode. These results suggest the possibility of targeting the damping of modes of certain azimuthal order by geometric variations of the liner or of the connection gap. The findings of this study provide a deeper understanding of the mechanisms that drive the can-to-can acoustic communication, and open the path for devising passive damping strategies aimed at stabilizing specific modes in can-annular combustors.

A Non-Compact Effective Impedance Model for Can-to-Can Acoustic Communication: Analysis and Optimization of Damping Mechanisms

Can-annular combustors can feature azimuthal instabilities even if the acoustic coupling between the individual cans is weak. Recently, various studies have focused on modeling the acoustic communication between adjacent cans in can-annular systems. In this study, a coupling model is presented that, in contrast to previous models, includes the effect of density fluctuations, mean flow, and dissipative effects at the connection gaps. By assuming plane acoustic waves inside each can and exploiting the discrete rotational symmetry of the can-annular system, the acoustic can-to-can interaction can be represented by an effective Bloch-type impedance. A single can modeled with the effective impedance at the downstream end emulates the acoustic response of the entire can-annular arrangement. We then propose the idea of installing a liner just upstream of the first turbine stage to damp azimuthal instabilities. By using the proposed can-to-can coupling model, we discuss in detail the effect that the impedance of the liner has on the effective reflection coefficient for different Bloch wavenumbers. In the low-frequency limit, we derive an analytical condition for achieving maximum damping at a specific Bloch-number. We show that the damping of azimuthal modes depends on the porosity of the liner, mean flow parameters and the Bloch-structure of the mode. These results suggest the possibility of targeting the damping of modes of certain azimuthal order by geometric variations of the liner or of the connection gap. As an exemplary application of the theory, we set up a network model of a generic industrial 12-can combustor and investigate a cluster of acoustic and thermoacoustic eigenvalues for a varying liner porosity. The findings of this study provide a deeper understanding of the mechanisms that drive the can-to-can acoustic communication, and open the path for devising passive damping strategies aimed at stabilizing specific modes in can-annular combustors.