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Shake table testing of a tuned mass damper inerter (Tmdi)-equipped structure and nonlinear dynamic modeling under harmonic excitations
This paper presents preliminary experimental results from a novel shaking table testing campaign investigating the dynamic response of a two-degree-of-freedom (2DOF) physical specimen with a grounded inerter under harmonic base excitation and contributes a nonlinear dynamic model capturing the behavior of the test specimen. The latter consists of a primary mass connected to the ground through a high damping rubber isolator (HDRI) and a secondary mass connected to the primary mass through a second HDRI. Further, a flywheel-based rack-and-pinion inerter prototype device is used to connect the secondary mass to the ground. The resulting specimen resembles the tuned mass damper inerter (TMDI) configuration with grounded inerter analytically defined and numerically assessed by the authors in a number of previous publications. Physical specimens with three different inerter coefficients are tested on the shake table under sine-sweep excitation with three different amplitudes. Experimental frequency response functions (FRFs) are derived manifesting a softening nonlinear behavior of the specimens and enhanced vibration suppression with increased inerter coefficient. Further, a 2DOF parametric nonlinear model of the specimen is established accounting for non-ideal inerter device behavior and its potential to characterize experimental response time-histories, FRFs, and force-displacement relationships of the HDRIs and of the inerter is verified
Numerical and Experimental Investigation of Machinery Isolation Featuring Gap-Type Nonlinear Rotational Inertial Mechanisms for Marine Applications
Transmitted noise and vibration from equipment and machinery is an ongoing and serious priority onboard marine vessels as noise and vibrations interfere with system operations and can compromise the functionality of the vessel. Vibration isolation systems have been widely studied for civil and mechanical applications because of the damage that can occur from extreme vibrations and excessive motion. Conventional vibration isolation systems often include components such as springs and dampers in the isolation layer, but researchers have begun to incorporate other devices including, linear rotational inertial mechanisms (RIMs), often known as inerters, to enhance traditional vibration isolation systems. The inerter is a mechanical device with two terminals in which the equal and opposite force produced is equal to a constant known as inertance multiplied by the relative acceleration between the two terminals. The inertance is a calculated value based on characteristics of the inerter including the geometry of its flywheel. When an inerter is incorporated in an isolation system, the inerter reduces the natural frequency of the system and reduces displacements, but also results in high-frequency transmitted forces, or loads induced back into the system. The high-frequency transmitted forces caused by inerters have encouraged the investigation of nonlinear rotational inertial mechanisms (NRIMs). The objective of this thesis was to investigate the behavior of NRIMs, with an emphasis on gap-type mechanisms, for use in machinery isolation in marine environments. A numerical study was performed to compare a conventional inerter with three different NRIMs. To experimentally investigate linear rotational inertial mechanisms and nonlinear rotational inertial mechanisms, a test apparatus was designed to analyze the effects of incorporating these devices in an isolation layer. A gap-type NRIM, referred to as the bushing-crown gap inerter that would engage and disengage a flywheel based on the primary mass displacement, was designed, fabricated, and tested to determine the effectiveness of the device. The test apparatus was tested without a RIM, with a linear RIM, and with a NRIM to compare responses. The bushing-crown gap inerter significantly reduced high-frequency transmitted forces compared to the RIM. The natural frequency of the isolation mode of the system increased slightly with the gap-type NRIM compared to the no RIM case. Additionally, the amplitude of the peak at the natural frequency was decreased compared to the no RIM case but was still slightly higher than the conventional inerter. The gap-type NRIM flywheel configuration has potential to reduce the natural frequency peak amplitude while avoiding high-frequency transmitted forces that is observed with the inerter when subjected to broadband loading. The results of this research indicate the potential of gap-type NRIMs and encourage further study of them