Dynamic response of fluid coupled coaxial cylinders to seismic excitation

Abstract

This study deals with the overall dynamic behavior of concentric flexible cylinders coupled with fluid in the annular region, subjected to both sinusoidal and seismic excitations. Three different combinations of acrylic cylinders are used, i.e., one outside cylinder and three inside cylinders with different inside diameters. The cylinders have the same thickness and length, which when paired with the outer cylinder provide three annular gap sizes. To determine the sinusoidal response of the system the outside cylinder is point‐excited and the natural frequencies and corresponding mode shapes are determined. For the seismic tests, the three coaxial setups are mounted on a rigid steel support frame. The support frame is suspended from the ceiling by flexible rods and excited by a hydraulic shaker with a simulated seismic input. Two distinct responses are observed, i.e., either in‐phase or out‐of‐phase breathing modes between the inner and outer cylinders. The out‐of‐phase modes approach some limiting value as the gap size decreases, while the in‐phase modes increase in frequency as the gap becomes smaller. Comparison between the inner and outer cylinder displacement responses from the seismic loadings shows the ratios of the averaged peak responses varies as a function of the gap between the inner and outer cylinder. Similar trends were observed on the spectral densities of the responses. It also appears that the gap size has a strong influence in the frequency response of the system.