Study of the Natural Frequencies of a Disc
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This thesis presents experimental methods designed to determine the natural frequencies of a disc in air and submerged in water. With the goal of showing the change in natural frequencies when the disc is submerged. In addition, analytical calculations and numerical simulations of the natural frequencies of the same disc are presented. The roving hammer experiment measures the vibration response of the disc to impacts on various positions. The frequency response experiment measures the vibration response to excitation from a piezoelectric patch glued to the disc. From these experiments the natural frequencies and the mode shapes are identified. The analytical calculation are based on table values of eigenvalues for an annular plate with clamped inside and free outside. The numerical simulations are performed in ANSYS with material properties of standard steel to simulate the disc in air, and with an increased density to simulate the disc submerged in water. The increased density is calculated based on results from the frequency response experiment.All methods found the natural frequencies of the disc gave similar results, with only minor differences. The roving hammer experiment found the natural frequencies of the disc in air, and indicated their mode shape. The frequency response experiment gave the frequency response function of the disc in both air and submerged in water, and by comparing of the functions the change in natural frequencies was shown. The analytical calculations and numerical simulations gave satisfactory results, but with slight over/under prediction of the natural frequencies.