Experimental Study on the Effects of Water Depth in Frequency Reduction of Slender Bodies

Modal Analysis for obtaining the shape modes and frequencies of a structure is a major step in design and analysis of marine structures. It is known that the natural frequency is reduced due to added mass effects under the water, and excitation near the natural frequencies may result in dangerous vibrations in the system. So understanding how the vibration property of a structure is changed under water is an important problem. In this paper modal analysis of a cylindrical structure under water and also in the air is investigated. The effects of water depth and water surface on the mode shapes and natural frequencies of the structure are studied via experimental and numerical methods. To this end three Aluminum cylindrical structures consist of 7 cabins with different diameters are constructed. Then modal experiments are performed in the air and also under water with 15 different depths from surface to 3- meter depth. Different test results are compared with the numerical/analytical results obtained from the ANSYS software and a semi-analytical method. The results show that there is a nearly good agreement between the numerical/analytical and experimental results; however some differences between the results are considered which should be studied further in future works.