WET VIBRATION OF LOW-ASPECT-RATIO AEROFOIL WING : SEMI-ANALYTICAL AND NUMERICAL APPROACH WITH EXPERIMENTAL INVESTIGATION

The free wet vibration characteristics of an idealized low aspect-ratio cantilever wing are studied semi-analytically, numerically, and experimentally. The wing is modeled as a tapered, hollow Kirchhoff's plate, with the chord-wise section as a symmetrical NACA0018 aerofoil. The chord length tapers from root to tip, over the span. The main aim is to set up a suitable radiation boundary problem for the vibrating cantilever wing, in order to semi-analytically generate the wet frequencies. The efficacy of the semi-analytical wet vibration approach is studied by comparing it with the other two approaches. The difficulties encountered are due to the hollow two-way tapered shape of the wing and the free edge boundary conditions on its three sides. The semi-analytical approach is based on Galerkin's method, which includes the modal superposition of two orthogonal beam modeshapes (Free-Free beam in chord wise and cantilever in span-wise directions). The plate modeshapes thus generated are further used in the 3D source distribution technique to calculate the fluid inertia, leading to a consistent drop in the natural frequencies. The cantilever wing has been fabricated and tested in-house. The underwater impact hammer test generates the wet natural frequencies. The free vibration frequencies are also verified numerically using ANSYS, and compared with experimental studies.