Experimental and Numerical Evaluation of Elevated Rectangular Water Tank Frequencies Using Digital Image Correlation

Key words:  fluid-structure interaction  natural frequency  rectangular tank  elevated tank  sloshing waves  DIC

Abstract:

Elevated, rectangular water tanks are an essential component of the water supply network and for emergency water storage. However, determining the dynamic response from seismic loads will require accurate estimates of their natural frequencies to help avoid damage or failure. Several numerical and analytical methodologies depend on assumptions that may not account for the complexity of fluid-structure interaction regarding rectangular geometries, indicating a strong need for solid experimental validation. This study pioneers the application of Digital Image Correlation (DIC), a non-contact optical technique, to measure the natural frequencies of a small-scale acrylic and Teflon tank, addressing this gap with a novel experimental approach. The method includes DIC using a high-speed camera, image analyzed by MATLAB, frequency analysis by Continuous Wavelet Transformation (CWT), and Fast Fourier Transformation (FFT); ANSYS finite element analysis, simplified models based on Eurocode and the Egyptian Code of Practice (ECP). Study results indicate that DIC is critical for attaining high accuracy, with maximum error differing by 2.92% for impulsive and 4.55% for convective frequencies from ANSYS and provides a better measure of dynamic response compared to contact-based measurements. Impulsive frequency decreased from 5.8724 Hz to 4.0085 Hz, and sloshing increased from 1.00 Hz to 1.84 Hz, as the water cover height varied from 0 cm to 9.8 cm. The Eurocode and ECP models describe acceptable errors of 7.36% and 9.21%, respectively. DIC showed higher accuracy, making it a useful tool for seismic design. This study improves the safety and reliability of designs for elevated water tanks in seismic regions with elevated seismic risk.