Local scour beneath a forced-vibrating circular cylinder in unidirectional current

• Main Author: Chok, Man Ming
• Other Authors: Chiew Yee Meng
• Format: Final Year Project
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Civil engineering::Water resources
• Online Access: http://hdl.handle.net/10356/76369
• Institution: Nanyang Technological University

This research is a continuation of study on the scour mechanism around a forced vibrating pipeline based on the research paper “Experimental study of scour around a forced vibrating pipeline in quiescent water” (Guan, Hsieh, Chiew, & Low, 2019). In this study, unidirectional current is introduced into the experiments to understand how the scour mechanisms around a vibrating cylinder differ from the case of no current. Due to the nature of the experimental setup, the effect due to impingement of cylinder on sand bed is significant and it is called the “Pounding Effect” in this study. Physical experiments were conducted for the visual observations of the scour and the phenomenon are reasoned using the scour mechanism theory as proposed by Guan et. al on forced vibrating cylinder and Hsieh et. al’s (Hsieh, Chiew, Wei, & Low, 2016) on “Piston Effect”. Five experiments were conducted in total. Test 1 and Test 2 are the main experiments for forced-vibration frequency f1 = 0.3 Hz and f2 = 0.96 Hz respectively in unidirectional current. Geometry data for the scour profile of these two tests were collected and analysed. Tests 3 to 5 were replications of test conducted based on Guan et. al’s and Hsieh et. al’s study to support the reasonings for Tests 1 and 2. Through reasoning and observations, the following findings were deduced: (i) the scour capacity around a forced vibrating pipeline increases when there is current, (ii) asymmetrical vortices are produced when there is current, (iii) the combined effect of current and vibrating frequency results in different scour shape, (iv) the pounding effect is related to the vibrating frequency whereby the effect is more significant at a higher vibrating frequency (v) the scour process can be divided into a 3-stage scour before reaching the equilibrium and it is affected by the position of the vibrating cylinder. It is recommended that for future experiments, the response amplitude should be kept constant for all vibrating frequencies to understand fully the effect of vibrating frequency on the scour mechanisms.