Local scour beneath a forced-vibrating cylinder

• Main Author: Zhang, Yulu
• Other Authors: Chiew Yee Meng
• Format: Final Year Project
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Civil engineering
• Online Access: http://hdl.handle.net/10356/78445
• Institution: Nanyang Technological University

This project investigates the impact of a forced-vibrating cylinder both qualitatively and quantitively on local scour in the unidirectional current, as a continual study on scour around a forced-vibrating cylinder in quiescent water (Guan et al., 2019). First, the scour mechanism was visually observed for 7 tests. Except for Test 2, the pounding effect due to the impingement of the cylinder on the sand bed occurred in the first 30 minutes for Test 3-7. Subsequently, the piston effect took over and dominated the remaining scouring process. The piston effect was described by Hsieh et al. (2016) and Chok (2018), when the cylinder is moving towards the sand bed, resulting in a “water jet” and suspending sand sediments. Second, the scour profile was measured between 6 discontinuous time intervals for 6 tests, except for test 1, which measured the maximum scour depth under a fixed pipeline and acted as a control test. Three-dimensional parameters were obtained during the scouring process and compared under 3 categories, where each category only allows one changing variables like amplitude, frequency and flow velocity. The following findings were inferred: (1) the dimensions of scour hole were developed quickly during the first 5 hours of testing, then progressed at a decreasing rate; (2) the relationship between the maximum scour depth and amplitude can be described by a normalized second-order polynomial equation with R^2=0.9734; (3) The half-length of scour hole at the downstream side and at the upstream side, and the horizontal location of the lowest point on the sand bed were represented by l_(s,r),l_(s,l),l_(s,0) respectively, as illustrated in Figure 3.4. L_(s,r),〖 l〗_(s,L),and l_(s,0) showed less dependency on the amplitude; (4) Both the scour depth d_s and l_(s,0) were positively related to the frequency, while width-related parameters (l_(s,r),l_(s,l)) showed less dependency on the frequency; (5) If the flow velocity increases, all four dimensional parameters of scour hole will also increase.Third, a normalized empirical equation was proposed to quantify a general relationship between the scour depth and other vibrating parameters, which are amplitude, frequency and flow velocity. The proposed equation was analyzed and compared with other published equations of Li et al. (2013) and Luan et al. (2015). Lastly, it is recommended that future experiments should focus on the influence of f,u_0 on the scour depth at equilibrium. The exponents and constant values in the proposed equation need to be modified with new test results.