A generalised study of 3D confined jet at a wall

• Main Author: Lim, Raine Jia Ying
• Other Authors: Lim Siow Yong
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Civil engineering
• Online Access: https://hdl.handle.net/10356/175833
• Institution: Nanyang Technological University

Scour, the erosion of seabed sediment by currents or flowing water, poses a significant challenge to the stability of port infrastructure. In berthing ports for large ships, the underlying sediment plays a critical role in resisting geotechnical failures like overturning or sliding of retaining walls. However, the powerful thrust generated by ship propellers can scour the seabed at the wall's toe, potentially weakening the sediment and compromising the stability of the wall. Understanding the scour profiles generated by ship thrusters is crucial for determining the necessity of seabed protection measures to safeguard retaining walls. This study investigated the influence of varying offset height and lateral distance of a water jet on scour development near a wall, employing a confined jet scour setup. Other factors, including jet diameter, jet velocity, sediment size, and duration, were maintained at established critical values to isolate the effects of offset height and lateral distance. The presence of the wall significantly influenced the resulting scour pattern. Jet deflection due to the wall interaction created a distinct scour profile with unevenness observed beyond the jet outlet. This unevenness likely originated from the recirculating flow patterns influenced by the formation of ridges. Additionally, the wall reflected water back onto the sediment bed, intensifying scour in this confined environment. The findings revealed an interesting observation regarding the influence of these two parameters: lateral distance of the jet exhibited minimal impact on the scour hole size. Conversely, increasing the offset height of the jet from the datum (reference level) resulted in larger scour holes. This observed trend regarding lateral distance deviates from expectations in some prior studies on unobstructed jets. Data analysis revealed limitations in existing predictive formulas for unobstructed jets, such as those proposed by Chow (2020) and Tan (2018). These formulas consistently overestimated scour depths by a factor of 2 to 4 when compared to the data collected in this study. While equation proposed by Chee (2022) were sufficient, a new approach was adopted to improve the predictive formulas. Data from this study was combined with relevant data from previous studies to create a comprehensive dataset. This combined dataset was then analysed using dimensional analysis and multiple regression analysis, leading to the development of new empirical equations. These new equations boast high correlation coefficients ranging from 0.927 to 0.986, indicating a strong relationship between the predicted and observed scour hole dimensions. These equations hold promise for estimating the maximum scour hole dimensions caused by 3D jets near a wall. However, further data collection encompassing a wider range of conditions is necessary to validate the general applicability of these proposed equations.