water Article Numerical Simulation of Local Scour around Three Cylindrical Piles in a Tandem Arrangement Jyh-Haw Tang and Aisyah Dwi Puspasari *   Citation: Tang, J.-H.; Puspasari, A.D. Numerical Simulation of Local Scour around Three Cylindrical Piles in a Tandem Arrangement. Water 2021, 13, 3623. https://doi.org/10.3390/ w13243623 Academic Editor: Giuseppe Pezzinga Received: 24 November 2021 Accepted: 13 December 2021 Published: 16 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Department of Civil Engineering, College of Engineering, Chung Yuan Christian University, 200 Chung Pei Road, Chung Li District, Taoyuan City 320, Taiwan; jyhhaw@cycu.edu.tw * Correspondence: aisyahdwipus@cycu.org.tw; Tel.: +886-902-447-765 Abstract: Scouring is one of the most common potential causes of bridge pile foundation failure with loss of life, economic and environmental impacts. Comprehensive studies on the numerical simulation of local scour around pile groups are still limited. This paper presents a numerical simulation using Flow-3D software to calculate the maximum sediment scour depth and investigate the mechanism around the groups of three cylinders in a tandem arrangement. A validation using the experimental study was carried out to confirm the reliability of the present numerical model. By using the Van Rijn transport rate equation and RNG k-ε turbulence model, the results of time evolution of scour depth and bed elevation contour show good agreement with the experimental study. The numerical simulation of three cylinders in a tandem arrangement were conducted with pile spacing ratios, G/D of 2 and 3. The local scour is affected by the horseshoe vortex from the downflow driven by the downward pressure gradient and rotates in front of the pile and the high bed shear stress, triggered by flow acceleration. The deepest maximum local scour depth is always obtained by the front pile as a shield pile, followed by the piles behind. The trend of the maximum local scour depth in a tandem arrangement is in accordance with the experimental studies and has a better agreement than previous numerical studies with the same model setup. This means that the numerical model used to simulate pile groups is accurate and capable of calculating the depth of sediment scour. Keywords: numerical model; local scour; pile foundations; tandem; Flow-3D 1. Introduction Currently, the use of offshore pile foundations has become commonplace [1–3], es- pecially for bridge construction due to its structural efficiency, low cost, and ease of construction [4–6]. In bridge construction, pile foundations are used to transmit the load of the superstructure and substructure to a lower level, which is a stronger soil layer in the soil mass [7]. However, several issues, including the scour phenomenon, must be addressed to ensure the safety and stability of the offshore pile foundation which will directly affect the bridge structure [8–10]. Scouring on bridge foundations is a natural phenomenon that occurs because the presence of pile foundations affects the flow pattern and increases turbulence around the pile foundation [11–13]. The blocked current in front of the pile triggers the formation of a horseshoe vortex which erodes and transports the bed particles, then begins to gen- erate the scour [12–15]. Reported by Lagasse et al. [16], in the United States, there are 488,750 bridges spanning rivers with scour-related bridge failures, costing $30 million per year. Furthermore, over the last 30 years, about 823 total bridge failures were identified and about 60% were caused by hydraulic failure modes including scour in pile foundations [17]. One of the deadliest bridge collapse tragedies due to excessive scouring under bridge pile foundations was the collapse of the Schoharie Creek Bridge, NY State Thruway on 5 April 1987 as shown in Figure 1a, which claimed five vehicles which fell into the river, Water 2021, 13, 3623. https://doi.org/10.3390/w13243623 https://www.mdpi.com/journal/water