Local scour around submerged tandem and offset cylinders due to change in alignment angle Abhijit Rout BTech, MTech Doctoral Research Fellow, School of Infrastructure, Indian Institute of Technology Bhubaneswar, Bhubaneswar, Odisha, India Arindam Sarkar BE, ME, PhD Assistant Professor, School of Infrastructure, Indian Institute of Technology Bhubaneswar, Bhubaneswar, Odisha, India (corresponding author: asarkar@iitbbs.ac.in) A study on scour around submerged tandem and offset cylinders was carried out with the aim of characterising the scour process and the influence of various parameters on the variation of scour depth over time. An experimental campaign was undertaken in a laboratory flume over a shallow range of submergence ratios (1·95–2·60) and alignment angles between the tandem cylinders (0°–40°). The formation of dunes in between tandem cylinders was observed for experiments with reduced flow velocity. The scour depths around both cylinders increase with the increase of Froude number and decrease with an increase in the submergence ratio and sediment size. The scour depth around the downstream cylinder increases with increase of alignment angle. Equations to representi the various combinations of cylinders and arrangements have been proposed. On the basis of the experimental results, adjustment factors and a timescale are proposed. The degree of deviation and adjustment factors to correlate scour depth around tandem and single cylinders have been determined. The time variation is represented using a timescale that decreases with increase of densimetric Froude number. Notation a i exponents, i = 1, 2, 3, 4 (M 0 L 0 T 0 ) D diameter of circular structures or side of square structures (L) D 1 diameter of upstream cylinder (L) D 2 diameter of downstream cylinder (L) d depth of water (L) d s scour depth (L) d st scour depth at time t (L) d s1 scour depth around upstream cylinder (L) d s2 scour depth around downstream cylinder (L) d 16 16% finer sediment size (L) d 50 median sediment size (L) d 84 84% finer sediment size (L) ˆ d non-dimensional depth of water (M 0 L 0 T 0 ) ˜ d s non-dimensional scour depth (M 0 L 0 T 0 ) ˆ d 50 non-dimensional median sediment size (M 0 L 0 T 0 ) F Froude number, U/( gd ) 0·5 (M 0 L 0 T 0 ) F d densimetric Froude number, U/(Δgd 50 ) 0·5 (M 0 L 0 T 0 ) g gravitational acceleration (L/T 2 ) h height of structure (L) ˆ h non-dimensional height of structure (M 0 L 0 T 0 ) K constant (M 0 L 0 T 0 ) S submergence ratio (M 0 L 0 T 0 ) s relative density of sediments (M 0 L 0 T 0 ) T timescale (T) ˆ T non-dimensional timescale (M 0 L 0 T 0 ) t time (T) U mean approach flow velocity (L/T) U c critical mean flow velocity (L/T) ˆ x non-dimensional longitudinal distance (M 0 L 0 T 0 ) ˆ y non-dimensional transverse distance (M 0 L 0 T 0 ) ˆ z non-dimensional vertical distance (M 0 L 0 T 0 ) α angular alignment (M 0 L 0 T 0 ) Δ s – 1 (M 0 L 0 T 0 ) θ Shields parameter (M 0 L 0 T 0 ) ν kinematic viscosity of water (L 2 /T) ρ mass density of water (M/L 3 ) ρ s mass density of sediments (M/L 3 ) σ g geometric standard deviation (M 0 L 0 T 0 ) τ c h threshold bed shear stress on horizontal bed (M 0 /(L/T 2 )) 1. Introduction Swift-flowing natural streams or rivers scour the erodible bed around hydraulic structures due to the enhanced shear stresses induced by the fluid–structure interaction. The problem of scour around single partially submerged piers of various shapes and sizes have been extensively studied (Ashtiani and Kordkandi, 2013; Dey, 1997a, 1997b; Khosronejad et al., 2012; Melville and Coleman, 2000; Sheppard et al., 2014). Various circumstances, such as high floods and tides, among others, can completely submerge the well foundations of bridge piers under construction and offshore structures (Amini et al., 2012; Dey et al., 2008; Mohamed, 2013; Sarkar, 2014). In this regard, studies have reported scour characteristics around submerged cylinders and various other structures (Dey et al., 2008; Euler and Herget, 2011; Zhao et al., 2010). Furthermore, considering the structural aspects, bridges are also designed on a group of piers (Amini et al., 2012; Ashtiani and Beheshti, 2006; Ashtiani et al., 2010; Coleman, 2005; 14 Cite this article Rout A and Sarkar A (2020) Local scour around submerged tandem and offset cylinders due to change in alignment angle. Proceedings of the Institution of Civil Engineers – Water Management 173(1): 14–30, https://doi.org/10.1680/jwama.18.00030 Water Management Research Article Paper 1800030 Received 19/04/2018; Accepted 24/01/2019; Published online 26/02/2019 ICE Publishing: All rights reserved Keywords: hydraulics & hydrodynamics/ hydrology & water resource Downloaded by [ Indian Institute of Technology - Bhubaneswar] on [01/09/20]. Copyright © ICE Publishing, all rights reserved.