Experimental study on turbulent flow around a pair of cylinders placed along of symmetry in a rectangular open channel

Abstract

Piers for hydraulic structures such as bridges, barrages and regulators are often built in open channels which acts as bluff bodies. Piers causes obstruction to the flow, thereby affecting the flow pattern.The turbulent flow behavior around a pair of cylinders which acts like bluff bodies with varyingcenter to center spacing placed along the plane of symmetry (POS) are investigated in a rectangular flume at a fixed discharge and depth of water in this study. Mean and turbulent flow fields between two cylinders along with wake of downstream (d/s) cylinders were investigated using Acoustic Doppler Velocimeter (ADV).Analysis of longitudinal normalized velocity profiles in-between cylinders reveal that the near wake region is identical at each scenario whereasthe far wake region is affected by the downstream cylinder in 3D(D=diameter of cylinder) spacing and has zero interruption from downstream cylinder in 6D and 9D spacing. However, the flow gets affected as it approaches the d/s cylinder for 6D and 9D respectively.Analysis of longitudinal normalized velocity profiles downstream of the second cylinder states that upstream (u/s) cylinder has great influence on the both near and far wake of downstream cylinder. The length of the near wake or the recirculation zone increases with the increase in c/c(center to center) spacing of cylinders. In the far wake region, the rate of flow development to achieve cross sectional mean velocity becomes slower with the increment of c/c spacing. In the near wake of midstream, the recirculation is stronger near the surface than the bottom, whereas the reverse is true for the near wake of downstream of second cylinder. However, in overall, the recirculation in the midstream is stronger than the that in the downstream. Wall wake similarity in the far wake regionis investigated and the profiles and observation shows that the results at downstream follow wake similarity up to shorter longitudinal distance for all 3 cases although the number of sections in which follow wake similarity reduces with increased c/c spacing, since the size of the near wake region is becomes larger with increasing c/c spacing of cylinders. The range of magnitude and position of peak of turbulent intensity in longitudinal, transverse and vertical directions are similar in three scenarios in midstream. The longitudinal turbulent intensity and vertical turbulent is almost two-third and half respectively of transverse intensity. The maximum turbulent kinetic energy for all three cases occurs near the end of their respective recirculation zones. The measurement of turbulent shear stress is relatively chaotic than the turbulent intensities. The maximum of turbulent shear stresses occurs within the recirculation zone for all cases.Wake similarity profile in terms of near wake on a horizontal plane at mid-depth is drawn for test results taken from both upstream pier and downstream pier measurements. The findings confirm Balachandar’s equation justifying the validity of this experiment.Therefore, these results will be useful in validation of depth averaged as well as CFD modeling of flow affected by similar obstructions, specially these data will help in simulating flows more precisely around the bridge piers. Furthermore, the obtained data in this experimentation are useful for simulation of the flows around bluff bodies in open channels as it will help in simulating flows more precisely around the bridge piers