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Abstract
Threedimensional (3D) wake transition for flow past a square cylinder aligned with sides perpendicular and parallel to the approaching flow is investigated using direct numerical simulation. The secondary wake instability, namely a Mode A instability, occurs at a Reynolds number (Re) of 165.7. A gradual wake transition from Mode A∗ (i.e. Mode A with vortex dislocations) to Mode B is observed over a range of from 185 to 210, within which the probability of occurrence of vortex dislocations decreases monotonically with increasing Re. The characteristics of the StrouhalReynolds number relationship are analysed. At the onset of Mode A∗, a sudden drop of the 3D Strouhal number from its twodimensional counterpart is observed, which is due to the subcritical nature of the Mode A∗ instability. A continuous 3D StrouhalReynolds number curve is observed over the mode swapping regime, since Mode A∗ and Mode B have extremely close vortex shedding frequencies and therefore only a single merged peak is observed in the frequency spectrum. The existence of hysteresis for the Mode A and Mode B wake instabilities is examined. The unconfined Mode A and Mode B wake instabilities are hysteretic and nonhysteretic, respectively. However, a spanwise confined Mode A could be nonhysteretic. It is proposed that the existence of hysteresis at a wake instability can be identified by examining the sudden/gradual variation of the 3D flow properties at the onset of the wake instability, with sudden and gradual variations corresponding to hysteretic (subcritical) and nonhysteretic (supercritical) flows, respectively.
Original language  English 

Pages (fromto)  102127 
Number of pages  26 
Journal  Journal of Fluid Mechanics 
Volume  842 
DOIs  
Publication status  Published  10 May 2018 
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 2 Finished

Hydrodynamic Forces on Small Diameter Pipelines Laid on Natural Seabed
Cheng, L., Draper, S., An, H., Zhao, M., White, D. & Fogliani, N.
1/01/15 → 31/12/17
Project: Research

Effect of natural seabed on hydrodynamics around cylindrical structures
1/01/15 → 11/01/18
Project: Research