Coherent structure dynamics and sediment erosion mechanisms around an in‐stream rectangular cylinder at low and moderate angles of attack

[1] Local scour around elongated in-stream structures (e.g., high–aspect ratio rectangular bridge piers) is mainly driven by the interactions between the erodible bed and the large-scale coherent structures generated by the presence of the flow obstruction. The present investigation uses eddy-resolving numerical simulations to study the mean flow and turbulence structure around a high–aspect ratio rectangular cylinder placed in a flat bed channel. Simulations are conducted for three angles of attack (α = 0°, 15°, and 30°) at a channel Reynolds number of 2.4 × 105. This paper focuses on the dynamics of the large-scale coherent structures forming around the rectangular cylinder and their role in controlling sediment entrainment at conditions corresponding to the start of the scour process. Simulation results show that most of the sediment is entrained from the bed by the eddies shed inside the separated shear layers (SSLs), by the legs of the necklace vortices, and by the strongly accelerated flow on the outer side of the SSLs. For α = 0° and 15°, the horseshoe vortex (HV) system plays a relatively minor role in the entrainment of sediment in front of the cylinder, and the passage of the wake vortices (rollers) results in a small amplification of the bed friction velocity. In contrast, for α = 30°, the unsteady dynamics of the main necklace vortices part of the HV system and of the roller vortices results in a significant amplification of the instantaneous bed friction velocity. The mean flux of sediment entrained from the bed calculated on the basis of the mean flow field is found to underestimate by 2–3 times the same quantity when estimated more correctly on the basis of the instantaneous flow fields. The primary reason for this underestimation is sediment entrainment in regions where the mean flow bed friction velocity is smaller than the critical value for entrainment. Quantitative information on the extent of the regions of high values of the bed friction velocity and its standard deviation and on the sediment entrainment flux as a function of the angle of attack is essential to guide scour protection measures around the in-stream noncircular obstructions.

[1]  George Constantinescu,et al.  Analysis of the flow and mass transfer processes for the incompressible flow past an open cavity with a laminar and a fully turbulent incoming boundary layer , 2006, Journal of Fluid Mechanics.

[2]  L. Biferale,et al.  Dynamics and statistics of heavy particles in turbulent flows , 2006, nlin/0601027.

[3]  Van Rijn,et al.  Sediment transport; Part I, Bed load transport , 1984 .

[4]  Bijan Dargahi,et al.  Controlling Mechanism of Local Scouring , 1990 .

[5]  George Constantinescu,et al.  Detached Eddy Simulation Investigation of Turbulence at a Circular Pier with Scour Hole , 2009 .

[6]  G. Constantinescu,et al.  An investigation of the flow and scour mechanisms around isolated spur dikes in a shallow open channel: 1. Conditions corresponding to the initiation of the erosion and deposition process , 2008 .

[7]  Gokhan Kirkil,et al.  Flow and turbulence structure around an in‐stream rectangular cylinder with scour hole , 2010 .

[8]  Willi H. Hager,et al.  Down-flow and horseshoe vortex characteristics of sediment embedded bridge piers , 2006 .

[9]  F. Sotiropoulos,et al.  Lagrangian model of bed-load transport in turbulent junction flows , 2011, Journal of Fluid Mechanics.

[10]  B. Sumer,et al.  Numerical and experimental investigation of flow and scour around a circular pile , 2005, Journal of Fluid Mechanics.

[11]  G. Constantinescu,et al.  Structure of turbulent flow at a river confluence with momentum and velocity ratios close to 1: Insight provided by an eddy‐resolving numerical simulation , 2011 .

[12]  G. Constantinescu,et al.  The structure of turbulent flow in an open channel bend of strong curvature with deformed bed: Insight provided by detached eddy simulation , 2011 .

[13]  George Constantinescu,et al.  An investigation of the dynamics of coherent structures in a turbulent channel flow with a vertical sidewall obstruction , 2009 .

[14]  P. Spalart.  Detached-Eddy Simulation , 2009 .

[15]  Subhasish Dey,et al.  Kinematics of horseshoe vortex development in an evolving scour hole at a square cylinder , 2008 .

[16]  Marian Muste,et al.  Similitude of Large-Scale Turbulence in Experiments on Local Scour at Cylinders , 2006 .

[17]  António H. Cardoso,et al.  Local scour at vertical‐wall abutments under clear‐water flow conditions , 2006 .

[18]  F. Sotiropoulos,et al.  On the bimodal dynamics of the turbulent horseshoe vortex system in a wing-body junction , 2007 .

[19]  P. Spalart,et al.  A New Version of Detached-eddy Simulation, Resistant to Ambiguous Grid Densities , 2006 .

[20]  Subhasish Dey,et al.  Characteristics of Horseshoe Vortex in Developing Scour Holes at Piers , 2007 .

[21]  George Constantinescu,et al.  Nature of flow and turbulence structure around an in‐stream vertical plate in a shallow channel and the implications for sediment erosion , 2009 .

[22]  B. Melville,et al.  Scale Effect in Pier-Scour Experiments , 1998 .

[23]  Yi-Ju Chou,et al.  A model for the simulation of coupled flow-bed form evolution in turbulent flows , 2010 .

[24]  Philippe R. Spalart,et al.  Trends in turbulence treatments , 2000 .

[25]  George Constantinescu,et al.  The structure of turbulent flow at a river confluence with a momentum ratio close to one: insight provided by an eddy-resolving simulation , 2010 .

[26]  George Constantinescu,et al.  3D Calculations of Equilibrium Conditions in Loose-Bed Open Channels with Significant Suspended Sediment Load , 2010 .

[27]  Y. Dubief,et al.  On coherent-vortex identification in turbulence , 2000 .

[28]  George Constantinescu,et al.  A 3D non-hydrostatic model to predict flow and sediment transport in loose-bed channel bends , 2008 .

[29]  W. Graf,et al.  Experiments on flow around a cylinder; the velocity and vorticity fields , 1998 .

[30]  R. Simpson,et al.  Time-depeiident and time-averaged turbulence structure near the nose of a wing-body junction , 1990, Journal of Fluid Mechanics.

[31]  K. Richards,et al.  The theoretical foundations and potential for large-eddy simulation (LES) in fluvial geomorphic and sedimentological research , 2005 .

[32]  W. Rodi.  Comparison of LES and RANS calculations of the flow around bluff bodies , 1997 .

[33]  Emmett M. Laursen,et al.  Scour Around Bridge Piers and Abutments , 1956 .

[34]  George Constantinescu,et al.  Assessment of Predictive Capabilities of Detached Eddy Simulation to Simulate Flow and Mass Transport Past Open Cavities , 2007 .

[35]  James R. Forsythe,et al.  Numerical Investigation of Flow Past a Prolate Spheroid , 2002 .

[36]  Van Rijn,et al.  Sediment Pick‐Up Functions , 1984 .

[37]  Sujit K. Bose,et al.  Clear water scour at circular piers : a model , 1995 .

[38]  Mete Koken,et al.  Coherent structures around isolated spur dikes at various approach flow angles , 2011 .

[39]  Ugo Piomelli,et al.  A dynamic stochastic forcing method as a wall-layer model for large-eddy simulation , 2006 .

[40]  C. Paola,et al.  Skin friction behind isolated hemispheres and the formation of obstacle marks , 1986 .

[41]  Herve Morvan,et al.  Bed-Shear Stress Characteristics of a Simple, Prismatic, Rectangular Channel , 2008 .

[42]  G. Constantinescu,et al.  Numerical investigations of flow over a sphere in the subcritical and supercritical regimes , 2004 .

[43]  Jørgen Fredsøe,et al.  Influence of turbulence on bed load sediment transport , 2003 .