Coherent Structures in a Channel with Groyne Fields: A Numerical Investigation Using LES

George Constantinescu; Andrew McCoy; Larry Weber

doi:10.1061/40792(173)400

The flow fields around two vertical spanwise groynes (spur dikes) in an open channel with lateral walls are investigated using Large Eddy Simulation (LES). The interest in this flow is motivated by its similarity to flow around groyne fields in natural rivers which are used to stabilize river banks and play and an important role in restoring fish habitat and enhancing the bio-diversity of flora and fauna in rivers. This work focuses on the investigation of the main phenomena present at flow around groynes from the point of view of the dynamics of coherent structures. This flow is characterized by massive separation, high adverse pressure gradients and presence of highly unsteady vortices. Simulation of the dynamically important flow structures encountered around groyne fields require methods that are capable of accurately capturing the wide range of eddies present in such flows such as LES which is used in the present investigation. The main flow features include the formation of a horseshoe vortex (HV) system at the base of the upstream groyne that plays an important role in the scouring process, the coherent structures in the area between the groynes (embayment), the detached shear layers originating at the groyne tips, the recirculation regions upstream the first groyne and downstream the second one. The simulations capture the structure of the HV system as it wraps around the groyne including the complex dynamics of the unsteady vortices that are part of the HV system. The formation and subsequent merging of vortices emanating from the detached shear layer originating at the upstream groyne tip and their subsequent interaction with the vortical structures inside the embayment is investigated and their spectral content is analyzed. The statistically averaged 3D vortical structures located in the upstream recirculation zone, within the embayment and detached shear layer, and downstream of it are visualized using 2D and 3D streamlines. The flow dynamics in these areas is described. The differences between the instantaneous coherent structures observed in LES and the mean structures are also highlighted and explained. It is shown that trying to understand the flow physics just based on the mean structures (this is all what we get from a steady RANS simulation) may be insufficient, some important vortices may not be captured as a result of the averaging process, while some other averaged flow structures do not really exist in the instantaneous flow, rather they are just a reflection of the averaging process when unsteady vortices are present.

Coherent Structures in a Channel with Groyne Fields: A Numerical Investigation Using LES

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