Search

The interaction between surface gravity waves and a vertical wall-mounted rigid cylindrical structure such as a pier or vegetation involves various interdependent physical phenomena including the scouring process and the vertical mixing and horizontal dispersion of materials. As a step toward understanding this interaction, detailed flow fields of sinusoidal oscillatory flow passing a vertical wall-mounted cylinder are numerically investigated for three different wave conditions. With a moderately wide range of the Keulegan–Carpenter number from 6 to 20, numerical simulations are systematically performed, and the data are extensively investigated to determine the dynamic characteristics of the oscillatory flow past a wall-mounted cylinder. In this study, three-dimensional unsteady incompressible Navier–Stokes equations are solved using the open-source software, OpenFOAM®. Grid-convergence tests are conducted, and the undisturbed oscillating Stokes boundary-layer determined by numerical simulations is validated by a good agreement with the analytical solution. Flow details in the form of profiles, streamlines, and contours of calculated turbulence fields are presented. Coherent structure dynamics is illustrated using iso-surfaces of the Q-criterion. The synthesis of various flow variables presents a mechanistic view of the bed shear and processes responsible for scour near the cylinder-wall junction. [ABSTRACT FROM AUTHOR]

This study applies the Dynamic Mode Decomposition (DMD) to better understand and model the oscillatory flow across a vertical wall-mounted cylinder. At different Keulegan–Carpenter numbers, three-dimensional direct numerical simulations are performed to provide the flow details such as the snapshots of the coherent structures around the cylinder, vorticity fields, and bed shear stress. The selected fields are decomposed into dynamic modes. The characteristic flow features with relevant information including spatial mode shape, frequency, and mode amplitude are systematically investigated. The time series of flow fields is also reconstructed using the DMD analysis and compared against the original data to assess the efficacy of information in the low-dimensional system. The results show that the DMD analysis can capture the dynamic and nonlinear features of the oscillatory flow past the vertical wall-mounted cylinder and also efficiently reconstruct the relevant fields with reasonable accuracy. It provides a basis for the data-driven model of scour near the cylinder–wall junction relevant to coastal engineering applications. [ABSTRACT FROM AUTHOR]

In wall-bounded time-periodic flows, nonlinearity, associated with higher harmonic term(s) in velocity and/or acceleration outside the boundary layer, can significantly change the wall turbulence compared with that in the linear Stokes Boundary Layer. A significant feature of a nonlinear wall-bounded turbulent time-periodic flow is the formation of a net current which has not yet been mechanistically explained. This study investigates the effects of asymmetric velocity outside the boundary layer on wall turbulence and net current formation through Direct Numerical Simulations of a smooth-walled planar channel driven by the Second-order Stokes Wave. Simulation results suggest that net current characteristics depend on whether developed turbulence is present. When turbulence is developed, asymmetric viscous length scale is found to be the primary reason of the net current whereby a vertical offset between negative and positive Reynolds shear stress profiles, associated with forward and reverse flows, respectively, is created in a cycle. After averaging over a cycle, residual Reynolds shear stress, which drives the net current, is observed to be within the offset layer. [ABSTRACT FROM AUTHOR]

Recent turbulence-resolving simulations of fine sediment transport in the oscillatory bottom boundary layer (OBBL) revealed the existence of a diverse range of flow regimes over muddy seabeds. Transitions between these flow regimes are caused by different degrees of sediment-induced stable density stratification in the OBBL. These transitions have critical implications for the role of wave resuspension in the delivery of fine sediment and hydrodynamic dissipation over muddy seabeds. This study further investigates the effect of Newtonian rheology, parameterized as a concentration-dependent effective viscosity, on turbulence modulation and the transition from turbulent to laminar states. Assuming small particle Stokes number, the equilibrium approximation is adopted to simplify the Eulerian two-phase flow governing equations. The resulting simplified equations are solved with a high-accuracy hybrid scheme in an idealized OBBL. A sixth-order centered compact finite difference is implemented in the vertical direction to solve the governing equations with a flow-dependent viscosity while the pseudospectral method is retained for two horizontal directions. At Stokes Reynolds number , simulations reveal that when rheology is incorporated, the enhanced effective viscosity can further attenuate the flow turbulence in addition to the well-known sediment-induced stable density stratification. Through the enhanced viscosity, the velocity gradient very near the bed is significantly reduced, which leads to much weaker turbulent production and the onset of laminarization. Although the sediment-induced stable density stratification is known to cause laminarization of bottom boundary layer flows, our preliminary finding that the enhanced viscosity via rheological stresses encourages the flow laminarization may be useful in explaining the field-observed large wave dissipation rate over muddy seabeds during the waning stage of a storm. [ABSTRACT FROM AUTHOR]

Gravity currents from instantaneous sources down a slope were modeled with classic thermal theory, which has formed the basis for many subsequent studies. Considering entrainment of ambient fluid and conservation of total buoyancy, thermal theory predicted the height, length, and velocity of the gravity current head. In this study, the problem with direct numerical simulations was re-investigated, and the results compared with thermal theory. The predictions based on thermal theory are shown to be appropriate only for the acceleration phase, not for the entire gravity current motion. In particular, for the current head forms on a 10° slope produced from an instantaneous buoyancy source, the contained buoyancy in the head is approximately 58% of the total buoyancy at most and is not conserved during the motion as assumed in thermal theory. In the deceleration phase, the height and aspect ratio of the head and the buoyancy contained within it may all decrease with downslope distance. Thermal theory relies on the increase in the mass of the current head through entrainment as the major mechanism for deceleration and, therefore, tends to underpredict the front velocity in the deceleration phase. [ABSTRACT FROM AUTHOR]