Celalettin E. Ozdemir

High‐resolution numerical modeling of wave‐supported gravity‐driven mudflows

[1]xa0Wave-supported gravity-driven mudflow has been identified as a major offshore fine sediment transport mechanism of terrestrial sediment into the coastal ocean. This transport process essentially occurs within the wave boundary layer. In this study, wave-supported gravity-driven mudflow is investigated via a wave-phase-resolving high-resolution numerical model for fluid mud transport. The model results are verified with field observation of sediment concentration and near-bed flow velocities at Po prodelta. The characteristics of wave-supported gravity-driven mudflows are diagnosed by varying the bed erodibility, floc properties (fractal dimension), and rheological stresses in the numerical simulations. Model results for moderate concentration suggest that using an appropriately specified fractal dimension, the dynamics of wave-supported gravity-driven mudflow can be predicted without explicitly incorporating rheological stress. However, incorporating rheological stress makes the results less sensitive to prescribed fractal dimension. For high-concentration conditions, it is necessary to incorporate rheological stress in order to match observed intensity of downslope gravity-driven current. Model results are further analyzed to evaluate and calibrate simple parameterizations. Analysis suggests that when neglecting rheological stress, the drag coefficient decreases with increasing wave intensity and seems to follow a power law. However, when rheological stress is incorporated, the resulting drag coefficient is more or less constant (around 0.0013) for different wave intensities. Model results further suggest the bulk Richardson number has a magnitude smaller than 0.25 and is essentially determined by the amount of available soft mud (i.e., the erodibility), suggesting a supply limited condition for unconsolidated mud.

A numerical investigation of fine particle laden flow in an oscillatory channel: the role of particle-induced density stratification

Studying particle-laden oscillatory channel flow constitutes an important step towards understanding practical application. This study aims to take a step forward in our understanding of the role of turbulence on fine-particle transport in an oscillatory channel and the back effect of fine particles on turbulence modulation using an Eulerian–Eulerian framework. In particular, simulations presented in this study are selected to investigate wave-induced fine sediment transport processes in a typical coastal setting. Our modelling framework is based on a simplified two-way coupled formulation that is accurate for particles of small Stokes number ( St ). As a first step, the instantaneous particle velocity is calculated as the superposition of the local fluid velocity and the particle settling velocity while the higher-order particle inertia effect neglected. Correspondingly, only the modulation of carrier flow is due to particle-induced density stratification quantified by the bulk Richardson number, Ri . In this paper, we fixed the Reynolds number to be Re Δ = 1000 and varied the bulk Richardson number over a range ( Ri = 0, 1 × 10 −4 , 3 × 10 −4 and 6 × 10 −4 ). The simulation results reveal critical processes due to different degrees of the particle–turbulence interaction. Essentially, four different regimes of particle transport for the given Re Δ are observed: (i) the regime where virtually no turbulence modulation in the case of very dilute condition, i.e. Ri ~ 0; (ii) slightly modified regime where slight turbulence attenuation is observed near the top of the oscillatory boundary layer. However, in this regime a significant change can be observed in the concentration profile with the formation of a lutocline; (iii) regime where flow laminarization occurs during the peak flow, followed by shear instability during the flow reversal. A significant reduction in the oscillatory boundary layer thickness is also observed; (iv) complete laminarization due to strong particle-induced stable density stratification.

A numerical investigation of lutocline dynamics and saturation of fine sediment in the oscillatory boundary layer

[1]xa0Understanding the state of the muddy seabed is critical to sediment transport, hydrodynamic dissipation and seabed properties. However, this endeavor is challenging because the availability and settling velocity of sediment in muddy environments are highly variable. For a given Reynolds number, typical of fine sediment settling in a moderately energetic muddy shelf, recent 3D numerical simulations have revealed four distinct regimes of wave-induced fine sediment transport. These regimes depend on the availability (or concentration) of sediment and range from well-mixed condition to the formation of lutocline, and eventually a complete flow laminarization. By keeping the sediment availability unchanged, this study further demonstrates the existence of these flow regimes for a range of sediment settling velocities. Simulation results suggest that when settling velocity is larger, the location of the lutocline becomes lower (closer to the bed) and the flow eventually laminarizes when there is further increase in the settling velocity. Hence, the dynamics of lutocline is clearly related to the transition between these flow regimes. The vertical flow structure in the presence of lutocline is revealed through the budgets of sediment flux and turbulent kinetic energy. The suppressed mixing in the lutocline layer is further illustrated from a new perspective, i.e., the budgets of turbulent suspension and concentration fluctuation variance. The concept of saturation, commonly used for tidal boundary layer, is extended here for wave boundary layer.

Gravity Currents from Instantaneous Sources Down a Slope

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 o...

Direct numerical simulations of transition and turbulence in smooth-walled Stokes boundary layer

Stokes boundary layer (SBL) is a time-periodic canonical flow that has several environmental, industrial, and physiological applications. Understanding the hydrodynamic instability and turbulence in SBL, therefore, will shed more light on the nature of such flows. Unlike its steady counterpart, the flow in a SBL varies both in space and time, which makes hydrodynamic instability and transition from laminar to turbulent state highly complicated. In this study, we utilized direct numerical simulations (DNS) to understand the characteristics of hydrodynamic instability, the transition from laminar to turbulent state, and the characteristics of intermittent turbulence in a smooth SBL for ReΔ in the range of 500–1000. Simulation results show that nonlinear growth plays a critical role on the instability at ReΔ=500 and 600. However, the nonlinear growth does not warrant sustainable transition to turbulence and the outcome is highly dependent on the amplitude and spatial distribution of the initial velocity dist...

On the transport modes of fine sediment in the wave boundary layer due to resuspension/deposition: A turbulence‐resolving numerical investigation

Previous field observations revealed that the wave boundary layer is one of the main conduits delivering fine sediments from the nearshore to continental shelves. Recently, a series of turbulence-resolving simulations further demonstrated the existence of a range of flow regimes due to different degrees of sediment-induced density stratification controlled by the sediment availability. In this study, we investigate the scenario in which sediment availability is governed by the resuspension/deposition from/to the bed. Specifically, we focus on how the critical shear stress of erosion and the settling velocity can determine the modes of transport. Simulations reveal that at a given wave intensity, which is associated with more energetic muddy shelves and a settling velocity of about 0.5 mm/s, three transport modes, ranging from the well-mixed transport (mode I), two-layer like transport with the formation of lutocline (mode II), and laminarized transport (mode III) are obtained as the critical shear stress of erosion reduces. Moreover, reductions in the settling velocity also yield similar transitions of transport modes. We also demonstrate that the onset of laminarization can be well explained by the reduction of wave-averaged bottom stress to about 0.39 Pa due to attenuated turbulence by sediments. A 2-D parametric map is proposed to characterize the transition from one transport mode to another as a function of the critical shear stress and the settling velocity at a fixed wave intensity.

Numerical Investigation of Turbulence Modulation by Sediment-Induced Stratification and Enhanced Viscosity in Oscillatory Flows

AbstractRecent 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 v...

Turbulence‐resolving, two‐phase flow simulations of wave‐supported gravity flows: A conceptual study

Discoveries over the last three decades have shown that wave-supported gravity flows (WSGFs) are among the participating physical processes that carry substantial amount of fine sediments across low-gradient shelves. Therefore, understanding the full range of mechanisms responsible for such gravity flows is likely to shed light on the dynamics of subaqueous delta and clinoform development. As wave-induced boundary layer turbulence is the major agent to suspend sediments in WSGFs, the scale of WSGFs in the water column is also bounded by the wave-induced boundary layer thickness which is on the order of decimeters. Therefore, in order to explore the details of participating physical mechanisms, especially that due to turbulence-sediment interaction, highly resolved and accurate numerical models or measurements in the laboratory and the field are required. In this study, the dynamics of WSGFs is investigated by using turbulence-resolving, two-phase flow simulations that utilize Direct Numerical Simulations (DNS). The effect of variable sediment loading, slope, and wave orbital velocity is investigated via 21 simulations.

NUMERICAL SIMULATION OF FINE SEDIMENT TRANSPORT IN WAVE BOUNDARY LAYER

Wave-induced sediment transport is recognized as one of the main mechanisms that play part in the morphodynamics and sediment source to sink. In this study, we identify and quantify the role of turbulence-sediment interactions in determining the diverse bed state using turbulence-resolving numerical simulations of fine sediment transport in a oscillatory bottom boundary layer. The formation and the dynamics of lutocline waves observe at muddy shelves, sediment burst events observed during flow reversal in sandy nearshore and at muddy shelves, and relaminarization of wave boundary layer due to concentrated fluid mud are the few examples that are well captured in a series of simulations in this study. The laminarization process is often assumed in two-layer (or multi-layer) models for wave-mud interactions. However, the criteria for laminarization in a wave boundary layer as a function of Reynolds number, settling velocity and sediment concentration have not yet been established. In addition, the accuracy of the waveresolving Reynolds-averaged (RANS) wave boundary layer sediment transport models in predicting the state of the muddy seabed under the influence of variable parameters has not yet been evaluated. It is demonstrated here that RANS model can predict the formation of lutocline from low to moderately concentration condition. However, when sediment concentration is further increased, sediment burst events and laminarization of the wave boundary layer cannot be reproduced without further modification to the conventional turbulence closure.