An Eulerian two-phase model for steady sheet flow using large-eddy simulation methodology

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Advances in Water Resources 111 (2018): 205-223, doi:10.1016/j.advwatres.2017.11.016.A three-dimensional Eulerian two-phase flow model for sediment transport in sheet flow conditions is presented. To resolve turbulence and turbulence-sediment interactions, the large-eddy simulation approach is adopted. Specifically, a dynamic Smagorinsky closure is used for the subgrid fluid and sediment stresses, while the subgrid contribution to the drag force is included using a drift velocity model with a similar dynamic procedure. The contribution of sediment stresses due to intergranular interactions is modeled by the kinetic theory of granular flow at low to intermediate sediment concentration, while at high sediment concentration of enduring contact, a phenomenological closure for particle pressure and frictional viscosity is used. The model is validated with a comprehensive high-resolution dataset of unidirectional steady sheet flow (Revil-Baudard et al., 2015, Journal of Fluid Mechanics, 767, 1–30). At a particle Stokes number of about 10, simulation results indicate a reduced von Kármán coefficient of κ ≈ 0.215 obtained from the fluid velocity profile. A fluid turbulence kinetic energy budget analysis further indicates that the drag-induced turbulence dissipation rate is significant in the sheet flow layer, while in the dilute transport layer, the pressure work plays a similar role as the buoyancy dissipation, which is typically used in the single-phase stratified flow formulation. The present model also reproduces the sheet layer thickness and mobile bed roughness similar to measured data. However, the resulting mobile bed roughness is more than two times larger than that predicted by the empirical formulae. Further analysis suggests that through intermittent turbulent motions near the bed, the resolved sediment Reynolds stress plays a major role in the enhancement of mobile bed roughness. Our analysis on near-bed intermittency also suggests that the turbulent ejection motions are highly correlated with the upward sediment suspension flux, while the turbulent sweep events are mostly associated with the downward sediment deposition flux.This study was supported by National Science Foundation (OCE-1635151; OCE- 958 1537231) and Office of Naval Research (N00014-16-1-2853). J. Chauchat was supported by the Region Rhones-Alpes (COOPERA project and Explora Pro grant) and the French national programme EC2CO-LEFE MODSED. The authors would also like to acknowledge the support from the program on "Fluid-Mediated Particle Transport in Geophysical Flows" at the Kavli Institute for Theoretical Physics, Santa Barbara, USA

A turbulence-resolving numerical investigation of wave-supported gravity flows

Author Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 125(2), (2020): e2019JC015220, doi:10.1029/2019JC015220.Wave‐supported gravity flows (WSGFs) have been identified as a key process driving the offshore delivery of fine sediment across continental shelves. However, our understanding on the various factors controlling the maximum sediment load and the resulting gravity current speed remains incomplete. We adopt a new turbulence‐resolving numerical model for fine sediment transport to investigate the formation, evolution, and termination of WSGFs. We consider the simplest scenario in which fine sediments are supported by the wave‐induced fluid turbulence at a low critical shear stress of erosion over a flat sloping bed. Under the energetic wave condition reported on the Northern California Coast with a shelf slope of 0.005, simulation results show that WSGFs are transitionally turbulent and that the sediment concentration cannot exceed 30 kg/m urn:x-wiley:jgrc:media:jgrc23843:jgrc23843-math-0001 (g/L) due to the attenuation of turbulence by the sediment‐induced stable density stratification. Wave direction is found to be important in the resulting gravity current intensity. When waves are in cross‐shelf direction, the downslope current has a maximum velocity of 1.2 cm/s, which increases to 2.1 cm/s when waves propagate in the along‐shelf direction. Further analysis on the wave‐averaged momentum balance confirms that when waves are parallel to the slope (cross‐shelf) direction, the more intense wave‐current interaction results in larger wave‐averaged Reynolds shear stress and thus in a smaller current speed. Findings from this study suggest that the more intense cross‐shelf gravity current observed in the field may be caused by additional processes, which may enhance the sediment‐carrying capacity of flow, such as the ambient current or bedforms.This study is supported by NSF (OCE‐1537231 and OCE‐1924532) and Office of Naval Research (N00014‐17‐1‐2796). Numerical simulations presented in this study were carried out using the Mills and Canviness clusters at University of Delaware, and the SuperMIC cluster at Louisiana State University via XSEDE (TG‐OCE100015). Z. Cheng would like to express thanks for the support of a postdoctoral scholarship from Woods Hole Oceanographic Institution. The source code and the case setup to reproduce the same results are publicly available via the repository maintained by GitHub: https://github.com/yueliangyi/TURBID (source code) and https://github.com/yueliangyi/TURBID/tree/master/spike/wave_supported_gravity_flow (case setup), respectively.2020-08-0

Direct numerical simulations of instability and boundary layer turbulence under a solitary wave

Author Posting. © Cambridge University Press, 2013. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 731 (2013): 545-578, doi:10.1017/jfm.2013.361 .A significant amount of research effort has been made to understand the boundary layer instability and the generation and evolution of turbulence subject to periodic/oscillatory flows. However, little is known about bottom boundary layers driven by highly transient and intermittent free-stream flow forcing, such as solitary wave motion. To better understand the nature of the instability mechanisms and turbulent flow characteristics subject to solitary wave motion, a large number of direct numerical simulations are conducted. Different amplitudes of random initial fluctuating velocity field are imposed. Two different instability mechanisms are observed within the range of Reynolds number studied. The first is a short-lived, nonlinear, long-wave instability which is observed during the acceleration phase, and the second is a broadband instability that occurs during the deceleration phase. Transition from a laminar to turbulent state is observed to follow two different breakdown pathways: the first follows the sequence of $K$-type secondary instability of a near-wall boundary layer at comparatively lower Reynolds number and the second one follows a breakdown path similar to that of free shear layers. Overall characteristics of the flow are categorized into four regimes as: (i) laminar; (ii) disturbed laminar; (iii) transitional; and (iv) turbulent. Our categorization into four regimes is consistent with earlier works. However, this study is able to provide more specific definitions through the instability characteristics and the turbulence breakdown process.This study is supported by National Science Foundation (CMMI-1135026; OCE- 1130217; OCE-1131016).2014-08-2

Wave-induced sediment transport and onshore sandbar migration

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Coastal Engineering 53 (2006): 817-824, doi:10.1016/j.coastaleng.2006.04.003.The 25-m onshore migration of a nearshore sandbar observed over a 5-day period near Duck, NC is simulated with a simplified, computationally efficient, wave-resolving singlephase model. The modeled sediment transport is assumed to occur close to the seabed and to be in phase with the bottom stress. Neglected intergranular stresses and fluid-granular interactions, likely important in concentrated flow, are compensated for with an elevated (relative to that appropriate for a clear fluid) model roughness height that gives the best fit to the observed bar migration. Model results suggest that when mean-current-induced transport is small, wave-induced transport leads to the observed onshore bar migration. Based on the results from the simplified phase-resolving model, a wave-averaged, energetics-type model (e.g., only moments of the near-bottom velocity field are required) with different friction factors for oscillatory and mean flows is developed that also predicts the observed bar migration. Although the assumptions underlying the models differ, the similarity of model results precludes determination of the dominant mechanisms of sediment transport during onshore bar migration.Supported by the Office of Naval Research, the National Science Foundation, the Army Research Office, and the Woods Hole Oceanographic Institution Coastal Ocean Institute

Eddy interaction model for turbulent suspension in Reynolds-averaged Euler-Lagrange simulations of steady sheet flow

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Advances in Water Resources 111 (2018): 435-451, doi:10.1016/j.advwatres.2017.11.019.A Reynolds-averaged Euler–Lagrange sediment transport model (CFDEM-EIM) was developed for steady sheet flow, where the inter-granular interactions were resolved and the flow turbulence was modeled with a low Reynolds number corrected turbulence closure modified for two-phase flows. To model the effect of turbulence on the sediment suspension, the interaction between the turbulent eddies and particles was simulated with an eddy interaction model (EIM). The EIM was first calibrated with measurements from dilute suspension experiments. We demonstrated that the eddy-interaction model was able to reproduce the well-known Rouse profile for suspended sediment concentration. The model results were found to be sensitive to the choice of the coefficient, C0, associated with the turbulence-sediment interaction time. A value was suggested to match the measured concentration in the dilute suspension. The calibrated CFDEM-EIM was used to model a steady sheet flow experiment of lightweight coarse particles and yielded reasonable agreements with measured velocity, concentration and turbulence kinetic energy profiles. Further numerical experiments for sheet flow suggested that when C0 was decreased to C0  1.0). Additional simulations for a range of Shields parameters between 0.3 and 1.2 confirmed that CFDEM-EIM was capable of predicting sediment transport rates similar to empirical formulations. Based on the analysis of sediment transport rate and transport layer thickness, the EIM and the resulting suspended load were shown to be important when the fall parameter is less than 1.25.Z. Cheng and T.-J. Hsu were supported by the U.S. Office of Naval Research (N00014- 16-1-2853) and National Science Foundation (OCE- 1537231). J. Chauchat was supported by the Région Rhones-Alpes (COOPERA project and Explora Pro grant) and the French national programme EC2CO-LEFE MODSED. J. Calantoni was supported under base funding to the U.S. Naval Research Laboratory from the U.S. Office of Naval Research. The authors would also like to acknowledge the support from the program on "Fluid- Mediated Particle Transport in Geophysical Flows" at the Kavli Institute for Theoretical Physics, Santa Barbara, USA

Hydrodynamic and sediment transport modeling of New River Inlet (NC) under the interaction of tides and waves

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 120 (2015): 4028–4047, doi:10.1002/2014JC010425.The interactions between waves, tidal currents, and bathymetry near New River Inlet, NC, USA are investigated to understand the effects on the resulting hydrodynamics and sediment transport. A quasi-3-D nearshore community model, NearCoM-TVD, is used in this integrated observational and modeling study. The model is validated with observations of waves and currents at 30 locations, including in a recently dredged navigation channel and a shallower channel, and on the ebb tidal delta, for a range of flow and offshore wave conditions during May 2012. In the channels, model skills for flow velocity and wave height are high. Near the ebb tidal delta, the model reproduces the observed rapid onshore (offshore) decay of wave heights (current velocities). Model results reveal that this sharp transition coincides with the location of the breaker zone over the ebb tidal delta, which is modulated by semidiurnal tides and by wave intensity. The modulation of wave heights is primarily owing to depth changes rather than direct wave-current interaction. The modeled tidally averaged residual flow patterns show that waves play an important role in generating vortices and landward-directed currents near the inlet entrance. Numerical experiments suggest that these flow patterns are associated with the channel-shoal bathymetry near the inlet, similar to the generation of rip currents. Consistent with other inlet studies, model results suggest that tidal currents drive sediment fluxes in the channels, but that sediment fluxes on the ebb tidal delta are driven primarily by waves.Funding was provided by the Office of Naval Research (N00014-13-1–0120 and N00014-14-1-0586) and the Office of the Assistant Secretary of Defense for Research and Engineering.2015-12-0

A numerical study of onshore ripple migration using a Eulerian two-phase model

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 126(2), (2021): e2020JC016773, https://doi.org/10.1029/2020JC016773.A new modeling methodology for ripple dynamics driven by oscillatory flows using a Eulerian two‐phase flow approach is presented in order to bridge the research gap between near‐bed sediment transport via ripple migration and suspended load transport dictated by ripple induced vortices. Reynolds‐averaged Eulerian two‐phase equations for fluid phase and sediment phase are solved in a two‐dimensional vertical domain with a k‐ε closure for flow turbulence and particle stresses closures for short‐lived collision and enduring contact. The model can resolve full profiles of sediment transport without making conventional near‐bed load and suspended load assumptions. The model is validated with an oscillating tunnel experiment of orbital ripple driven by a Stokes second‐order (onshore velocity skewed) oscillatory flow with a good agreement in the flow velocity and sediment concentration. Although the suspended sediment concentration far from the ripple in the dilute region was underpredicted by the present model, the model predicts an onshore ripple migration rate that is in very good agreement with the measured value. Another orbital ripple case driven by symmetric sinusoidal oscillatory flow is also conducted to contrast the effect of velocity skewness. The model is able to capture a net offshore‐directed suspended load transport flux due to the asymmetric primary vortex consistent with laboratory observation. More importantly, the model can resolve the asymmetry of onshore‐directed near‐bed sediment flux associated with more intense boundary layer flow speed‐up during onshore flow cycle and sediment avalanching near the lee ripple flank which force the onshore ripple migration.This study is supported by National Science Foundation (Grant no. OCE‐1635151) and Strategic Environmental Research and Development Program (Grant no. MR20‐1478).2021-06-2