No Thumbnail Available
Now showing 1 - 6 of 6
PreprintEddy interaction model for turbulent suspension in Reynolds-averaged Euler-Lagrange simulations of steady sheet flow( 2017-10-31) Cheng, Zhen ; Chauchat, Julien ; Hsu, Tian-Jian ; Calantoni, JosephA 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 < 3, the simulation under-predicted the amount of suspended sediment in the dilute region and the Schmidt number is over-predicted (Sc > 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.
ArticleA numerical study of onshore ripple migration using a Eulerian two-phase model(American Geophysical Union, 2020-12-29) Salimi‐Tarazouj, Ali ; Hsu, Tian-Jian ; Traykovski, Peter A. ; Cheng, Zhen ; Chauchat, JulienA 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.
ArticleSedFoam-2.0 : a 3-D two-phase flow numerical model for sediment transport(Copernicus Publications on behalf of the European Geosciences Union, 2017-11-30) Chauchat, Julien ; Cheng, Zhen ; Nagel, Tim ; Bonamy, Cyrille ; Hsu, Tian-JianIn this paper, a three-dimensional two-phase flow solver, SedFoam-2.0, is presented for sediment transport applications. The solver is extended from twoPhaseEulerFoam available in the 2.1.0 release of the open-source CFD (computational fluid dynamics) toolbox OpenFOAM. In this approach the sediment phase is modeled as a continuum, and constitutive laws have to be prescribed for the sediment stresses. In the proposed solver, two different intergranular stress models are implemented: the kinetic theory of granular flows and the dense granular flow rheology μ(I). For the fluid stress, laminar or turbulent flow regimes can be simulated and three different turbulence models are available for sediment transport: a simple mixing length model (one-dimensional configuration only), a k − ε, and a k − ω model. The numerical implementation is demonstrated on four test cases: sedimentation of suspended particles, laminar bed load, sheet flow, and scour at an apron. These test cases illustrate the capabilities of SedFoam-2.0 to deal with complex turbulent sediment transport problems with different combinations of intergranular stress and turbulence models.
PreprintAn Eulerian two-phase model for steady sheet flow using large-eddy simulation methodology( 2017-11-08) Cheng, Zhen ; Hsu, Tian-Jian ; Chauchat, JulienA 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.
ArticleA numerical study of sheet flow under monochromatic nonbreaking waves using a free surface resolving Eulerian two‐phase flow model(John Wiley & Sons, 2018-07-05) Kim, Yeulwoo ; Cheng, Zhen ; Hsu, Tian-Jian ; Chauchat, JulienWe present a new methodology that is able to concurrently resolve free surface wavefield, bottom boundary layer, and sediment transport processes throughout the entire water column. The new model, called SedWaveFoam, is developed by integrating an Eulerian two‐phase model for sediment transport, SedFoam, and a surface wave solver, InterFoam/waves2Foam, in the OpenFOAM framework. SedWaveFoam is validated with a large wave flume data for sheet flow driven by monochromatic nonbreaking waves. To isolate the effect of free surface, SedWaveFoam results are contrasted with one‐dimensional‐vertical SedFoam results, where the latter represents the oscillating water tunnel condition. Results demonstrate that wave‐averaged total sediment fluxes in both models are onshore‐directed; however, this onshore transport is significantly enhanced under surface waves. Onshore‐directed near‐bed sediment flux is driven by a small mean current mainly associated with velocity skewness. More importantly, progressive wave streaming drives onshore transport mostly in suspended load region due to an intrawave sediment flux. Further analysis suggests that the enhanced onshore transport in suspended load is due to a “wave‐stirring” mechanism, which signifies a nonlinear interaction between waves, streaming currents, and sediment suspension. We present some preliminary efforts to parameterize the wave‐stirring mechanism in intrawave sediment transport formulations.
ArticleA turbulence-resolving numerical investigation of wave-supported gravity flows(American Geophysical Union, 2020-02-04) Yue, Liangyi ; Cheng, Zhen ; Hsu, Tian-JianWave‐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.