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Yi-Ju Chou
Institute of Applied Mechanics, National Taiwan University, Taipei 10617, Taiwan

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Journal article
Published: 05 July 2021 in Journal of Computational Physics
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This paper presents a single-pressure-field two-fluid model with finite-volume discretization to solve the equations of motion of compressible multiphase flows. To capture the discontinuities caused by shock waves and fluid interfaces, we propose a generalized discontinuity sharpening technique that combines the conventional monotonic upstream scheme for conservation law (MUSCL) and tangent of hyperbola interface capturing (THINC) schemes. In addition, a slope ratio-weighted parameter, ζ, is used to control the proportion of values reconstructed by MUSCL and THINC, and we show that the present method can retain sharp interfaces when the value of the parameter β in the THINC scheme is set ranging from 1.6 to 3.0. Fluxes across various interfaces are evaluated using a hybrid AUSMD-type flux algorithm, where the mass flux and pressure induced on the cell faces are calculated using an approximate Riemann solver. The accuracy and robustness of the proposed method are validated by solving a series of one- and two-dimensional single-phase flows. Furthermore, complex wave patterns arising from two-dimensional shock bubble/water-column interactions are examined, which indicate that compared with the existing schemes applied to two-fluid modeling, the proposed scheme significantly sharpens the interfaces and captures more details of the flow features. Finally, simulations of a three-dimensional example of the liquid jet crossflow are conducted. The proposed scheme shows more details of the fluid interface, including the interfacial instabilities on the windward side of the liquid jet and droplet formation due to the breakup phenomenon in the downstream of the crossflow, than the existing schemes.

ACS Style

Te-Yao Chiu; Yang-Yao Niu; Yi-Ju Chou. Accurate hybrid AUSMD type flux algorithm with generalized discontinuity sharpening reconstruction for two-fluid modeling. Journal of Computational Physics 2021, 443, 110540 .

AMA Style

Te-Yao Chiu, Yang-Yao Niu, Yi-Ju Chou. Accurate hybrid AUSMD type flux algorithm with generalized discontinuity sharpening reconstruction for two-fluid modeling. Journal of Computational Physics. 2021; 443 ():110540.

Chicago/Turabian Style

Te-Yao Chiu; Yang-Yao Niu; Yi-Ju Chou. 2021. "Accurate hybrid AUSMD type flux algorithm with generalized discontinuity sharpening reconstruction for two-fluid modeling." Journal of Computational Physics 443, no. : 110540.

Journal article
Published: 16 June 2021 in Physics of Fluids
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Numerical simulations are conducted to study instabilities and the associated convective motion of particle-laden layers settling in continuously stratified environments. We show that when the background density stratification is insignificant relative to the bulk excessive density of the particle-laden layer, the unstable motions of the particle-laden interface are mainly driven by Rayleigh–Taylor instability but become double-diffusive convection when the background stratification is relatively significant. Our results agree with theoretical prediction based on linear stability analysis. However, in the Rayleigh–Taylor instability regime, the motion of particle-laden plumes can be further suppressed by the background density stratification while the plumes reach the height of neutral buoyancy. This leads to the second stage of flow development, in which secondary instability occurs at the plumes' tip in the form of double-diffusive convection. Due to the change in the background density gradient within the plumes' head, the occurrence of secondary instability is accompanied by a shift of the dominant mode, which is particularly significant in cases with a high background Prandtl number (i.e., salinity-induced stratification). The theoretical argument on the mode shift is based on previous linear stability analysis for the two-layer structured background density gradient provided. The ratio between the particles' settling velocity and velocity scaling for the developed local density gradient at the plumes' tip then allows us to distinguish and predict whether the final convective motion is driven mainly by double-diffusive or settling-driven buoyancy-dominant convection.

ACS Style

Che-Jung Cheng; Yi-Ju Chou. Numerical study of instabilities of particle-laden fronts in continuously stratified environments. Physics of Fluids 2021, 33, 064107 .

AMA Style

Che-Jung Cheng, Yi-Ju Chou. Numerical study of instabilities of particle-laden fronts in continuously stratified environments. Physics of Fluids. 2021; 33 (6):064107.

Chicago/Turabian Style

Che-Jung Cheng; Yi-Ju Chou. 2021. "Numerical study of instabilities of particle-laden fronts in continuously stratified environments." Physics of Fluids 33, no. 6: 064107.

Journal article
Published: 27 April 2020 in Journal of Fluid Mechanics
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We conduct Eulerian–Lagrangian simulations to study double-diffusive sedimentation in stratified flows. The results show the pattern of double-diffusive sedimentation and the transition to the pattern of Rayleigh–Taylor instability when the size of particles increases. In cases of double-diffusive sedimentation, our simulation results show little variation in the temperature-to-particle flux ratio among cases with various particle sizes and initial concentrations, which is consistent with previous theoretical derivations and experimental observations. The energy budget is analysed to show that the settling enhancement is a result of the thermal effect combined with shear dissipation and that the thermal contribution decreases as the size increases. Based on the balance of the energy budget, velocity scaling was derived for the quasi-steady state in the thermally controlled region, which can be used to characterize the plumes’ final velocity of double-diffusive sedimentation. Moreover, adopting some values from the simulation results yields a velocity criterion with which to distinguish different sedimentation patterns. Finally, we investigate changes in the particle-laden plumes below the region of the apparent temperature gradient at which secondary instabilities occur in the form of significant horizontal flow motion. We show that the resulting initial shift of the dominant modes can be approximated with the existing theoretical analysis of collective instabilities for salt fingers. A simple scaling argument for the change in the total cross-sectional area of particle-laden plumes is presented, which is then used to scale the resulting enhanced sedimentation.

ACS Style

Chen-Yen Hung; Yang-Yao Niu; Yi-Ju Chou. Numerical study of double-diffusive sedimentation in thermally stratified fluid. Journal of Fluid Mechanics 2020, 893, 1 .

AMA Style

Chen-Yen Hung, Yang-Yao Niu, Yi-Ju Chou. Numerical study of double-diffusive sedimentation in thermally stratified fluid. Journal of Fluid Mechanics. 2020; 893 ():1.

Chicago/Turabian Style

Chen-Yen Hung; Yang-Yao Niu; Yi-Ju Chou. 2020. "Numerical study of double-diffusive sedimentation in thermally stratified fluid." Journal of Fluid Mechanics 893, no. : 1.

Journal article
Published: 17 December 2019 in Journal of Fluid Mechanics
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We conduct numerical simulations to investigate the formation and evolution of drops and vortex rings of particle-laden fingers in double-diffusive convection in stably stratified environments. We show that the temporal evolution can be divided into double diffusion, acceleration and deceleration phases. The acceleration phase is a result of the vanishing temperature perturbation in the drop during the descent in the layer of uniform temperature. The drop decelerates because it transforms into a vortex ring. A theoretical drag model is presented to predict the speed of the spherical drop with the low drop Reynolds number. By formulating the boundary condition based on the vorticity, our drag model gives a more general form of the drag coefficient for small spherical drops and shows good agreement in predicting the drag coefficient. Drops with five particle sizes are compared, and it is found that although the greater vertical settling enhances vertical transport, the final state differs little among the various sizes. Comparison of our drag model with the simulation results under various bulk conditions and previous experimental results shows good model predictability. Finally, a comparison with the salt-finger case shows that the diffusive nature of the dissolved scalar field, along with the wake effect, can result in an apparent loss of mass from the drop and a permanent presence of the connection between the drop and its parent finger. This makes the observed detachment of the particle-laden drop much less likely in the salt-finger case.

ACS Style

Yi-Ju Chou; Chen-Yen Hung; Chien-Fu Chen. Formation of drops and rings in double-diffusive sedimentation. Journal of Fluid Mechanics 2019, 884, 1 .

AMA Style

Yi-Ju Chou, Chen-Yen Hung, Chien-Fu Chen. Formation of drops and rings in double-diffusive sedimentation. Journal of Fluid Mechanics. 2019; 884 ():1.

Chicago/Turabian Style

Yi-Ju Chou; Chen-Yen Hung; Chien-Fu Chen. 2019. "Formation of drops and rings in double-diffusive sedimentation." Journal of Fluid Mechanics 884, no. : 1.

Journal article
Published: 01 April 2019 in Powder Technology
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ACS Style

Yen-Cheng Chang; Te-Yao Chiu; Chen-Yen Hung; Yi-Ju Chou. Three-dimensional Eulerian-Lagrangian simulation of particle settling in inclined water columns. Powder Technology 2019, 348, 80 -92.

AMA Style

Yen-Cheng Chang, Te-Yao Chiu, Chen-Yen Hung, Yi-Ju Chou. Three-dimensional Eulerian-Lagrangian simulation of particle settling in inclined water columns. Powder Technology. 2019; 348 ():80-92.

Chicago/Turabian Style

Yen-Cheng Chang; Te-Yao Chiu; Chen-Yen Hung; Yi-Ju Chou. 2019. "Three-dimensional Eulerian-Lagrangian simulation of particle settling in inclined water columns." Powder Technology 348, no. : 80-92.

Journal article
Published: 24 January 2019 in Water
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In field-scale modeling, when the resuspension of sediment is modeled using a hydrodynamic model, a standard and common approach is to add a resuspension flux as the bottom boundary condition in the transport model. In this study, we show that the way of simply imposing an empirical bottom erosion formula as the flux is actually unrealistic. Its inability to stabilize the sediment concentration can cause excessive suspension fluxes in some extreme cases. Moreover, we present a modified erosion/deposition formula to model the resuspension of sediment. The formulation is based on volume conservation in the presence of erosion/deposition near the bottom. By taking into account the prescribed dry density of the bed material, the proposed formulation is able to produce realistic near-bed concentrations while ensuring model stability. The formulation is then tested in a one-dimensional vertical model and field modeling cases using a three-dimensional coastal circulation model. We show that the modified formulation is particularly important in modeling mud resuspension subject to the large bottom stress, which can be a result of waves or a strong river discharge.

ACS Style

Yi-Ju Chou; Yun-Chuan Shao; Yi-Hao Sheng; Che-Jung Cheng. Stabilized Formulation for Modeling the Erosion/Deposition Flux of Sediment in Circulation/CFD Models. Water 2019, 11, 197 .

AMA Style

Yi-Ju Chou, Yun-Chuan Shao, Yi-Hao Sheng, Che-Jung Cheng. Stabilized Formulation for Modeling the Erosion/Deposition Flux of Sediment in Circulation/CFD Models. Water. 2019; 11 (2):197.

Chicago/Turabian Style

Yi-Ju Chou; Yun-Chuan Shao; Yi-Hao Sheng; Che-Jung Cheng. 2019. "Stabilized Formulation for Modeling the Erosion/Deposition Flux of Sediment in Circulation/CFD Models." Water 11, no. 2: 197.

Journal article
Published: 29 June 2018 in Journal of Geophysical Research: Oceans
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A suspended sediment transport model is implemented in the unstructured‐grid SUNTANS model and applied to study fine‐grained sediment transport in South San Francisco Bay. The model enables calculation of suspension of bottom sediment based on combined forcing of tidal currents and wind waves. We show that accurate results can be obtained by employing two size classes which are representative of micro‐ and macroflocs in the Bay. A key finding of the paper is that the critical calibration parameter is the ratio of the erosion of the micro to macroflocs from the bed. Different values of this erosion ratio are needed on the shallow shoals and deeper channels because of the different nature of the sediment dynamics in these regions. Application of a spatially‐variable erosion ratio and critical shear stress for erosion is shown to accurately reproduce observed suspended sediment concentration at four field sites located along a cross‐channel transect. The results reveal a stark contrast between the behavior of the suspended sediment concentration on the shoals and in the deep channel. Waves are shown to resuspend sediments on the shoals, although tidal and wind‐generated currents are needed to mix the thin wave‐driven suspensions into the water column. The contribution to the suspended sediment concentration in the channel by transport from the shoals is similar in magnitude to that due to local resuspension. However, the local contribution is in phase with strong bottom currents which resuspend the sediments, while the contribution from the shoals peaks during low‐water slack tide.

ACS Style

Yi‐Ju Chou; Kurt S. Nelson; Rusty C. Holleman; Oliver Fringer; Mark T. Stacey; Jessica R. Lacy; Stephen G. Monismith; Jeffrey R. Koseff. Three‐Dimensional Modeling of Fine Sediment Transport by Waves and Currents in a Shallow Estuary. Journal of Geophysical Research: Oceans 2018, 123, 4177 -4199.

AMA Style

Yi‐Ju Chou, Kurt S. Nelson, Rusty C. Holleman, Oliver Fringer, Mark T. Stacey, Jessica R. Lacy, Stephen G. Monismith, Jeffrey R. Koseff. Three‐Dimensional Modeling of Fine Sediment Transport by Waves and Currents in a Shallow Estuary. Journal of Geophysical Research: Oceans. 2018; 123 (6):4177-4199.

Chicago/Turabian Style

Yi‐Ju Chou; Kurt S. Nelson; Rusty C. Holleman; Oliver Fringer; Mark T. Stacey; Jessica R. Lacy; Stephen G. Monismith; Jeffrey R. Koseff. 2018. "Three‐Dimensional Modeling of Fine Sediment Transport by Waves and Currents in a Shallow Estuary." Journal of Geophysical Research: Oceans 123, no. 6: 4177-4199.

Journal article
Published: 01 April 2018 in Ocean Modelling
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A three-dimensional nonhydrostatic coastal model SUNTANS is used to study hyperpycnal plumes on sloping continental shelves with idealized domain setup. The study aims to examine the nonhydrostatic effect of the plunging hyperpycnal plume and the associated flow structures on different shelf slopes. The unstructured triangular grid in SUNTANS allows for local refinement of the grid size for regions in which the flow varies abruptly, while retaining low-cost computation using the coarse grid resolution for regions in which the flow is more uniform. These nonhydrostatic simulations reveal detailed three-dimensional flow structures in both transient and steady states. Via comparison with the hydrostatic simulation, we show that the nonhydrostatic effect is particularly important before plunging, when the plume is subject to significant changes in both the along-shore and vertical directions. After plunging, where the plume becomes an undercurrent that is more spatially uniform, little difference is found between the hydrostatic and nonhydrostatic simulations in the present gentle- and mild-slope cases. A grid-dependence study shows that the nonhydrostatic effect can be seen only when the grid resolution is sufficiently fine that the calculation is not overly diffusive. A depth-integrated momentum budget analysis is then conducted to show that the flow convergence due to plunging is an important factor in the three-dimensional flow structures. Moreover, it shows that the nonhydrostatic effect becomes more important as the slope increases, and in the steep-slope case, neglect of transport of the vertical momentum during plunging in the hydrostatic case further leads to an erroneous prediction for the undercurrent.

ACS Style

Chien-Yung Tseng; Yi-Ju Chou. Nonhydrostatic simulation of hyperpycnal river plumes on sloping continental shelves: Flow structures and nonhydrostatic effect. Ocean Modelling 2018, 124, 33 -47.

AMA Style

Chien-Yung Tseng, Yi-Ju Chou. Nonhydrostatic simulation of hyperpycnal river plumes on sloping continental shelves: Flow structures and nonhydrostatic effect. Ocean Modelling. 2018; 124 ():33-47.

Chicago/Turabian Style

Chien-Yung Tseng; Yi-Ju Chou. 2018. "Nonhydrostatic simulation of hyperpycnal river plumes on sloping continental shelves: Flow structures and nonhydrostatic effect." Ocean Modelling 124, no. : 33-47.

Journal article
Published: 06 July 2017 in Journal of Fluid Mechanics
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We conduct numerical simulations using the Eulerian–Lagrangian approach to investigate the formation of the leaking, finger, and stable-settling modes in convective sedimentation when a sediment-laden fluid layer descends through a sharply stratified ambient flow. We show that the temporal evolution of the sedimentation process for the leaking mode can be divided into three stages, including (in temporal order) Rayleigh–Taylor instability, convection, and leaking stages. The presence of sheet-like descending plumes of suspended particles is an important characteristic of the leaking mode, which marks the existence of the leaking stage. For larger particles, the motion is more dominated by gravitational settling and less affected by buoyancy-induced flow motion. The resulting lack of the leaking stage for the larger-particle case leads to persistent finger-like plumes of suspended particles, known as the finger mode. The stable-settling mode occurs when the particles are large and the concentration is dilute such that flow motion due to Rayleigh–Taylor instability has no effect on the particle motion, and the convective motion of suspended particles is insignificant. For the third stage of the leaking mode, which is also the final stationary state, we derive the criterion for the occurrence of the leaking pattern from a scaling argument of the viscous boundary layer. The criterion is further confirmed by the present simulation results and previous laboratory experiments. Through analysis of the energy budget and the vertical flux, we show that although the settling of individual particles is accelerated, the presence of the sheet-like descending plumes in the leaking mode does not contribute to an efficient settling enhancement compared with the finger mode and the Rayleigh–Taylor instability, i.e., the cases with no background stratification. This implies a negative effect on the settling enhancement for small suspended particles when a stable background density stratification exists. In addition, simulations using the equilibrium Eulerian description for the suspended particles are also conducted to examine the difference between the present Lagrangian particle approach and the conventional Eulerian model.

ACS Style

Yun-Chuan Shao; Chen-Yen Hung; Yi-Ju Chou. Numerical study of convective sedimentation through a sharp density interface. Journal of Fluid Mechanics 2017, 824, 513 -549.

AMA Style

Yun-Chuan Shao, Chen-Yen Hung, Yi-Ju Chou. Numerical study of convective sedimentation through a sharp density interface. Journal of Fluid Mechanics. 2017; 824 ():513-549.

Chicago/Turabian Style

Yun-Chuan Shao; Chen-Yen Hung; Yi-Ju Chou. 2017. "Numerical study of convective sedimentation through a sharp density interface." Journal of Fluid Mechanics 824, no. : 513-549.

Journal article
Published: 01 April 2016 in Physics of Fluids
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In this study, we investigate Rayleigh-Taylor instability in which the density stratification is caused by the suspension of particles in liquid flows using the conventional single-phase model and Euler-Lagrange (EL) two-phase model. The single-phase model is valid only when the particles are small and number densities are large, such that the continuum approximation applies. The present single-phase results show that the constant settling of the particle concentration restricts the lateral development of the vortex ring, which results in a decrease of the rising speed of the Rayleigh-Taylor bubbles. The EL model enables the investigation of particle-flow interaction and the influence of particle entrainment, resulting from local non-uniformity in the particle distribution. We compare bubble dynamics in the single-phase and EL cases, and our results show that the deviation between the two cases becomes more pronounced when the particle size increases. The main mechanism responsible for the deviation is particle entrainment, which can only be resolved in the EL model. We provide a theoretical argument for the small-scale local entrainment resulting from the local velocity shear and non-uniformity of the particle concentration. The theoretical argument is supported by numerical evidence. Energy budget analysis is also performed and shows that potential energy is released due to the interphase drag and buoyant effect. The buoyant effect, which results in the transformation of potential energy into kinetic energy and shear dissipation, plays a key role in settling enhancement. We also find that particle entrainment increases the shear dissipation, which in turn enhances the release of potential energy.

ACS Style

Yi-Ju Chou; Yun-Chuan Shao. Numerical study of particle-induced Rayleigh-Taylor instability: Effects of particle settling and entrainment. Physics of Fluids 2016, 28, 043302 .

AMA Style

Yi-Ju Chou, Yun-Chuan Shao. Numerical study of particle-induced Rayleigh-Taylor instability: Effects of particle settling and entrainment. Physics of Fluids. 2016; 28 (4):043302.

Chicago/Turabian Style

Yi-Ju Chou; Yun-Chuan Shao. 2016. "Numerical study of particle-induced Rayleigh-Taylor instability: Effects of particle settling and entrainment." Physics of Fluids 28, no. 4: 043302.

Journal article
Published: 01 October 2015 in Journal of Computational Physics
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This article presents a two-way coupled Euler-Lagrange model to simulate the suspension of fine particles in liquid flows. The goal is to develop a three-dimensional numerical model that is capable of replicating the detailed features of particle-laden turbulent flow. A two-phase fractional-step projection method is developed to ensure mixture incompressibility by solving a modified Poisson equation for pressure, which in turn affects the particle motion through the pressure gradient. An efficient particle-moving algorithm that exchanges particles between Eulerian meshes is developed that automatically retains the necessary particle information at each time step and does not require any Lagrangian particle tracking. A soft-sphere particle collision model is employed to avoid excessive particle overlap and to achieve the random closed packing limit for depositing particles. Since particles are all inherently localized in Eulerian meshes, efficient particle searches can be achieved when calculating particle-particle collision. This model is then used to simulate the gravitational settling of particles, and the results confirm the effect of mixture incompressibility and demonstrate that the model is capable of reaching the random closed packing limit. Numerical examples for the flow problems of particle-induced stratification are conducted, and the model is able to reveal the detailed features of particle-laden flows. Sensitivity on the grid resolution and deviations from the existing single-phase model results are also discussed.

ACS Style

Yi-Ju Chou; Shih-Hung Gu; Yun-Chuan Shao. An Euler–Lagrange model for simulating fine particle suspension in liquid flows. Journal of Computational Physics 2015, 299, 955 -973.

AMA Style

Yi-Ju Chou, Shih-Hung Gu, Yun-Chuan Shao. An Euler–Lagrange model for simulating fine particle suspension in liquid flows. Journal of Computational Physics. 2015; 299 ():955-973.

Chicago/Turabian Style

Yi-Ju Chou; Shih-Hung Gu; Yun-Chuan Shao. 2015. "An Euler–Lagrange model for simulating fine particle suspension in liquid flows." Journal of Computational Physics 299, no. : 955-973.