ON THE DEVELOPMENT OF LATTICE BOLTZMANN MODELING FOR LIQUID-SOLID PHASE TRANSITIONS IN FREE SURFACE FLOWS

Side-by-side comparison of horizontal subsurface flow and free water surface flow constructed wetlands and artificial neural network (ANN) modelling approach

In this paper we will present an algorithm to perform free surface flow simulations with the lattice Boltzmann method on adaptive grids. This reduces the required computational time by more than a factor of three for simulations with large volumes of fluid. To achieve this, the simulation of large fluid regions is performed with coarser grid resolutions. We have developed a set of rules to dynamically adapt the coarse regions to the movement of the free surface, while ensuring the consistency of all grids. Furthermore, the free surface treatment is combined with a Smagorinsky turbulence model and a technique for adaptive time steps to ensure stable simulations. The method is validated by comparing the position of the free surface with an uncoarsened simulation. It yields speedup factors of up to 3.85 for a simulation with a resolution of 4803 cells and three coarser grid levels, and thus enables efficient and stable simulations of free surface flows, e.g. for highly detailed physically based animations of fluids.

This paper reports on the progress of the liquid-solid phase transition modeling of water in open channel flow by using the lattice Boltzmann method with the immersed boundary modification. The phase transition in a fluid flow has a moving interface between the liquid and solid state, which leads complicated treatments in existing numerical models. By applying the immersed boundary modification in the lattice Boltzmann method and the non-iterative enthalpy approach for the separation of the states, the moving boundary of the melting or solidification front is solved without any difficulty. The ice bed and the submerged ice cover under dynamic flow conditions is exercised to demonstrate the model performance. The model is extremely suitable in the formulation in terms of its simple and compact framework extendable to any dimensions.

A three-dimensional non-hydrostatic coupled model for free surface – Subsurface variable – Density flows

Three-dimensional numerical simulations of strongly nonlinear free surface flows are performed by lattice Boltzmann method (LBM), which features a number of performance-related advantages, particularly concerning data locality and parallel computing. A Multi-Passage-Interface (MPI) multicore processors parallelized free surface LBM solver is applied for the present three-dimensional numerical simulations. A Smagorinsky LES turbulent model serves to capture the small-scale turbulent structures of the flow. Experiments on dam breaking from previous articles are used to compare and verify two-dimensional (2D) and three-dimensional (3D) LBM model. A new experimental setup is also developed in order to observe the three-dimensionality effect. The findings demonstrated that the free surface LBM simulation agrees well with the experiments.

In this work, the modelling of horizontal two-phase flows within the two-fluid Euler–Euler approach is investigated. A modified formulation of the morphology detection functions within the Algebraic Interfacial Area Density (AIAD) model is presented in combination with different models for the drag force acting on a sheared gas–liquid interface. In the case of free surface flows, those closure laws are often based on experimental correlations whose applicability is limited to certain flow regimes. It is investigated here whether the implementation of the modified blending functions in ANSYS CFX avoids this limitation. The influence of the new functions on the prediction of turbulence parameters in free surface flows is also examined quantitatively for the k-ω and k-ε two-equation turbulence models. Transient simulations of the WENKA counter-current stratified two-phase flow experiment were performed for validation. A prediction of the correct flow pattern as observed in the experiment improved dramatically when a turbulence damping term was included in the standard two-equation models. Using the k-ω and a modified k-ε turbulence model with damping terms close to the interface, better agreement with the experimental data was achieved. The morphology detection mechanism of the unified blending functions within the AIAD is seen as an improvement with respect to the detection of sharp interfaces. Satisfactory quantitative agreement is achieved for the modified free surface drag. Furthermore, it is demonstrated that turbulence dampening has to be accounted for in both turbulence models to qualitatively reproduce the mean flow and turbulence quantities from the experiment.

Nonlinear solutions of free surface flow under a sluice gate are studied. Upstream, the fluid is assumed to be infinitely deep, and the gate is at an angle to the horizontal axis. The flow emerges from the gate and produces a uniform flow far downstream. The problem is solved numerically by a boundary element method derived from an integral equation along the free surface. An analytical function is constructed, related to the upstream flow, so that the integral equation is solvable. As a result, a free surface flow with smooth detachment from the edge of the gate is obtained for relatively large upstream Froude numbers. As the Froude number decreases, a free surface with back flow near the edge of the gate occurs, until at a certain Froude number a stagnation point occurs. References D. D. Frangmeier, T. S. Strelkoff, Solution for gravity flow under a sluice gate, ASCE J. Engng. Mech. Div. , 94 , 1968, 153--176. B. E. Lorock, Gravity-affected flow from planer sluice gate, ASCE J. Engng. Mech. Div. , 96 , 1969, 1211--1226. Y. K. Chung, Solution of flow under sluice gates, ASCE J. Engng. Mech. Div. , 98 , 1972, 121--140. J. Asavanant, J.-M. Vanden-Broeck, Nonlinear free surface flows emerging from vessels and flows under a sluice gate, J. Austral. Math. Soc. , 38 , 1996, 63--86. doi:10.1017/S0334270000000473 J.-M. Vanden-Broeck, Numerical calculations of the free-surface flow under a sluice gate, J. Fluid Mech. , 330 , 1996, 339--347. http://journals.cambridge.org/abstract_S0022112096003849 B. J. Binder, J.-M. Vanden-Broeck, Free surface flows past surfboards and sluice gates, Eur. J. Appl. Maths. , 16 , 2005, 601--619 doi:10.1017/S0956792505006443 B. J. Binder, J.-M. Vanden-Broeck, The effect of disturbances on the flows under a sluice gate and past an inclined plate, J. Fluid Mech. , 576 , 2007, 475--490 doi:10.1017/S0022112007004806 J.-M. Vanden-Broeck, E. O. Tuck, Flow near the interaction of a free surface with a vertical wall, SIAM J. Appl. Math. , 54 , 1994, 1--33. doi:10.1137/S0036139992240838 L. H. Wiryanto, J. Widjaja, B. Supriyanto, Free-surface flow under a sluice gate from deep water, Bul. Malay. Math. Soc. , 34 , 2011, 601--609. http://www.emis.de/journals/BMMSS/vol34_3_17.html L. H. Wiryanto, E. O. Tuck, A back-turning jet formed by a uniform shallow stream hitting a vertical wall, Proc. Int. Conf. on Dif. Eqs. , Kluwer Academic Press, 1997. L. H. Wiryanto, E. O. Tuck, An open-channel flow meeting a barrier and forming one or two jets, J. Austral. Math. Soc. , 41 , 2000, 458--472. doi:10.1017/S0334270000011760 L. H. Wiryanto, Small Froude number solutions of flow caused by a line source, J. Indones. Math. Soc. , 6 , 2000, 1--6. G. C. Hocking, L. K. Forbes, A note on the flow induced by a line sink beneath a free surface, J. Austral. Math. Soc. B , 32 , 1991, 251--260. doi:10.1017/S0956792507006924 G. C. Hocking, L. K. Forbes, Subcritical free-surface flow caused by a line source in a fluid of finite depth, J. Eng. Math. , 26 , 1992, 455--466. doi:10.1007/BF00042763 L. H. Wiryanto, Zero gravity of a jet emerging from a slit, J. Indones. Math. Soc. , 12 , 2006, 89--98. L. M. Milne-Thomson, Theoretical hydrodynamics . Dover Publications, Inc., 1968, 274--281.

Free surface flow and heat transfer characteristics of liquid metal Galinstan at low flow velocity

Towards non-intrusive reduced order 3D free surface flow modelling

Three pilot-scale horizontal-subsurface flow (H-SSF), surface flow (SF), and free water surface flow (FWS) constructed wetland system designs were installed and evaluated to determine the effectiveness of constructed wetlands to treat tertiary effluent wastewater generated from Paşaköy Advanced Biological Wastewater Treatment Plant (PABWWTP). The average ammonia-nitrogen (NH+ 4) removal efficiencies of % 49 to 52% were observed in all three system designs. Average NO− 3 removal efficiency was approximately 58% in the SF system design, suggesting that this design had the highest denitrification rate. Total phosphorus (TP) removal efficiency was the highest (approximately 60%) in the h-SSF system, possibly due to substrate (gravel). Systems were capable of removing approximately 94% of fecal coliform (FC) bacteria. Removal rate coefficients (k 20, 1/day) in the H-SSF, the FWS and the SF systems were estimated for NH+ 4, NO− 3, TP and FC. The average nutrient removal efficiencies were found higher in the summer periods except for the FC. The results of statistical analyses indicated that nutrient removals were affected by temperature, hydraulic residence time (HRT), and nitrogen loading rate (NLR).

Potential of constructed wetlands in treating the eutrophic water: Evidence from Taihu Lake of China

Constructed wetlands (CWs) are an eco-technology for wastewater treatment and are applied worldwide. Due to the regular influx of pollutants, CWs can release considerable quantities of greenhouse gases (GHGs), ammonia (NH3), and other atmospheric pollutants, such as volatile organic compounds (VOCs) and hydrogen sulfide (H2S), etc., which will aggravate global warming, degrade air quality and even threaten human health. However, there is a lack of systematic understanding of factors affecting the emission of these gases in CWs. In this study, we applied meta-analysis to quantitatively review the main influencing factors of GHG emission from CWs; meanwhile, the emissions of NH3, VOCs, and H2S were qualitatively assessed. Meta-analysis indicates that horizontal subsurface flow (HSSF) CWs emit less CH4 and N2O than free water surface flow (FWS) CWs. The addition of biochar can mitigate N2O emission compared to gravel-based CWs but has the risk of increasing CH4 emission. Polyculture CWs stimulate CH4 emission but pose no influence on N2O emission compared to monoculture CWs. The influent wastewater characteristics (e.g., C/N ratio, salinity) and environmental conditions (e.g., temperature) can also impact GHG emission. The NH3 volatilization from CWs is positively related to the influent nitrogen concentration and pH value. High plant species richness tends to reduce NH3 volatilization and plant composition showed greater effects than species richness. Though VOCs and H2S emissions from CWs do not always occur, it should be a concern when using CWs to treat wastewater containing hydrocarbon and acid. This study provides solid references for simultaneously achieving pollutant removal and reducing gaseous emission from CWs, which avoids the transformation of water pollution into air contamination.

This study was conducted to investigate the cadmium removal in the free water surface and subsurface flow constructed wetland systems under different environmental conditions. Removal efficiencies of the chemical oxygen demand were found to be in the range of 79–92% for the two wetland systems. The removal efficiencies for total kjeldahl nitrogen, total phosphorus, total suspended solid, and volatile suspended solid were in the range of 64–91, 62–90, 68–91, and 50–84%, respectively. Influents were obtained by mixing a synthetic wastewater with CdCl2.H2O at four concentrations 1, 5, 10, and 20mg∕L, respectively, for the four experimental runs. The mean effluent cadmium concentrations varied between 0.02 and 0.16mg∕L. Cadmium removal efficiencies were 98.6–99.4% in free water surface, and 99.3–99.9% in the subsurface flow wetland systems. The subsurface flow wetland showed slightly better performance than the free water surface wetland in terms of various pollutants’ removal. However, the two systems had similar performances in the cadmium removal.

This study addresses a particular phenomenon in open channel flows for which the basic assumption of hydrostatic pressure distribution is essentially invalid, and expands previous suggestions to flows where streamline curvature is significant. The proposed model incorporates the effects of the vertical curvature of the streamline and steep slope, in making the pressure distribution non-hydrostatic, and overcomes the accuracy problem of the Saint-Venant equations when simulating curvilinear free surface flow problems. Furthermore, the model is demonstrated to be a higher-order one-dimensional model that includes terms accounting for wave-like variations of the free surface on a constant slope channel. Test results of predicted flow surface and pressure profiles for flow in a channel transition from mild to steep slopes, transcritical flow over a short-crested weir and flow with dual free surfaces are compared with experimental data and previous numerical results. A good agreement is attained between the experimental and computed results. The overall simulation results reveal the satisfactory performance of the proposed model in simulating rapidly varied gravity-driven flows with predominant non-hydrostatic pressure distribution effects. This study suggests that a higher-order pressure equation should be used for modelling the pressure distribution of a curvilinear flow in a steeply sloping channel.

Free surface flow over square bars at different Reynolds numbers