Wet Bed Research Articles

Numerical simulation of plunging wave breaker impact by a modified Turbulent WCSPH method

In this paper, the mesh-less weakly compressible smoothed particle hydrodynamics (WCSPH) method was used to solve the continuity and momentum equations with laminar viscosity and SPS turbulent model. To correct the pressure field and improve the accuracy of the free surface profile, modification of kernel and gradient of kernel was implemented to WCSPH model. The modified method, namely mSPH-T-K, was also equipped with periodic smoothing of the density using the modified kernel. To validate the modified model, the pressure field and wave front position of the 2D dam break flow were compared with those of experimental data, standard SPH method and mSPH-T method, which is the turbulence SPH method without modification of kernel and its gradients. Furthermore, the dam break on a wet bed was performed by the model. The results showed the well agreement of the modified model results of free surface with this benchmark. Finally, transformation of plunging breaker wave on a ramp was modeled with both mSPH-T and modified model. The results showed that the modified model has a good agreement with experimental data.

Hydrology and bedload transport relationships for sand-bed streams in the Ngarradj Creek catchment, northern Australia

Summary Rainfall, discharge and bedload were measured at three gauging stations (East Tributary, Upper Swift Creek and Swift Creek) in the Ngarradj Creek catchment at Jabiluka, Northern Territory, Australia. Hand-held, pressure difference, Helley-Smith bedload samplers were used to measure bedload fluxes for the 1998/1999, 1999/2000, 2000/2001 and 2001/2002 wet seasons. Rainfall is strongly seasonal over the Ngarradj Creek catchment, being concentrated in the wet season between November and April. Mean annual point rainfall between 1998 and 2007 for the water year, September to August, inclusive varied over the Ngarradj Creek catchment from 1731 ± 98 mm (SE) to 1754 ± 116 mm. Between 190 and 440 mm of rainfall are required before streamflow commences in December in most years. Streamflow persists until at least April. Mean annual runoff, as a percentage of mean annual rainfall, decreases slightly with increasing catchment area. Bedload ratings were calculated for four data sets. Significant bedload ratings were defined as those that were not only statistically significant (ρ ⩽ 0.05) but also explained at least 0.60 of the variance in bedload flux. For the three stations, twenty-three bedload ratings complied with the above criteria. Sixteen equations were accepted for East Tributary, four bedload ratings were accepted for Upper Swift Creek and three bedload ratings were accepted for Swift Creek. Significant bedload ratings were established between bedload flux and discharge, unit bedload flux and discharge, transport rate of unsuspended bedload by immersed weight per unit width and time and both unit and excess unit stream power, and finally, adjusted submersed bedload weight and both unit and excess unit stream power for raw and log10-transformed data. ‘Censored data sets’ were compiled for Upper Swift Creek and Swift Creek to include only bedload fluxes measured when there was no apparent scour or fill so that there were no changes in sand supply from the catchment (i.e. equilibrium conditions). Bedload sediments are similar at all sites. There is little difference in grain size statistics between wet season bedload and dry season bed material. The differences which were significant suggest that most of the bed material is transported as bedload during the wet season. Size selective transport occurs at all three gauging stations with bedload being better sorted than bed material and the coarsest fraction (Cobble gravel) is mobile only during extreme events.

Discretization of Multidimensional Mathematical Equations of Dam Break Phenomena Using a Novel Approach of Finite Volume Method

This paper was concerned to simulate both wet and dry bed dam break problems. A high-resolution finite volume method (FVM) was employed to solve the one-dimensional (1D) and two-dimensional (2D) shallow water equations (SWEs) using an unstructured Voronoi mesh grid. In this attempt, the robust local Lax-Friedrichs (LLxF) scheme was used for the calculating of the numerical flux at cells interfaces. The model named V-Break was run under the asymmetry partial and circular dam break conditions and then verified by comparing the model outputs with the documented results. Due to a precise agreement between those output and documented results, the V-Break could be considered as a reliable method for dealing with shallow water (SW) and shock problems, especially those having discontinuities. In addition, statistical observations indicated a good conformity between the V-Break and analytical results clearly.

Modeling of Shallow-Water Equations by Using Implicit Higher-Order Compact Scheme with Application to Dam-Break Problem

The paper deals with the unsteady two-dimensional (2D) non-linear shallow-water equations (SWE) in conservation-law form to capture the fluid flow in transition. Numerical simulations of dam-break flood wave in channel transitions have been performed for inviscid and incompressible flow by using two new implicit higher-order compact (HOC) schemes. The algorithm is second order accurate in time and fourth order accurate in space, on the nine-point stencil using third order non-centered difference at the wall boundaries. To solve the algebraic system, bi-conjugate gradient stabilized method (BiCGStab) with preconditioning has been employed. Although, both the schemes are able to capture both transient and steady state solution of shallow water equations, the scheme expressed in conservative law form is unconditionally stable. The model results have been validated for dam-break problem and compared with the experimental data for dry and wet bed conditions. The model results are found to be in good agreement with the experimental observations. The proposed scheme is useful to solve to capture flow transition with minimal number of nodal points, particularly for hyperbolic system.

Sculpting sandcastles grain by grain: Self-assembled sand towers

We study the spontaneous formation of granular towers produced when dry sand is poured on a wet sand bed. When the liquid content of the bed exceeds a threshold value W*, the impacting grains have a nonzero probability to stick on the wet grains due to instantaneous liquid bridges created during the impact. The trapped grains become wet by the capillary ascension of water and the process continues, giving rise to stable narrow towers. The growth velocity is determined by the surface liquid content which decreases exponentially as the tower height augments. This self-assembly mechanism (only observed in the funicular and capillary regimes) could theoretically last while the capillary rise of water is possible; however, the structure collapses before reaching this limit. The collapse occurs when the weight of the tower surpasses the cohesive stress at its base. The cohesive stress increases as the liquid content of the bed is reduced. Consequently, the highest towers are found just above W*.

Sediment entrainment by debris flows: In situ measurements from the headwaters of a steep catchment

Debris flows can dramatically increase their volume, and hence their destructive potential, by entraining sediment. Yet quantitative constraints on rates and mechanics of sediment entrainment by debris flows are limited. Using an in situ sensor network in the headwaters of a natural catchment we measured flow and bed properties during six erosive debris‐flow events. Despite similar flow properties and thicknesses of bed sediment entrained across all events, time‐averaged entrainment rates were significantly faster for bed sediment that was saturated prior to flow arrival compared with rates for sediment that was dry. Bed sediment was entrained from the sediment‐surface downward in a progressive fashion and occurred during passage of dense granular fronts as well as water‐rich, inter‐surge flow.En massefailure of bed sediment along the sediment‐bedrock interface was never observed. Large‐magnitude, high‐frequency fluctuations in total normal basal stress were dissipated within the upper 5 cm of bed sediment. Within this near surface layer, concomitant fluctuations in Coulomb frictional resistance are expected, irrespective of the influence of pore fluid pressure or fluctuations in shear stress. If the near‐surface sediment was wet as it was overridden by a flow, additional large‐magnitude, high‐frequency pore pressure fluctuations were measured in the near‐surface bed sediment. These pore pressure fluctuations propagated to depth at subsonic rates and in a diffusive manner. The depth to which large excess pore pressures propagated was typically less than 10 cm, but scaled as (D/fi)0.5, in which D is the hydraulic diffusivity and fiis the frequency of a particular pore pressure fluctuation. Shallow penetration depths of granular‐normal‐stress fluctuations and excess pore pressures demonstrate that only near‐surface bed sediment experiences the full dynamic range of effective‐stress fluctuations, and as a result, can be more easily entrained than deeper sediment. These data provide robust tests for mechanical models of entrainment and demonstrate that a debris flow over wet bed sediment will be larger than the same flow over dry bed sediment.

Using Artificial Neural Network to Predict the Pressure Drop in a Rotating Packed Bed

Although rotating beds are good equipments for intensified separations and multiphase reactions, but the fundamentals of its hydrodynamics are still unknown. In the wide range of operating conditions, the pressure drop across an irrigated bed is significantly lower than dry bed. In this regard, an approach based on artificial intelligence, that is, artificial neural network (ANN) has been proposed for prediction of the pressure drop across the rotating packed beds (RPB). The experimental data sets used as input data (280 data points) were divided into training and testing subsets. The training data set has been used to develop the ANN model while the testing data set was used to validate the performance of the trained ANN model. The results of the predicted pressure drop values with the experimental values show a good agreement between the prediction and experimental results regarding to some statistical parameters, for example (AARD% = 4.70, MSE = 2.0 × 10−5 and R2 = 0.9994). The designed ANN model can estimate the pressure drop in the countercurrent flow rotating packed bed with unexpected phenomena for higher pressure drop in dry bed than in wet bed. Also, the designed ANN model has been able to predict the pressure drop in a wet bed with the good accuracy with experimental.

Efficacy of oxygen-supplying capacity of Azolla in a controlled life support system

Azolla shows high growth and propagation rates, strong photosynthetic O 2-releasing ability and high nutritional value. It is suitable as a salad vegetable and can be cultured on a multi-layered wet bed. Hence, it possesses potential as a fresh vegetable, and to release O 2 and absorb CO 2 in a Controlled Ecological Life Support System in space. In this study, we investigated the O 2-providing characteristics of Azolla in a closed chamber under manned, controlled conditions to lay a foundation for use of Azolla as a biological component in ground simulation experiments for space applications. A closed test chamber, representing a Controlled Ecological Life Support System including an Azolla wet-culture device, was built to measure the changes in atmospheric O 2 and CO 2 concentrations inside the chamber in the presence of coexisting Azolla, fish and men. The amount of O 2 consumed by fish was 0.0805–0.0831 L kg −1 h −1 and the level of CO 2 emission was 0.0705–0.0736 L kg −1 h −1; O 2 consumption by the two trial volunteers was 19.71 L h −1 and the volume of respiration-released CO 2 was 18.90 L h −1. Under 7000–8000 Lx artificial light and Azolla wet-culture conditions, human and fish respiration and Azolla photosynthesis were complementary, thus the atmospheric O 2 and CO 2 concentrations inside chamber were maintained in equilibrium. The increase in atmospheric CO 2 concentration in the closed chamber enhanced the net photosynthesis efficiency of the Azolla colony. This study showed that Azolla has strong photosynthetic O 2-releasing ability, which equilibrates the O 2 and CO 2 concentrations inside the chamber in favor of human survival and verifies the potential of Azolla for space applications.

Incompressible smoothed particle hydrodynamics for free-surface flows: A generalised diffusion-based algorithm for stability and validations for impulsive flows and propagating waves

The incompressible smoothed particle hydrodynamics (ISPH) method with projection-based pressure correction has been shown to be highly accurate and stable for internal flows and, importantly for many problems, the pressure field is virtually noise-free in contrast to the weakly compressible SPH approach (Xu et al., 2009 [31]). However for almost inviscid fluids instabilities at the free surface occur due to errors associated with the truncated kernels. A new algorithm is presented which remedies this issue, giving stable and accurate solutions to both internal and free-surface flows. Generalising the particle shifting approach of Xu et al. (2009) [31], the algorithm is based upon Fick’s law of diffusion and shifts particles in a manner that prevents highly anisotropic distributions and the onset of numerical instability. The algorithm is validated against analytical solutions for an internal flow at higher Reynolds numbers than previously, the flow due to an impulsively started plate and highly accurate solutions for wet bed dam break problems at zero and small times. The method is then validated for progressive regular waves with paddle motion defined by linear theory. The accurate predictions demonstrate the effectiveness of the algorithm in stabilising solutions and minimising the surface instabilities generated by the inevitable errors associated with truncated kernels. The test cases are thought to provide a more thorough quantitative validation than previously undertaken.

Melatonin treatment in children with therapy-resistant monosymptomatic nocturnal enuresis

ObjectiveTo evaluate the effects of exogenous melatonin on the frequency of wet nights, on the sleep-wake cycle, and on the melatonin profile in children with therapy-resistant MNE. Patients and methods24 patients were included. Patients had to maintain a diary including time of sleep and arousal, and whether they had a dry or a wet bed in the morning. We measured baseline melatonin profiles in saliva. Hereafter, patients were randomized to synthetic melatonin or placebo. After 3 and 6 months we evaluated the frequency of enuresis and the melatonin profiles. Results11 patients were randomized to melatonin, 13 to placebo. We evaluated melatonin profiles of 7 patients in the melatonin group and of 8 in the placebo group. We observed a change in profile in the melatonin group, but we did not observe a difference in the sleep-wake cycle or the frequency of wet nights in either group. ConclusionThis is the first time exogenous melatonin has been evaluated in the treatment of MNE. Although we observed a change in melatonin profile after the use of exogenous melatonin, we did not observe a change in enuresis frequency or in the sleep-wake cycle of this select group of patients.

Simulation of dam-break problem by a robust flux-vector splitting approach in Cartesian grid

A robust and effective flux-vector splitting method is utilized to simulate dam-break problems based on a finite-volume method on a Cartesian grid. The method combines the effectiveness of Flux-Difference Splitting (FDS) methods and the robustness of Flux-Vector Splitting (FVS) methods to precisely estimate the numerical flux at each cell interface. A nonlinear second-order term, which is evaluated in local characteristics field, is added to decomposed fluxes to enhance the accuracy of the scheme. Several classical dam-break problems including 1D dam-break problem, hypothetical 2D dam-break problem with non-symmetric breach, circular dam-break problem and 2D dam-break problem with a baffle in the floodplain have been tested by the model. The results show very satisfactory agreement with analytical solutions and other numerical methods. The paper shows the accuracy, effectiveness and robustness of the hybrid FVS and FDS methods, both over wet and dry bed.

Similarity solution of dam-break flow on horizontal frictionless channel

Compared with the Blasius similarity approach for the boundary layer theory, a similarity solution for dam-break flow on a horizontal channel with zero friction is considered by the theory of scale transformation. Time and space coordinates as the two independent variables are substituted by one independent dimensionless similarity variable, so that the governing partial differential equations can be transformed into a system of ordinary differential equations. The analytical similarity dam-break solution on a dry bed and the multinomial algebraic equation of shock wave on a wet bed are proposed for a prismatic channel of triangular cross-section. Basing on the similarity characteristic of dam-break flow, the flood propagation at any time and space can be described by a dimensionless curve. The dimensionless positions of strong and weak discontinuities for dam-break shock wave do not change with time, and the description of the dam-break hydrograph is therefore simplified.

Positive feedback and momentum growth during debris-flow entrainment of wet bed sediment

Debris flows typically occur when intense rainfall or snowmelt triggers landslides or extensive erosion on steep, debris-mantled slopes. The flows can then grow dramatically in size and speed as they entrain material from their beds and banks, but the mechanism of this growth is unclear. Indeed, momentum conservation implies that entrainment of static material should retard the motion of the flows if friction remains unchanged. Here we use data from large-scale experiments to assess the entrainment of bed material by debris flows. We find that entrainment is accompanied by increased flow momentum and speed only if large positive pore pressures develop in wet bed sediments as the sediments are overridden by debris flows. The increased pore pressure facilitates progressive scour of the bed, reduces basal friction and instigates positive feedback that causes flow speed, mass and momentum to increase. If dryer bed sediment is entrained, however, the feedback becomes negative and flow momentum declines. We infer that analogous feedbacks could operate in other types of gravity-driven mass flow that interact with erodible beds. The mechanisms by which debris flows acquire mass and momentum as they entrain material are unclear. Large-scale experiments suggest that the pore pressure of wet bed sediment increases as the flow moves over the bed, leading to reduced friction and progressive scouring of the base.

On particle-based simulation of a dam break over a wet bed

This research presents particle-based simulations of a dam break on a wet bed by standard and improved versions of three particle methods, namely moving particle semi-implicit (MPS), incompressible smoothed particle hydrodynamics (ISPH) and weakly compressible smoothed particle hydrodynamics (WCSPH). The improved versions of these three methods are corrected MPS/ISPH (CMPS/CISPH) with a higher-order source term and WCSPH with a moving least square (MLS) density re-initialization. Direct comparisons of test photos and their corresponding simulation snapshots are made in terms of the reproduced free surface profile and simulated mixing processes. The work highlights potential capabilities of particle methods in reproducing detailed features of a dam break on a wet bed. A new viscosity reduction function is proposed for an efficient reproduction of backward breaking by WCSPH with an artificial viscosity term. The effect of bed friction is taken into account by applying a frictional drag force term usually applied in dissipative particle dynamics.

Continuous classification in a centrifugal fluidized bed apparatus

The optimization of a novel wet centrifugal fluidized bed classifier is discussed in this paper. Experiments in a pilot-scale apparatus and CFD calculations for the pure liquid flow reveal that the Coriolis force induces a strong tangential flow relative to the rotating vessel. Based on this finding a discharge mechanism for the coarse fraction has been developed which consists of a simple nozzle and thus allows a continuous and stable operation of the classifier. The measured pressure drop characteristic of the coarse particle bed reveals the range of fluidizing velocities which are optimal for the performance of classification. With the optimized classifier sharp classifications with a strong reduction of the misclassification of fines were achieved in different experiments with quartz, limestone and glass beads. Down to cut sizes of 5 μm sharp classifications were achieved.

Accurate Particle Methods for Refined Simulation of Complicated Breaking Waves

The paper presents particle-based simulations of a dam break on a wet bed by standard and improved versions of three particle methods, namely, MPS, ISPH and WCSPH methods. The improved versions of the three mentioned methods are CMPS-HS, CISPH-HS and WCSPH-MLS methods. Direct comparisons of experimental photos and their corresponding simulation snapshots have been made in terms of the reproduced free-surface profile as well as the simulated mixing processes. In order to obtain further improved simulations, in this paper, we propose a new viscosity reduction function in WCSPH-MLS calculation, and apply a frictional force term in CMPS-HS method.

Modeling Dam Break Behavior over a Wet Bed by a SPH Technique

Dam break evolution over dry and wet beds is analyzed with a smoothed particle hydrodynamics model. The model is shown to accurately fit both experimental dam break profiles and the measured velocities. In addition, the model allows one to study different propagation regimes during the dam break evolution. In particular, different dissipation mechanisms were identified: bottom friction and wave breaking. Although breaking dominates over wet beds at the beginning of the movement, bottom friction becomes the main dissipation mechanism in the long run.

Numerical and experimental investigation of heat and mass transfer in unsaturated porous media with low convective drying intensity

AbstractThe heat and mass transfer in an unsaturated wet cylindrical bed packed with quartz particles was investigated theoretically and experimentally for relatively low convective drying rates. The medium was dried by blowing dry air over the top of the porous bed which was insulated by impermeable, adiabatic material on the bottom and sides. Local thermodynamic equilibrium was assumed in the mathematical model describing the multi‐phase flow in the unsaturated porous medium using the energy and mass conservation equations for heat and mass transfer during the drying. The drying model included convection and capillary transport of the moisture, and convection and diffusion of the gas. The wet and dry regions were coupled with a dynamic boundary condition at the evaporation front. The numerical results indicated that the drying process could be divided into three periods: the initial temperature rise period, the constant drying rate period, and the reduced drying rate period. The numerical results agreed well with the experimental data, verifying that the mathematical model can evaluate the drying performance of porous media for low drying rates. ©2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(5): 290–312, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20205

Well-Balanced Scheme between Flux and Source Terms for Computation of Shallow-Water Equations over Irregular Bathymetry

Although many numerical techniques such as approximate Riemann solvers can be used to analyze subcritical and supercritical flows modeled by hyperbolic-type shallow-water equations, there are some difficulties in practical applications due to the numerical unbalance between source and flux terms. In this study, a revised surface gradient method is proposed that balances source and flux terms. The new numerical model employs the MUSCL-Hancock scheme and the HLLC approximate Riemann solver. Several verifications are conducted, including analyses of transcritical steady-state flows, unsteady dam break flows on a wet and dry bed, and flows over an irregular bathymetry. The model consistently returns accurate and reasonable results comparable to those obtained through analytical methods and laboratory experiments. The revised surface gradient method may be a simple but robust numerical scheme appropriate for solving hyperbolic-type shallow-water equations over an irregular bathymetry.

Improved implementation of the HLL approximate Riemann solver for one-dimensional open channel flows

Several new techniques are proposed to overcome the deficiencies in the conventional formulation of the approximate Riemann solvers for onedimensional open channel flows, which include numerical imbalance and inaccuracy in the solution of discharge. The former arises in the case of irregular geometry and the latter in the presence of a hydraulic jump. These new techniques include: (1) adopting the form of the saint venant equations that include both gravity and pressure in one source term; (2) using water surface level as one of the primitive variables, in stead of cross-sectional area; (3) defining discharge at interface and evaluating it according to the flux obtained by the HLL Riemann solver; and (4) estimating water surface gradient based on piecewise linearly reconstructed variables in the second-order scheme. The performance of the resulting schemes is evaluated by means of theoretical analysis and various test examples, including ideal dam-break flows with dry and wet bed, hydraulic jump, steady flow over bump with hydraulic jump, wave interactions, tidal flow in an open channel, and wave propagation. it is demonstrated that the schemes have excellent numerical balance and mass conservation property and are capable of satisfactorily reproducing various open channel flows.