scholarly journals Channel and shoal development in a short tidal embayment: an idealized model study

2011 ◽  
Vol 677 ◽  
pp. 503-529 ◽  
Author(s):  
MIRIAM C. ter BRAKE ◽  
HENK M. SCHUTTELAARS
Keyword(s):  
Deposition Parameter ◽  
Initial Formation ◽  
Sediment Fluxes ◽  
Advective Flux ◽  
Model Studies ◽  
Idealized Model ◽  
Model Study ◽  
Equilibrium Profile ◽  
Tidal Embayment ◽  
Uniform Equilibrium

In many tidal embayments, complex patterns of channels and shoals are observed. To gain a better understanding of these features, an idealized model, that describes the interaction of water motion, sediment transport and bed evolution in a semi-enclosed, rectangular basin, is developed and analysed. To explain the initial formation of channels and shoals, two-dimensional perturbations superposed on a laterally uniform equilibrium bottom are studied. These perturbations evolve due to convergences of various residual suspended sediment fluxes: a diffusive flux, a flux related to the bed topography, an advective flux resulting from internally generated overtides and an advective flux due to externally prescribed overtides. For most combinations of these fluxes, perturbations start to grow if the bottom friction is strong enough. Their growth is mainly a result of convergences of diffusive and topographically induced sediment fluxes. Advective contributions due to internally generated overtides enhance this growth. If only diffusive sediment fluxes are considered, the underlying equilibrium is always unstable. This can be traced back to the depth dependence of the deposition parameter. Contrary to the results of previous idealized models, the channels and shoals always initiate in the shallow, landward areas. This is explained by the enhanced generation (compared to that in previous models) of frictional torques in shallow regions. The resulting initial channel–shoal formation compares well with results found in complex numerical model studies. The instability mechanism and the location of the initial formation of bottom patterns do not change qualitatively when varying parameters. Changes are mainly related to differences in the underlying equilibrium profile due to parameter variations.

scholarly journals Multi-model study of mercury dispersion in the atmosphere: Vertical distribution of mercury species

2016 ◽  
Author(s):  
Johannes Bieser ◽  
Franz Slemr ◽  
Jesse Ambrose ◽  
Carl Brenninkmeijer ◽  
Steve Brooks ◽  
...  
Keyword(s):  
Vertical Distribution ◽  
Atmospheric Chemistry ◽  
Lower Troposphere ◽  
Elemental Mercury ◽  
Mercury Species ◽  
Substantial Impact ◽  
Model Studies ◽  
Model Study ◽  
The Impact ◽  
The Vertical Distribution

Abstract. Atmospheric chemistry and transport of mercury play a key role in the global mercury cycle. However, there are still considerable knowledge gaps concerning the fate of mercury in the atmosphere. This is the second part of a model inter-comparison study investigating the impact of atmospheric chemistry and emissions on mercury in the atmosphere. While the first study focused on ground based observations of mercury concentration and deposition, here we investigate the vertical distribution and speciation of mercury from the planetary boundary layer to the lower stratosphere. So far, there have been few model studies investigating the vertical distribution of mercury, mostly focusing on single aircraft campaigns. Here, we present a first comprehensive analysis based on various aircraft observations in Europe, North America, and on inter-continental flights. The investigated models proved to be able to reproduce the distribution of total and elemental mercury concentrations in the troposphere including inter-hemispheric trends. One key aspect of the study is the investigation of mercury oxidation in the troposphere. We found that different chemistry schemes were better at reproducing observed oxidized mercury (RM) patterns depending on altitude. High RM concentrations in the upper troposphere could be reproduced with oxidation by bromine while elevated concentrations in the lower troposphere were better reproduced by OH and ozone chemistry. However, the results were not always conclusive as the physical and chemical parametrizations in the chemistry transport models also proved to have a substantial impact on model results.

The Prediction of Combined Sewer Overflow Performance from Models – A Case Study

1994 ◽  
Vol 29 (1-2) ◽  
pp. 373-382 ◽  
Author(s):  
Peter D. Hedges
Keyword(s):  
Storm Events ◽  
Operating Characteristics ◽  
Combined Sewer Overflows ◽  
Model Studies ◽  
Original Proposal ◽  
Model Study ◽  
Storm Flow ◽  
Combined Sewer ◽  
The Uk ◽  
Main Programme

Hydrodynamic separators have been employed as combined sewer overflows (CSOs) in the UK since the early 1960s. Since little was known of their operating characteristics, Severn Trent Water Authority contracted Aston University to monitor the performance of a separator constructed during 1986-87 at James Bridge, Walsall, UK. Unfortunately only three overflow events were fully documented during the monitoring period, which was particularly dry. One separator was therefore artificially charged by pumping from the adjacent river to simulate storm flow conditions. A model study was not included in the original proposal, but one was established to complement the main programme since the characteristics of hydrodynamic separators were poorly understood. The arrangement of the field site is described, and the results from the monitored storm events, pump tests and model studies are summarised. Characteristics derived from the model study are combined with sewage settling velocity grading curves to predict the performance of the prototype hydrodynamic separator for one of the observed storm events. Given the limitations inherent in the sewage data used, a comparison of the predicted and observed separator efficiencies demonstrates the potential of this approach for the design and selection of CSOs for specific locations.

scholarly journals Multi-model study of mercury dispersion in the atmosphere: vertical and interhemispheric distribution of mercury species

2017 ◽  
Vol 17 (11) ◽  
pp. 6925-6955 ◽  
Author(s):  
Johannes Bieser ◽  
Franz Slemr ◽  
Jesse Ambrose ◽  
Carl Brenninkmeijer ◽  
Steve Brooks ◽  
...  
Keyword(s):  
Atmospheric Chemistry ◽  
Lower Troposphere ◽  
Elemental Mercury ◽  
Mercury Species ◽  
Substantial Impact ◽  
Model Studies ◽  
Model Study ◽  
High Concentrations ◽  
Intercomparison Study ◽  
The Impact

Abstract. Atmospheric chemistry and transport of mercury play a key role in the global mercury cycle. However, there are still considerable knowledge gaps concerning the fate of mercury in the atmosphere. This is the second part of a model intercomparison study investigating the impact of atmospheric chemistry and emissions on mercury in the atmosphere. While the first study focused on ground-based observations of mercury concentration and deposition, here we investigate the vertical and interhemispheric distribution and speciation of mercury from the planetary boundary layer to the lower stratosphere. So far, there have been few model studies investigating the vertical distribution of mercury, mostly focusing on single aircraft campaigns. Here, we present a first comprehensive analysis based on various aircraft observations in Europe, North America, and on intercontinental flights. The investigated models proved to be able to reproduce the distribution of total and elemental mercury concentrations in the troposphere including interhemispheric trends. One key aspect of the study is the investigation of mercury oxidation in the troposphere. We found that different chemistry schemes were better at reproducing observed oxidized mercury patterns depending on altitude. High concentrations of oxidized mercury in the upper troposphere could be reproduced with oxidation by bromine while elevated concentrations in the lower troposphere were better reproduced by OH and ozone chemistry. However, the results were not always conclusive as the physical and chemical parameterizations in the chemistry transport models also proved to have a substantial impact on model results.

Initial formation of channels and shoals in a short tidal embayment

1999 ◽  
Vol 386 ◽  
pp. 15-42 ◽  
Author(s):  
H. M. SCHUTTELAARS ◽  
H. E. DE SWART
Keyword(s):  
Sediment Transport ◽  
Evolution Equation ◽  
Transport Model ◽  
Bottom Friction ◽  
Small Scale ◽  
Model Parameters ◽  
Load Transport ◽  
Equilibrium Profile ◽  
Tidal Embayment ◽  
The Stability

It is demonstrated, by using a simple model, that bedforms in a short tidal embayment can develop due to a positive feedback between tidal currents, sediment transport and bedforms. The water motion is modelled by the depth integrated shallow water equations. The system is forced by a prescribed free-surface elevation at the entrance of the embayment. For the sediment dynamics a diffusively dominated suspended load transport model is considered. Tidal averaging is used to obtain the bottom profiles at the long morphological time scale.The stability of a constantly sloping equilibrium bottom profile is studied for various combinations of the model parameters. It turns out that without a mechanism that generates vorticity this equilibrium profile is stable. In that case small-scale perturbations can at most become marginally stable if no bedload term in the bottom evolution equation is incorporated. If vorticity is generated, in our model by bottom friction torques, the basic state is unstable. The spatial patterns of the unstable modes and their growth rates depend, among other things, on the strength of the bottom friction, the width of the embayment and the grain size: if the sediment under consideration consists of large particles, the equilibrium will be more stable than when smaller particles are considered. Without a diffusive term in the bed evolution equation, small-scale perturbations become unstable. To avoid this physically unrealistic behaviour bedload terms are included in the sediment transport. Furthermore, it is shown that using an asymptotic expansion for the concentration as given in earlier literature is only valid for small or moderate mode numbers and the technique is extended to large mode numbers. A physical interpretation of the results is also given.

scholarly journals MOVABLE-BED MODEL STUDIES OF PERCHED BEACH CONCEPT

1972 ◽  
Vol 1 (13) ◽  
pp. 60
Author(s):  
C.B. Chatham
Keyword(s):  
Three Dimensional ◽  
Normal Wave ◽  
Beach Erosion ◽  
Model Studies ◽  
Model Study ◽  
Wide Range ◽  
Final Design ◽  
Storm Wave ◽  
Wave Conditions ◽  
Movable Bed

Hydraulic model studies were conducted to aid in ascertaining the technical feasibility and optimum design factors of the perched beach concept. Among these were two-dimensional, movable-bed studies to determine an estimate of the amount of sand which would be lost seaward over the submerged toe structure by normal and storm wave action, the optimum elevation of the submerged toe structure, and the length of a stone blanket required to reduce seaward migration of sand to a minimum. The model beach was subjected to test waves until equilibrium was reached for a wide range of wave conditions for both the existing beach and the perched beach. Test results indicate that (a) little or no beachfill material will be lost seaward of the toe structure for normal wave conditions but the larger storm waves may cause erosion of the perched beach, (b) the installation of a stone blanket shoreward of the toe structure will reduce the amount of beach erosion, (c) if the beach fill is extended a sufficient distance seaward, the toe structure serves no useful purpose, and (d) a three-dimensional movable-bed model study is feasible and is necessary to determine the final design features of a perched beach.

Artifacts associated with acoustic rhinometric assessment of infants and young children: a model study

1994 ◽  
Vol 77 (6) ◽  
pp. 2558-2563 ◽  
Author(s):  
J. E. Buenting ◽  
R. M. Dalston ◽  
T. L. Smith ◽  
A. F. Drake
Keyword(s):  
Nasal Cavity ◽  
Acoustic Rhinometry ◽  
Area Measurement ◽  
Model Studies ◽  
Model Study ◽  
Accurate Indication ◽  
Cavity Configuration ◽  
Series Of Experiments ◽  
Potential Impact ◽  
Infants And Young Children

The present study was undertaken to determine in model studies whether currently available acoustic rhinometry instrumentation might be used to analyze the nasal cavity configuration of infants and children. A simple nasal cavity model was constructed using eight Lucite inserts that were placed between standard nosepieces provided by the manufacturer and a 35-cm-long polyvinyl chloride pipe closed at its distal end. To simulate the nasal valve, the inserts were 12 mm in length and had apertures ranging in diameter from 2 to 9 mm. A series of experiments was conducted to evaluate the accuracy with which the acoustic rhinometer measured the size of each insert aperture and the configuration of the model system distal to that aperture. Transmission losses caused errors in the area measurement of the insert aperture and the tube distal to the insert. When the insert aperture was < 6 mm in diameter (0.28 cm2), the aperture area was overestimated by > 10%, whereas the area of the distal tube was underestimated by > 10%. As a result of response lags, the acoustic rhinometer also failed to provide an accurate indication of insert length. Finally, oscillation artifacts caused estimates of the distal pipe area to fluctuate. These three systematic errors are described, and their potential impact on acoustic rhinometry in children is discussed.

Enhancement of transient electromagnetic soundings: A metallic model study

Geophysics ◽  
1984 ◽  
Vol 49 (7) ◽  
pp. 895-901 ◽  
Author(s):  
W. E. Doll ◽  
T. V. Skibicky ◽  
C. S. Clay
Keyword(s):  
Half Space ◽  
Short Pulse ◽  
Early Time ◽  
Image Transmission ◽  
Matched Filters ◽  
Magnetic Dipoles ◽  
Model Studies ◽  
Transient Electromagnetic ◽  
Model Study ◽  
Reflected Signal

Transient electromagnetic (TEM) soundings can be enhanced by additions of matched filtered signals from different coil separations. Results of laboratory metallic model studies are used to demonstrate this technique. A short pulse of current is transmitted from one coil. For each of the signals received in the other coil, the half‐space response (surface wave) is subtracted from the total signal to determine the reflected signal. These reflected signals are passed through matched filters corresponding to the proper separation and a possible layer depth. All of the matched filtered signals corresponding to a particular possible depth are added, and the summed output is stored. The filtering and summing process is repeated for a set of possible interface depths. The sum of matched filtered signals for the correct depth has a peak value of 1 and is symmetric about the peak. We believe that short‐range early‐time values of the transient response can be used to estimate the conductivity of the first layer. This estimate is used for numerical computations of half‐space responses. We have used several magnetic dipoles to represent the finite‐sized coils to show good agreement between theoretical and experimental half‐spaces. Theoretical reflection signals for matched filters can be computed numerically. The stacking of matched filtered signals is an electromagnetic analog of the common‐depth‐point (CDP) seismic technique. It should reduce the effects of nearsurface inhomogeneities and improve the resolution. As an incidental experiment, we show comparisons of the reflected signal and the “image transmission.” The reflected and transmitted signals are essentially the same and support the concept of hybrid‐ray theory for electromagnetic sounding.

Finite element analysis of the impact of the properties of dental wedge materials on functional features

2021 ◽  
Vol 112 (1) ◽  
pp. 32-41
Author(s):  
M. Czerwiński ◽  
J. Żmudzki ◽  
K. Kwieciński ◽  
M. Kowalczyk
Keyword(s):  
Finite Element Analysis ◽  
Finite Element ◽  
High Plasticity ◽  
Element Analysis ◽  
Expanded Polytetrafluoroethylene ◽  
Model Studies ◽  
Model Study ◽  
The Matrix ◽  
Functional Features ◽  
The Impact

Purpose: Defect of the interproximal wall of the tooth is filled with use the shaped matrix and wedge which seals bottom margin during filling. Better fit of the wedge and equalization of the pressure forces on the matrix is achieved by the compliance of the wedge structure through cuts and perforations and the use of silicone materials and unidirectionally expanded polytetrafluoroethylene (ePTFE). The work presents a methodology for model studies of the mechanics of dental wedges in order to evaluate and compare the impact of wedge materials on functional features. The hypothesis of the work was that the mechanical properties of ePTFE determine the effectiveness of the dental wedge. Design/methodology/approach: Effect of modulus of elasticity and friction coefficient of wedge and matrix materials on the functional features of the wedge was studied on the way Finite Element Analysis (FEA). Simulation included contact sliding between wedge and matrix what was simulated in nonlinear large displacements regime. The sealing evaluation criterion was the pressure distribution between the wedge and matrix below the lower edge of the defect. Displacement values were the criterion for the loss of convexity as a result of matrix deformation. Findings: The material for the wedge should be characterized by a low coefficient of friction, low elasticity (ensuring high compliance of the wedge) and at the same time the ability to large permanent deformations, which allows for plastic shaping of the matrix from the side of the defect in order to achieve the required wall convexity and the tangent point. Research limitations/implications: Results show tendency of phenomena in limitation to model simplification of the interdental gap and the ideal adhesion of the matrix to the tooth and linear elasticity of materials. Practical implications: The material that best meets the requirements is unidirectionally expanded polytetrafluoroethylene, which has one of the lowest coefficients of friction and very high plasticity necessary to shape the matrix from the inside of the cavity. Originality/value: Methodology of model study and criteria of functional characteristics of dental wedge was presented.

Model study on trapping of protein selenenic acids by utilizing a stable synthetic congener

2019 ◽  
Vol 43 (18) ◽  
pp. 6830-6833
Author(s):  
Shohei Sase ◽  
Ryutaro Kimura ◽  
Ryosuke Masuda ◽  
Kei Goto
Keyword(s):  
Model Compound ◽  
Model Studies ◽  
Model Study

Model studies on the trapping reaction of protein selenenic acids were carried out with a stable primary-alkyl model compound.

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