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Détails du magazine
Format
Magazine
eISSN
1338-4333
Première publication
28 Mar 2009
Période de publication
4 fois par an
Langues
Anglais

Chercher

Volume 63 (2015): Edition 4 (December 2015)

Détails du magazine
Format
Magazine
eISSN
1338-4333
Première publication
28 Mar 2009
Période de publication
4 fois par an
Langues
Anglais

Chercher

10 Articles
access type Accès libre

The Limit Deposit Velocity model, a new approach

Publié en ligne: 20 Oct 2015
Pages: 273 - 286

Résumé

Abstract

In slurry transport of settling slurries in Newtonian fluids, it is often stated that one should apply a line speed above a critical velocity, because blow this critical velocity there is the danger of plugging the line. There are many definitions and names for this critical velocity. It is referred to as the velocity where a bed starts sliding or the velocity above which there is no stationary bed or sliding bed. Others use the velocity where the hydraulic gradient is at a minimum, because of the minimum energy consumption. Most models from literature are one term one equation models, based on the idea that the critical velocity can be explained that way.

Here the following definition is used: The critical velocity is the line speed below which there may be either a stationary bed or a sliding bed, depending on the particle diameter and the pipe diameter, but above which no bed (stationary or sliding) exists, the Limit Deposit Velocity (LDV). The way of determining the LDV depends on the particle size, where 5 regions are distinguished.

These regions for sand and gravel are roughly; very small particles up to 0.014–0.040 mm (d < δv), small particles from δv–0.2 mm, medium particles in a transition region from 0.2–2.00 mm, large particles > 2 mm and very large particles > 0.015·Dp. The lower limit of the LDV is the transition between a sliding bed and heterogeneous transport. The new model is partly based on physics and correlates well with experiments from literature.

Mots clés

  • Critical velocity
  • Limit Deposit Velocity
  • Slurry transport
  • Newtonian fluid
access type Accès libre

Analysis of the influence of input data uncertainties on determining the reliability of reservoir storage capacity

Publié en ligne: 20 Oct 2015
Pages: 287 - 294

Résumé

Abstract

The paper contains a sensitivity analysis of the influence of uncertainties in input hydrological, morphological and operating data required for a proposal for active reservoir conservation storage capacity and its achieved values. By introducing uncertainties into the considered inputs of the water management analysis of a reservoir, the subsequent analysed reservoir storage capacity is also affected with uncertainties. The values of water outflows from the reservoir and the hydrological reliabilities are affected with uncertainties as well. A simulation model of reservoir behaviour has been compiled with this kind of calculation as stated below. The model allows evaluation of the solution results, taking uncertainties into consideration, in contributing to a reduction in the occurrence of failure or lack of water during reservoir operation in low-water and dry periods.

Mots clés

  • Uncertainties
  • Reliability
  • Reservoir storage capacity
  • Monte Carlo method
  • Mean monthly flows
  • Evaporation
  • Elevation–volume curve
  • Elevation–area curve
access type Accès libre

Effects of relative submergence and bed slope on sediment incipient motion under decelerating flows

Publié en ligne: 20 Oct 2015
Pages: 295 - 302

Résumé

Abstract

This paper presents the results of an experimental study to quantify the effects of bed slope and relative submergence on incipient motion of sediment under decelerating flows. Experiments were conducted in an experimental tilting-flume of 8 m long 0.4 m wide and 0.6 m deep with glass-walls. Three uniform sediments with median grain sizes of 0.95, 1.8 and 3.8 mm and three bed slopes of 0.0075, 0.0125 and 0.015 were used under decelerating flow. The main conclusion is that the Shields diagram, which is commonly used to evaluate the critical shear stress, is not suitable to predict the critical shear stress under decelerating flows.

Mots clés

  • Fluvial hydraulics
  • Open channel flow
  • River beds
  • Sediment transport
  • Shear stress
  • Streamflow
access type Accès libre

Experimental study on the rheological behaviour of coal ash slurries

Publié en ligne: 20 Oct 2015
Pages: 303 - 310

Résumé

Abstract

Extensive experimental investigations were carried out to evaluate the rheological behaviour of fly ash (FA) slurry without and with the addition of bottom ash (BA) and BA slurry without and with the addition of FA. The FA slurries exhibited Bingham behaviour at solid mass concentrations ranging from 60–65% and mixing proportions from 10– 40%. A substantial reduction in yield stress was observed except for mixing proportion of 40% on which the yield stress and viscosity were increased drastically for all solid concentrations. Hence, it can be concluded that the yield stress and viscosity of FA slurry were very much influenced by adding BA up to the mixing proportion of 30%. The rheological behaviour of BA slurries with and without the addition of FA in proportions of 10–50% was investigated and exhibited Newtonian behaviours for solid mass concentrations ranging from 30–50% without and with the addition of FA. The viscosity increases with increasing the solid concentrations and proportion of FA. Based on these experimental data, a correlation was developed to predict the relative viscosity of BA slurries as a function of solid volume fraction and FA mass proportion of 0–50% and the RMSE and R2 values showed good agreement between the experimental and the predicted data.

Mots clés

  • Coal ash slurry
  • Newtonian fluid
  • Yield stress
  • Plastic viscosity
  • Relative viscosity
access type Accès libre

MHD stagnation point flow of Jeffrey fluid by a radially stretching surface with viscous dissipation and Joule heating

Publié en ligne: 20 Oct 2015
Pages: 311 - 317

Résumé

Abstract

The steady stagnation-point flow of an electrically conducting fluid due to convectively heated stretched disk in the radial direction is considered. Effects of viscous dissipation and Joule heating are present. Mathematical modelling is based upon constitutive relations of Jeffrey fluid. The governing partial differential equations are first transformed into the coupled system of ordinary differential equations and then solved for the convergent series solutions. Numerical values of skin friction coefficient and local Nusselt number are also computed and analysed.

Mots clés

  • MHD stagnation point flow
  • Jeffrey fluid
  • Viscous dissipation
  • Joule heating
access type Accès libre

Experimental investigation of internal structure of open-channel flow with intense transport of sediment

Publié en ligne: 20 Oct 2015
Pages: 318 - 326

Résumé

Abstract

Gravity-driven open-channel flows carrying coarse sediment over an erodible granular deposit are studied. Results of laboratory experiments with artificial sediments in a rectangular tilting flume are described and analyzed. Besides integral quantities such as flow rate of mixture, transport concentration of sediment and hydraulic gradient, the experiments include measurements of the one-dimensional velocity distribution across the flow. A vertical profile of the longitudinal component of local velocity is measured across the vertical axis of symmetry of a flume cross section using three independent measuring methods. Due to strong flow stratification, the velocity profile covers regions of very different local concentrations of sediment from virtually zero concentration to the maximum concentration of bed packing. The layered character of the flow results in a velocity distribution which tends to be different in the transport layer above the bed and in the sediment-free region between the top of the transport layer and the water surface. Velocity profiles and integral flow quantities are analyzed with the aim of evaluating the layered structure of the flow and identifying interfaces in the flow with a developed transport layer above the upper plane bed.

Mots clés

  • Acoustic anemometry
  • Plane bed
  • Solid-liquid flow
  • Tilting flume experiment
access type Accès libre

Impacts of bridge piers on the initiation of ice cover – an experimental study

Publié en ligne: 20 Oct 2015
Pages: 327 - 333

Résumé

Abstract

Ice jams in northern rivers during winter period significantly change the flow conditions due to the extra boundary of the flow. Moreover, with the presence of bridge piers in the channel, the flow conditions can be further complicated. Ice cover often starts from the front of bridge piers, extending to the upstream. With the accumulation of ice cover, ice jam may happen during early spring, which results in the notorious ice jam flooding. In the present study, the concentration of flowing ice around bridge piers has been evaluated based on experiments carried out in laboratory. The critical condition for the initiation of ice cover around bridge piers has been investigated. An equation for the critical floe concentration was developed. The equation has been validated by experimental data from previous studies. The proposed model can be used for the prediction of formation of ice cover in front of a bridge pier under certain conditions.

Mots clés

  • Arch-shaped congestion
  • Bridge pier
  • Ice cover
  • Ice floe concentration
  • Pier distance
access type Accès libre

3-D CFD simulations of hydrodynamics in the Sulejow dam reservoir

Publié en ligne: 20 Oct 2015
Pages: 334 - 341

Résumé

Abstract

This paper reports the processes by which a single-phase 3-D CFD model of hydrodynamics in a 17-km-long dam reservoir was developed, verified and tested. A simplified VOF model of flow was elaborated to determine the effect of wind on hydrodynamics in the lake. A hexahedral mesh with over 17 million elements and a k-ω SST turbulence model were defined for single-phase simulations in steady-state conditions. The model was verified on the basis of the extensive flow measurements (StreamPro ADCP, USA). Excellent agreement (average error of less than 10%) between computed and measured velocity profiles was found. The simulation results proved a strong effect of wind on hydrodynamics in the lake, especially on the development of the water circulation pattern in the lacustrine zone.

Mots clés

  • CFD
  • 3-D hydrodynamic model
  • Effect of wind
  • Dam reservoir
access type Accès libre

Changes in runoff in two neighbouring catchments in the Bohemian Forest related to climate and land cover changes

Publié en ligne: 20 Oct 2015
Pages: 342 - 352

Résumé

Abstract

There is public concern that large-scale disturbances to forest cover caused by insects and storm winds in the Bohemian Forest could intensify high water flows and enhance the expected flooding risks predicted in current regional climate change scenarios. We analysed stream discharge in Upper Vydra and Große Ohe, neighbouring catchments in the Bohemian Forest, the largest contiguous forested area in Central Europe. Upper Vydra, in the Šumava National Park, and Große Ohe (including the Upper Große Ohe headwater catchment in the Bavarian Forest National Park) are similar in size, but differ in land use cover and the extent of disturbed Norway spruce stands. Publicly available runoff and meteorological data (1978–2011) were examined using non-parametric trend and breakpoint analysis. Together with mapped vegetation cover changes, the results were used to address the following questions: 1) are there significant changes in the hydrological cycle and, if so, do these changes relate to 2) the extent and expansion of disturbance in forests stands and/or 3) altered precipitation dynamics and thermal conditions?

We found no marked overall change in annual runoff or in annual or seasonal precipitation, but a significant increase in high flows in March. This overall trend related to the marked warming in late winter and early spring (+~4 K in April, p < 0.01), irrespective of altitude and slope position. It significantly shifted the end of the snow cover period by more than three weeks to the beginning/middle of April depending on altitude, and intensified snow melt.

In the Upper Große Ohe catchment, a significant decrease in catchment balance, the difference between the long term precipitation and runoff (–72 mm, 11%) was found when the loss of tree cover reached 30% of catchment area. Diminished evapotranspiration losses from severely disturbed stands increased groundwater recharge during summer and caused a significant rise in low flows in autumn.

However, observed increases in late winter high flows were due to warming only. They could be further intensified by the increasing winter precipitation predicted under present climate change scenarios, and would therefore increase the risk of flooding at lower elevations.

Mots clés

  • Runoff
  • Climate change
  • Forest disturbance
access type Accès libre

Dissolved oxygen and water temperature dynamics in lowland rivers over various timescales

Publié en ligne: 20 Oct 2015
Pages: 353 - 363

Résumé

Abstract

The impact of floodplain hydrology on the in-stream dissolved oxygen dynamics and the relation between dissolved oxygen and water temperature are investigated. This has been done by examining the time series of dissolved oxygen and water temperature coupled with meteorological and hydrological data obtained from two lowland rivers having contrasting hydrological settings. Spectral analysis of long-term oxygen variations in a vegetated river revealed a distinct scaling regime with slope ‘–1’ indicating a self-similar behaviour. Identical slopes were obtained for water temperature and water level. The same power-law behaviour was observed for an unvegetated river at small timescales revealing the underlying scaling behaviour of dissolved oxygen regime for different types of rivers and over various time scales. The results have shown that the oxygenation of a vegetated river is strongly related to its thermal regime and flow conditions. Moreover, analysis of short-term fluctuations in the unvegetated river demonstrated that physical factors such as rainfall and backwaters play a substantial role in the functioning of this ecosystem. Finally, the results show that the relation between water temperature and dissolved oxygen concentration at the diurnal timescale exhibits a looping behaviour on the variable plot. The findings of this study provide an insight into the sensitivity of rivers to changing hydro-physical conditions and can be useful in the assessment of environmental variability.

Mots clés

  • Dissolved oxygen
  • Water temperature
  • Hysteresis
  • Spectral analysis
  • Self-similarity
  • Anastomosing river
  • Backwaters
10 Articles
access type Accès libre

The Limit Deposit Velocity model, a new approach

Publié en ligne: 20 Oct 2015
Pages: 273 - 286

Résumé

Abstract

In slurry transport of settling slurries in Newtonian fluids, it is often stated that one should apply a line speed above a critical velocity, because blow this critical velocity there is the danger of plugging the line. There are many definitions and names for this critical velocity. It is referred to as the velocity where a bed starts sliding or the velocity above which there is no stationary bed or sliding bed. Others use the velocity where the hydraulic gradient is at a minimum, because of the minimum energy consumption. Most models from literature are one term one equation models, based on the idea that the critical velocity can be explained that way.

Here the following definition is used: The critical velocity is the line speed below which there may be either a stationary bed or a sliding bed, depending on the particle diameter and the pipe diameter, but above which no bed (stationary or sliding) exists, the Limit Deposit Velocity (LDV). The way of determining the LDV depends on the particle size, where 5 regions are distinguished.

These regions for sand and gravel are roughly; very small particles up to 0.014–0.040 mm (d < δv), small particles from δv–0.2 mm, medium particles in a transition region from 0.2–2.00 mm, large particles > 2 mm and very large particles > 0.015·Dp. The lower limit of the LDV is the transition between a sliding bed and heterogeneous transport. The new model is partly based on physics and correlates well with experiments from literature.

Mots clés

  • Critical velocity
  • Limit Deposit Velocity
  • Slurry transport
  • Newtonian fluid
access type Accès libre

Analysis of the influence of input data uncertainties on determining the reliability of reservoir storage capacity

Publié en ligne: 20 Oct 2015
Pages: 287 - 294

Résumé

Abstract

The paper contains a sensitivity analysis of the influence of uncertainties in input hydrological, morphological and operating data required for a proposal for active reservoir conservation storage capacity and its achieved values. By introducing uncertainties into the considered inputs of the water management analysis of a reservoir, the subsequent analysed reservoir storage capacity is also affected with uncertainties. The values of water outflows from the reservoir and the hydrological reliabilities are affected with uncertainties as well. A simulation model of reservoir behaviour has been compiled with this kind of calculation as stated below. The model allows evaluation of the solution results, taking uncertainties into consideration, in contributing to a reduction in the occurrence of failure or lack of water during reservoir operation in low-water and dry periods.

Mots clés

  • Uncertainties
  • Reliability
  • Reservoir storage capacity
  • Monte Carlo method
  • Mean monthly flows
  • Evaporation
  • Elevation–volume curve
  • Elevation–area curve
access type Accès libre

Effects of relative submergence and bed slope on sediment incipient motion under decelerating flows

Publié en ligne: 20 Oct 2015
Pages: 295 - 302

Résumé

Abstract

This paper presents the results of an experimental study to quantify the effects of bed slope and relative submergence on incipient motion of sediment under decelerating flows. Experiments were conducted in an experimental tilting-flume of 8 m long 0.4 m wide and 0.6 m deep with glass-walls. Three uniform sediments with median grain sizes of 0.95, 1.8 and 3.8 mm and three bed slopes of 0.0075, 0.0125 and 0.015 were used under decelerating flow. The main conclusion is that the Shields diagram, which is commonly used to evaluate the critical shear stress, is not suitable to predict the critical shear stress under decelerating flows.

Mots clés

  • Fluvial hydraulics
  • Open channel flow
  • River beds
  • Sediment transport
  • Shear stress
  • Streamflow
access type Accès libre

Experimental study on the rheological behaviour of coal ash slurries

Publié en ligne: 20 Oct 2015
Pages: 303 - 310

Résumé

Abstract

Extensive experimental investigations were carried out to evaluate the rheological behaviour of fly ash (FA) slurry without and with the addition of bottom ash (BA) and BA slurry without and with the addition of FA. The FA slurries exhibited Bingham behaviour at solid mass concentrations ranging from 60–65% and mixing proportions from 10– 40%. A substantial reduction in yield stress was observed except for mixing proportion of 40% on which the yield stress and viscosity were increased drastically for all solid concentrations. Hence, it can be concluded that the yield stress and viscosity of FA slurry were very much influenced by adding BA up to the mixing proportion of 30%. The rheological behaviour of BA slurries with and without the addition of FA in proportions of 10–50% was investigated and exhibited Newtonian behaviours for solid mass concentrations ranging from 30–50% without and with the addition of FA. The viscosity increases with increasing the solid concentrations and proportion of FA. Based on these experimental data, a correlation was developed to predict the relative viscosity of BA slurries as a function of solid volume fraction and FA mass proportion of 0–50% and the RMSE and R2 values showed good agreement between the experimental and the predicted data.

Mots clés

  • Coal ash slurry
  • Newtonian fluid
  • Yield stress
  • Plastic viscosity
  • Relative viscosity
access type Accès libre

MHD stagnation point flow of Jeffrey fluid by a radially stretching surface with viscous dissipation and Joule heating

Publié en ligne: 20 Oct 2015
Pages: 311 - 317

Résumé

Abstract

The steady stagnation-point flow of an electrically conducting fluid due to convectively heated stretched disk in the radial direction is considered. Effects of viscous dissipation and Joule heating are present. Mathematical modelling is based upon constitutive relations of Jeffrey fluid. The governing partial differential equations are first transformed into the coupled system of ordinary differential equations and then solved for the convergent series solutions. Numerical values of skin friction coefficient and local Nusselt number are also computed and analysed.

Mots clés

  • MHD stagnation point flow
  • Jeffrey fluid
  • Viscous dissipation
  • Joule heating
access type Accès libre

Experimental investigation of internal structure of open-channel flow with intense transport of sediment

Publié en ligne: 20 Oct 2015
Pages: 318 - 326

Résumé

Abstract

Gravity-driven open-channel flows carrying coarse sediment over an erodible granular deposit are studied. Results of laboratory experiments with artificial sediments in a rectangular tilting flume are described and analyzed. Besides integral quantities such as flow rate of mixture, transport concentration of sediment and hydraulic gradient, the experiments include measurements of the one-dimensional velocity distribution across the flow. A vertical profile of the longitudinal component of local velocity is measured across the vertical axis of symmetry of a flume cross section using three independent measuring methods. Due to strong flow stratification, the velocity profile covers regions of very different local concentrations of sediment from virtually zero concentration to the maximum concentration of bed packing. The layered character of the flow results in a velocity distribution which tends to be different in the transport layer above the bed and in the sediment-free region between the top of the transport layer and the water surface. Velocity profiles and integral flow quantities are analyzed with the aim of evaluating the layered structure of the flow and identifying interfaces in the flow with a developed transport layer above the upper plane bed.

Mots clés

  • Acoustic anemometry
  • Plane bed
  • Solid-liquid flow
  • Tilting flume experiment
access type Accès libre

Impacts of bridge piers on the initiation of ice cover – an experimental study

Publié en ligne: 20 Oct 2015
Pages: 327 - 333

Résumé

Abstract

Ice jams in northern rivers during winter period significantly change the flow conditions due to the extra boundary of the flow. Moreover, with the presence of bridge piers in the channel, the flow conditions can be further complicated. Ice cover often starts from the front of bridge piers, extending to the upstream. With the accumulation of ice cover, ice jam may happen during early spring, which results in the notorious ice jam flooding. In the present study, the concentration of flowing ice around bridge piers has been evaluated based on experiments carried out in laboratory. The critical condition for the initiation of ice cover around bridge piers has been investigated. An equation for the critical floe concentration was developed. The equation has been validated by experimental data from previous studies. The proposed model can be used for the prediction of formation of ice cover in front of a bridge pier under certain conditions.

Mots clés

  • Arch-shaped congestion
  • Bridge pier
  • Ice cover
  • Ice floe concentration
  • Pier distance
access type Accès libre

3-D CFD simulations of hydrodynamics in the Sulejow dam reservoir

Publié en ligne: 20 Oct 2015
Pages: 334 - 341

Résumé

Abstract

This paper reports the processes by which a single-phase 3-D CFD model of hydrodynamics in a 17-km-long dam reservoir was developed, verified and tested. A simplified VOF model of flow was elaborated to determine the effect of wind on hydrodynamics in the lake. A hexahedral mesh with over 17 million elements and a k-ω SST turbulence model were defined for single-phase simulations in steady-state conditions. The model was verified on the basis of the extensive flow measurements (StreamPro ADCP, USA). Excellent agreement (average error of less than 10%) between computed and measured velocity profiles was found. The simulation results proved a strong effect of wind on hydrodynamics in the lake, especially on the development of the water circulation pattern in the lacustrine zone.

Mots clés

  • CFD
  • 3-D hydrodynamic model
  • Effect of wind
  • Dam reservoir
access type Accès libre

Changes in runoff in two neighbouring catchments in the Bohemian Forest related to climate and land cover changes

Publié en ligne: 20 Oct 2015
Pages: 342 - 352

Résumé

Abstract

There is public concern that large-scale disturbances to forest cover caused by insects and storm winds in the Bohemian Forest could intensify high water flows and enhance the expected flooding risks predicted in current regional climate change scenarios. We analysed stream discharge in Upper Vydra and Große Ohe, neighbouring catchments in the Bohemian Forest, the largest contiguous forested area in Central Europe. Upper Vydra, in the Šumava National Park, and Große Ohe (including the Upper Große Ohe headwater catchment in the Bavarian Forest National Park) are similar in size, but differ in land use cover and the extent of disturbed Norway spruce stands. Publicly available runoff and meteorological data (1978–2011) were examined using non-parametric trend and breakpoint analysis. Together with mapped vegetation cover changes, the results were used to address the following questions: 1) are there significant changes in the hydrological cycle and, if so, do these changes relate to 2) the extent and expansion of disturbance in forests stands and/or 3) altered precipitation dynamics and thermal conditions?

We found no marked overall change in annual runoff or in annual or seasonal precipitation, but a significant increase in high flows in March. This overall trend related to the marked warming in late winter and early spring (+~4 K in April, p < 0.01), irrespective of altitude and slope position. It significantly shifted the end of the snow cover period by more than three weeks to the beginning/middle of April depending on altitude, and intensified snow melt.

In the Upper Große Ohe catchment, a significant decrease in catchment balance, the difference between the long term precipitation and runoff (–72 mm, 11%) was found when the loss of tree cover reached 30% of catchment area. Diminished evapotranspiration losses from severely disturbed stands increased groundwater recharge during summer and caused a significant rise in low flows in autumn.

However, observed increases in late winter high flows were due to warming only. They could be further intensified by the increasing winter precipitation predicted under present climate change scenarios, and would therefore increase the risk of flooding at lower elevations.

Mots clés

  • Runoff
  • Climate change
  • Forest disturbance
access type Accès libre

Dissolved oxygen and water temperature dynamics in lowland rivers over various timescales

Publié en ligne: 20 Oct 2015
Pages: 353 - 363

Résumé

Abstract

The impact of floodplain hydrology on the in-stream dissolved oxygen dynamics and the relation between dissolved oxygen and water temperature are investigated. This has been done by examining the time series of dissolved oxygen and water temperature coupled with meteorological and hydrological data obtained from two lowland rivers having contrasting hydrological settings. Spectral analysis of long-term oxygen variations in a vegetated river revealed a distinct scaling regime with slope ‘–1’ indicating a self-similar behaviour. Identical slopes were obtained for water temperature and water level. The same power-law behaviour was observed for an unvegetated river at small timescales revealing the underlying scaling behaviour of dissolved oxygen regime for different types of rivers and over various time scales. The results have shown that the oxygenation of a vegetated river is strongly related to its thermal regime and flow conditions. Moreover, analysis of short-term fluctuations in the unvegetated river demonstrated that physical factors such as rainfall and backwaters play a substantial role in the functioning of this ecosystem. Finally, the results show that the relation between water temperature and dissolved oxygen concentration at the diurnal timescale exhibits a looping behaviour on the variable plot. The findings of this study provide an insight into the sensitivity of rivers to changing hydro-physical conditions and can be useful in the assessment of environmental variability.

Mots clés

  • Dissolved oxygen
  • Water temperature
  • Hysteresis
  • Spectral analysis
  • Self-similarity
  • Anastomosing river
  • Backwaters

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