Onset of sediment motion under waves and currents

1987 ◽  
Vol 14 (2) ◽  
pp. 196-206 ◽  
Author(s):  
Michael C. Quick ◽  
Kristopher Kingston ◽  
Shenglong Lei
Keyword(s):  
Maximum Velocity ◽  
Wave Theory ◽  
Roughness Height ◽  
General Criterion ◽  
Flow Conditions ◽  
Threshold Criterion ◽  
Velocity Condition ◽  
Waves And Currents ◽  
Average Current ◽  
Sediment Threshold

Experiments have been made to study the resultant flow field and bed shear stress produced by the combination of surface waves and steady currents. Comparison has been made between the sediment threshold flow conditions for steady flow, for waves alone, and for combined waves and currents. Experiments, using a laser Doppler anemometer and sediments sieved into narrow size ranges, show that a similar maximum velocity condition, close to the bed, causes onset of motion for all the conditions tested. This maximum velocity can be calculated from the Manning equation, the log velocity law, and second-order wave theory, if it is assumed that the near-bed maximum wave and average current velocity at one roughness height above the bed combine linearly. It is demonstrated that the Shields threshold criterion can be extended to both the wave and combined wave plus current situations. The maximum velocity at approximately one roughness height above the bed is therefore shown to be a good general criterion, which can also be used to estimate the probability of sediment motion. Key words: waves and currents, sediment transport, coastal erosion.

Correlation Between LDA and Ultrasound Heart Catheter Measurements in a Stenosed Arterial Model

1995 ◽  
Vol 117 (1) ◽  
pp. 103-106 ◽  
Author(s):  
D. Liepsch ◽  
A. Poll ◽  
R. Blasini
Keyword(s):  
Cross Section ◽  
Laser Doppler Anemometry ◽  
Water Solution ◽  
Maximum Velocity ◽  
Symmetric Model ◽  
Axially Symmetric ◽  
Flow Conditions ◽  
Future Studies ◽  
Glycerine Water ◽  
Arterial Model

Ultrasound heart catheters are used to measure the velocity in coronary arteries. However, the act of introducing a catheter into the vessel disturbs the very flow being measured. We used laser Doppler anemometry to measure the velocity distribution in an axially symmetric model, both with and without a catheter inserted. The catheter reduced the center-line velocity by as much as 60 percent at a distance of 2 mm downstream from the catheter, and by as much as 25 percent at a distance of 10 mm. This means the velocity measured with an ultrasound catheter does not show the maximum velocity of the undisturbed flow in the tube center. In the constriction, however, the measured velocities with the LDA and ultrasound catheter are almost the same. Thus, catheter measurements in the stenosis achieve accurate results. The velocity profile in the stenosed areas is flattened over nearly the whole cross section. The velocity is extremely reduced only close to the wall. The measurements outside of the stenosis lead to large differences which need to be studied carefully in the future. The disturbed flow finally disappeared 15 mm downstream of the catheter. The measurements were done at steady flow using a glycerine water solution with a dynamic viscosity of 4.35m Pas. In future studies, these experiments will be repeated for pulsatile flow conditions using non-Newtonian blood-like fluids.

scholarly journals Influence of Rigid Emerged Vegetation in a Channel Bend on Bed Topography and Flow Velocity Field: Laboratory Experiments

Water ◽  
2019 ◽  
Vol 12 (1) ◽  
pp. 118 ◽  
Author(s):  
Hossein Hamidifar ◽  
Alireza Keshavarzi ◽  
Paweł M. Rowiński
Keyword(s):  
Laboratory Experiments ◽  
Flow Characteristics ◽  
Maximum Velocity ◽  
Flow Conditions ◽  
Mean Flow ◽  
Bed Topography ◽  
Channel Bend ◽  
Bed Erosion ◽  
Flow Hydraulics ◽  
The Mean

Trees have been used extensively by river managers for improving the river environment and ecology. The link between flow hydraulics, bed topography, habitat availability, and organic matters is influenced by vegetation. In this study, the effect of trees on the mean flow, bed topography, and bed shear stress were tested under different flow conditions. It was found that each configuration of trees produced particular flow characteristics and bed topography patterns. The SR (single row of trees) model appeared to deflect the maximum velocity downstream of the bend apex toward the inner bank, while leading the velocity to be more uniformly distributed throughout the bend. The entrainment of sediment particles occurred toward the area with higher values of turbulent kinetic energy (TKE). The results showed that both SR and DR (double rows of trees) models are effective in relieving bed erosion in sharp ingoing bends. The volume of the scoured bed was reduced up to 70.4% for tests with trees. This study shows the effectiveness of the SR model in reducing the maximum erosion depth.

A Comprehensive Study of Asymmetric Micro-Droplet Splitting in T-Junction

2019 ◽  
Author(s):  
Way Lee Cheng ◽  
Reza Sadr ◽  
Arum Han
Keyword(s):  
Pressure Gradient ◽  
Cfd Simulation ◽  
General Criterion ◽  
Flow Conditions ◽  
Functional Feature ◽  
Splitting Ratio ◽  
Junction Geometry ◽  
Break Up ◽  
Gradient Ratio ◽  
Underlying Mechanisms

Abstract Splitting a single droplet into two unequal portions using a microfluidic T-junction has been an important functional feature of many modern lab-on-a-chip devices. A recent study introduced a general criterion for asymmetric droplet break-up in the range of intermediate Capillary numbers. The current work attempts to analyze, in more details, the different underlying mechanisms governing the asymmetric break-up process. In particular, this work focuses on the relationship between the break-up mechanism versus the splitting ratio of the daughter droplets. CFD simulation is used to closely monitor the effect of different fluid properties on the evolution of droplet break-up process. The splitting ratio under different flow conditions is characterized. Four mechanisms for primary droplet break-up are defined as follows: break-up with permanent obstruction, unstable break-up, breakup with tunnels and non-breakup. In particular, the main focus of this study is on the unstable break-up mechanisms where is very likely results to a much-deviated splitting ratio. Typically, yet unexpectedly, the resulting splitting ratio is often larger than the pressure gradient ratio in the T-junction. However, the two ratios are approximately equals to each other under a limited set of flow conditions. It has been observed that the splitting ratio could be more than double the pressure gradient ratio of the T-junction. The break-up is observed to be in the permanent obstruction mode if the splitting ratio is about the same magnitude as the pressure gradient ratio. The effects of the T-junction geometry on the break-up will also be examined.

Flow Patterns and Bed Deformation in a Meandering Channel with Different Flow Conditions

2013 ◽  
Vol 405-408 ◽  
pp. 2292-2296
Author(s):  
Dong Dong Jia ◽  
Peng Fei Hei ◽  
Xue Jun Shao ◽  
Xi Bao Zhang
Keyword(s):  
Velocity Distribution ◽  
Transport Model ◽  
Maximum Velocity ◽  
Flow Conditions ◽  
Meandering Channel ◽  
Bed Deformation ◽  
Meandering Channels ◽  
Small Flow ◽  
Flow And Sediment Transport ◽  
Deformation Patterns

The velocity distribution and bed deformation patterns in a meandering channel with consecutive bends were investigated by using both an experimental study and a 3-D flow and sediment transport model. The simulated results from the 3-D model agree well with the experimental data. The flow structure and bed deformation patterns in channel bends with different flow conditions were analyzed. Modeled results demonstrated that the velocity distribution and bed deformation in the meandering channel depend mostly on the flow regimes. For the case of small flow discharge, the maximum velocity occurs in the vicinity of concave bank. For the case of large discharge, the maximum velocity occurs near the center part of the channel. Consequently, bed deformation patterns in meandering channels will be influenced by the flow conditions.

scholarly journals THE INTERACTION OF WAVES AND CURRENTS OVER A LONGSHORE BAR

1986 ◽  
Vol 1 (20) ◽  
pp. 116 ◽  
Author(s):  
I.A. Svendsen ◽  
J. Buhr Hansen
Keyword(s):  
Wave Height ◽  
Wave Motion ◽  
Surf Zone ◽  
Wave Theory ◽  
Runge Kutta Method ◽  
Waves And Currents ◽  
Set Up ◽  
The Waves ◽  
Wave Properties ◽  
A Current

A two-dimensional model for waves and steady currents in the surf zone is developed. It is based on a depth integrated and time averaged version of the equations for the conservation of mass, momentum, and wave energy. A numerical solution is described based on a fourth order Runge-Kutta method. The solution yields the variation of wave height, set-up, and current in the surf zone, taking into account the mass flux in the waves. In its general form any wave theory can be used for the wave properties. Specific results are given using the description for surf zone waves suggested by Svendsen (1984a), and in this form the model is used for the wave motion with a current on a beach with a longshore bar. Results for wave height and set-up are compared with measurements by Hansen & Svendsen (1986).

scholarly journals A dynamic bidirectional coupled surface flow model for flood inundation simulation

2021 ◽  
Vol 21 (2) ◽  
pp. 497-515
Author(s):  
Chunbo Jiang ◽  
Qi Zhou ◽  
Wangyang Yu ◽  
Chen Yang ◽  
Binliang Lin
Keyword(s):  
Flow Model ◽  
Wave Theory ◽  
Surface Flow ◽  
Flood Inundation ◽  
Flow Conditions ◽  
Local Flow ◽  
Benchmark Tests ◽  
Proposed Model ◽  
The World ◽  
Future Management

Abstract. Flood disasters frequently threaten people and property all over the world. Therefore, an effective numerical model is required to predict the impacts of floods. In this study, a dynamic bidirectional coupled hydrologic–hydrodynamic model (DBCM) is developed with the implementation of characteristic wave theory, in which the boundary between these two models can dynamically adapt according to local flow conditions. The proposed model accounts for both mass and momentum transfer on the coupling boundary and was validated via several benchmark tests. The results show that the DBCM can effectively reproduce the process of flood propagation and also account for surface flow interaction between non-inundation and inundation regions. The DBCM was implemented for the floods simulation that occurred at Helin Town located in Chongqing, China, which shows the capability of the model for flood risk early warning and future management.

Effect of bed roughness on 1-D entropy velocity distribution in open channel flow

2013 ◽  
Vol 46 (1) ◽  
pp. 1-10 ◽  
Author(s):  
Michele Greco
Keyword(s):  
Open Channel ◽  
Measurement Data ◽  
Maximum Velocity ◽  
Open Channel Flow ◽  
Small Scale ◽  
Flow Depth ◽  
Flow Conditions ◽  
Bed Roughness ◽  
Logarithmic Relationship ◽  
The Relationship

A theoretic-analytical formulation, based on entropy velocity profile law and classical relationships for uniform flow and friction factor, is proposed enlightening the general logarithmic relationship existing between the parameter Φ(M), defined as mean cross section velocity over maximum velocity, and the ratio water depth/bed roughness (D/d). The relationship Φ(M)–D/d has been applied to a relevant set of experimental velocity measurement data collected both in laboratory and in field, showing different behaviour between small scale and large-intermediate roughness flows. In particular, the roughness influence becomes remarkable whenever shallow water flow conditions occur, that is when the ratio between the flow depth and the roughness height is less than 4, while Φ(M) tends to be constant as the value of D/d increases.

scholarly journals SCOUR DURATION AT STRUCTURES AT STEADY FLOW CONDITIONS

2015 ◽  
Author(s):  
Oskars LAUVA ◽  
Boriss GJUNSBURGS
Keyword(s):  
Relative Depth ◽  
Contraction Rate ◽  
Clear Water ◽  
Flow Conditions ◽  
Local Flow ◽  
Equilibrium Time ◽  
Sediment Movement ◽  
Flow Modification ◽  
Threshold Criterion ◽  
Movement Parameters

One of the reasons of the bridge crossings failure or damage in flow is the unpredicted depth of scour near foundations. The aim of this study is to find the equilibrium time near elliptical guide banks. Analysis of the literature shows that there are no methods or formulas to calculate equilibrium time of scour near elliptical guide banks. In the formulas used in calculating equilibrium time at piers or abutments, different parameters are not taken into consideration. These parameters include: contraction rate of the flow, Froude number, bed layering, sediment movement parameters, local flow modification, ratio relative local and critical velocities, and relative depth. The differential equation of the bed sediment movement in clear water was used and method for computing equilibrium time of scour near elliptical guide banks was elaborated. New hydraulic threshold criterion is proposed for the calculation of equilibrium time of scour. Computer modeling results were compared with equilibrium time of scour which were calculated by the presented method and they were in agreement.

scholarly journals Experimental Investigation on Characteristics of Sand Waves with Fine Sand under Waves and Currents

Water ◽  
2019 ◽  
Vol 11 (3) ◽  
pp. 612 ◽  
Author(s):  
Zhenlu Wang ◽  
Bingchen Liang ◽  
Guoxiang Wu
Keyword(s):  
Large Scale ◽  
Wave Length ◽  
Wave Theory ◽  
Fine Sand ◽  
Sand Wave ◽  
Wave Steepness ◽  
Sand Waves ◽  
Wave Characteristics ◽  
Waves And Currents ◽  
Wave Nonlinearity

A series of physical experiments was conducted to study the geometry characteristics and evolution of sand waves under waves and currents. Large scale bedforms denoted as sand waves and small bedforms represented by ripples were both formed under the experimental hydrodynamic conditions. Combining the experimental data with those from previous research, the characteristics of waves and currents and measured sand waves were listed. Small amplitude wave theory and Cnoidal wave theory were used to calculate the wave characteristics depending on different Ursell numbers, respectively. The results show good agreement between the dimensionless characteristics of sand waves and the dimensionless wave characteristics with a smaller wave steepness. When the wave steepness is large, the results seem rather scattered which may be affected by the wave nonlinearity. Sand wave steepness hardly changed with bed shear stress. A simple linear relationship can be found between sand wave length and wave steepness. It is easy to evaluate the sand wave characteristics from the measured wave data.

scholarly journals PROBABILITY OF EXCEEDING THE CRITICAL SHEAR STRESS FOR SAND MOTION IN SPECIFIC WAVE AND CURRENT CONDITIONS

2012 ◽  
Vol 1 (33) ◽  
pp. 4
Author(s):  
Pierre-Yves Henry ◽  
Alf Tørum ◽  
Øivind Artsen ◽  
Dag Myrhaug ◽  
Muk Chen Ong
Keyword(s):  
Shear Stress ◽  
Critical Shear Stress ◽  
Bed Shear Stress ◽  
Random Waves ◽  
Flow Conditions ◽  
Flume Experiments ◽  
Natural Sand ◽  
Waves And Currents ◽  
Mobility Parameter ◽  
A Current

This study is focusing on the threshold of sand motion under random waves combined with a following current. The analysis is based on some flume experiments realized over a natural sand bed for different flow conditions (waves and currents). The main result comes as a map of the probability to exceed the threshold of sand motion, as a function of a wave and a current mobility parameter. These observations are compared to methods predicting the bed shear stress using an equivalent monochromatic wave, and links between the probability of exceeding the critical shear stress for initiation of sand motion and the calculated maximum bed shear stress are found.

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