A numerical method for simulations of cohesive, porous sediments

2021 ◽  
Author(s):  
Alexander Metelkin ◽  
Bernhard Vowinckel
Keyword(s):  
Computational Model ◽  
Stokes Equations ◽  
Spatial Scales ◽  
Flow Characteristics ◽  
Hydrodynamic Forces ◽  
Cohesive Sediments ◽  
Flow Conditions ◽  
Mean Flow ◽  
Local Flow ◽  
Clay Particles

<p><span>The dynamics of cohesive sediments under various flow conditions </span><span>are </span><span>of special interest in the framework of aquatic ecosystems. Being one of the main sources of transport for minerals and organic matter, the constituents of cohesive sediments are the source of food for many aquatic organisms. Due to the additional complexity of physical mechanisms, there are only a few simulation techniques for cohesive sediments, which do not cover all spatial scales. The primary cohesive clay particles are platelets smaller than 2 μm, which is small enough to experience Brownian motion. Composed together under the influence of van der Waals forces, they shape rounded aggregates also known as microflocs that are rather stable. These microflocs can form fragile, larger macroflocs with complex shapes exceeding 100 μm in size. Owing to the huge difference in the spatial scales, it is almost impossible to simulate macroflocs as the assembly of primary clay particles in the context of cohesive sediment transport modeling. In contrast to separate sediment grains, microflocs represent porous aggregates. </span><span>T</span><span>o perform direct numerical simulations of microflocs transported in a viscous fluid flow, we are developing a computational model for immersed porous particles. The model resolves the flow outside and inside porous aggregates and accurately computes the hydrodynamic forces on the microflocs. The simulation of macroflocs is also attainable by employing </span><span>cohesive</span><span> forces between microflocs, which assembles them into bigger aggregates with the propensity of breaking up under high shear rates. Our computational model solves the system of Navier - Stokes equations directly with an additional Darcy term inside the porous aggregate. Using this approach, it becomes feasible to consider the influence of the flow inside porous media, so that we can study its impact on the mean flow characteristics depending on the properties of the porous flocs. The hydrodynamic forces are calculated implicitly based on the pressure and shear stress distribution. By comparison with methods that use Stokes-based drag coefficients, our approach allows estimating the influence of local flow conditions and the presence of neighboring aggregates on the resulting fluid force.</span></p><p><span> </span></p>

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.

Hydrodynamic Forces Acting on Objects in a Moonpool

2014 ◽  
Author(s):  
Leiv Aspelund ◽  
Bjørnar Pettersen ◽  
Jan Visscher ◽  
Tor-Bjørn Idsøe Næss
Keyword(s):  
Cross Sections ◽  
Traditional Approach ◽  
Fluid Velocity ◽  
Flow Characteristics ◽  
Hydrodynamic Forces ◽  
Flow Conditions ◽  
Piv Measurements ◽  
Wave Elevation ◽  
Linear Damping ◽  
Heave Motion

Traditionally, it has often been assumed that the flow conditions in a moonpool are only moderately altered when an object is introduced therein. Moreover, the hydrodynamic forces acting on the object has typically been estimated by Morison’s equation for small volume structures, using the fluid kinematics of the empty moonpool as a basis and applying correction factors for the confined flow conditions, as for an object in a tube or a channel. To investigate the validity of the traditional approach, an experimental study on the forces acting on objects in a moonpool was performed at NTNU/MARINTEK in Trondheim, Norway in 2013. The experiments were done using a simplified 2-dimensional moonpool model which was given a forced heave motion. Two objects, both with square cross sections but of different sizes, were put inside the moonpool one at the time. The resulting wave elevations inside the moonpool and the forces acting on the objects were recorded and analyzed. To get a deeper understanding of the flow characteristics in the moonpool, PIV measurements were used to obtain the fluid velocity fields. The experiments revealed that even moderately sized objects (relative to the size of the moonpool) change the fluid motions in the moonpool to a large extent; the overall wave elevation amplitude is strongly reduced and the resonance period is altered. A consequence of this is that there is a large discrepancy between the hydrodynamic forces acting on the objects measured in the experiments and the forces calculated using the traditional approach. The PIV results showed the formation of vortices at the inlet of the moonpool and at the edges of the objects, which is the main source of non-linear damping of the wave elevation inside a moonpool.

High Resolution PIV and CH-PLIF Measurements and Analysis of a Shear Layer Stabilized Flame

2015 ◽  
Author(s):  
C. W. Foley ◽  
I. Chterev ◽  
J. Seitzman ◽  
T. Lieuwen
Keyword(s):  
High Resolution ◽  
Shear Layer ◽  
Flow Velocity ◽  
Near Field ◽  
Flame Stabilization ◽  
Flow Conditions ◽  
Mean Flow ◽  
Local Flow ◽  
High Bulk ◽  
Flow Velocities

Understanding the mechanisms and physics of flame stabilization and blowoff of premixed flames is critical towards the design of high velocity combustion devices. In the high bulk flow velocity situation typical of practical combustors, the flame anchors in shear layers where the local flow velocities are much lower. Within the shear layer, fluid strain deformation rates are very high and the flame can be subjected to significant stretch levels. The main goal of this work was to characterize the flow and stretch conditions that a premixed flame experiences in a practical combustor geometry and to compare these values to calculated extinction values. High resolution, simultaneous PIV and CH-PLIF measurements are used to capture the flame edge and near-field stabilization region. When approaching lean limit extinction conditions, we note characteristic changes in the stretch and flow conditions experienced by the flame. Most notably, the flame becomes less critically stretched when fuel/air ratio is decreased. However, at these lean conditions, the flame is subject to higher mean flow velocities at the edge, suggesting less favorable flow conditions are present at the attachment point of the flame as blowoff is approached. These measurements suggest that blowoff of the flame from the shear layer is not directly stretch extinction induced, but rather the result of an imbalance between the speed of the flame edge and local tangential flow velocity.

scholarly journals A unified multiscale vision of behavioral crowds

2019 ◽  
Vol 30 (01) ◽  
pp. 1-22 ◽  
Author(s):  
Bouchra Aylaj ◽  
Nicola Bellomo ◽  
Livio Gibelli ◽  
Alessandro Reali
Keyword(s):  
Computational Model ◽  
Complex Interaction ◽  
Emotional States ◽  
Complex Environments ◽  
Flow Conditions ◽  
Unified Framework ◽  
Hydrodynamic Models ◽  
Local Flow ◽  
Microscopic Scale ◽  
Modeling Principles

This paper proposes a multiscale vision to human crowds which provides a consistent description at the three possible modeling scales, namely, microscopic, mesoscopic, and macroscopic. The proposed approach moves from interactions at the microscopic scale and shows how the same modeling principles lead to kinetic and hydrodynamic models. Hence, a unified framework is developed which permits to derive models at each scale using the same principles and similar parameters. This approach can be used to simulate crowd dynamics in complex environments composed of interconnected areas, where the most appropriate scale of description can be selected for each area. This offers a pathway to the development of a multiscale computational model which has the capability to optimize the granularity of the description depending on the pedestrian local flow conditions. An important feature of the modeling at each scale is that the complex interaction between emotional states of walkers and their motion is taken into account.

scholarly journals Large eddy simulations in 2030 and beyond

2014 ◽  
Vol 372 (2022) ◽  
pp. 20130320 ◽  
Author(s):  
U Piomelli
Keyword(s):  
Stokes Equations ◽  
Academic Community ◽  
Large Eddy Simulations ◽  
Navier Stokes ◽  
Academic Environment ◽  
Flow Conditions ◽  
Local Flow ◽  
Navier Stokes Equations ◽  
The Past ◽  
Large Eddy

Since its introduction, in the early 1970s, large eddy simulations (LES) have advanced considerably, and their application is transitioning from the academic environment to industry. Several landmark developments can be identified over the past 40 years, such as the wall-resolved simulations of wall-bounded flows, the development of advanced models for the unresolved scales that adapt to the local flow conditions and the hybridization of LES with the solution of the Reynolds-averaged Navier–Stokes equations. Thanks to these advancements, LES is now in widespread use in the academic community and is an option available in most commercial flow-solvers. This paper will try to predict what algorithmic and modelling advancements are needed to make it even more robust and inexpensive, and which areas show the most promise.

Broadband liner impedance eduction for multimodal acoustic propagation in the presence of a mean flow

2016 ◽  
Vol 11 (2) ◽  
pp. 150-155
Author(s):  
R. Troian ◽  
D. Dragna ◽  
C. Bailly ◽  
M.-A. Galland
Keyword(s):  
Numerical Model ◽  
Flow Characteristics ◽  
Acoustic Propagation ◽  
Rational Fraction ◽  
Physical Parameters ◽  
Mean Flow ◽  
Linearized Euler Equations ◽  
Grazing Flow ◽  
Propagation Medium ◽  
Difference Time

Modeling of acoustic propagation in a duct with absorbing treatment is considered. The surface impedance of the treatment is sought in the form of a rational fraction. The numerical model is based on a resolution of the linearized Euler equations by finite difference time domain for the calculation of the acoustic propagation under a grazing flow. Sensitivity analysis of the considered numerical model is performed. The uncertainty of the physical parameters is taken into account to determine the most influential input parameters. The robustness of the solution vis-a-vis changes of the flow characteristics and the propagation medium is studied.

scholarly journals Experimental study on the mean flow characteristics of a supersonic multiple jet configuration

2021 ◽  
Vol 108 ◽  
pp. 106377
Author(s):  
Mohammed Faheem ◽  
Aqib Khan ◽  
Rakesh Kumar ◽  
Sher Afghan Khan ◽  
Waqar Asrar ◽  
...  
Keyword(s):  
Experimental Study ◽  
Flow Characteristics ◽  
Mean Flow ◽  
The Mean

scholarly journals Quantifying the Effects of Wave—Current Interactions on Tidal Energy Resource at Sites in the English Channel Using Coupled Numerical Simulations

Energies ◽  
2021 ◽  
Vol 14 (12) ◽  
pp. 3625
Author(s):  
Jon Hardwick ◽  
Ed B. L. Mackay ◽  
Ian G. C. Ashton ◽  
Helen C. M. Smith ◽  
Philipp R. Thies
Keyword(s):  
Wave Model ◽  
Tidal Energy ◽  
English Channel ◽  
Flow Conditions ◽  
Mean Flow ◽  
Radiation Stress ◽  
Large Wave ◽  
Stress Gradients ◽  
The Mean ◽  
Flow Power

Numerical modeling of currents and waves is used throughout the marine energy industry for resource assessment. This study compared the output of numerical flow simulations run both as a standalone model and as a two-way coupled wave–current simulation. A regional coupled flow-wave model was established covering the English Channel using the Delft D-Flow 2D model coupled with a SWAN spectral wave model. Outputs were analyzed at three tidal energy sites: Alderney Race, Big Roussel (Guernsey), and PTEC (Isle of Wight). The difference in the power in the tidal flow between coupled and standalone model runs was strongly correlated to the relative direction of the waves and currents. The net difference between the coupled and standalone runs was less than 2.5%. However, when wave and current directions were aligned, the mean flow power was increased by up to 7%, whereas, when the directions were opposed, the mean flow power was reduced by as much as 9.6%. The D-Flow Flexible Mesh model incorporates the effects of waves into the flow calculations in three areas: Stokes drift, forcing by radiation stress gradients, and enhancement of the bed shear stress. Each of these mechanisms is discussed. Forcing from radiation stress gradients is shown to be the dominant mechanism affecting the flow conditions at the sites considered, primarily caused by dissipation of wave energy due to white-capping. Wave action is an important consideration at tidal energy sites. Although the net impact on the flow power was found to be small for the present sites, the effect is site specific and may be significant at sites with large wave exposure or strong asymmetry in the flow conditions and should thus be considered for detailed resource and engineering assessments.

Comparison of LBM and FVM in the estimation of LAD stenosis

2021 ◽  
pp. 095441192110169
Author(s):  
Jian Liu ◽  
Yong Yu ◽  
Chenqi Zhu ◽  
Yu Zhang
Keyword(s):  
Stokes Equations ◽  
Flow Simulation ◽  
Diagnostic Methods ◽  
Computational Time ◽  
Speed Ratio ◽  
Maximum Speed ◽  
Fractional Flow ◽  
Local Flow ◽  
Non Invasive ◽  
Flow Reserve

The finite volume method (FVM)-based computational fluid dynamics (CFD) technology has been applied in the non-invasive diagnosis of coronary artery stenosis. Nonetheless, FVM is a time-consuming process. In addition to FVM, the lattice Boltzmann method (LBM) is used in fluid flow simulation. Unlike FVM solving the Navier–Stokes equations, LBM directly solves the simplified Boltzmann equation, thus saving computational time. In this study, 12 patients with left anterior descending (LAD) stenosis, diagnosed by CTA, are analysed using FVM and LBM. The velocities, pressures, and wall shear stress (WSS) predicted using FVM and LBM for each patient is compared. In particular, the ratio of the average and maximum speed at the stenotic part characterising the degree of stenosis is compared. Finally, the golden standard of LAD stenosis, invasive fractional flow reserve (FFR), is applied to justify the simulation results. Our results show that LBM and FVM are consistent in blood flow simulation. In the region with a high degree of stenosis, the local flow patterns in those two solvers are slightly different, resulting in minor differences in local WSS estimation and blood speed ratio estimation. Notably, these differences do not result in an inconsistent estimation. Comparison with invasive FFR shows that, in most cases, the non-invasive diagnosis is consistent with FFR measurements. However, in some cases, the non-invasive diagnosis either underestimates or overestimates the degree of stenosis. This deviation is caused by the difference between physiological and simulation conditions that remains the biggest challenge faced by all CFD-based non-invasive diagnostic methods.

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