scholarly journals Secondary currents induced mixing at channel confluences

2017 ◽  
Vol 44 (12) ◽  
pp. 1071-1083 ◽  
Author(s):  
Xue Chen ◽  
David Z. Zhu ◽  
Peter M. Steffler
Keyword(s):  
Three Dimensional ◽  
Flow Dynamics ◽  
Side Channel ◽  
Mixing Rate ◽  
Secondary Current ◽  
Secondary Currents ◽  
Secondary Circulations ◽  
Mixing Rates ◽  
Large Discharge ◽  
Channel Confluence

Channel confluence is a common feature in river systems. The flow dynamics associated with channel confluence are highly three-dimensional with strong flow circulations and secondary currents and can result in enhanced river mixing downstream. In this study, a three-dimensional numerical model was employed to estimate the secondary currents induced streamline curvatures and the resulting mixing rate at channel confluences with different junction angles and discharge ratios. The results show that while twin secondary circulations are found at channel confluence, their contribution to the mixing depends on their local positions with respect to the river streams. With the secondary current growing downstream, the mixing rate is accelerated, in particular for the cases with the side channel perpendicular to the main channel and having a relatively large discharge. Turbulent diffusion can contribute up to about half of the rapid mixing. The mixing rates for different simulation cases are examined.

scholarly journals Vertical Mixing and Heat Fluxes Conditioned by a Seismically Imaged Oceanic Front

2021 ◽  
Vol 8 ◽  
Author(s):  
Kathryn L. Gunn ◽  
Alex Dickinson ◽  
Nicky J. White ◽  
Colm-cille P. Caulfield
Keyword(s):  
Water Mass ◽  
Vertical Mixing ◽  
Three Dimensional ◽  
Heat Fluxes ◽  
Water Masses ◽  
Mixing Rate ◽  
Southwest Atlantic ◽  
Cross Sectional ◽  
Mean Values ◽  
Mixing Rates

The southwest Atlantic gyre connects several distinct water masses, which means that this oceanic region is characterized by a complex frontal system and enhanced water mass modification. Despite its significance, the distribution and variability of vertical mixing rates have yet to be determined for this system. Specifically, potential conditioning of mixing rates by frontal structures, in this location and elsewhere, is poorly understood. Here, we analyze vertical seismic (i.e., acoustic) sections from a three-dimensional survey that straddles a major front along the northern portion of the Brazil-Falkland Confluence. Hydrographic analyses constrain the structure and properties of water masses. By spectrally analyzing seismic reflectivity, we calculate spatial and temporal distributions of the dissipation rate of turbulent kinetic energy, ε, of diapycnal mixing rate, K, and of vertical diffusive heat flux, FH. We show that estimates of ε, K, and FH are elevated compared to regional and global mean values. Notably, cross-sectional mean estimates vary little over a 6 week period whilst smaller scale thermohaline structures appear to have a spatially localized effect upon ε, K, and FH. In contrast, a mesoscale front modifies ε and K to a depth of 1 km, across a region of O(100) km. This front clearly enhances mixing rates, both adjacent to its surface outcrop and beneath the mixed layer, whilst also locally suppressing ε and K to a depth of 1 km. As a result, estimates of FH increase by a factor of two in the vicinity of the surface outcrop of the front. Our results yield estimates of ε, K and FH that can be attributed to identifiable thermohaline structures and they show that fronts can play a significant role in water mass modification to depths of 1 km.

Three Dimensional Numerical Simulation of Vortex Structures in Barak River

2015 ◽  
Vol 772 ◽  
pp. 120-124
Author(s):  
S. Kiran ◽  
Upendra Kumar ◽  
Amit Kumar Dey
Keyword(s):  
Turbulence Model ◽  
Three Dimensional ◽  
Free Water ◽  
Velocity Profiles ◽  
Navier Stokes ◽  
Navier Stokes Equation ◽  
Secondary Current ◽  
Secondary Currents ◽  
Outer Bank ◽  
Barak River

Natural channel have complex three dimensional flow structures particularly at the outer bank cell due to the combined effects of secondary currents and higher velocity profiles. In this paper Computational fluid dynamics is used to study the meandering bend of the Barak River. Numerical modeling is done using Reynolds averaged continuity and Navier Stokes equation. These equations are solved by finite volume method. Appropriate representation of counter-rotating secondary flow in the channel bend requires both the suitable treatment of the free water surface and a turbulence model that can resolve the anisotropy of turbulence. Hence the volume of fluid method (VOF) was used to model the free surface and reynolds stress turbulence model (RSM) has been used to close the RANS equations. Higher velocity profiles were prominent at the outer bank. Skew induced stream wise vorticity was observed close to the outer bank which confirms the existence of corner induced secondary current. The vortices formed were found to be of Prandtl’s first kind.

Effects of Scalloping on the Mixing Mechanisms of Forced Mixers With Highly Swirling Core Flow

2013 ◽  
Vol 135 (7) ◽  
Author(s):  
Alex Wright ◽  
Zhijun Lei ◽  
Ali Mahallati ◽  
Mark Cunningham ◽  
Julio Militzer
Keyword(s):  
Separation Bubble ◽  
Three Dimensional ◽  
Navier Stokes ◽  
Suction Surface ◽  
Mixing Rate ◽  
Reduced Pressure ◽  
Core Flow ◽  
Flow Swirl ◽  
Swirl Angle ◽  
Mixing Rates

This paper presents a detailed experimental and computational investigation of the effects of scalloping on the mixing mechanisms of a scaled 12-lobe turbofan mixer. Scalloping was achieved by eliminating approximately 70% of the lobe sidewall area. Measurements were made downstream of the mixer in a co-annular wind tunnel, and the simulations were carried out using an unstructured Reynolds averaged Navier–Stokes (RANS) solver, Numeca FINE/Hexa, with k-ω SST model. In the core flow, the swirl angle was varied from 0 deg to 30 deg. At high swirl angles, a three-dimensional separation bubble was formed on the lobe's suction surface penetration region and resulted in the generation of a vortex at the lobe valley. The valley vortex quickly dissipated downstream. The mixer lobes removed most of the swirl, but scalloped lobes removed less swirl in the region of the scalloped notch. The residual swirl downstream of the scalloped mixer interacted with the vortices and improved mixing rates compared to the unscalloped mixer. Core flow swirl up to 10 deg provided improved mixing rates and reduced pressure and thrust losses for both mixers. As core flow swirl increased beyond 10 deg, the mixing rate continued to improve, but pressure and thrust losses declined compared to the zero swirl case. Lobe scalloping, in high swirl conditions, resulted in better mixing and improved pressure loss over the unscalloped mixer but at the expense of reduced thrust.

Methods for Enhanced Turbulence Mixing in Supersonic Shear Flows

1994 ◽  
Vol 47 (6S) ◽  
pp. S188-S192 ◽  
Author(s):  
E. J. Gutmark ◽  
K. C. Schadow ◽  
K. H. Yu
Keyword(s):  
Compressible Flows ◽  
Three Dimensional ◽  
Shear Layers ◽  
Mixing Rate ◽  
Planar Shear ◽  
Wide Range ◽  
Turbulence Mixing ◽  
Mixing Rates ◽  
Performance Penalty ◽  
Supersonic Shear Layers

Supersonic shear layers have inherent low mixing rates due to compressibility effects. Their mixing rate relative to subsonic shear layers can be up to 5 times lower. Several important technological applications which require intense mixing of supersonic flows gave impetus to research aimed to develop methods to enhance mixing in compressible flows with minimal performance penalty. This paper reviews some of these methods applied to both planar shear layers and jets. The methods are arranged in several categories: passive and active control of shear layer instabilities, three dimensional jets, generation of axial vorticity and shock interaction with shear layers. The paper concludes by discussing the importance of the wide range of length-scales in which turbulent mixing occurs.

Plane turbulent jets in a bounded fluid layer

1992 ◽  
Vol 241 ◽  
pp. 587-614 ◽  
Author(s):  
T. Dracos ◽  
M. Giger ◽  
G. H. Jirka
Keyword(s):  
Experimental Investigation ◽  
Cross Section ◽  
Fluid Layer ◽  
Three Dimensional ◽  
Turbulent Jets ◽  
Two Dimensional ◽  
Vortical Structures ◽  
Secondary Currents ◽  
Fluid Layers ◽  
Bounding Surfaces

An experimental investigation of plane turbulent jets in bounded fluid layers is presented. The development of the jet is regular up to a distance from the orifice of approximately twice the depth of the fluid layer. From there on to a distance of about ten times the depth, the flow is dominated by secondary currents. The velocity distribution over a cross-section of the jet becomes three-dimensional and the jet undergoes a constriction in the midplane and a widening near the bounding surfaces. Beyond a distance of approximately ten times the depth of the bounded fluid layer the secondary currents disappear and the jet starts to meander around its centreplane. Large vortical structures develop with axes perpendicular to the bounding surfaces of the fluid layer. With increasing distance the size of these structures increases by pairing. These features of the jet are associated with the development of quasi two-dimensional turbulence. It is shown that the secondary currents and the meandering do not significantly affect the spreading of the jet. The quasi-two-dimensional turbulence, however, developing in the meandering jet, significantly influences the mixing of entrained fluid.

Flow dynamics and enhanced mixing in a converging–diverging channel

2016 ◽  
Vol 807 ◽  
pp. 167-204 ◽  
Author(s):  
S. W. Gepner ◽  
J. M. Floryan
Keyword(s):  
Three Dimensional ◽  
Travelling Wave ◽  
Flow Dynamics ◽  
Streamwise Vortices ◽  
Dimensional Model ◽  
Three Dimensional Model ◽  
Wave Instability ◽  
Unsteady Separation ◽  
Flow Stabilization ◽  
Diverging Channel

An analysis of flows in converging–diverging channels has been carried out with the primary goal of identifying geometries which result in increased mixing. The model geometry consists of a channel whose walls are fitted with spanwise grooves of moderate amplitudes (up to 10 % of the mean channel opening) and of sinusoidal shape. The groove systems on each wall are shifted by half of a wavelength with respect to each other, resulting in the formation of a converging–diverging conduit. The analysis is carried out up to Reynolds numbers resulting in the formation of secondary states. The first part of the analysis is based on a two-dimensional model and demonstrates that increasing the corrugation wavelength results in the appearance of an unsteady separation whose onset correlates with the onset of the travelling wave instability. The second part of the analysis is based on a three-dimensional model and demonstrates that the flow dynamics is dominated by the centrifugal instability over a large range of geometric parameters, resulting in the formation of streamwise vortices. It is shown that the onset of the vortices may lead to the elimination of the unsteady separation. The critical Reynolds number for the vortex onset initially decreases as the corrugation amplitude increases but an excessive increase leads to the stream lift up, reduction of the centrifugal forces and flow stabilization. The flow dynamics under such conditions is again dominated by the travelling wave instability. Conditions leading to the formation of streamwise vortices without interference from the travelling wave instability have been identified. The structure and the mixing properties of the saturated states are discussed.

scholarly journals Quantifying blood flow dynamics during cardiac development: demystifying computational methods

2018 ◽  
Vol 373 (1759) ◽  
pp. 20170330 ◽  
Author(s):  
Katherine Courchaine ◽  
Sandra Rugonyi
Keyword(s):  
Blood Flow ◽  
Congenital Heart ◽  
Cardiac Development ◽  
Three Dimensional ◽  
Flow Dynamics ◽  
Cardiovascular Development ◽  
Flow Conditions ◽  
Congenital Heart Malformations ◽  
Blood Flow Dynamics ◽  
Altered Blood

Blood flow conditions (haemodynamics) are crucial for proper cardiovascular development. Indeed, blood flow induces biomechanical adaptations and mechanotransduction signalling that influence cardiovascular growth and development during embryonic stages and beyond. Altered blood flow conditions are a hallmark of congenital heart disease, and disrupted blood flow at early embryonic stages is known to lead to congenital heart malformations. In spite of this, many of the mechanisms by which blood flow mechanics affect cardiovascular development remain unknown. This is due in part to the challenges involved in quantifying blood flow dynamics and the forces exerted by blood flow on developing cardiovascular tissues. Recent technologies, however, have allowed precise measurement of blood flow parameters and cardiovascular geometry even at early embryonic stages. Combined with computational fluid dynamics techniques, it is possible to quantify haemodynamic parameters and their changes over development, which is a crucial step in the quest for understanding the role of mechanical cues on heart and vascular formation. This study summarizes some fundamental aspects of modelling blood flow dynamics, with a focus on three-dimensional modelling techniques, and discusses relevant studies that are revealing the details of blood flow and their influence on cardiovascular development. This article is part of the Theo Murphy meeting issue ‘Mechanics of development’.

Geometry Effects in Eulerian/Granular Simulation of a Turbulent FCC Riser with a (kg-?g)-KTGF Model

2010 ◽  
Vol 8 (1) ◽  
Author(s):  
Hamid Reza Nazif ◽  
Hassan Basirat Tabrizi ◽  
Farhad A Farhadpour
Keyword(s):  
Cross Section ◽  
Large Scale ◽  
Rectangular Cross Section ◽  
Three Dimensional ◽  
Circular Cross Section ◽  
Good Prediction ◽  
Model Predictions ◽  
Secondary Circulations ◽  
Sharp Corners ◽  
Granular Simulation

Three-dimensional, transient turbulent particulate flow in an FCC riser is modeled using an Eulerian/Granular approach. The turbulence in the gas phase is described by a modified realizable (kg-?g) closure model and the kinetic theory of granular flow (KTGF) is employed for the particulate phase. Separate simulations are conducted for a rectangular and a cylindrical riser with similar dimensions. The model predictions are validated against experimental data of Sommerfeld et al (2002) and also compared with the previously reported LES-KTGF simulations of Hansen et al (2003) for the rectangular riser. The (kg-?g)-KTGF model does not perform as well as the LES-KTGF model for the riser with a rectangular cross section. This is because, unlike the more elaborate LES-KTGF model, the simpler (kg-?g)-KTGF model cannot capture the large scale secondary circulations induced by anisotropic turbulence at the corners of the rectangular riser. In the cylindrical geometry, however, the (kg-?g)-KTGF model gives good prediction of the data and is a viable alternative to the more complex LES-KTGF model. This is not surprising as the circulations in the riser with a circular cross section are due to the curvature of the walls and not due to the presence of sharp corners.

scholarly journals Modeling the Influence of the Penetration Channel’s Shape on Plasma Parameters When Handling Highly Concentrated Energy Sources

2017 ◽  
Vol 2017 ◽  
pp. 1-8
Author(s):  
Dmitriy N. Trushnikov ◽  
Ekaterina S. Salomatova ◽  
Igor I. Bezukladnikov ◽  
Igor L. Sinani ◽  
K. P. Karunakaran
Keyword(s):  
Electron Beam Welding ◽  
Gaussian Function ◽  
Three Dimensional ◽  
Energy Sources ◽  
Plasma Parameters ◽  
Statistical Probability ◽  
Secondary Current ◽  
Thermal Fields ◽  
Sustained Discharge ◽  
A Current

In our work to formulate a scientific justification for process control methods when processing materials using concentrated energy sources, we develop a model that can calculate plasma parameters and the magnitude of the secondary waveform of a current from a non-self-sustained discharge in plasma as a function of the geometry of the penetration channel, thermal fields, and the beam’s position within the penetration channel. We present the method and a numeric implementation whose first stage involves the use of a two-dimensional model to calculate the statistical probability of the secondary electrons’ passage through the penetration channel as a function of the interaction zone’s depth. Then, the discovered relationship is used to numerically calculate how the secondary current changes as a distributed beam moves along a three-dimensional penetration channel. We demonstrate that during oscillating electron beam welding the waveform has the greatest magnitude during interaction with the upper areas of the penetration channel and diminishes with increasing penetration channel depth in a way that depends on the penetration channel’s shape. When the surface of the penetration channel is approximated with a Gaussian function, the waveform decreases nearly exponentially.

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