Vortical Structures and Mixing Characteristics of Flow in Randomly Packed Porous Media During Transition to Turbulence

2021 ◽  
Author(s):  
Reza M. Ziazi ◽  
James A. Liburdy
Keyword(s):  
Porous Media ◽  
Scale Effects ◽  
Turbulent Transport ◽  
Flow Characteristics ◽  
Transition Process ◽  
Industrial Applications ◽  
Transition To Turbulence ◽  
Vortical Structures ◽  
Macro Scale ◽  
Mixing Characteristics

Abstract Transition to turbulence in randomly arranged porous media is observed in nature and industrial applications. The flow characteristics of these flows during transition are not well identified. This work describes the parameters influencing on overall mixing during the transition process from the perspective of scale of vortical structures and dispersion characteristics by addressing the following questions: (a) what are the dominant mechanisms evolution of scale of vortices, and (b) how does the inertial effects of vortical structures enhance the flow transport properties through tortuosity and dispersion. Time-resolved PIV is used to investigate the flow in the macro-scale Reynolds numbers from 100 to 1000 to show the pore- versus macro-scale effects on the scale of the flow dispersion, and their contribution in interpreting the overall flow mixing. Lagrangian mixing characteristics based on Eulerian local pore velocity variances is used to demonstrate the bed characteristics for flow in randomly distributed porous media flows. The dispersion asymptotically approaches 0.085 % of VintDH longitudinally which shows the turbulent transport is increased by enhancing the Reynolds number that matches very well with the literature.

The effect of small-amplitude convective disturbances on the size and bursting of a laminar separation bubble

2011 ◽  
Vol 671 ◽  
pp. 1-33 ◽  
Author(s):  
OLAF MARXEN ◽  
DAN S. HENNINGSON
Keyword(s):  
Large Scale ◽  
Separation Bubble ◽  
Transition Process ◽  
Transition To Turbulence ◽  
Laminar Separation Bubble ◽  
Mean Flow ◽  
Fixed Frequency ◽  
Laminar Separation ◽  
Vortical Structures ◽  
Turbulent Transition

Short laminar separation bubbles can develop on a flat plate due to an externally imposed pressure gradient. Here, these bubbles are computed by means of direct numerical simulations. Laminar–turbulent transition occurs in the bubble, triggered by small disturbance input with fixed frequency, but varying amplitude, to keep the bubbles short. The forcing amplitudes span a range of two orders of magnitude. All resulting bubbles differ with respect to their mean flow, linear-stability characteristics and distance between transition and mean reattachment locations. Mechanisms responsible for these differences are analysed in detail. Switching off the disturbance input or reducing it below a certain, very small threshold causes the short bubble to grow continuously. Eventually, it no longer exhibits typical characteristics of a short laminar separation bubble. Instead, it is argued that bursting has occurred and the bubble displays characteristics of a long-bubble state, even though this state was not a statistically steady state. This hypothesis is backed by a comparison of numerical results with measurements. For long bubbles, the transition to turbulence is not able to reattach the flow immediately. This effect can lead to the bursting of a short bubble, which remains short only when sufficiently large disturbances are convected into the bubble. Large-scale spanwise-oriented vortices at transition are observed for short but not for long bubbles. The failure of the transition process to reattach the flow in the long-bubble case is ascribed to this difference in transitional vortical structures.

scholarly journals Comparison of Macro-Scale Porosity Implementations for CFD Modelling of Wave Interaction with Thin Porous Structures

2021 ◽  
Vol 9 (2) ◽  
pp. 150
Author(s):  
Anna Feichtner ◽  
Ed Mackay ◽  
Gavin Tabor ◽  
Philipp R. Thies ◽  
Lars Johanning
Keyword(s):  
Porous Media ◽  
Large Scale ◽  
Scale Effects ◽  
Computational Cost ◽  
Wave Interaction ◽  
Volume Of Fluid ◽  
Pressure Jump ◽  
Porous Structures ◽  
Cfd Modelling ◽  
Macro Scale

Computational fluid dynamics (CFD) modelling of wave interaction with thin perforated structures is of interest in a range of engineering applications. When large-scale effects such as forces and the overall flow behaviour are of interest, a microstructural resolution of the perforated geometry can be excessive or prohibitive in terms of computational cost. More efficiently, a thin porous structure can be represented by its macro-scale effects by means of a quadratic momentum source or pressure-drop respectively. In the context of regular wave interaction with thin porous structures and within an incompressible, two-phase Navier–Stokes and volume-of-fluid framework (based on interFoam of OpenFOAM®), this work investigates porosity representation as a porous surface with a pressure-jump condition and as volumetric isotropic and anisotropic porous media. Potential differences between these three types of macro-scale porosity implementations are assessed in terms of qualitative flow visualizations, velocity profiles along the water column, the wave elevation near the structures and the horizontal force on the structures. The comparison shows that all three types of implementation are capable of reproducing large-scale effects of the wave-structure interaction and that the differences between all obtained results are relatively small. It was found that the isotropic porous media implementation is numerically the most stable and requires the shortest computation times. The pressure-jump implementation requires the smallest time steps for stability and thus the longest computation times. This is likely due to the spurious local velocities at the air-water interface as a result of the volume-of-fluid interface capturing method combined with interFoam’s segregated pressure-velocity coupling algorithm. This paper provides useful insights and recommendations for effective macro-scale modelling of thin porous structures.

Mass Transport Modelling in Porous Media Using Delay Differential Equations

2006 ◽  
Vol 258-260 ◽  
pp. 586-591
Author(s):  
António Martins ◽  
Paulo Laranjeira ◽  
Madalena Dias ◽  
José Lopes
Keyword(s):  
Porous Media ◽  
Differential Equations ◽  
Mass Transport ◽  
Delay Differential Equations ◽  
Dispersion Model ◽  
Flow Characteristics ◽  
Convective Transport ◽  
Transport Modelling ◽  
Flow Field Characteristics ◽  
Delay Differential

In this work the application of delay differential equations to the modelling of mass transport in porous media, where the convective transport of mass, is presented and discussed. The differences and advantages when compared with the Dispersion Model are highlighted. Using simplified models of the local structure of a porous media, in particular a network model made up by combining two different types of network elements, channels and chambers, the mass transport under transient conditions is described and related to the local geometrical characteristics. The delay differential equations system that describe the flow, arise from the combination of the mass balance equations for both the network elements, and after taking into account their flow characteristics. The solution is obtained using a time marching method, and the results show that the model is capable of describing the qualitative behaviour observed experimentally, allowing the analysis of the influence of the local geometrical and flow field characteristics on the mass transport.

scholarly journals Study of an array of two circular jets impinging on a flat surface

2018 ◽  
Vol 32 ◽  
pp. 01021
Author(s):  
Ştefan-Mugur Simionescu ◽  
Nilesh Dhondoo ◽  
Corneliu Bălan
Keyword(s):  
High Speed ◽  
Solid Wall ◽  
Flow Characteristics ◽  
Navier Stokes ◽  
Vortical Structures ◽  
Air Jets ◽  
Circular Jets ◽  
2D Flow ◽  
Rans Modeling ◽  
Two Jets

In this study, the flow characteristics of an array of two circular, laminar air jets impinging on a smooth solid wall are experimentally and numerically investigated. Direct visualizations using high speed/resolution camera are performed. The evolution of the vortical structures in the area where the jet is deflected from axial to radial direction is emphasized, as well as the interaction between the two jets. A set of CFD numerical simulations in 2D flow domains are performed by using the commercial software Fluent, in the context of Reynolds-averaged Navier-Stokes (RANS) modeling. The numerical resultsare compared and validated with the experiments. The vorticity number is computed and plotted at two different positions from the jet nozzle, and a study of its distribution gives a clue on how the jets are interacting with each other in the proximity of the solid wall.

Experimental and Numerical Investigations on Flow Characteristics of the KVLCC2 at 30° Drift Angl

2015 ◽  
Author(s):  
Moustafa Abdel-Maksoud ◽  
Volker Müller ◽  
Tao Xing ◽  
Serge Toxopeus ◽  
Frederick Stern ◽  
...  
Keyword(s):  
Numerical Methods ◽  
Turbulence Modeling ◽  
Flow Characteristics ◽  
Flow Structures ◽  
Local Flow ◽  
Drift Angle ◽  
Vortical Structures ◽  
Ship Model ◽  
University Of Technology ◽  
Global And Local

Investigations of flow characteristics around ship hulls at large drift angle are very important for understanding the motion behavior of ships during maneuvers. At large drift angles, the flow is dominated by strong vortical structures and complex three-dimensional separations. An accurate prediction of these flow structures is still a challenge for modern computational fluid dynamics (CFD) solvers. Hull forms with high block coefficients are blunt and have strong curvatures, which leads to large area flow separations over smooth surfaces. These areas are sensitive to the relative angle between the flow and the ship motion direction. The paper is concerned with a collaborative computational study of the flow behavior around a double model of KVLCC2 at 30 degrees drift angle and Fr=0 condition, including analysis of numerical methods, turbulence modeling and grid resolution, and their effects on the mean flow and separation onset as well as formation of the vortical structures. This research is an outcome of a multi-year collaboration of five research partners from four countries. The overall approach adopted for the present study combines the advantages of CFD and EFD with the ultimate goal of capturing the salient details of the flow around the bluff hull form. The experiments were performed at the low - speed wind tunnel of the Hamburg University of Technology (TUHH). The main features of the global and local flow were captured in the experimental study. To determine the global flow characteristics, two different flow visualization techniques were used. The first one is a smoke test, which allows the visualization of vortex structures in vicinity of the ship model. The second test is a classic oil film method, which yields the direction of the limiting wall streamlines on the surface of the model. The analysis of the experimental results helped identify the separation zones on the ship model. To resolve the local flow-fields, LDA and PIV measurements were carried out in a selected number of measuring sections. Subsequently, the EFD and CFD results for the global and local flow structures were compared and analyzed. The numerical simulations were carried out by 5 institutions: Iowa Institute of Hydraulic Research of the University of Iowa (IIHR), USA, Maritime Research Institute Netherlands (MARIN), The Netherlands, Hamburg University of Technology (TUHH), Germany, Naval Surface Warfare Center, Carderock Division (NSWCCD) West Bethesda, USA and Swedish Defense Research Agency (FOI), Sweden. For the comparison with the experimental results, seven submissions of steady and unsteady CFD results are included in the present study. The participating codes include CFDShip-Iowa, ReFRESCO, FreSCo+, Edge, OpenFOAM (FOI) and NavyFoam. The size of the computational grids varies between 11 and 202 million control volumes or nodes. The influence of turbulence modeling on the predicted flow is studied by a wide variety of models such as isotropic eddy viscosity models of k-w family, Explicit Algebraic Reynolds Stress Model (EARSM), hybrid RANS-LES (DES), and LES. Despite notable differences in the grid resolutions, numerical methods, and turbulence models, the global features of the flow are closely captured by the computations. Noticeable differences among the computations are found in the details of the local flow such as the vortex strength and the location and extent of the flow separations.

Modeling Two-Phase Flow in Porous Media Including Fluid-Fluid Interfacial Area

2008 ◽  
Author(s):  
Jennifer Niessner ◽  
S. Majid Hassanizadeh ◽  
Dustin Crandall
Keyword(s):  
Porous Media ◽  
Capillary Pressure ◽  
Two Phase Flow ◽  
Interfacial Area ◽  
Flow In Porous Media ◽  
Extended Model ◽  
Phase Flow ◽  
Two Phase ◽  
Specific Interfacial Area ◽  
Macro Scale

We present a new numerical model for macro-scale two-phase flow in porous media which is based on a physically consistent theory of multi-phase flow. The standard approach for modeling the flow of two fluid phases in a porous medium consists of a continuity equation for each phase, an extended form of Darcy’s law as well as constitutive relationships for relative permeability and capillary pressure. This approach is known to have a number of important shortcomings and, in particular, it does not account for the presence and role of fluid–fluid interfaces. An alternative is to use an extended model which is founded on thermodynamic principles and is physically consistent. In addition to the standard equations, the model uses a balance equation for specific interfacial area. The constitutive relationship for capillary pressure involves not only saturation, but also specific interfacial area. We show how parameters can be obtained for the alternative model using experimental data from a new kind of flow cell and present results of a numerical modeling study.

scholarly journals Instability and Transition of a Boundary Layer over a Backward-Facing Step

Fluids ◽  
2022 ◽  
Vol 7 (1) ◽  
pp. 35
Author(s):  
Ming Teng ◽  
Ugo Piomelli
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Velocity Distribution ◽  
Adverse Pressure Gradient ◽  
Transition Process ◽  
Transition To Turbulence ◽  
Recirculation Region ◽  
Backward Facing Step ◽  
Freestream Velocity ◽  
Displacement Thickness

The development of secondary instabilities in a boundary layer over a backward-facing step is investigated numerically. Two step heights are considered, h/δo*=0.5 and 1.0 (where δo* is the displacement thickness at the step location), in addition to a reference flat-plate case. A case with a realistic freestream-velocity distribution is also examined. A controlled K-type transition is initiated using a narrow ribbon upstream of the step, which generates small and monochromatic perturbations by periodic blowing and suction. A well-resolved direct numerical simulation is performed. The step height and the imposed freestream-velocity distribution exert a significant influence on the transition process. The results for the h/δo*=1.0 case exhibit a rapid transition primarily due to the Kelvin–Helmholtz instability downstream of step; non-linear interactions already occur within the recirculation region, and the initial symmetry and periodicity of the flow are lost by the middle stage of transition. In contrast, case h/δo*=0.5 presents a transition road map in which transition occurs far downstream of the step, and the flow remains spatially symmetric and temporally periodic until the late stage of transition. A realistic freestream-velocity distribution (which induces an adverse pressure gradient) advances the onset of transition to turbulence, but does not fundamentally modify the flow features observed in the zero-pressure gradient case. Considering the budgets of the perturbation kinetic energy, both the step and the induced pressure-gradient increase, rather than modify, the energy transfer.

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