scholarly journals A Theoretical Investigation of Flow Topologies in Bubble- and Droplet-Affected Flows

Fluids ◽  
2019 ◽  
Vol 4 (3) ◽  
pp. 117 ◽  
Author(s):  
Josef Hasslberger ◽  
Svenja Marten ◽  
Markus Klein
Keyword(s):  
Boundary Condition ◽  
Boundary Conditions ◽  
Incompressible Flows ◽  
Near Field ◽  
Creeping Flow ◽  
Focal Region ◽  
Field Boundary ◽  
Flow Topology ◽  
Local Flow ◽  
Topology Analysis

A local flow topology analysis was conducted for laminar particle-affected flows. Based on the invariants of the velocity gradient tensor, all possible flow structures can be categorized into two focal and two nodal topologies for incompressible flows. The underlying field descriptions for bubble- and droplet-affected flows in the creeping flow regime were determined analytically for two different boundary conditions. A nodal-to-focal-to-nodal transition can be observed in both phases and the focal topologies are predominant in the interior phase. It was also found that the topology distribution in the interior phase is independent of the dynamic viscosity ratio and the boundary conditions, which is not the case in the exterior phase. The focal region in the exterior phase extends to infinity for the far-field boundary condition, whereas it is bounded to a tire-like zone attached to the bubble or droplet for the near-field boundary condition. Furthermore, the existence of a narrow band of intermediate nodal topologies was demonstrated analytically, which raises the question on the origin of this behavior. To complement the findings about the flow topology classification, the strengths of the underlying vorticity and invariant fields are discussed, including their dependency on the considered phase and boundary condition.

scholarly journals Flow topologies in primary atomization of liquid jets: a direct numerical simulation analysis

2018 ◽  
Vol 859 ◽  
pp. 819-838 ◽  
Author(s):  
Josef Hasslberger ◽  
Sebastian Ketterl ◽  
Markus Klein ◽  
Nilanjan Chakraborty
Keyword(s):  
Numerical Simulation ◽  
Reynolds Number ◽  
Direct Numerical Simulation ◽  
Incompressible Flows ◽  
Flow Structures ◽  
Liquid Jets ◽  
Small Scale ◽  
Flow Topology ◽  
Local Flow ◽  
Turbulent Length Scale

The local flow topology analysis of the primary atomization of liquid jets has been conducted using the invariants of the velocity-gradient tensor. All possible small-scale flow structures are categorized into two focal and two nodal topologies for incompressible flows in both liquid and gaseous phases. The underlying direct numerical simulation database was generated by the one-fluid formulation of the two-phase flow governing equations including a high-fidelity volume-of-fluid method for accurate interface propagation. The ratio of liquid-to-gas fluid properties corresponds to a diesel jet exhausting into air. Variation of the inflow-based Reynolds number as well as Weber number showed that both these non-dimensional numbers play a pivotal role in determining the nature of the jet break-up, but the flow topology behaviour appears to be dominated by the Reynolds number. Furthermore, the flow dynamics in the gaseous phase is generally less homogeneous than in the liquid phase because some flow regions resemble a laminar-to-turbulent transition state rather than fully developed turbulence. Two theoretical models are proposed to estimate the topology volume fractions and to describe the size distribution of the flow structures, respectively. In the latter case, a simple power law seems to be a reasonable approximation of the measured topology spectrum. According to that observation, only the integral turbulent length scale would be required as an input for the a priori prediction of the topology size spectrum.

Developing Consistent Inlet Thermal Boundary Condition in Micro/Nano Scale Channels With Heat Transfer

2008 ◽  
Author(s):  
Masoud Darbandi ◽  
Shidvash Vakilipour
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Boundary Conditions ◽  
Incompressible Flows ◽  
Thermal Boundary Condition ◽  
Constant Wall Temperature ◽  
Transfer Factors ◽  
The Real ◽  
Nano Scale ◽  
Inlet Boundary Condition

In this work, we present a more realistic inlet boundary condition to simulate compressible and incompressible flows through micro and nano channels considering consistent momentum and heat transfer specifications there. At solid walls, a constant wall temperature with suitable jump is applied as the wall thermal boundary condition; however, two types of thermal inlet boundary conditions are investigated at the inlet. We firstly examine the classical inlet boundary condition, which specifies a uniform temperature distribution right at the real inlet. Alternatively, we apply the same boundary condition but at a fictitious place far upstream of the real channel inlet. To validate our results, the results obtained after employing the former boundary conditions are evaluated against other available numerical results and Lattice Boltzmann solutions. In this validation, the friction factor and Nusselt number are treated as the most important hydrodynamics and heat transfer factors to be appraised. Next, we present the results after applying the second type of inlet boundary conditions and compare them with those of applying the first type of boundary condition. This study suggests an innovation in micro/nano scale heat transfer treatment close to the channel inlets.

scholarly journals FAR FIELD BOUNDARY CONDITIONS FOR INCOMPRESSIBLE FLOWS COMPUTATION

2018 ◽  
Vol 8 (3) ◽  
pp. 690-709
Author(s):  
Charles-Henri Bruneau ◽  
◽  
Sra Tancogne ◽  
Keyword(s):  
Boundary Conditions ◽  
Incompressible Flows ◽  
Far Field ◽  
Field Boundary

Numerical Simulation of the Flow in the Row Labyrinth Seal of an Axial Turbine Using a Low-Mach Preconditioning Technique

2016 ◽  
Author(s):  
Jens Fiedler ◽  
Anton Weber ◽  
Arianna Bosco ◽  
Karl Engel ◽  
Francesca di Mare
Keyword(s):  
Boundary Conditions ◽  
Incompressible Flows ◽  
Stokes Equations ◽  
Mass Conservation ◽  
Labyrinth Seal ◽  
Current Work ◽  
Low Flow ◽  
Navier Stokes ◽  
Flow Topology ◽  
Fully Implicit

Labyrinth seals on shrouded blades are an effective way for reducing efficiency penalties, as compared to free ended blades. Due to the difficulties of gaining optical access to cavity regions, mostly pressure measurements are available in the literature, from which the details of the flow must be inferred. The use of numerical tools can provide insight in the flow topology and therefore help obtaining a better understanding of the factors (geometric, thermodynamic and aerodynamic) which can affect the performance of the machine. Whilst in the main passage relatively high Mach numbers are to be found (0.3–1.3), the flow field in the cavities is dominated by extremely low flow speeds with strong recirculation patterns. The treatments of such flows, where large disparities between the acoustic and convective speed exist, are known to be highly problematic if density-based solution methods are employed. As the flow conditions approach the incompressibility limit a degradation of the convergence behaviour can be observed, leading, potentially to incorrect solutions. In order to overcome these problems preconditioning methods can be conveniently applied to the Navier-Stokes equations. In the current work the formulation of a fully implicit local preconditioning method with domain control of the Mach number dependency is presented. Numerical simulations of turbomachine components are generally performed on truncated domains. In order to prevent unphysical reflections at open boundaries and interfaces non-reflecting boundary conditions have been developed, e.g. [6, 8]. As reported in the available literature, low Mach preconditioning can cause stability problems and strongly impair the quality of the results especially in proximity of the domain’s boundaries. As shown in [14, 21] an appropriate scaling treatment of the boundary conditions is also required to alleviate such issues. In the current work non-reflecting boundary conditions, based on the formulation of Giles [6, 8], have been suitably modified to work reliably also in the limit of incompressible flows. To prove the robustness and accuracy of the algorithm implemented in the DLR’s CFD code TRACE, a canonical testcase representing an abstraction of the flow topology found in a labyrinth seal, such as a lid-driven cavity, is shown. Finally, the simulation of the steady flow in a multistage, shrouded low-pressure turbine is presented. For this, a classic RANS approach has been adopted using the k-ω model to illustrate the effectiveness of the developed method in a typical industrial application. Of particular significance and interest is the analysis of the mass conservation properties of the numerical scheme attained at mixing planes between rotor and stator and at non-matching grid interfaces, denoted as “zonal” and “zonal-mixed” interfaces.

Nonreciprocal Acoustics Using Programmable Boundary Conditions: From Boundary Control and Active Metamaterials to the Acoustic Diode

2017 ◽  
Author(s):  
Sami Karkar ◽  
Manuel Collet
Keyword(s):  
Boundary Condition ◽  
Boundary Conditions ◽  
Boundary Control ◽  
Near Field ◽  
Plane Waves ◽  
Acoustic Properties ◽  
Frequency Range ◽  
Active Components ◽  
Control Approach ◽  
Acoustic Diode

In this communication, we first introduce the concept of programmable boundary conditions, and then use it to design a nonreciprocal acoustic device: an effective, broadband, acoustic diode. Previous works showed that, using sufficiently small transducers, an active acoustic metasurface can be realized: a smart active acoustic skin with tunable acoustic properties. Using distributed control, these properties can be adapted or reconfigured in real-time. Or, it can even depend on the acoustic field itself, allowing for a programming of the (meta)surface properties: a programmable boundary condition. For instance, a partial derivative equation depending on the acoustic quantities can be imposed, in a discretized form, at the surface of such a programmable boundary. This type of non-standard boundary conditions have been shown to provide the necessary basis for nonreciprocal propagation for a plane wave interacting with a boundary with non grazing incidence, ie. for wavevectors that possess a component normal to the boundary. This restriction may appear problematic when the wavevector is then parallel to the boundary, e.g. when dealing with plane waves in a 1D waveguide, as in an acoustic diode. An acoustic diode, or acoustic isolator, is a nonreciprocal device that let acoustic power pass only in one direction, hence breaking the reciprocity of normal acoustic propagation. We propose a new model of acoustic diode, based on active components: a continuous, distributed source inside the domain. However, based on the modeling of parietal sources in ducts, in the low frequency range, we show that the boundary control approach and the distributed domain sources are equivalent. The only difference is that, in the case of the programmable boundary condition, the near-field of the boundary also contains a component normal to the boundary. Hence our acoustic diode can be realized in practice using programmable boundary conditions. Moreover, the acoustic diode is effective on a broad frequency range, since it can work both on the fundamental mode (plane waves) and on higher-order mode of the waveguide.

Analytic near-field boundary conditions for transonic flow computations

AIAA Journal ◽  
1987 ◽  
Vol 25 (6) ◽  
pp. 884-886 ◽  
Author(s):  
P. Niederdrenk ◽  
E. Wedemeyer
Keyword(s):  
Boundary Conditions ◽  
Transonic Flow ◽  
Near Field ◽  
Field Boundary

Sturm-Liouville Problem for Stationary Differential Operator with Nonlocal Two-Point Boundary Conditions

2006 ◽  
Vol 11 (1) ◽  
pp. 47-78 ◽  
Author(s):  
S. Pečiulytė ◽  
A. Štikonas
Keyword(s):  
Boundary Condition ◽  
Differential Operator ◽  
Boundary Conditions ◽  
Nonlocal Boundary Condition ◽  
Nonlocal Boundary ◽  
Liouville Problem ◽  
Complex Case ◽  
Qualitative Behavior ◽  
Point Boundary ◽  
Sturm Liouville

The Sturm-Liouville problem with various types of two-point boundary conditions is considered in this paper. In the first part of the paper, we investigate the Sturm-Liouville problem in three cases of nonlocal two-point boundary conditions. We prove general properties of the eigenfunctions and eigenvalues for such a problem in the complex case. In the second part, we investigate the case of real eigenvalues. It is analyzed how the spectrum of these problems depends on the boundary condition parameters. Qualitative behavior of all eigenvalues subject to the nonlocal boundary condition parameters is described.

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