Comparison of Various Pressure Based Boundary Conditions for Three-Dimensional Subsonic DSMC Simulation

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Niraj Shah ◽  
Abhimanyu Gavasane ◽  
Amit Agrawal ◽  
Upendra Bhandarkar
Keyword(s):  
Boundary Conditions ◽  
Cell Size ◽  
Pressure Ratio ◽  
Three Dimensional ◽  
Direct Simulation Monte Carlo ◽  
Statistical Error ◽  
Nonequilibrium Effect ◽  
Time Required ◽  
Characteristic Scheme ◽  
Simulation Time

Three-dimensional (3D) direct simulation Monte Carlo (DSMC) has been used to simulate flow in a straight microchannel using an in-house parallelized code. In the present work, a comparative study of seven boundary conditions is carried out with respect to time required for achieving steady-state, accuracy in predicting the specified pressure at the boundaries, and the total simulation time required for attaining a statistical error within one percent. The effect of changing the Knudsen number, pressure ratio (PR), and cross aspect ratio (CAR) on these parameters is also studied. The presence of a boundary is seen to affect the simulated pressure in a cell when compared to the specified pressure, the difference being highest for corner cells and least for cells away from walls. All boundary conditions tested work well at the inlet boundary; however, similar results are not obtained at the outlet boundary. For the same cell size, the schemes that employ first- and second-order corrections lead to a smaller pressure difference compared to schemes applying no corrections. The best predictions can be obtained by using first-order corrections with finer cell size close to the boundary. For most of the simulated cases, the boundary condition employing the characteristic scheme with nonequilibrium effect leads to the minimum simulation time. Considering the nonequilibrium effect, prediction of inlet and outlet pressures and the speed of simulation, the characteristic scheme with nonequilibrium effect performs better than all the other schemes, at least over the range of parameters investigated herein.

Numerical Investigations for Leakage and Windage Heating in Straight-Through Labyrinth Seals

2015 ◽  
Vol 138 (1) ◽  
Author(s):  
Kali Charan Nayak ◽  
Pradip Dutta
Keyword(s):  
Cell Size ◽  
Turbulence Model ◽  
Gas Turbines ◽  
Pressure Ratio ◽  
Three Dimensional ◽  
Labyrinth Seal ◽  
Radial Clearance ◽  
Labyrinth Seals ◽  
Turbine Engine ◽  
Honeycomb Cell

The ability to quantify leakage flow and windage heating for labyrinth seals with honeycomb lands is critical in understanding gas turbine engine system performance and predicting its component life. Variety of labyrinth seal configurations (number of teeth, stepped or straight, honeycomb cell size) are in use in gas turbines, and for each configuration, there are many geometric factors that can impact a seal's leakage and windage characteristics. This paper describes the development of a numerical methodology aimed at studying the effect of honeycomb lands on leakage and windage heating. Specifically, a three-dimensional computational fluid dynamics (CFD) model is developed utilizing commercial finite volume-based software incorporating the renormalization group (RNG) k-ε turbulence model with modified Schmidt number. The modified turbulence model is benchmarked and fine-tuned based on several experiments. Using this model, a broad parametric study is conducted by varying honeycomb cell size, pressure ratio (PR), and radial clearance for a four-tooth straight-through labyrinth seal. The results show good agreement with available experimental data. They further indicate that larger honeycomb cells predict higher seal leakage and windage heating at tighter clearances compared to smaller honeycomb cells and smooth lands. However, at open seal clearances larger honeycomb cells have lower leakage compared to smaller honeycomb cells.

Volute CFD Modeling Evaluation for Centrifugal Compressors

2014 ◽  
Author(s):  
Anil Samale ◽  
Jorge E. Pacheco
Keyword(s):  
Cfd Simulation ◽  
Guide Vane ◽  
Pressure Ratio ◽  
Cfd Modeling ◽  
Inlet Guide Vane ◽  
Centrifugal Compressors ◽  
Auxiliary Equipment ◽  
Flow Field Characteristics ◽  
Time Required ◽  
Simulation Time

Significant effort has been spent over the years to improve the accuracy and reduce the Computational Fluid Dynamics (CFD) simulation time required to predict performance for centrifugal compressors. Most of the emphasis has been on modeling the impeller and diffuser components. This paper presents an evaluation of volute modeling targeted at reducing simulation time while increasing the accuracy of the results. Providing accurate predictions of performance and operating range is critical to the equipment users as it allows reduction in design margins for plant equipment dependent on compressor performance (i.e. drivers, intercoolers and other auxiliary equipment). The volute is the component that collects the flow from the diffuser and guides it into the discharge nozzle. Due to the circumferential variation (tongue) in the geometry, this component has to be modeled in its entirety (360 degrees); which results in very large grid sizes. The impeller and diffuser, normally modeled as a sector or pie slice, result in significantly smaller meshes. The volute models require large numbers of computing nodes to be solved and tend to have convergence issues. The investigation, with the objective of reducing the amount of time required to run these simulations and improve the convergence of the runs, evaluated several mesh configurations that focused on grid density (element count), element aspect ratio and use of inflation layers. The domain evaluated consisted of several stationary components and one rotating component. The model started at the inlet guide vane section followed by the impeller, vanned diffuser, volute, and discharge nozzle. Commercial software ANSYS CFX was used to develop the meshes using tetrahedral/prism elements and complete steady-state CFD analyses. Detailed flow field characteristics (total and static pressure, velocity streamlines, etc.) and key performance parameters (loss coefficient, pressure ratio, etc.) were compared for the various configurations evaluated. In addition, experimental measurements were used to validate the CFD results. The configuration that resulted in the shortest cycle time with the best performance accuracy was selected as optimum. Accuracy is paramount for performance prediction and reduction in simulation time will allow more volute iterations to be investigated, which would help improve volute performance in centrifugal compressors.

scholarly journals Evaluation of an Euler Code With Non-Reflecting Boundary Conditions for the Analysis of 3D Flow in Radial Impellers

1994 ◽  
Author(s):  
Antonios Fatsis ◽  
René Van den Braembussche
Keyword(s):  
Boundary Conditions ◽  
Pressure Ratio ◽  
Time Integration ◽  
Three Dimensional ◽  
Centrifugal Impeller ◽  
Rotational Flow ◽  
Fourier Decomposition ◽  
High Pressure Ratio ◽  
Convergence To Steady State ◽  
Space Discretization

The aim of the present paper is the investigation of the influence of non-reflecting boundary conditions on the calculation of the three-dimensional inviscid steady rotational flow inside a high pressure ratio centrifugal impeller. The space discretization of the Euler equations was done using the finite volume technique and the multistep Runge-Kutta scheme was used for the time integration. Nonreflecting boundary conditions using Fourier decomposition are applied at inflow and outflow boundaries. Computations show that convergence to steady state is reached faster when non-reflecting boundary conditions are used compared to the reflecting boundary conditions. Studying the influence of the non-reflecting boundary conditions on the accuracy of the flowfield prediction, it is concluded that the location of the upstream boundary has a negligible influence on the solution even in the case where a shock is crossing the boundary.

scholarly journals A Numerical Study of Sub-Millisecond Integrated Mix-and-Inject Microfluidic Devices for Sample Delivery at Synchrotron and XFELs

2021 ◽  
Vol 11 (8) ◽  
pp. 3404
Author(s):  
Majid Hejazian ◽  
Eugeniu Balaur ◽  
Brian Abbey
Keyword(s):  
Molecular Dynamics ◽  
Flow Rate ◽  
Numerical Study ◽  
Three Dimensional ◽  
Microfluidic Devices ◽  
Liquid Jets ◽  
Mixing Efficiency ◽  
X Ray Diffraction ◽  
Cross Sectional ◽  
Time Required

Microfluidic devices which integrate both rapid mixing and liquid jetting for sample delivery are an emerging solution for studying molecular dynamics via X-ray diffraction. Here we use finite element modelling to investigate the efficiency and time-resolution achievable using microfluidic mixers within the parameter range required for producing stable liquid jets. Three-dimensional simulations, validated by experimental data, are used to determine the velocity and concentration distribution within these devices. The results show that by adopting a serpentine geometry, it is possible to induce chaotic mixing, which effectively reduces the time required to achieve a homogeneous mixture for sample delivery. Further, we investigate the effect of flow rate and the mixer microchannel size on the mixing efficiency and minimum time required for complete mixing of the two solutions whilst maintaining a stable jet. In general, we find that the smaller the cross-sectional area of the mixer microchannel, the shorter the time needed to achieve homogeneous mixing for a given flow rate. The results of these simulations will form the basis for optimised designs enabling the study of molecular dynamics occurring on millisecond timescales using integrated mix-and-inject microfluidic devices.

scholarly journals Charge algebra in Al(A)dSn spacetimes

2021 ◽  
Vol 2021 (5) ◽  
Author(s):  
Adrien Fiorucci ◽  
Romain Ruzziconi
Keyword(s):  
Boundary Conditions ◽  
Dirichlet Boundary ◽  
Three Dimensional ◽  
Dirichlet Boundary Conditions ◽  
Boundary Structure ◽  
Asymptotic Symmetry ◽  
Renormalization Procedure ◽  
Field Dependent ◽  
Odd Dimensions

Abstract The gravitational charge algebra of generic asymptotically locally (A)dS spacetimes is derived in n dimensions. The analysis is performed in the Starobinsky/Fefferman-Graham gauge, without assuming any further boundary condition than the minimal falloffs for conformal compactification. In particular, the boundary structure is allowed to fluctuate and plays the role of source yielding some symplectic flux at the boundary. Using the holographic renormalization procedure, the divergences are removed from the symplectic structure, which leads to finite expressions. The charges associated with boundary diffeomorphisms are generically non-vanishing, non-integrable and not conserved, while those associated with boundary Weyl rescalings are non-vanishing only in odd dimensions due to the presence of Weyl anomalies in the dual theory. The charge algebra exhibits a field-dependent 2-cocycle in odd dimensions. When the general framework is restricted to three-dimensional asymptotically AdS spacetimes with Dirichlet boundary conditions, the 2-cocycle reduces to the Brown-Henneaux central extension. The analysis is also specified to leaky boundary conditions in asymptotically locally (A)dS spacetimes that lead to the Λ-BMS asymptotic symmetry group. In the flat limit, the latter contracts into the BMS group in n dimensions.

scholarly journals Boundary conditions in topological AdS4/CFT3

2021 ◽  
Vol 2021 (2) ◽  
Author(s):  
Pietro Benetti Genolini ◽  
Matan Grinberg ◽  
Paul Richmond
Keyword(s):  
Field Theory ◽  
Free Energy ◽  
Boundary Conditions ◽  
Smooth Solution ◽  
Three Dimensional ◽  
Field Theories ◽  
Legendre Transformation ◽  
Generating Functional ◽  
Holographic Duals

Abstract We revisit the construction in four-dimensional gauged Spin(4) supergravity of the holographic duals to topologically twisted three-dimensional $$ \mathcal{N} $$ N = 4 field theories. Our focus in this paper is to highlight some subtleties related to preserving supersymmetry in AdS/CFT, namely the inclusion of finite counterterms and the necessity of a Legendre transformation to find the dual to the field theory generating functional. Studying the geometry of these supergravity solutions, we conclude that the gravitational free energy is indeed independent from the metric of the boundary, and it vanishes for any smooth solution.

scholarly journals Bootstrapping boundary-localized interactions

2020 ◽  
Vol 2020 (12) ◽  
Author(s):  
Connor Behan ◽  
Lorenzo Di Pietro ◽  
Edoardo Lauria ◽  
Balt C. van Rees
Keyword(s):  
Boundary Condition ◽  
Scalar Field ◽  
Boundary Conditions ◽  
Parameter Space ◽  
Three Dimensional ◽  
Conformal Boundary ◽  
Dimensional Boundary ◽  
Dirichlet And Neumann Conditions ◽  
Boundary Fields ◽  
Boundary Dynamics

Abstract We study conformal boundary conditions for the theory of a single real scalar to investigate whether the known Dirichlet and Neumann conditions are the only possibilities. For this free bulk theory there are strong restrictions on the possible boundary dynamics. In particular, we find that the bulk-to-boundary operator expansion of the bulk field involves at most a ‘shadow pair’ of boundary fields, irrespective of the conformal boundary condition. We numerically analyze the four-point crossing equations for this shadow pair in the case of a three-dimensional boundary (so a four-dimensional scalar field) and find that large ranges of parameter space are excluded. However a ‘kink’ in the numerical bounds obeys all our consistency checks and might be an indication of a new conformal boundary condition.

Sign in / Sign up

Export Citation Format

Share Document