PowerPoint Presentation

The Impact of Unsteadiness on Uncertainty in Automotive Aerodynamics Simulation using OpenFOAMBy: Akshat SrivastavaMSc Computational Fluid DynamicsSupervisor: Dr. Panagiotis TsoutsanisSeptember 2016

www.cranfield.ac.uk

#Presentation OverviewOpenFOAMCase structureOverview of solverComputational domainMesh generation processRefinementsMesh dependence studyFrequency spectrum analysisInstantaneous flowsMean flow parametersMean flow statisticsFuture work and recommendationsIntroductionTurbulence ModelingSoftware and MethodologyComputational MethodologyResults and conclusionObjective

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IntroductionSphere is taken as a test case in this thesis work.Even though geometry is straightforward, the flow around sphere is very complex to analyse.Mechanism for transition in shear layer, behaviour and quantitative estimation of wake structure is still rare for the considered Reynolds number of this thesis.The vortex shedding or the first mode is identified to the large-scale shedding in the wake.Second high frequency mode connected with the small-scale shear layer Kelvin-Helmholtz instability on the fringe of the recirculation zone.Small-scale instability causes transition to turbulence in a long run.A frequency lower than large-scale frequency is also present which linked to the shrinkage and enlargement of recirculation bubble.

#ObjectivesIntroduction continueUsing an open source numerical tool.Turbulence model which can capture the turbulent flow and is less expensive.Selection of suitable initial and boundary conditions.Total time period selection by considering the low-frequency component.Validation of results for instantaneous

#Turbulence ModellingThe Improved Delayed Detached Eddy Simulation (IDDES), combines the advantages of the wall modelled LES (WMLES) and the DDES capabilities.IDDES was developed to avoid the log-layer mismatch (LLM), between RANS and LES regime.Classical LESThe computational domain is divided into three subdomainsAway from the wall

Region close to the wall

Region between the above limiting cases

, Simple DES

, IDDES

#OpenFOAMSoftware and MethodologyOpenFOAM refers to Open Field Operation And Manipulation, which is basically a collection of libraries.It provides serval utilities which help in mesh generation, pre processing and post processing.The solver used in this thesis work is pimpleFOAM which combines the PISO and SIMPLE algorithm.

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Case structure in OpenFOAMSubdirectory: 0Contains Initial and Boundary ConditionsPUnuSGSnuTildaPressure flow fieldVelocity flow fieldSubgridscale viscosityTurbulent variable, SASubdirectory: constantContains Mesh data and transport modelpolyMeshtransportPropertiesRASPropertiesLESPropertiesDirectory contains mesh informationTransport model and fluid type is selectedClosure model (SA) and constants are setLES model and constants are selectedSubdirectory: systemContains solver settings, schemes, etc.controlDictfvSolutionfvSchemesTimestep, write control, functions : force calculation and averaging, sampling, etc.All solver settings, tolerance, etc.Numerical schemes are selectedturbulencePropertiesTurbulence model used (LES)

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StartInitial u, p, F, turb Update Turbulence Momentumpredictor uUpdate HSolve PUpdate F and uNon-orthogonal Update

PISOcorrector PIMPLE correctorNext time stepNONONOYESYESYESU, P, f, turbulence

#Example of log file in OpenFOAM from simulation

#Computational MethodologyThe sphere diameter is 1m and situated at (0, 0, 0) coordinate.Computational domain in upward and radial direction is 4.5D which corresponds to only 0.1% change in velocity in comparison to free-stream velocity.In downward direction it extends up to 25D

Time step is selected using convection time which is 50 time smaller than it to capture low-frequency fluctuation accurately, it is 0.00029. Total time of simulation is 9.048 seconds

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Mesh GenerationComputational Methodology continue1. Patch geometry creation in Salome.2. Export of each patch into separated STL files.3. Add of patch names in each STL file.4. Concatenation of all STL files into a single one.5. Conversion of the single STL file into FMS format using the surfaceToFMS command of cfMesh.6. Modification of patch types (wall, patch, symmetry) in the FMS file.7. Preparation of the meshDict file.8. Generation of the mesh using the cartesianMesh command from cfMesh.

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Computational Methodology continue

Refinements

#Computational Methodology continue

Cartesian mesh results

Structured mesh results

#Mesh Dependence StudyComputational Methodology continue

Three mesh were generated with y+ of 1 by using the additionalRefinmentLevels dictionary of cfMesh, while global maximum element size remain unchanged.The table represents the statistics by averaging 540 tU/D time units since initial 75 tU/D time units are taken to pass the transition stage.Errors were calculated by considering the drag coefficient of experimental data (by Schlichting). Strouhal number and Separation angle by Achenbach.

#Computational Methodology continue

#Computational Methodology continue

Coarse MeshMedium MeshFine Mesh

#Frequency Spectrum AnalysisResults and ConclusionLarge time span provides data to analyse the existence of low-frequency fluctuations.Probes are step up using probe utility in OpenFOAM.The probes which are presented in thesis are; P1, P2, P9 and P4.Frequencies are computed via FFT with Tecplot for cross-stream velocity component.

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Results and Conclusion continue

P1P1

With increase in distance from the sphere, the energy content decreases drastically.Due to turbulence occur close to the sphere at this Reynolds numberP9P4

#Results and Conclusion continue

P1P2P9P4Kelvin-Helmholtz instability which is associated with separating shear layer is recorded as 0.700 Besides these two frequency, a much lower frequency then large scale vortex shedding is recorded and is represented by fm here, it value is 0.038Low-frequency amplitude of sphere is much lower then other bluff bodies recorded earlier.This low-frequency is an attribute of shrinkage and enlargement of recirculation bubble.

#Results and Conclusion continue

P2P9Probes P2 and P9 are selected which are just after the mean recirculation region.To analyse these two probes, cross correlation is performed.When P9 observe negative or close to zero value then it is indicative of enlargement of recirculation region.The cross-correlation starts with negative value which indicates 180 degree phase difference.

#Instantaneous flowResults and Conclusion continue

X-Y planeX-Z plane

#Mean flow parameterResults and Conclusion continue

#Mean flow statisticsResults and Conclusion continue

#Results and Conclusion continueProfile of mean streamwise velocity at three locations.At x/D=3, the flow is in recovery zone.Only difference is observed at last location.Compared with experimental results of Kim & Durbin.

#Results and Conclusion continueMean streamwise and cross-streamwise velocity profiles compared with DNS results at Re=3700

#Results and Conclusion continue

Mean streamwise stressMean cross-streamwise stressMean shear stressMean turbulent kinetic energy

#Future work and recommendationsError encountered during initial set up of simulation is the meshing software, latest version of OpenFOAM would be preferred.Hybrid RANS-LES method is not working in the region of separation accurately, comparison with other models would be preferred.For IDDES simulation, mesh is a crucial part, hence a perfect mesh for DES simulations should be made with cfMesh or any other meshing software.Comparison with other solvers and turbulence models can provide solid verification and validation of this thesis work.Long simulation time is still a problem, some other methods are available in literature which could reduce the calculation time of such flow statistics.

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#Miscellaneous slides

#LES principlesTurbulence ModellingLES computes the large scales which contains the most of energy and models the small scale which contributes fraction of total energy.Energy spectrum is divided into three sub-regions:Energetic scaleInertial subrangeDissipative scale

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Turbulence Modelling continue.Filtering is applied in LES to write any variable as a contribution of large and small scale

Standard filter applied in OpenFOAM is the implicit top-hat filter.Eddy viscosity model is utilised in LES.

The filtered Navier-Stokes equation is given by:

Spalart-Allmaras is used as a turbulence closure model

#Hybrid RANS-LES modelsTurbulence Modelling continue.They are divided into two methodsZonal hybrid methodBlended hybrid methodThesis used the blended hybrid method where smooth transition is made between different regions.Common type of blended hybrid method is DES (Detached Eddy Simulation), switching between RANS and LES depends on the local grid resolution.Original DES combines the standard Spalart-Allmaras RANS model with it Sub-Grid Scale (SGS) counter part.

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Turbulence Modelling continue.Even though DES is very promising, it still encounters mainly two drawbacks.Grid Induces Separation (GIS) : When wall bounded flows have thick boundary layer and small separation regions.Modeled Stress Depletion (MSD) : Arises when SGS model of the DES originates deep in the boundary layer.

The Delayed Detached Eddy Simulation (DDES) was formulated in order to avoid the appearance of the MSD.

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