Presentation on theme: "Computational Fluid Dynamics (CFD) U7AEA29"— Presentation transcript:
1Computational Fluid Dynamics (CFD) U7AEA29 Dr. S. Senthil KumarAssociate ProfessorDept. of Aeronautical EngineeringVel Tech Dr. RR & Dr. SR Technical UniversityAvadi, Chennai.
2Outline What is CFD? Why use CFD? Where is CFD used? Physics Modeling NumericsCFD processResources
3What is CFD? What is CFD and its objective? Computational Fluid DynamicsHistorically Analytical Fluid Dynamics (AFD) and EFD (Experimental Fluid Dynamics) was used. CFD has become feasible due to the advent of high speed digital computers.Computer simulation for prediction of fluid-flow phenomena.The objective of CFD is to model the continuous fluids with Partial Differential Equations (PDEs) and discretize PDEs into an algebra problem (Taylor series), solve it, validate it and achieve simulation based design.
4Why use CFD? Why use CFD? Analysis and Design Simulation-based design instead of “build & test”More cost effectively and more rapidly than with experimentsCFD solution provides high-fidelity database for interrogation of flow fieldSimulation of physical fluid phenomena that are difficult to be measured by experimentsScale simulations (e.g., full-scale ships, airplanes)Hazards (e.g., explosions, radiation, pollution)Physics (e.g., weather prediction, planetary boundary layer, stellar evolution)Knowledge and exploration of flow physics
5Where is CFD used? (Aerospace) AppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSportsF18 Store SeparationWing-Body InteractionHypersonic Launch Vehicle
7Where is CFD used? (Appliances) AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSportsSurface-heat-flux plots of the No-Frost refrigerator and freezer compartments helped BOSCH-SIEMENS engineers to optimize the location of air inlets.
9Where is CFD used? (Biomedical) AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSportsMedtronic Blood PumpTemperature and natural convection currents in the eye following laser heating.
10Twin-screw extruder modeling Where is CFD used? (Chemical Processing)Where is CFD used?AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSportsPolymerization reactor vessel - prediction of flow separation and residence time effects.Twin-screw extruder modelingShear rate distribution in twin-screw extruder simulation
11Where is CFD used? (HVAC&R) AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSportsParticle traces of copier VOC emissions colored by concentration level fall behind the copier and then circulate through the room before exiting the exhaust.Streamlines for workstation ventilationFlow pathlines colored by pressure quantify head loss in ductworkMean age of air contours indicate location of fresh supply air
12Where is CFD used? (Hydraulics) AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSports
13Where is CFD used? (Marine) AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSports
14Where is CFD used? (Oil & Gas) AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSportsVolume fraction of gasFlow vectors and pressure distribution on an offshore oil rigVolume fraction of oilVolume fraction of waterAnalysis of multiphase separatorFlow of lubricating mud over drill bit
15Flow around cooling towers Flow pattern through a water turbine. Where is CFD used? (Power Generation)Flow in a burnerWhere is CFD used?AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSportsFlow around cooling towersPathlines from the inlet colored by temperature during standard operating conditionsFlow pattern through a water turbine.
16Where is CFD used? (Sports) AerospaceAppliancesAutomotiveBiomedicalChemical ProcessingHVAC&RHydraulicsMarineOil & GasPower GenerationSports
17PhysicsCFD codes typically designed for representation of specific flow phenomenonViscous vs. inviscid (no viscous forces) (Re)Turbulent vs. laminar (Re)Incompressible vs. compressible (Ma)Single- vs. multi-phase (Ca)Thermal/density effects and energy equation (Pr, g, Gr, Ec)Free-surface flow and surface tension (Fr, We)Chemical reactions, mass transferetc…
19Navier-Stokes Equation Claude-Louis NavierGeorge Gabriel Stokes
20Modeling Mathematical representation of the physical problem Some problems are exact (e.g., laminar pipe flow)Exact solutions only exist for some simple cases. In these cases nonlinear terms can be dropped from the N-S equations which allow analytical solution.Most cases require models for flow behavior [e.g., Reynolds Averaged Navier Stokes equations (RANS) or Large Eddy Simulation (LES) for turbulent flow]Initial —Boundary Value Problem (IBVP), include: governing Partial Differential Equations (PDEs), Initial Conditions (ICs) and Boundary Conditions (BCs)
21Governing Equations (B,S,& L) (Equations based on “average” velocity)Continuityx - Equation of motion
22Numerics / Discretization Computational solution of the IBVPMethod dependent upon the model equations and physicsSeveral components to formulationDiscretization and linearizationAssembly of system of algebraic equationsSolve the system and get approximate solutions
23Finite Differences Methods of Solution Direct methods representationTruncation errorMethods of SolutionDirect methodsIterative methodsCramer’s Rule, Gauss eliminationLU decompositionJacobi method, Gauss-SeidelMethod, SOR method
24Numeric Solution (Finite Differences) jmaxj+1jTaylor’s Series Expansionu i,j = velocity of fluidj-1oi-1ii+1imaxxDiscrete Grid Points
25CFD process Geometry description Specification of flow conditions and propertiesSelection of modelsSpecification of initial and boundary conditionsGrid generation and transformationSpecification of numerical parametersFlow solutionPost processing: Analysis, and visualization
26Geometry description Typical approaches Make assumptions and simplificationsCAD/CAE integrationEngineering drawingsCoordinates include Cartesian system (x,y,z), cylindrical system (r, θ, z), and spherical system(r, θ, Φ)
27Selection of models for flow field Direct Numerical Simulations (DNS) is to solve the N-S equations directly without any modeling. Grid must be fine enough to resolve all flow scales. Applied for laminar flow and rare be used in turbulent flow.Reynolds Averaged Navier-Stokes (NS) equations (RANS) is to perform averaging of NS equations and establishing turbulent models for the eddy viscosity. Too many averaging might damping vortical structures in turbulent flowsLarge Eddy Simulation (LES), Smagorinsky’ constant model and dynamic model. Provide more instantaneous information than RANS did. Instability in complex geometriesDetached Eddy Simulation (DES) is to use one single formulation to combine the advantages of RANS and LES.
28Domain for bottle filling problem. CFD - how it worksAnalysis begins with a mathematical model of a physical problem.Conservation of matter, momentum, and energy must be satisfied throughout the region of interest.Fluid properties are modeled empirically.Simplifying assumptions are made in order to make the problem tractable (e.g., steady-state, incompressible, inviscid, two-dimensional).Provide appropriate initial and boundary conditions for the problem.Filling NozzleBottleDomain for bottle filling problem.
29Mesh for bottle filling problem. CFD - how it works (2)CFD applies numerical methods (called discretization) to develop approximations of the governing equations of fluid mechanics in the fluid region of interest.Governing differential equations: algebraic.The collection of cells is called the grid.The set of algebraic equations are solved numerically (on a computer) for the flow field variables at each node or cell.System of equations are solved simultaneously to provide solution.The solution is post-processed to extract quantities of interest (e.g. lift, drag, torque, heat transfer, separation, pressure loss, etc.).Mesh for bottle filling problem.
30DiscretizationDomain is discretized into a finite set of control volumes or cells. The discretized domain is called the “grid” or the “mesh.”General conservation (transport) equations for mass, momentum, energy, etc., are discretized into algebraic equations.All equations are solved to render flow field.unsteadyconvectiondiffusiongenerationFluid region of pipe flow discretized into finite set of control volumes (mesh).control volume
31Design and create the grid Should you use a quad/hex grid, a tri/tet grid, a hybrid grid, or a non-conformal grid?What degree of grid resolution is required in each region of the domain?How many cells are required for the problem?Will you use adaption to add resolution?Do you have sufficient computer memory?trianglequadrilateraltetrahedronpyramidprism or wedgehexahedronarbitrary polyhedron
32Tri/tet vs. quad/hex meshes For simple geometries, quad/hex meshes can provide high-quality solutions with fewer cells than a comparable tri/tet mesh.For complex geometries, quad/hex meshes show no numerical advantage, and you can save meshing effort by using a tri/tet mesh.
33Hybrid mesh example Valve port grid. Specific regions can be meshed with different cell types.Both efficiency and accuracy are enhanced relative to a hexahedral or tetrahedral mesh alone.Hybrid mesh for an IC engine valve porttet meshhex meshwedge mesh
35Set up the numerical model For a given problem, you will need to:Select appropriate physical models.Turbulence, combustion, multiphase, etc.Define material properties.Fluid.Solid.Mixture.Prescribe operating conditions.Prescribe boundary conditions at all boundary zones.Provide an initial solution.Set up solver controls.Set up convergence monitors.
36Initial and boundary conditions For steady/unsteady flowIC should not affect final solution, only convergence path, i.e. iteration numbers needed to get the converged solution.Robust codes should start most problems from very crude IC, . But more reasonable guess can speed up the convergence.Boundary conditionsNo-slip or slip-free on the wall, periodic, inlet (velocity inlet, mass flow rate, constant pressure, etc.), outlet (constant pressure, velocity convective, buffer zone, zero-gradient), and non-reflecting (compressible flows, such as acoustics), etc.
37Compute the solutionThe discretized conservation equations are solved iteratively. A number of iterations are usually required to reach a converged solution.Convergence is reached when:Changes in solution variables from one iteration to the next are negligible.Residuals provide a mechanism to help monitor this trend.Overall property conservation is achieved.The accuracy of a converged solution is dependent upon:Appropriateness and accuracy of the physical models.Grid resolution and independence.Problem setup.
38Numerical parameters & flow solution Typical time history of residualsThe closer the flow field to the converged solution, the smaller the speed of the residuals decreasing.Solution converged, residuals donot change after more iterations
39Post-processing Analysis, and visualization Calculation of derived variablesVorticityWall shear stressCalculation of integral parameters: forces, momentsVisualization (usually with commercial software)Simple X-Y plotsSimple 2D contours3D contour carpet plotsVector plots and streamlines (streamlines are the lines whose tangent direction is the same as the velocity vectors)Animations (dozens of sample pictures in a series of time were shown continuously)
40Examine the resultsVisualization can be used to answer such questions as:What is the overall flow pattern?Is there separation?Where do shocks, shear layers, etc. form?Are key flow features being resolved?Are physical models and boundary conditions appropriate?Numerical reporting tools can be used to calculate quantitative results, e.g:Lift, drag, and torque.Average heat transfer coefficients.Surface-averaged quantities.
44Advantages of CFD Relatively low cost. Using physical experiments and tests to get essential engineering data for design can be expensive.CFD simulations are relatively inexpensive, and costs are likely to decrease as computers become more powerful.Speed.CFD simulations can be executed in a short period of time.Quick turnaround means engineering data can be introduced early in the design process.Ability to simulate real conditions.Many flow and heat transfer processes can not be (easily) tested, e.g. hypersonic flow.CFD provides the ability to theoretically simulate any physical condition.
45Limitations of CFD Physical models. CFD solutions rely upon physical models of real world processes (e.g. turbulence, compressibility, chemistry, multiphase flow, etc.).The CFD solutions can only be as accurate as the physical models on which they are based.Numerical errors.Solving equations on a computer invariably introduces numerical errors.Round-off error: due to finite word size available on the computer. Round-off errors will always exist (though they can be small in most cases).Truncation error: due to approximations in the numerical models. Truncation errors will go to zero as the grid is refined. Mesh refinement is one way to deal with truncation error.
46Fully Developed Inlet Profile Limitations of CFD (2)Boundary conditions.As with physical models, the accuracy of the CFD solution is only as good as the initial/boundary conditions provided to the numerical model.Example: flow in a duct with sudden expansion. If flow is supplied to domain by a pipe, you should use a fully-developed profile for velocity rather than assume uniform conditions.poorbetterFully Developed Inlet ProfileComputational DomainUniform Inlet Profile
47Software and resources CFD software was built upon physics, modeling, numerics.Two types of available softwareCommercial (e.g., FLUENT, CFX, Star-CD)Research (e.g., CFDSHIP-IOWA, U2RANS)More information on CFD can be got on the following website:CFD Online:CFD softwareFLUENT:CFDRC:Computational Dynamics:CFX/AEA:Grid generation softwareGridgen:GridPro:HypermeshVisualization softwareTecplot:Fieldview: