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Fluent and Gambit Workshop


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A workshop on Fluent and Gambit conducted at NIT Trichy.

A workshop on Fluent and Gambit conducted at NIT Trichy.

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  • We obviously cannot cover details of FE in general
  • Transcript

    • 1. FLUENT & GAMBIT ALCHEMY 2010 S G Sriram Ragav and N Khalid Ahmed Department of Chemical Engineering National Institute of Technology, Tiruchirrappalli – 620 015
    • 2. WHAT IS CFD...??
      • Computational Fluid Dynamics:
        • simulation of fluid engineering systems using modeling (mathematical physical problem formulation) and numerical methods (discretization methods, solvers, numerical parameters, and grid generations, etc.) ‏
      • Knowledge and exploration of flow physics
    • 3. WHERE IS CFD USED...??
      • Aerospace, Automotive, Biomedical, Chemical Processing, Hydraulics, Marine, Oil & Gas, Power Generation, Sports
      Aerospace F18 Store Separation Automotive Biomedical Temperature and natural convection currents in the eye following laser heating. Polymerization reactor vessel - prediction of flow separation and residence time effects. Chemical Processing Hydraulics HVAC Sports Power Generation Flow around cooling towers Flow of lubricating mud over drill bit Oil & Gas Streamlines for workstation ventilation
    • 4. MODELLING
      • Mathematical physics problem formulation in terms of a continuous initial boundary value problem (IBVP) ‏
      • Includes :
        • Geometry and domain
        • Coordinates
        • Governing equations
        • Flow conditions
        • Initial and Boundary conditions
        • Selection of models for different applications
    • 5. Math Modeling and Simulation of Physical Processes
      • Define the physical problem
      • Create mathematical model
        • PDEs, ODEs, Algebraic equations
        • Define the initial and boundary conditions
      • Create a discrete numerical model
        • Discretize the domain, Generate the grid
        • Solve the system
      • Analyze the errors in the model
        • Consistency, Stability and convergence analysis
      • Simple geometries - Easily created by few geometric parameters - e.g. circular pipe
      • Complex geometries - Created by partial differential equations or importing the database of the geometry(e.g. airfoil) into commercial software
      • Domain - size and shape
        • Note : The three coordinates: Cartesian system (x,y,z), cylindrical system (r, θ , z), and spherical system(r, θ, Φ) should be appropriately chosen for a better resolution of the geometry (e.g. cylindrical for circular pipe).
      • Navier Stokes Equation
      • Continuity Equation
    • 8. Methods Of Solution:
      • FVM (Finite volume methods)
      • FEM (Finite element methods)
      • FDM (Finite Difference methods) – Very primitive and poor solution qualities.
    • 9. Finite Element Basics
      • Mesh of nodes and elements
      • Commonly used for structural simulations
      • Relate nodal displacements to nodal forces
      • K = element property matrix (stiffness)
      • q = vector of unknowns (nodal displacements)
      • Q = vector of nodal forcing parameters
      • Boundary values provide Q, Elements provide K
      • Solve for q
    • 10. Finite Volume Basics
      • Commonly used for fluid flows simulations
      • Subdivide spatial domain into cells
      • Maintain an approximation of over each cell
      • In each timestep we update the approximation of q for each cell using an approximation to the flux through the boundary of the cell
      • Explicit time stepping may be performed
    • 11. FVM Vs FEM
      • FVM
      • Converts PDEs to algebraic equations
      • Unstructured grid
      • Defined from conservation laws. Uses divergence theorem.
      • Nodes present any where in the control volume.
      • FEM
      • Converts PDEs to ODEs and algebraic equations.
      • Structured grid
      • Nodes at the vertices of the grid.
    • 12. Types of Solution Strategies
      • Implicit
      • Explicit
    • 13. Implicit Vs Explicit
      • Implicit
      • A parameter is always a function of knowns and unknowns
      • Solves simultaneous Equations
      • Highly convergent
      • Large time steps
      • Computationally expensive
      • Uses Matrix equations
      • Explicit
      • A parameter is always a function of knowns
      • Low convergence
      • Very small time steps
      • Uses algebraic difference equations
      • Based on the physics of fluids, different categories exist:
        • Viscous or inviscid (Re) ‏
        • External flow or internal flow (wall bounded or not) ‏
        • Turbulent vs laminar (Re) ‏
        • Incompressible vs. compressible (Ma) ‏
        • Single- vs. multi-phase (Ca) ‏
        • Thermal/density effects (Pr, g, Gr, Ec) ‏
        • Free-surface flow (Fr) and surface tension (We) ‏
        • Chemical reactions and combustion (Pe, Da) ‏
      • Initial conditions (ICS, steady/unsteady flows) ‏
        • D o not affect final results and only affect convergence path, i.e. number of iterations (steady) or time steps (unsteady) need to reach converged solutions.
        • More reasonable guess can speed up the convergence
      • Boundary conditions
        • No-slip or slip-free on walls, periodic, inlet (velocity inlet, mass flow rate, constant pressure, etc.), outlet (constant pressure, velocity convective, numerical beach, zero-gradient), and non-reflecting (for compressible flows, such as acoustics), etc.
    • 16. BOUNDARY CONDITIONS No-slip walls: u=0,v=0 v=0, dp/dr=0,du/dr=0 Inlet ,u=c,v=0 Outlet, p=c o r x Axisymmetric
      • Discretize the continuous Initial Boundary Value Problems (IBVPs) algebraic equations using numerical methods.
      • Assemble the system of algebraic equations and solve the system to get approximate solutions
      • Numerical Methods:
        • Discretization methods
        • Solvers and numerical parameters
        • Grid generation and transformation
        • High Performance Computation (HPC) and post-processing
      • Grids can either be structured (hexahedral) or unstructured (tetrahedral) - depending upon type of discretization scheme and application
        • Scheme:
          • Finite Difference – Structured
          • Finite Volume or Finite Element – Structured or Unstructed
      structured unstructured
    • 19. CFD PROCESS
      • Purposes of CFD codes will be different for different applications: investigation of bubble-fluid interactions for bubbly flows, study of wave induced massively separated flows for free-surface, etc.
      • Depend on the specific purpose and flow conditions of the problem, different CFD codes can be chosen for different applications (aerospace, marines, combustion, multi-phase flows, etc.)‏
      • Once purposes and CFD codes chosen, “ CFD process ” is the steps to set up the IBVP problem and run the code:
        • Geometry
        • Physics
        • Mesh
        • Solve
        • Reports
        • Post Processing
    • 20. Viscous Model Boundary Conditions Initial Conditions Convergent Limit Contours Precisions (single/ double) ‏ Numerical Scheme Vectors Streamlines Verification Geometry Select Geometry Geometry Parameters Physics Mesh Solve Post-Processing Compressible ON/OFF Flow properties Unstructured (automatic/ manual) ‏ Steady/ Unsteady Forces Report (lift/drag, shear stress, etc) ‏ XY Plot Domain Shape and Size Heat Transfer ON/OFF Structured (automatic/ manual) ‏ Iterations/ Steps Validation Reports
        • Menu is laid out such that order of operation is generally left to right.
          • Import and scale mesh file.
          • Select physical models.
          • Define material properties.
          • Prescribe operating conditions.
          • Prescribe boundary conditions.
          • Provide an initial solution.
          • Set solver controls.
          • Set up convergence monitors.
          • Compute and monitor solution.
        • Post-Processing
          • Feedback into Solver
          • Engineering Analysis
      • GUI commands have a corresponding TUI command.
        • Advanced commands are only available through TUI.
        • ‘ Enter’ displays command set at current level.
        • ‘ q’ moves up one level.
      • Components are defined in preprocessor
        • Cell = control volume into which domain is broken up
          • computational domain is defined by mesh that represents the fluid and solid regions of interest.
        • Face = boundary of a cell
        • Edge = boundary of a face
        • Node = grid point
        • Zone = grouping of nodes, faces, and/or cells
          • Boundary data assigned to face zones.
          • Material data and source terms assigned to cell zones.
      Simple 2D mesh Simple 3D mesh face cell node edge node face cell cell center
      • All physical dimensions initially assumed to be in meters .
        • Scale grid accordingly.
      • Other quantities can also be scaled. independent of other units used.
        • Fluent defaults to SI units.
      • Physical models may require inclusion of additional materials and dictates which properties need to be defined.
      • Material properties defined in Materials Panel.
        • Single-Phase, Single Species Flows
          • Define fluid/solid properties
          • Real gas model (NIST’s REFPROP) ‏
        • Multiple Species (Single Phase) Flows
          • Mixture Material concept employed
            • Mixture properties (composition dependent) defined separately from constituent’s properties.
            • Constituent properties must be defined.
          • PDF Mixture Material concept
            • PDF lookup table used for mixture properties.
              • Transport properties for mixture defined separately.
            • Constituent properties extracted from database.
        • Multiple Phase Flows (Single Species) ‏
          • Define properties for all fluids and solids.
      • Materials are assigned to cell zone where assignment method depends upon models selected:
        • Single-Phase, Single Species Flows
          • Assign material to fluid zone(s) in Fluid Panel.
        • Multiple Species (Single Phase) Flows
          • Assign mixture material to fluid zones in Species Model Panel or in Pre-PDF.
          • All fluid zones consist of ‘mixture’.
        • Multiple Phase Flows (Single Species) ‏
          • Primary and secondary phases selected in Phases Panel.
            • from Define menu
          • All fluid zones consist of ‘mixture’.
      • Many post-processing tools are available.
      • Post-Processing functions typically operate on surfaces.
        • Surfaces are automatically created from zones.
        • Additional surfaces can be created.
      • Example: an Iso-Surface of constant grid coordinate can be created for viewing data within a plane.
      • Fluent calculates field variable data at cell centers.
      • Node values of the grid are either:
        • calculated as the average of neighboring cell data, or,
        • defined explicitly (when available) with boundary condition data.
      • Node values on surfaces are interpolated from grid node data.
      • Data files store:
        • data at cell centers
        • node value data for primitive variables at boundary nodes.
      • Enable Node Values to interpolate field data to nodes.
      • Grid adaption adds more cells where needed to resolve the flow field without pre-processor .
      • Fluent adapts on cells listed in register.
        • Registers can be defined based on:
          • Gradients of flow or user-defined variables
          • Iso-values of flow or user-defined variables
          • All cells on a boundary
          • All cells in a region
          • Cell volumes or volume changes
          • y + in cells adjacent to walls
        • To assist adaption process, you can:
          • Combine adaption registers
          • Draw contours of adaption function
          • Display cells marked for adaption
          • Limit adaption based on cell size and number of cells:
    • 31. ADAPTATION EXAMPLE – 2D PLANAR SHELL 2D planar shell - initial grid
      • Adapt grid in regions of high pressure gradient to better resolve pressure jump across the shock.
      2D planar shell - contours of pressure initial grid
    • 32. ADAPTATION EXAMPLE – FINAL GRID & SOLUTION 2D planar shell - contours of pressure final grid 2D planar shell - final grid
      • To define a problem that results in a unique solution, you must specify information on the dependent (flow) variables at the domain boundaries.
        • Specifying fluxes of mass, momentum, energy, etc. into domain.
      • Defining boundary conditions involves:
        • identifying the location of the boundaries (e.g., inlets, walls, symmetry) ‏
        • supplying information at the boundaries
      • The data required at a boundary depends upon the boundary condition type and the physical models employed.
      • You must be aware of the information that is required of the boundary condition and locate the boundaries where the information on the flow variables are known or can be reasonably approximated .
        • Poorly defined boundary conditions can have a significant impact on your solution.
      • Boundary Condition Types of External Faces
        • General : Pressure inlet, Pressure outlet
        • Incompressible : Velocity inlet, Outflow
        • Compressible flows : Mass flow inlet, Pressure far-field
        • Special : Inlet vent, outlet vent, intake fan, exhaust fan
        • Other : Wall, Symmetry, Periodic, Axis
      • Boundary Condition Types of Cell ‘Boundaries’
        • Fluid and Solid
      • Boundary Condition Types of Double-Sided Face ‘Boundaries’
        • Fan, Interior, Porous Jump, Radiator, Walls
      inlet outlet wall interior Orifice_plate and orifice_plate-shadow
      • Zones and zone types are initially defined in pre-processor.
      • To change zone type for a particular zone:
        • Define  Boundary Conditions...
        • Choose the zone in Zone list.
          • Can also select boundary zone using right mouse button in Display Grid window.
        • Select new zone type in Type list.
      • Explicitly assign data in BC panels.
        • To set boundary conditions for particular zone:
          • Choose the zone in Zone list.
          • Click Set ... button
        • Boundary condition data can be copied from one zone to another.
      • Boundary condition data can be stored and retrieved from file.
        • file  write-bc and file  read-bc
      • Boundary conditions can also be defined by UDFs and Profiles.
      • Profiles can be generated by:
        • Writing a profile from another CFD simulation
        • Creating an appropriately formatted text file with boundary condition data .
      • Specify Velocity by:
        • Magnitude, Normal to Boundary
        • Components
        • Magnitude and Direction
      • Velocity profile is uniform by default
      • Intended for incompressible flows.
        • Static pressure adjusts to accommodate prescribed velocity distribution.
        • Total (stagnation) properties of flow also varies.
        • Using in compressible flows can lead to non-physical results.
      • Can be used as an outlet by specifying negative velocity.
        • You must ensure that mass conservation is satisfied if multiple inlets are used.
      • Specify:
        • Total Gauge Pressure
          • Defines energy to drive flow.
          • Doubles as back pressure (static gauge) for cases where back flow occurs.
            • Direction of back flow determined from interior solution.
        • Static Gauge Pressure
          • Static pressure where flow is locally supersonic; ignored if subsonic
          • Will be used if flow field is initialized from this boundary.
        • Total Temperature
          • Used as static temperature for incompressible flow.
        • Inlet Flow Direction
      Incompressible flows: Compressible flows:
      • Note: Gauge pressure inputs are required.
        • Operating pressure input is set under: Define  Operating Conditions
      • Suitable for compressible and incompressible flows.
        • Pressure inlet boundary is treated as loss-free transition from stagnation to inlet conditions.
        • Fluent calculates static pressure and velocity at inlet
        • Mass flux through boundary varies depending on interior solution and specified flow direction.
      • Can be used as a “free” boundary in an external or unconfined flow.
      • Specify static gauge pressure
        • Interpreted as static pressure of environment into which flow exhausts.
        • Radial equilibrium pressure distribution option available.
        • Doubles as inlet pressure ( total gauge ) for cases where backflow occurs.
      • Backflow
        • Can occur at pressure outlet during iterations or as part of final solution.
        • Backflow direction is assumed to be normal to the boundary.
        • Backflow boundary data must be set for all transport variables.
        • Convergence difficulties minimized by realistic values for backflow quantities.
      • Suitable for compressible and incompressible flows
        • Pressure is ignored if flow is locally supersonic.
      • Can be used as a “free” boundary in an external or unconfined flow.
    • 41. OUTFLOW
      • No pressure or velocity information is required.
        • Data at exit plane is extrapolated from interior.
        • Mass balance correction is applied at boundary.
      • Flow exiting Outflow boundary exhibits zero normal diffusive flux for all flow variables.
        • Appropriate where exit flow is close to fully developed condition.
      • Intended for incompressible flows.
        • Cannot be used with a Pressure Inlet; must use velocity inlet.
          • Combination does not uniquely set pressure gradient over whole domain.
        • Cannot be used for unsteady flows with variable density.
      • Poor rate of convergence when back flow occurs during iteration.
        • Cannot be used if back flow is expected in final solution.
      • Used to bound fluid and solid regions.
      • In viscous flows, no-slip condition enforced at walls:
        • Tangential fluid velocity equal to wall velocity.
        • Normal velocity component = 0
        • Shear stress can also be specified.
      • Thermal boundary conditions:
        • several types available
        • Wall material and thickness can be defined for 1-D or shell conduction heat transfer calculations.
      • Wall roughness can be defined for turbulent flows.
        • Wall shear stress and heat transfer based on local flow field.
      • Translational or rotational velocity can be assigned to wall.
      • Symmetry Boundary
        • Used to reduce computational effort in problem.
        • No inputs required.
        • Flow field and geometry must be symmetric:
          • Zero normal velocity at symmetry plane
          • Zero normal gradients of all variables at symmetry plane
          • Must take care to correctly define symmetry boundary locations.
        • Can be used to model slip walls in viscous flow
      • Axis Boundary
        • Used at centerline for 2D axisymmetric problems.
        • No inputs required.
      symmetry planes
      • Used to reduce computational effort in problem.
      • Flow field and geometry must be either translationally or rotationally periodic.
      • For rotationally periodic boundaries:
        •  p = 0 across periodic planes.
        • Axis of rotation must be defined in fluid zone.
      • For translationally periodic boundaries:
        •  p can be finite across periodic planes.
          • Models fully developed conditions.
          • Specify either mean  p per period or net mass flow rate.
          • Periodic boundaries defined in Gambit are translational.
      Translationally periodic planes 2D tube heat exchanger flow Rotationally periodic planes
    • 45. CELL ZONES - FLUID
      • Fluid zone = group of cells for which all active equations are solved.
      • Fluid material input required.
        • Single species, phase.
      • Optional inputs allow setting of source terms:
        • mass, momentum, energy, etc.
      • Define fluid zone as laminar flow region if modeling transitional flow.
      • Can define zone as porous media.
      • Define axis of rotation for rotationally periodic flows.
      • Can define motion for fluid zone.
    • 46. CELL ZONES - SOLID
      • “ Solid” zone = group of cells for which only heat conduction problem solved.
        • No flow equations solved
        • Material being treated as solid may actually be fluid, but it is assumed that no convection takes place.
      • Only required input is material type
        • So appropriate material properties used.
      • Optional inputs allow you to set volumetric heat generation rate (heat source).
      • Need to specify rotation axis if rotationally periodic boundaries adjacent to solid zone.
      • Can define motion for solid zone
    • 48. What is GAMBIT ?
      • A single, integrated preprocessor for CFD analysis:
        • Geometry construction and import
          • Using STEP, Parasolid, IGES, etc. import
            • Cleanup and modification of imported data and volume
        • Mesh generation for all Fluent solvers
          • Structured and Unstructured hexahedral, tetrahedral, pyramid, and prisms.
        • Mesh quality examination
        • Mesh Export
        • Boundary zone assignment
    • 49. Types of Grids in Gambit
      • Structured
      • Unstructured
    • 50. Structured Grids
      • Topologically rectangular
      • This means that the mesh volume is a quadrilateral in 2d or a hexahedron in 3d
      • Each mesh volume is linked only to its immediate neighbors
      • But the edges can be mapped around curves and mesh volumes don’t have to be the same size
      • Reduces storage and CPU requirements
      • Solid regions like tank baffles can be generated by blanking those mesh points which overlap the solid region and making them “dead zones”. Fiddly but still generates a good grid
      • dead zones waste storage.
    • 51. Unstructured Grids
      • Mesh volumes can be linked to any other volume in the domain
      • And can be any shape
      • less computationally efficient than a structured grid
      • but can still read a structured grid topology (often still the best)
      • can use non-conformal grids
      • introduces flexbility but this flexbility creates problems
    • 52. Unstructured Grid Topologies
    • 53. Gambit - bottom up approach
      • create vertex’s
      • link vertex’s to make edges
      • create faces from edges
      • create volumes from faces (3d)
      • mesh edges
      • mesh faces
      • mesh volumes (3d)
    • 54. GUI Main Menu bar Global Control Operation toolpad Command line Description window
    • 55. Operation Tool Pads
        • Vertex
        • Edge
        • Face
        • Volume
        • Group
        • Boundary Layer
        • Edge
        • Face
        • Volume
        • Group
          • Boundary Types Boundary Entity
          • Continuum Types Continuum Entity
      Coordinate Systems Sizing Function G/Turbo User-Defined Tools
    • 56. Graphical User Interface
      • Command :
        • Input of (non-GUI) commands , e.g.,
          • reset : deletes all mesh and geometry in the current model
          • reset mesh : deletes mesh, keeps geometry
      • Description
        • Gives a short description of all global function buttons and screen areas
      • Transcript
        • Output from GAMBIT is printed here as well as in ident .trn
        • Transcript window can be expanded using arrow button in top right corner
    • 57. Global Control
    • 58. Mouse Operations
        • You can toggle between picking with or without “Shift”:
          • Keep right mouse button down while doing a “left-click”
          • The cursor now changes to another symbol
          • Now, Pick/Next/Accept do not need a “Shift”
          • The Rotation/Translation/Zoom now needs a “Shift”
    • 59. Thank You