Fluent Software Training TRN99 003 Boundary Conditions C

Fluent Software Training TRN-99 -003 Boundary Conditions C 1 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Outline u u Overview Inlet and Outlet Boundaries Velocity n n u u Pressure Boundaries and others. . . Wall, Symmetry, Periodic and Axis Boundaries Internal Cell Zones Fluid n n u Profiles Turbulence Parameters Porous Media Moving Cell Zones Solid Internal Face Boundaries C 2 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Overview u Boundary Conditions: u outlet e. g. , mass, momentum, and energy Fluid/Solid regions represented by cell zones. u orifice_plate and orifice_plate-shadow Specify fluxes into computational domain. u Boundaries direct motion of flow. Boundary Conditions are a required component of mathematical model. orifice (interior) Material and Source terms are assigned to cell zones. Boundaries and internal surfaces are represented by face zones. Boundary data are assigned to face zones. C 3 wall inlet fluid Example: Face and Cell zones associated with Pipe Flow through orifice plate © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Setting Boundary Conditions u u 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. n u u Select new zone type in Type list. To set boundary conditions for particular zone: u Can also select boundary zone using right mouse button in Display Grid window. 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 write-bc file read-bc C 4 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Flow Inlets and Outlets u Wide range of boundary conditions types permit flow to enter and exit solution domain: General n n n u Pressure inlet Pressure outlet Compressible flows n n Incompressible n u Velocity inlet Outflow Mass flow inlet Pressure far-field Special n Inlet vent, outlet vent, intake fan, exhaust fan Boundary data required depends on physical models selected. General guidelines: Select boundary location and shape such that flow either goes in or out. n Should not observe large gradients in direction normal to boundary. n Not necessary, but will typically observe better convergence. Indicates incorrect set-up. Minimize grid skewness near boundary. C 5 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Velocity Inlets u u Defines velocity vector and scalar properties of flow at inlet boundaries. Useful when velocity profile is known at inlet. u uniform profile is default Intended for incompressible flows. Total (stagnation) properties of flow are not fixed. n u Stagnation properties vary to accommodate prescribed velocity distribution. Using in compressible flows can lead to non-physical results. Avoid placing velocity inlet too close to a solid obstruction. Can force the solution to be non-physical, e. g. , imposes velocity field, etc. , at boundary that may not be intended. C 6 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Using Profiles u Alternative to UDF’s for defining boundary profiles. u Profiles can be generated by: u Writing a profile from another CFD simulation Creating an appropriately formatted text file with location information and boundary condition data. Profiles can be manipulated through: u Profiles can define spatial and time varying boundary conditions. Define Profiles data applied to boundary through ‘hooks’. C 7 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Determining Turbulence Parameters u u When turbulent flow enters domain at inlet, outlet, or at a far-field boundary, FLUENT 5 requires boundary values for: Turbulent kinetic energy k Turbulence dissipation rate Four methods available for specifying turbulence parameters: Set k and explicitly u Set turbulence intensity and turbulence length scale Set turbulence intensity and turbulent viscosity ratio Set turbulence intensity and hydraulic diameter Intensity and length scale depend on conditions upstream, e. g. : Exhaust of a turbine Intensity = 20 % Length scale = 1 - 10 % of blade span Downstream of perforated plate or screen Intensity = 10 % Length scale = screen/hole size Fully-developed flow in a duct or pipe Intensity = 5 % Length scale = hydraulic diameter C 8 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Pressure Boundary Conditions u pressure level Pressure boundary conditions require gauge pressure inputs: gauge pressure u Operating pressure input is set under: u Define Operating Conditions Useful when: flow rate and/or velocity is not known (e. g. , buoyancy-driven flows). “free” boundary in an external or unconfined flow needs to be defined. C 9 absolute pressure operating pressure vacuum © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Pressure Inlet Boundary (1) u Defines total pressure, temperature, and other scalar quantities at flow inlets. incompressible flows u Supersonic/Initial Gauge Pressure: u Defines static pressure at boundary for locally supersonic flows. Used, if necessary, to initialize flow field for incompressible flows. Total temperature: must be defined for compressible flows. is used, if necessary, to set static temperature for incompressible flows. C 10 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Pressure Inlet Boundary (2) u Flow Direction must be defined. u Suitable for compressible and incompressible flows. u Can get non-physical results if you don’t specify a reasonable direction. Pressure inlet boundary is treated as loss-free transition from stagnation to inlet conditions. Mass flux through boundary varies depending on interior solution and specified flow direction. Outflow can occur at pressure inlet boundaries. Flow direction taken from interior solution. Exhaust static pressure is defined by value specified for gauge total pressure wherever outflow occurs. C 11 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Pressure Outlet Boundary (1) u Defines static (gauge) pressure at the outlet boundary. u u Interpreted as static pressure of environment into which flow exhausts. Radial equilibrium pressure distribution option available. Backflow can occur at pressure outlet boundaries: during solution process or as part of solution. Backflow is assumed to be normal to the boundary. Convergence difficulties minimized by realistic values for backflow quantities. Value specified for static pressure used as total pressure wherever backflow occurs. C 12 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Pressure Outlet Boundary (2) u For incompressible flows: u For compressible flows: u The static pressure input defines the boundary pressure All other flow quantities are extrapolated from the interior. The static pressure input is ignored if locally supersonic. All flow quantities are extrapolated from interior. Pressure Outlet must be used when problem is set up with Pressure Inlet. C 13 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Outflow Boundary u u u Flow exiting domain at Outflow boundary has zero normal gradients for all flow variables except pressure. FLUENT extrapolates required information from interior. Useful when: u Details of flow velocity and pressure not known prior to solution of flow problem. Appropriate where exit flow is close to fully developed condition. Note: Use of Pressure Outlet (instead of Outflow) often results in better rate of convergence when backflow occurs during iteration. C 14 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Restrictions on Outflow Boundaries u Outflow Boundaries cannot be used: with compressible flows. with the Pressure Inlet boundary condition (use Velocity Inlet instead): n u Combination does not uniquely set a pressure gradient over the whole domain. in unsteady flows with variable density. Do not use outflow boundaries where: Flow enters domain Gradients in flow direction are significant Conditions downstream of exit plane impact flow in domain outflow condition ill-posed C 15 outflow condition not obeyed outflow condition closely obeyed © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Modeling Multiple Exits u Using Outflow boundary condition: Mass flow divided equally among all outflow boundaries by default. Flow Rate Weighting (FRW) set to 1 by default. For uneven flow distribution: n n u specify Flow Rate Weighting for each outflow boundary: mi=FRWi/ FRWi. static pressure varies among exits to accommodate flow distribution. Can also use Pressure Outlet boundaries to define exits. velocity-inlet (v, T 0) or pressure-inlet (p 0, T 0) FRW 1 velocity inlet FRW 2 pressure-outlet (ps)1 pressure-outlet (ps)2 C 16 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Other Inlet/Outlet Boundary Conditions u Mass Flow Inlet u Pressure Far Field u Available when density is calculated from the ideal gas law. Used to model free-stream compressible flow at infinity, with free-stream Mach number and static conditions specified. Exhaust Fan/Outlet Vent u Used in compressible flows to prescribe mass flow rate at inlet. Not required for incompressible flows. Model external exhaust fan/outlet vent with specified pressure jump/loss coefficient and ambient (discharge) pressure and temperature. Inlet Vent/Intake Fan Model inlet vent/external intake fan with specified loss coefficient/ pressure jump, flow direction, and ambient (inlet) pressure and temperature. C 17 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Wall Boundaries u u Used to bound fluid and solid regions. In viscous flows, no-slip condition enforced at walls: u Thermal boundary conditions: u several types available. Wall material and thickness can be defined for 1 -D or in-plane thin plate heat transfer calculations. Wall roughness can be defined for turbulent flows. u Tangential fluid velocity equal to wall velocity. Normal velocity component = 0 Wall shear stress and heat transfer based on local flow field. Translational or rotational velocity can be assigned to wall. Shear stress can also be specified. C 18 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Symmetry Boundaries u u Used to reduce computational effort in problem. Flow field and geometry must be symmetric: n n u No inputs required. u Zero normal velocity at symmetry plane Zero normal gradients of all variables at symmetry plane Must take care to correctly define symmetry boundary locations. Also used to model slip walls in viscous flow symmetry planes C 19 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Periodic Boundaries u Used when physical geometry of interest and expected pattern of flow/thermal solution have periodically repeating nature. u Reduces computational effort in problem. Two types available in FLUENT 5. p = 0 across periodic planes. n Rotationally or translationally periodic. s p is finite across periodic planes. n n n Rotationally periodic boundaries require axis of rotation be defined in fluid zone. Translationally periodic only. Models fully developed conditions. Specify either mean p period or net mass flow rate. By default, periodic boundaries defined in Gambit are assumed to be translational in FLUENT 5. C 20 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Periodic Boundaries: Examples p = 0: p > 0: 4 tangential inlets computational domain flow direction Rotationally periodic boundaries Streamlines in a 2 D tube heat exchanger Translationally periodic boundaries C 21 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Axis Boundaries u Used: u At centerline (y=0) of an axisymmetric grid Where multiple grid lines meet at a point in a 3 D O-type grid Specify: No inputs required AXIS boundary C 22 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Cell Zones: Fluid u u Fluid zone = group of cells for which all active equations are solved. Fluid material input required. u Optional inputs allow setting of source terms: u u Single species, phase. 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. C 23 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Porous Media Conditions u Porous zone modeled as special type of fluid zone. u Enable Porous Zone option in Fluid panel. Pressure loss in flow determined via user inputs of resistance coefficients to lumped parameter model. Used to model flow through porous media and other “distributed” resistances, e. g. , Packed beds Filter papers Perforated plates Flow distributors Tube banks C 24 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Moving Zones u Single Zone Problems: Rotating Reference Frame Model n n u define zone as Moving Reference Frame limited applicability Multiple Zone Problems: Each zone defined as moving reference frame: n Multiple Reference Frame Model s n Mixing Plane Model s least accurate, least demanding on CPU field data are averaged at the outlet of one zone and used as inlet boundary data to adjacent zone. Each zone defined as Moving Mesh: n Sliding Mesh Model s s s must also define interface. Mesh positions are calculated; time-accurate simulations relative motion must be tangential (no normal translation) C 25 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Cell Zones: Solid u “Solid” zone = group of cells for which only heat conduction problem solved. u u Material being treated as solid may actually be fluid, but it is assumed that no convection takes place. Only required input is material type u u u No flow equations solved 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 C 26 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Internal Face Boundaries u Defined on cell faces u Do not have finite thickness Provide means of introducing step change in flow properties. Used to implement physical models representing: Fans Radiators Porous jump n Preferable over porous media- exhibits better convergence behavior. Interior wall C 27 © Fluent Inc. 11/1/2020

Fluent Software Training TRN-99 -003 Summary u u u Zones are used to assign boundary conditions. Wide range of boundary conditions permit flow to enter and exit solution domain. Wall boundary conditions used to bound fluid and solid regions. Repeating boundaries used to reduce computational effort. Internal cell zones used to specify fluid, solid, and porous regions. Internal face boundaries provide way to introduce step change in flow properties. C 28 © Fluent Inc. 11/1/2020