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1、Boundary Conditions,Defining Boundary Conditions,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: ident
2、ifying 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
3、 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.,Locating Boundaries: Example,Three possible approaches in locating inlet boundaries: 1. Upstream of manif
4、old Can use uniform profile Properly accounts for mixing Non-premixed reaction models Requires more cells 2. Nozzle inlet plane Non-premixed reaction models Requires accurate profile data 3. Nozzle outlet plane Premixed reaction model Requires accurate profile,General Guidelines,General guidelines:
5、If possible, select boundary location and shape such that flow either goes in or out. Not necessary, but will typically observe better convergence. Should not observe large gradients in direction normal to boundary. Indicates incorrect set-up. Minimize grid skewness near boundary. Introduces error e
6、arly in calculation.,Available Boundary Condition Types,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:
7、 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,Changing Boundary Condition Types,Zones a
8、nd 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.,Setting Boundary Condition Data
9、,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
10、 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.,Velocity Inlet,Specify Velocity by: Magnitude, Normal to Boundary Components Magnitude and Dir
11、ection 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 speci
12、fying negative velocity. You must ensure that mass conservation is satisfied if multiple inlets are used.,Pressure Inlet (1),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 in
13、terior 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,Pressure Inlet (2),Note: Gauge pressur
14、e 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 fl
15、ux through boundary varies depending on interior solution and specified flow direction.Can be used as a “free” boundary in an external or unconfined flow.,Mass Flow Inlet,Specify: (a) Mass Flow Rate or (b) Mass Flux (a) implies uniform mass flux (b) can be defined by profiles/UDF Static Gauge Pressu
16、re 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 Intended for compressible; can be used for incompressible flows. Total pre
17、ssure adjusts to accommodate mass flow inputs. More difficult to converge than with pressure inlet.,Pressure Outlet,Specify static gauge pressure Interpreted as static pressure of environment into which flow exhausts. Radial equilibrium pressure distribution option available. Doubles as inlet pressu
18、re (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.,
