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Boundary Conditions

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**Outline Purpose of Boundary Conditions Setting Boundary Conditions**

Flow Inlets and Exits Wall, Repeating, and Pole Boundaries Internal Cell Zones Internal Face Boundaries

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**Purpose of Boundary Conditions**

Boundaries direct and constrain motion of flow. Boundary Conditions are a required component of mathematical model. Specify fluxes into computational domain of: Mass Momentum Energy Boundary Conditions are assigned to Zones. Zones are a collection of cells (fluid or solid continuum) or cell faces (boundaries, interior surfaces). Surfaces are used for post-processing. Surfaces can correspond to Zones: Surfaces are automatically generated from cell face Zones. Surfaces corresponding to fluid and solid zones are not. out walls in K.E. of turbulence Dissipation rate of turbulence Species mass fractions

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**Setting Boundary Conditions**

To set boundary conditions for particular zone: Define Boundary Conditions... Choose the zone in Zone list. Click on selected zone type in Type list Click Set.. button Can also select boundary zone in graphics window using right mouse button. Useful if: Setting up problem for first time Two or more zones of same type in problem.

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**Flow Inlets and Exits - Introduction**

Wide range of boundary conditions permit flow to enter and exit solution domain. Types of boundary conditions for specification of flow inlets and exits: 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

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**Inlet and Outlet Boundaries**

Inlet and Outlet boundary conditions are available to specify fluxes for: Internal Flows: jet engine, reactor External Flows: aircraft in flight, natural convection flows General guidelines: Select inlet and outlet 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.

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Velocity Inlets (1) Defines velocity and scalar properties of flow at inlet boundaries. Useful when velocity or velocity profile is known at inlet. Intended for incompressible flows only. Total (or stagnation) properties of flow are not fixed. Stagnation properties vary to accommodate prescribed velocity distribution. Use in compressible flows non-physical result

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Velocity Inlets (2) User-defined functions (UDF) can be used to define spatial- and time- varying velocity profiles (magnitude and direction). If upstream flow comes from region of constant total energy and there are no losses (upstream), it may be easier to use the Pressure Inlet condition. Still need to specify direction of velocity vector. Don’t place 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.

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**Determining Turbulence Parameters**

When turbulent flow enters domain at inlet, outlet, or at a far-field boundary, FLUENT 5 requires boundary values for: Turbulent kinetic energy k Four methods available for specifying turbulence parameters: Set k and explicitly 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 = % 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 Turbulence dissipation rate

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**Pressure Preliminaries**

gauge pressure operating pressure pressure level absolute pressure vacuum Absolute pressure is referenced to a vacuum. Can be expressed relative to operating pressure as the gauge pressure: Static pressure is thermodynamic pressure (Stoke’s Hypothesis). Expressible as absolute or gauge pressure Boundary conditions require gauge pressure inputs.

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**Calculations Using Pressure**

Total (stagnation) pressure is defined as: Pressure at thermodynamic state which would exist if fluid were brought to rest (zero velocity) isentropically. 1/2 v2 is referred to as the dynamic pressure. Density can be calculated from the ideal gas law: For incompressible flow: For compressible flow: ptotal = pstatic + 1/2 v2

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**Setting Operating Pressure**

For compressible flows: Set operating pressure = 0 Treat gauge pressures as absolute pressures For incompressible, constant-density flows, operating pressure not used. For incompressible flows using ideal-gas law to determine density Select incompressible-ideal-gas in Define Materials... Set operating pressure close to mean pressure in problem.

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**Pressure Inlet Boundary (1)**

Defines total pressure, temperature, and other scalar quantities at flow inlets. Also requires direction of velocity vector to be defined. Can get non-physical results if you don’t specify reasonable direction for velocity vector. 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. Suitable for compressible and incompressible flows.

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**Pressure Inlet Boundary (2)**

Inflow stagnation properties are prescribed: Mechanical head of pressure/total pressure drives flow into computational domain. Mass flux varies depending on interior solution and direction specified for velocity vector. Note: Value specified for total pressure used as static pressure wherever outflow occurs. Total temperature set to static temperature for incompressible flows.

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**Pressure Outlet Boundary (1)**

Flow exits computational domain at fixed static pressure. Requires specification of static (gauge) pressure at outlet boundary. All other flow quantities at the pressure outlet boundary are extrapolated from the interior. Value of specified static pressure: used only while exit flow is subsonic. ignored for supersonic flow (pressure is extrapolated from flow in interior).

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**Pressure Outlet Boundary (2)**

“Backflow” conditions must be specified. Required for calculations if flow reverses direction at Pressure Outlet boundary during solution process. When backflow occurs, it is assumed to be normal to the boundary. Cannot specify the direction of the flow entering the domain, in contrast to pressure inlet boundary condition. Convergence difficulties minimized by realistic values for backflow quantities. Value specified for static pressure used as total pressure wherever backflow occurs. Pressure Outlet must be used when problem is set up with Pressure Inlet.

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Outflow Boundary Flow exiting domain at Outflow boundary has zero normal gradients for all flow variables except pressure. FLUENT extrapolates required information from interior. Useful when: 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.

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**Restrictions on Outflow Boundaries**

Outflow Boundaries cannot be used: with compressible flows. with the Pressure Inlet boundary condition (use Velocity Inlet instead): Combination does not uniquely set a pressure gradient over the whole domain. in unsteady flows with variable density. outflow condition ill-posed outflow condition not obeyed outflow condition obeyed outflow condition closely obeyed Do not use outflow boundaries where: Flow enters domain Gradients in flow direction are significant Conditions downstream of exit plane impact flow in domain

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**Modeling Multiple Exits**

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: 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. FRW2 velocity inlet FRW1 pressure-inlet (p0,T0) pressure-outlet (ps)2 velocity-inlet (v,T0) pressure-outlet (ps)1 or

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**Other Inlet/Outlet Boundary Conditions**

Mass Flow Inlet Used in compressible flows to prescribe mass flow rate at inlet. Not required for incompressible flows. Pressure Far Field 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 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.

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**Wall, Repeating, and Pole Boundaries**

Purpose of Boundary Conditions Setting Boundary Conditions Flow Inlets and Exits Wall, Repeating, and Pole Boundaries Wall Symmetry Periodic Axis Internal Cell Zones Internal Face Boundaries

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**Wall Boundaries Used to bound fluid and solid regions.**

In viscous flows, no-slip condition enforced at walls Tangential velocity component specified in terms of translational or rotational motion of wall boundary. Wall shear stress and heat transfer based on local flow field. Assumed to be rigid and impermeable Normal velocity component = 0 For accurate predictions of wall shear stress, be sure to resolve boundary layers in viscous flows.

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**Symmetry Boundaries Used to reduce computational effort in problem.**

Flow field and geometry must be symmetric: Zero normal velocity at symmetry plane Zero normal gradients of all variables at symmetry plane No inputs required. Must take care to correctly define symmetry boundary locations. Also used to model slip walls in viscous flow symmetry planes

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Periodic Boundaries 4 tangential inlets cyclic boundaries Used when physical geometry of interest and expected pattern of flow/thermal solution have periodically repeating nature. Reduces computational effort in problem. Two types available in FLUENT 5. Type 1: Does not allow pressure drop across periodic planes. Type 2: Periodic boundaries with pressure drop. Rotationally periodic I J Periodic at I=NI Periodic at I=1 Translationally periodic

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**Periodic Boundaries with Pressure Drop - Type 2**

MUST be translationally periodic Designed to model fully-developed conditions Fully-developed flow in pipes and ducts Tube banks Periodic heat transfer also possible Specify either: Mean pressure gradient per period Net mass flow rate computational domain Streamlines in a 2D tube heat exchanger flow direction

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**Axis Boundaries Used: Specify:**

At centerline (y=0) of an axisymmetric grid Where multiple grid lines meet at a point in a 3D O-type grid Specify: No inputs required AXIS boundary

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**Internal Cell Zones Purpose of Boundary Conditions**

Setting Boundary Conditions Flow Inlets and Exits Wall, Repeating, and Pole Boundaries Internal Cell Zones Fluid Porous Type of fluid zone Solid Internal Face Boundaries

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Fluid Conditions Fluid zone = group of cells for which all active equations are solved. Only required input is type of fluid material So appropriate material properties used Optional inputs allow setting of source terms: Heat Mass Momentum Can define motion for fluid zone If rotationally periodic boundaries adjacent to fluid zone, use rotation axis. Define fluid zone as laminar flow region if modeling transitional flow. Turbulence Species

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**Porous Media Conditions**

Porous zone modeled as special type of fluid zone. Enable Porous Zone option in Fluid panel. Pressure loss in flow determined via user inputs. Used to model flow through porous media and other “distributed” resistances: Packed beds Filter papers Perforated plates Flow distributors Tube banks

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Solid Conditions “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). Can define motion for solid zone Need to specify rotation axis if rotationally periodic boundaries adjacent to solid zone.

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**Internal Face Boundaries**

Defined on cell faces Do not have finite thickness Provide means of introducing step change in flow properties. Used to implement physical models representing: Fans Radiators Porous jump Thin porous membranes Interior wall

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**Summary 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.

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