FfC v5.0 06 Flow Physics

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February 17, 2009Inventory #002702

Introduction to FLUENT for CATIA V5

Flow Physics



Training Manual

FLUENT for CATIA V5 Version 5.0Introductory Training, December 2009

Fluent for Catia Support center http://www.fluentusers.com/ffc

Agenda• The Physical Model Definition Panel• Thermal Effects• Flow Type• Turbulence Models• Setting Default Turbulence Model• Compressibility and Natural Convection• Natural Convection• Species Transport and Cavitation• Setting Up Species Transport• Setting Up Cavitation• Setting Up VOF• Patching and Open Channel Flow• Periodic Flows• Moving Reference Frames: SRF and MRF• Setting up Moving Reference Frames• Sliding (Moving) Mesh Simulations



Training Manual



• The Physical Model Definition panel is accessed by double clicking the Physics Definition under Environment of the specification tree or the corresponding icon in the FFC workbench.

• The Physical Model Definition panel allows users to specify if:– Temperature effects need to be accounted.– Heat Conduction in solid materials is to be

considered.– Whether the flow is laminar, turbulent or

inviscid.– Specify the turbulence model if needed.– If natural convection or varying fluid

density is to be included.– Whether the calculation is steady state or

transient.– If the flow is single phase, involves

calculation of species transport or cavitation

The Physical Model Definition Panel



Training Manual



• Are temperature (thermal) effects considered?– If yes, energy conservation

equation is included and the fluid temperature is calculated.

– When temperature distribution (heat conduction) in solids is to be determined, turn on the “Include Solid” button

– If not, numerical calculation only considers velocities (mass and momentum conservation)

– For natural convection flows, use the Boussinesq approximation or incompressible ideal gas

Thermal Effects



Training Manual



• Is the flow laminar, turbulent or inviscid?– When the Reynolds number is

low, it is laminar – When the Reynolds number is

high, it is turbulent– When a strong swirling motion

of the flow is involved, one should model it as turbulent with strong swirl.

– If you want to neglect the effect of viscosity, i.e. perform an Euler calculation, choose the Inviscid flow.

Flow Type



Training Manual



Turbulence Models• The turbulence models available are:

– Standard k-epsilon: standard model for high Reynold’s number flows; robust, accurate for simple turbulent flows (pipes, plates etc.)

– Realizable k-epsilon: improves on standard model; accurate for flows with separation, flows impinging on surfaces and jet flows.

– RNG k-epsilon: increases range of standard model; accurate for flows with moderate swirl and separation.

– K-omega: useful for low Reynold’s number flows with transition between laminar and turbulent regimes; requires fine mesh near the walls.

– SST k-omega: Behaves like k-omega near walls and as k-epsilon away from the wall. Useful for flows with high turbulence production or transition from laminar to turbulent near walls.



Training Manual



• Advanced turbulence models are available panel in the Fluent Options/Customization tab.

• Turn on the Advanced Turbulence Models option.

• Select the default the turbulence model.– For most flows, the realizable

k-epsilon is a good choice.• See the FFC users guide for

more information on turbulence models.

Setting Default Turbulence Model



Training Manual



Compressibility and Natural Convection• Is the flow incompressible or

compressible?– In general low-speed flows (i.e.,

gaseous or liquid flows) can be treated as incompressible.

– Compressibility effect is important mainly for gaseous flows at higher speeds - local Mach number of the flow is higher than 0.3.

– When low-Mach number gaseous flows with variable density is modeled (e.g., temperature variation is significant, hence density is not a constant), incompressible ideal gas should be selected.

• Natural convection can be modeled by either Boussinesq or Incompressible ideal gas model.



Training Manual



Natural Convection• Boussinesq model introduces the

buoyancy term in the momentum equation(s), but treats density as a constant.

– The thermal expansion coefficient of the fluid needs to be specified in the Materials panel.

• Incompressible Ideal Gas varies the density as a function of temperature.

– The mean pressure is specified in the operating conditions panel.

• In the operating conditions panel, the gravitational acceleration vector has to be specified.

• See “Modeling Natural Convection Tutorial” for details in the case setup.



Training Manual



• The flow can be a single phase flow or involve species or Multiphase:– Single phase flow: only one material is present

in the flow volume. If the flow between the volumes is not mixing with each other/not interacting and separated by walls (immiscible), then we can use single phase flow solutions.

– Species Transport: A mixture of miscible gases or liquids is flowing through the flow volume. e.g: water vapor in air, methane and air mixing

– Multiphase: In multiphase flow, a phase can be defined as an identifiable class of material (gas, liquid and solid) that has a particular inertial response to and interaction with the flow and the potential field in which it is immersed. FfC offers two multiphase models.

• Cavitation: A liquid is present in the flow volume, which forms vapor bubbles when the pressure decreases below a critical pressure

• VOF: Can model two or more immiscible fluids. e.g. prediction of jet breakup, the motion of large bubbles in a liquid, the motion of liquid after a dam break, and the steady or transient tracking of any liquid-gas interface

Species Transport and Cavitation



Training Manual



• To set up Species:– Turn on Species Transport in

Model Definition.– Assign a mixture material to

the corresponding flow volume.

– Edit the mixture materials to add/remove materials.

– Set the Species boundary condition for each inlet and outlet.

Setting Up Species Transport



Training Manual



Setting Up Cavitation• To set up cavitation problems:

– Activate Cavitation in the Physical Modeling Definition panel.

– Create a User Defined material, and select the secondary gas phase.

– Click on the Cavitation item in Fluent Problem Setup and specify the cavitation parameters, including material.

– Set the Phase volume fraction for inlets and outlets for the secondary phase.



Training Manual



Setting Up the VOF model• To Set Up VOF problems

– Choose “Multiphase” in Physics Definition panel and

– “VOF” as the multiphase model– Add the secondary phases by

adding user materials– Set up “Secondary material” in the

Multiphase Definition panel (“VOF”) in Fluent Problem Set Up tree menu

– To add more secondary phases, right click on the menu (as shown)

– Select other VOF options like,• Implicit/Explicit Schemes • Interpolation schemes and

numerical controls• Open channels flows• Flat height initialization

– Enter surface tension value between the phases in phase interaction panel

FfC GUI panels for VOF set-up

Tank filling simulation in FfC

Right click here to add more secondary

phases



Training Manual



Patching and Open Channel Flows

Open Channel Boundary Conditions Flat height Initialization

Patching of Secondary Phases



Training Manual



Periodic Flow Modeling• Specify Periodic zone

– Right Click the Geometry definition.1 and select Periodic specification option.

– Select the two faces formed on the one side of the geometry (because of the symmetry operation) under Periodic faces and rest two faces on the other side under shadow faces.(as shown in figure)

– Under Periodic type select ‘rotational’ or ‘translational’

– Click Ok

Periodic planes



Training Manual



Modeling Rotating Machinery• Rotating machinery can be modeled

using the SRF (single rotating reference frame) and MRF (multiple rotating reference frame) or Sliding Mesh methods.

• SRF and MRF are methods applied to the modeling of moving zones, especially in rotating-machinery applications, in which steady-state approximations are obtained for problems involving rotating parts

• In MRF model, the face separating the rotating zone and the stationary zone must be a surface of revolution in the direction of the angular velocity, as shown in the schematic.

• In the Moving Mesh model, the rotating zone mesh moves with time in an unsteady calculation, and the fluid motion is calculated for each time step.

rotating zone

stationary zone

a surface ofrevolution

A schematic of Mixing Tank modelby using the MRF



Training Manual



• Identify the rotating zone (the volume to be attached to a rotating reference frame) by double clicking the zone (e.g., Flow Property.2)

• Enable the MRF model in that zone by selecting “Yes” for “Rotating Machinery” in Flow Property Definition panel for that zone.

• Click on to open the Motion of Rotating Machinery dialog box: rotation speed, angular velocity vector’s origin and direction are specified here accordingly.

• The button changes to after the specification

• Select Reference Zone (Frame) in the Fluent Solution.X tree

Setting up Moving Reference Frames



Training Manual



Setting Up Moving Mesh• To assign moving mesh motion:

– Select Unsteady in the Physics specification for time.

– Double click on the Flow or Solid property icon.

– Select Moving Mesh from the drop down list in front of Motion Type.

– Click the icon to open the Rotational Inputs panel .

– Specify the origin for axis of rotation.

– Select support for direction calculation or define the direction.

– Specify the direction of rotation either clockwise or Counter Clockwise.



Training Manual



Running Moving Mesh Calculations• For rotating machinery

calculations, the number of time steps as well as the time step size have to be chosen correctly.

– The time step size is critical for numerical stability and accuracy.

– The number of time steps is chosen so that the rotating zone goes through a number of revolutions and the solution develops periodicity.



Training Manual



Setting Unsteady Parameters • Solution parameters for

unsteady calculation– Double-click on Unsteady

Parameters.1 below the Fluent Problem Setup.1 feature in the specification tree to open the Unsteady Parameters dialog box

– Select appropriate time step size.• In one time step motion of sliding mesh

must not be greater then the length of the minimum mesh size on the edge of the moving region.

• Time step size must be less then the value obtained after dividing the minimum mesh size (say in m) by tangential velocity (m/sec).

– Specify total number of time steps.– Specify Max. iteration per time step

• This can be set by observing approximate number of iteration required to get converged solution for each time step.

– Specify data save frequency.

FfC v5.0 06 Flow Physics

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