fluent tut 12

Chapter 12: Using Multiple Reference Frames

This tutorial is divided into the following sections:

12.1. Introduction

12.2. Prerequisites

12.3. Problem Description

12.4. Setup and Solution

12.5. Summary

12.6. Further Improvements

12.1. Introduction

Many engineering problems involve rotating flow domains. One example is the centrifugal blower unit

that is typically used in automotive climate control systems. For problems where all the moving parts

(fan blades, hub and shaft surfaces, etc.) are rotating at a prescribed angular velocity, and the stationary

walls (e.g., shrouds, duct walls) are surfaces of revolution with respect to the axis of rotation, the entire

domain can be referred to as a single rotating frame of reference. However, when each of the several

parts is rotating about a different axis of rotation, or about the same axis at different speeds, or when

the stationary walls are not surfaces of revolution (such as the volute around a centrifugal blower wheel),

a single rotating coordinate system is not sufficient to immobilize" the computational domain so as

to predict a steady-state flow field. In such cases, the problem must be formulated using multiple refer-

ence frames.

In ANSYS FLUENT, the flow features associated with one or more rotating parts can be analyzed using

the multiple reference frame (MRF) capability. This model is powerful in that multiple rotating reference

frames can be included in a single domain. The resulting flow field is representative of a snapshot of

the transient flow field in which the rotating parts are moving. However, in many cases the interface

can be chosen in such a way that the flow field at this location is independent of the orientation of the

moving parts. In other words, if an interface can be drawn on which there is little or no angular depend-

ence, the model can be a reliable tool for simulating time-averaged flow fields. It is therefore very useful

in complicated situations where one or more rotating parts are present.

This tutorial illustrates the procedure for setting up and solving a problem using the MRF capability. As

an example, the flow field on a 2D section of a centrifugal blower will be calculated. Although this is a

general methodology that can be applied to cases where more than one reference frame is moving,

this example will be limited to a single rotating reference frame.

This tutorial demonstrates how to do the following:

Create mesh interfaces from interface-zones defined during meshing.

Specify different frames of reference for different fluid zones.

Set the relative velocity of each wall.

Calculate a solution using the pressure-based solver.

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of ANSYS, Inc. and its subsidiaries and affiliates.

12.2. Prerequisites

This tutorial is written with the assumption that you have completed one or more of the introductory

tutorials found in this manual:

Introduction to Using ANSYS FLUENT in ANSYS Workbench: Fluid Flow and Heat Transfer in a Mixing

Elbow (p. 1)

Parametric Analysis in ANSYS Workbench Using ANSYS FLUENT (p. 77)

Introduction to Using ANSYS FLUENT: Fluid Flow and Heat Transfer in a Mixing Elbow (p. 131)

This tutorial also assumes that you are familiar with the ANSYS FLUENT navigation pane and menu

structure. Some steps in the setup and solution procedure will not be shown explicitly.

In general, to solve problems using the MRF feature, you should be familiar with the concept of creating

multiple fluid zones in your mesh generator.

12.3. Problem Description

This problem considers a 2D section of a generic centrifugal blower. A schematic of the problem is

shown in Figure 12.1 (p. 524). The blower consists of 32 blades, each with a chord length of 13.5 mm.

The blades are located approximately 56.5 mm (measured from the leading edge) from the center of

rotation. The radius of the outer wall varies logarithmically from 80 mm to 146.5 mm. You will simulate

the flow under no load, or free-delivery conditions when inlet and outlet pressures are at ambient

conditions (0 Pa gauge). This corresponds to the maximum flow-rate of the blower when sitting in free

air. The blades are rotating with an angular velocity of 2500 rpm. The flow is assumed to be turbulent.

Figure 12.1 Schematic of the Problem

12.4. Setup and Solution

The following sections describe the setup and solution steps for this tutorial:

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12.4.1. Preparation

12.4.2. Step 1: Mesh

12.4.3. Step 2: General Settings

12.4.4. Step 3: Models

12.4.5. Step 4: Materials

12.4.6. Step 5: Cell Zone Conditions

12.4.7. Step 6: Boundary Conditions

12.4.8. Step 7: Mesh Interfaces

12.4.9. Step 8: Solution

12.4.10. Step 9: Postprocessing

12.4.1. Preparation

1. Extract the file multiple_rotating.zip from the ANSYS_Fluid_Dynamics_Tutori-al_Inputs.zip archive that is available from the Customer Portal.

Note

For detailed instructions on how to obtain the ANSYS_Fluid_Dynamics_Tutori-al_Inputs.zip file, please refer to Preparation (p. 3) in Introduction to Using ANSYSFLUENT in ANSYS Workbench: Fluid Flow and Heat Transfer in a Mixing Elbow (p. 1).

2. Unzip multiple_rotating.zip to your working directory.

The file, blower-2d.msh can be found in the multiple_rotating directory created after un-zipping the file.

3. Use FLUENT Launcher to start the 2D version of ANSYS FLUENT.

For more information about FLUENT Launcher, see Starting ANSYS FLUENT Using FLUENT Launcher in

the User's Guide.

Note

The Display Options are selected by default. Therefore, once you read in the mesh, it willbe displayed in the embedded graphics window.

12.4.2. Step 1: Mesh

1. Read the mesh file (blower-2d.msh).

File Read Mesh...

12.4.3. Step 2: General Settings

General

1. Check the mesh.

General Check



Setup and Solution

ANSYS FLUENT will perform various checks on the mesh and will report the progress in the console.

Make sure that the reported minimum volume is a positive number. It will also issue warnings about

unassigned interface zones. You do not need to take any action now. You will set up the mesh interfaces

in a later step.

2. Examine the mesh (Figure 12.2 (p. 526)).

The mesh consists of three fluid zones, fluid-casing, fluid-inlet, and fluid-rotor. These are reported

in the console when the mesh is read. In the Mesh Display dialog box, the fluid zones are reported as

interior zones default-interior, default-interior:013, and default-interior:015 respectively. The fluid

zone containing the blades will be solved in a rotational reference frame.

Figure 12.2 Mesh of the 2D Centrifugal Blower

The fluid zones are bounded by interface zones that appear in the mesh display in yellow. These interface

boundaries were used in the mesh generator to separate the fluid zones, and will be used to create

mesh interfaces between adjacent fluid zones when the boundary conditions are set later in this tutorial.

3. Set the units for angular velocity.



General Units...

In the problem description, angular velocity is specified in rpm rather than in the default unit of rad/s.

a. Select angular-velocity from the Quantities list and rpm in the Units list.

b. Close the Set Units dialog box.

4. Retain the default solver settings.

General

12.4.4. Step 3: Models

Models



Setup and Solution

1. Enable the standard - turbulence model.

Models Viscous Edit...

a. Select k-epsilon (2eqn) in the Model list.

b. Select Enhanced Wall Treatment in the Near-Wall Treatment list.

c. Click OK to close the Viscous Model dialog box.

12.4.5. Step 4: Materials

Materials

1. Retain the default properties for air.

Materials air Create/Edit...



Extra

If needed, you could modify the fluid properties for air or copy another material from

the database.

2. Click Close to close the Create/Edit Materials dialog box.

For details, see "Physical Properties" in the User's Guide.

12.4.6. Step 5: Cell Zone Conditions

Cell Zone Conditions



Setup and Solution

1. Define the boundary conditions for the rotational reference frame (fluid-rotor).

Cell Zone Conditions fluid-rotor Edit...



a. Enable Frame Motion.

b. Click the Reference Frame tab.

c. Retain the Rotation-Axis Origin default setting of (0,0).

This is the center of curvature for the circular boundaries of the rotating zone.

d. Enter -2500 rpm for Speed in the Rotational Velocity group box.

Note

The speed is entered as a negative value because the rotor is rotating clockwise

which is in the negative sense about the z-axis.

e. Click OK to close the Fluid dialog box.

Note

Since the other fluid zones are stationary, you do not need to set any boundary condi-

tions for them. If one or more of the remaining fluid zones were also rotating, you

would need to set the appropriate rotational speed for them.



Setup and Solution

Tip

In this example the names of the fluid-zones in the mesh file leave no ambiguity as to

which is the rotating fluid zone. In the event that you have a mesh without clear names,

you may have difficulty identifying the various fluid-zones. Unlike interior zones, the

fluid-zones cannot be individually selected and displayed from the Mesh Display dialogbox. However, you can use commands in the text interface to display them.

Cell Zone Conditions Display Mesh...

i. Click the interior zones, default-interior, default-interior:013, and default-interior:015in the Surfaces selection list to deselect them.

ii. Click Display.

Only the domain boundaries and interface zones will be displayed.

iii. Press the Enter key to get the > prompt.

iv. Type the commands, in the console, as shown.

> display

/display> zone-mesh()zone id/name(1) [()] 4zone id/name(2) [()]

The resulting display shows that the zone with ID 4 (in this case fluid-rotor) corresponds

to the rotating region.

v. Close the Mesh Display dialog box.

12.4.7. Step 6: Boundary Conditions

Boundary Conditions



1. Set the boundary conditions (see Figure 12.1 (p. 524)) for the flow inlet (inlet).

Boundary Conditions inlet Edit...



Setup and Solution

a. Enter 0 Pa for Gauge Total Pressure.

b. Select Intensity and Viscosity Ratio from the Specification Method drop-down list.

c. Enter 5 % for Turbulent Intensity.

d. Retain the default value of 10 for Turbulent Viscosity Ratio.

e. Click OK to close the Pressure Inlet dialog box.

Note

All pressures that you specify in ANSYS FLUENT are gauge pressures, relative to the

operating pressure specified in the Operating Conditions dialog box. By default, theoperating pressure is 101325 Pa.

For details, see Operating Pressure in the User's Guide.

2. Set the backflow turbulence parameters for the flow outlet (outlet) to the same values used for inlet.

Note

The backflow values are used only if reversed flow occurs at the outlet, but it is a good

idea to use reasonable values, even if you do not expect any backflow to occur.

3. Define the velocity of the wall zone representing the blades (blades) relative to the moving fluid zone.

Boundary Conditions blades Edit...



With fluid-rotor set to a rotating reference frame, blades becomes a moving wall.

a. Select Moving Wall in the Wall Motion list.

The Wall dialog box will expand to show the wall motion parameters.

b. Retain the default selection of Relative to Adjacent Cell Zone and select Rotational in the Motiongroup box.

c. Retain the default value of 0 rpm for (relative) Speed.

d. Click OK to close the Wall dialog box.

The Rotation-Axis Origin should be located at = 0 m and = 0 m. With these settings, the

blades will move at the same speed as the surrounding fluid.

12.4.8. Step 7: Mesh Interfaces

Recall that the fluid domain is defined as three distinct fluid zones. You must define mesh interfaces between

the adjacent fluid zones so that ANSYS FLUENT can solve the flow equations across the interfaces.

1. Set up the mesh interface between fluid-inlet and fluid-rotor.



Setup and Solution

Mesh Interfaces Create/Edit...

a. Enter int1 under Mesh Interface to name this interface definition.

b. Select interface-1 for Interface Zone 1 and interface-2 for Interface Zone 2.

You can use the Draw button to help identify the interface-zones.

c. Click Create in order to create the mesh interface, int1.

d. In a similar manner define a mesh interface called int2 between interface-3 and interface-4.

e. Close the Create/Edit Mesh Interfaces dialog box.

12.4.9. Step 8: Solution

1. Set the solution parameters.

Solution Methods



a. Select Coupled from Scheme drop-down list in the Pressure-Velocity Coupling group box.

b. Select Second Order Upwind for Turbulent Kinetic Energy and Turbulent Dissipation Ratein the Spatial Discretization group box.

The second-order scheme will provide a more accurate solution.

2. Ensure that plotting of residuals during the calculation is selected.

Monitors Residuals Edit...



Setup and Solution

a. Ensure that Plot is enabled in the Options group box.

b. Enter 5e-5 under Absolute Criteria for the continuity equation.

For this problem, the default value of 0.001 is insufficient for the flow rate in the blower to fullyconverge.

c. Click OK to close the Residual Monitors dialog box.

3. Create a surface monitor and plot the volume flow rate at the flow outlet.

Monitors (Surface Monitors) Create...



a. Enable the Plot and Write options for surf-mon-1.

Note

When the Write option is selected in the Surface Monitor dialog box, the massflow rate history will be written to a file. If you do not enable the Write option,the history information will be lost when you exit ANSYS FLUENT.

b. Select Volume Flow Rate from the Report Type drop-down list.

c. Select outlet from the Surfaces selection list.

d. Click OK in the Surface Monitor dialog box to enable the monitor.

4. Initialize the solution.

Solution Initialization



Setup and Solution

a. Retain the default selection of Hybrid Initialization in the Initialization Methods group box.

b. Click Initialize to initialize the solution.

Note

For flows in complex topologies, hybrid initialization will provide better initial ve-

locity and pressure fields than standard initialization. This in general will help in

improving the convergence behavior of the solver.

5. Save the case file (blower.cas.gz).

File Write Case...

6. Start the calculation by requesting 150 iterations.

Run Calculation

a. Enter 150 for Number of Iterations.

b. Click Calculate.



Early in the calculation, ANSYS FLUENT will report that there is reversed flow occurring at the exit.

This is due to the sudden expansion, which results in a recirculating flow near the exit.

The solution will converge in approximately 105 iterations (when all residuals have dropped

below their respective criteria).

Figure 12.3 History of Volume Flow Rate on outlet

The surface monitor history indicates that the flow rate at the outlet has ceased changing signi-

ficantly, further indicating that the solution has converged. The volume flow rate is approximately

3.26 m3

/s.

7. Save the case and data files (blower2.cas.gz and blower2.dat.gz).

File Write Case & Data...

Note

It is good practice to save the case file whenever you are saving the data. This will

ensure that the relevant parameters corresponding to the current solution data are

saved accordingly.

12.4.10. Step 9: Postprocessing

1. Display filled contours of static pressure (Figure 12.4 (p. 543)).

Graphics and Animations Contours Set Up...



Setup and Solution

a. Enable Filled in the Options group box.

b. Select Pressure... and Static Pressure from the Contours of drop-down lists.

c. Click Display and close the Contours dialog box (see Figure 12.4 (p. 543)).



Figure 12.4 Contours of Static Pressure

2. Display velocity vectors (Figure 12.5 (p. 545)).

Graphics and Animations Vectors Set Up...



Setup and Solution

a. Enter 10 for Scale.

By default, Auto Scale is chosen. This will automatically scale the length of velocity vectors relative

to the size of the smallest cell in the mesh. To increase the length of the scaled" vectors, set the

Scale factor to a value greater than 1.

b. Click Display and close the Vectors dialog box (see Figure 12.5 (p. 545)).



Figure 12.5 Velocity Vectors

The velocity vectors show an area of flow separation near the bottom of the outlet duct. You can zoom

in on this area and see the flow recirculation.

3. Report the mass flux at inlet and outlet.

Reports Fluxes Set Up...



Setup and Solution

a. Retain the selection of Mass Flow Rate in the Options group box.

b. Select inlet and outlet in the Boundaries selection list.

c. Click Compute.

The net mass imbalance should be no more than a small fraction (say, 0.5%) of the total flux

through the system. If a significant imbalance occurs, you should decrease your residual tolerances

by at least an order of magnitude and continue iterating.

The flux report will compute fluxes only for boundary zones.

d. Close the Flux Reports dialog box.

Use the Surface Integrals option to report fluxes on surfaces or planes.

Reports Surface Integrals Set Up...

12.5. Summary

This tutorial illustrates the procedure for setting up and solving problems with multiple reference frames

using ANSYS FLUENT. Although this tutorial considers only one rotating fluid zone, extension to multiple

rotating fluid zones is straightforward as long as you delineate each fluid zone.

Note that this tutorial was solved using the default absolute velocity formulation. For some problems

involving rotating reference frames, you may want to use the relative velocity formulation. See the

ANSYS FLUENT User's Guide for details.

12.6. Further Improvements

This tutorial guides you through the steps to reach an initial solution. You may be able to obtain a more

accurate solution by using an appropriate higher-order discretization scheme and by adapting the mesh.



Mesh adaption can also ensure that the solution is independent of the mesh. These steps are demon-

strated in Introduction to Using ANSYS FLUENT: Fluid Flow and Heat Transfer in a Mixing Elbow (p. 131).



Further Improvements

fluent tut 12

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