Manifold

Tutorial Flow in a 3D Manifold

Introduction

This tutorial illustrates the setup and solution of an unsteady manifold calculation usingFLUENT Flows within intake and exhaust manifolds are of great interest to automotiveengineers Computational fluid dynamics (CFD) can provide valuable information aboutthe nature of the manifold flow field including total pressure losses and flow splits for themanifold runners

Very often the flows of interest are also unsteady A typical example would be an exhaustmanifold for which the runner inlet flows vary with the engine cycle In this situation thelosses and flow splits vary as a function of time and it is possible for the time-averagedunsteady flow to be different from a steady-state solution using average inletoutlet condi-tions

The analysis of an unsteady manifold calculation will be carried out in two phases

1 Setup and solve a steady-state solution using constant inlet or outlet boundary con-ditions This solution will serve as the initial condition for the unsteady calculation

2 Setup and solve for the unsteady solution

In this tutorial you will learn how to

bull Read an existing grid file into FLUENT

bull Setup and run a 3D steady-state problem with multiple inletsoutlets

bull Use tabulated profile data to apply unsteady boundary conditions

bull Setup and run an unsteady solution using the previous steady-state result as an initialcondition

Prerequisites

This tutorial assumes that you are familiar with the FLUENT interface and have completedTutorial 1 from FLUENT 62 Tutorial Guide

Some of the basic steps in the setup and solution procedures will not be shown explicitly

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Problem Description

The manifold considered in this tutorial has two inlets for which a uniform velocity normalto the boundary will be prescribed The outlet is at a fixed pressure (0 gauge relativeto the environment) For the unsteady problem the velocity is specified as a function oftime through the use of tabulated data using the transient profile A user-defined function(UDF) can also be used and is required to define a varying transient profile

A 3D tetrahedral mesh containing 14184 cells was created for this problem The meshsize was kept small in this case in order to minimize the computational effort required forthe unsteady solution However all of the elements necessary to perform a more detailedanalysis on a larger mesh are available in the present example

Preparation

1 Copy the mesh file manifoldmshgz and the profile file tab dataprof to yourworking directory

2 Start the 3D version of FLUENT

Setup and Solution

Step 1 Grid

1 Read the grid file manifoldmshgz

File minusrarr Read minusrarrCase

(a) Select the file manifoldmsh by clicking on it under Files and then click OK

As FLUENT reads the grid file messages will appear in the console window reportingthe progress of the conversion FLUENT will report that 14184 wedge cells have beenread

2 Check the grid

Grid minusrarrCheck

FLUENT will perform various checks on the mesh and will report the progress in theconsole window Pay attention to the reported minimum volume and make sure thisis a positive number

3 Scale the grid

Grid minusrarrScale

(a) Under Unit Conversion select mm from the drop-down list to complete the phraseGrid Was Created In mm (millimeters)

(b) Click Change Length Units to set millimeters as the working units for length

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(c) Click Scale to scale the grid to millimeters

Confirm that the maximum x y and z values are 302405 mm 125613 mm and177843 mm respectively

The grid is now sized correctly and the working units for length are set to mil-limeters

(d) Close the panel

Note There is no need to change any other units in this problem because thedefault SI units will be used The choice of millimeters for the unit of lengthhas been made by the actions you have just taken If you want to change theworking units for length to something other than millimeters (say inches)you would have to use the Set Units panel in the Define pull-down menu

4 Display the grid (Figure 1)

Display minusrarrGrid

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(a) Click Display

GridFLUENT 62 (3d segregated lam)

Z

Y

X

Figure 1 The Tetrahedral Grid for the Manifold

Extra You can use the right mouse button to check which zone number correspondsto each boundary If you click the right mouse button on one of the boundariesin the graphics window its zone number name and type will be printed in theFLUENT console window This feature is useful when you have several zones ofthe same type and you want to distinguish between them quickly

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Step 2 Models

1 Keep the default solver settings

Define minusrarr Models minusrarrSolver

In this application we assume incompressible flow Hence we choose the segregatedsolver For the initial run we calculate a steady-state solution So all parameters taketheir default values

2 Enable the energy equation

Define minusrarr Models minusrarrEnergy

(a) Enable Energy Equation and click OK to close the panel

3 Enable the standard k-ε turbulence model

Define minusrarr Models minusrarrViscous

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

The original Viscous Model panel will expand when you do so

(b) Accept the default Standard model by clicking OK

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Step 3 Materials

1 Define the material data for the incompressible fluid

Define minusrarrMaterials

For this tutorial you will model air as an incompressible fluid Assume that thekinematic and thermodynamic properties are constant So no change is required inthe materials panel

Extra You can modify the fluid properties for the default material displayed in theMaterials panel or copy a material from the materials database The materialsdatabase is opened by clicking Fluent Database in the Materials panel If theproperties in the database are different from those you wish to use you can stilledit the values under Properties and click ChangeCreate to update your localcopy (The database will not be affected)

2 Close the Materials panel

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Step 4 Boundary Conditions

The manifold has two velocity inlet zones velocity-inlet-1 and velocity-inlet-2 and a singlepressure outlet zone pressure-outlet-1 The walls of the domain are contained in the zonewall-17 You will set up the problem for a steady-state solution as described below

Define minusrarrBoundary Conditions

1 Set the conditions for the fluid

(a) Select fluid-79 under Zone

(b) Click Set to open the Fluid panel

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(c) Click OK to accept the default option air for Material Name

2 Set the boundary conditions at the inlets

(a) Select velocity-inlet-1 under Zone and click the Set button

Hint If you are not sure of which inlet zone corresponds to the main inlet probethe grid display with the right mouse button The zone ID will be displayedin the FLUENT console window and the zone will automatically be selectedin the Zone list In 2D simulations it may be helpful to return to the GridDisplay panel and deselect the display of the fluid and interior zones beforeprobing with the mouse button for zone names

(b) Set the conditions for velocity-inlet-1 as shown in the Velocity Inlet panel

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(c) Click OK to select the values

3 Repeat the above operations for velocity-inlet-2 Enter the values as shown in thepanel

4 Set the boundary conditions for pressure-outlet-1

(a) In the Boundary Conditions panel select pressure-outlet-1

(b) Enter values as shown in the panel

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(c) Enter a value of 005 for the Backflow Turbulence Intensity (fraction) and 30 mmfor Backflow Hydraulic Diameter

(d) Click OK

FLUENT will use the backflow conditions only if the fluid is flowing into the compu-tational domain through the outlet Backflow might occur at some point during thesolution process You should set reasonable backflow conditions to prevent convergencefrom being adversely affected

5 Keep the default boundary conditions for wall-17

This makes the assumption that there is no heat loss through the walls of the manifold

6 Close the Boundary Conditions panel

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Step 5 Steady-State Solution

The manifold model works best when you start from a good initial flow field So firstcalculate the steady-state flow field

1 Set the solution parameters

Solve minusrarr Controls minusrarrSolution

(a) Retain the default values for the Under-Relaxation Factors

Note For this problem the default under-relaxation factors are satisfactory Ifthe solution diverges or the residuals display large oscillations you may needto have the under-relaxation factors from their default values See the FLU-ENT Userrsquos Guide for tips on how to adjust the under-relaxation parametersfor different situations

(b) Under Discretization choose the Standard scheme for Pressure Second Order Up-wind for Momentum and First Order Upwind for Energy

(c) Select SIMPLE for the Pressure-Velocity Coupling

This results in better convergence behavior

(d) Click OK to accept the settings

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2 Enable the display of residuals during the solution process

Solve minusrarr Monitors minusrarrResidual

(a) Select Plot under Options and click OK

For this calculation the convergence tolerance of the continuity equation was keptat 0001 You can reduce this value if necessary depending on the behavior ofthe solution

3 Define the monitoring parameters to enable the plotting of mass flow rate and area-weighted average static temperature at the outflow boundary

Solve minusrarr Monitors minusrarrSurface

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(a) Increase the value of Surface Monitors to 2

The top two rows of selections will be activated

(b) Activate the Plot Print and Write options for monitor-1

Note When the Write option is selected in the Surface Monitors panel the massflow rate history will be written to a file If you do not select the Write optionthe history information will be lost when you exit FLUENT

(c) In the drop-down list under Every select Iteration

(d) Click on Define to specify the surface monitor parameters in the Define SurfaceMonitor panel

i Select Mass Flow Rate from the Report Type drop-down list

ii Select pressure-outlet-1 under the Surfaces list

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iii Click OK to define the monitor

(e) In the Surface Monitors panel activate Plot Print and Write options for monitor-2

(f) Define the values for the second monitor as shown in the following panel

(g) Click OK in the Surface Monitors panel to enable the monitors

4 Save the case file

File minusrarr Write minusrarrCase

(a) Enter manifold-sscasgz under Case File and click OK

Always save your case file before beginning a calculation

Note While specifying the name if you give the gz extension FLUENT auto-matically saves the file in compressed format

5 Initialize the flow field at velocity-inlet-1

Solve minusrarr Initialize minusrarrInitialize

(a) Choose velocity-inlet-1 from the Compute From list and enter the values for thevelocity components as shown in the following panel

(b) Click Init and close the panel

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6 Request for 150 iterations

Solve minusrarrIterate

The solution should be sufficiently converged after about 100 iterations (Figure 2Figure 3 and Figure 4)

7 Check the mass flux balance

Report minusrarrFluxes

The residual history plot only indicates the solution convergence Check the netmass fluxes through the domain to ensure that mass is being conserved

(a) Select velocity-inlet-1 velocity-inlet-2 and pressure-outlet-1 under Boundaries Keepthe default Mass Flow Rate under Options and click on Compute

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Z

Y

X

Scaled ResidualsFLUENT 62 (3d segregated ske)

Iterations120100806040200

1e+02

1e+01

1e+00

1e-01

1e-02

1e-03

1e-04

1e-05

1e-06

1e-07

epsilonkenergyz-velocityy-velocityx-velocitycontinuityResiduals

Figure 2 Residual History for the Steady-State Manifold Calculation

Z

Y

X

Convergence history of Mass Flow Rate on presure-outlet-1FLUENT 62 (3d segregated ske)

Iteration

(kgs)RateFlow

Mass

120100806040200

-00200

-00250

-00300

-00350

-00400

-00450

-00500

-00550

-00600

monitor-1Monitors

Figure 3 Mass Flow Rate History

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Z

Y

X

Convergence history of Static Temperature on presure-outlet-1FLUENT 62 (3d segregated ske)

Iteration

(k)Average

WeightedArea

120100806040200

5000000

4800000

4600000

4400000

4200000

4000000

3800000

3600000

3400000

3200000

3000000

monitor-2Monitors

Figure 4 Static Temperature History

The net mass imbalance should be a small fraction (say 05) of the total fluxthrough the system If a significant imbalance occurs decrease the residual tol-erances by at least an order of magnitude and continue iterating

8 Save the data file

File minusrarr Write minusrarrData

(a) Enter manifold-ssdatgz under Data File and click OK

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Step 6 Postprocessing for Steady-State Solution

1 Display filled contours of static pressure

Display minusrarrContours

(a) Select Pressure and Static Pressure from the Contours of drop-down lists

(b) Select Filled under Options

(c) Select all surfaces except interior-80 under Surfaces

(d) Click on Display to view the contours of static pressure (Figure 5) and close thepanel

Use the left mouse button to manipulate the default view as shown in Figure 5To know the exact value of the pressure at a particular point click with theright mouse at that point and the values will be displayed in the FLUENT consolewindow

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Contours of Static Pressure (pascal)FLUENT 62 (3d segregated ske)

986e+02894e+02802e+02709e+02617e+02525e+02433e+02341e+02249e+02157e+02652e+01-269e+01-119e+02-211e+02-303e+02-395e+02-487e+02-579e+02-671e+02-763e+02-855e+02

Z

Y

X

Figure 5 Static Pressure for the Steady-State Manifold Solution

2 Display the pathlines

Display minusrarrPath Lines

Pathlines are the lines traveled by neutrally buoyant particles in equilibrium with thefluid motion They are an excellent tool for visualization of complex 3D flows

(a) Select Particle Variables and Particle ID from the Color by drop-down lists

(b) Select velocity-inlet-1 and velocity-inlet-2 under Release from Surfaces

(c) Set Step Size to 1 mm

(d) Click Display (Figure 6) and close the panel

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Path Lines Colored by Particle IDFLUENT 62 (3d segregated ske)

279e+02265e+02251e+02237e+02223e+02209e+02195e+02181e+02167e+02153e+02140e+02126e+02112e+02977e+01837e+01698e+01558e+01418e+01279e+01139e+01000e+00

Z

Y

X

Figure 6 Path Lines plot for the Steady-State Manifold Solution

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Step 7 Unsteady Solution

After the steady-state solution you will now set up the unsteady manifold problem Thetime dependent boundary data can be defined either by a user defined function (UDF) orby the use of a transient boundary profile A UDF must be used if you are interested indefining spatial variations at the boundary Two different transient profile data formats canbe used The tabulated format would be the simplest to read and write

1 Create the tabulated transient profile file

For this problem the tab dataprof file is provided for you It can be easily understoodby viewing the contents of the file in a text editor A boolean is included at the end ofthe first line to denote whether or not the behavior at the boundary is periodic

2 Read the transient profile file

(a) In the FLUENT console click ltEntergt to view the text commands list

(b) Type f or file

(c) Type read-transient-table or rtt and click ltEntergt

FLUENT will prompt you to enter a filename

(d) Enter the filename tab dataprof and click ltEntergt

FLUENT will respond with a message indicating that the data is being read

The profiles are shown in Figure 7 The velocity and temperature vary sinusoidallyand the cycles repeat every 008 seconds

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Figure 7 (a) Inlet Velocity Versus Time for Inlet-1 and Inlet-2 (b) Inlet Temperature VersusTime for Inlet-1 and Inlet-2

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3 Enable the unsteady solver

Define minusrarr Models minusrarrSolver

(a) Under Time select Unsteady

(b) Under Transient Controls enable Non-Iterative Time Advancement

The non-iterative solver increases speed and efficiency of calculations It providescontrol over the maximum number of sub-iterations for each individual equationFor details about Non-Iterative Time Advancement Scheme (NITA) please referto Section 2634 of FLUENT 62 Userrsquos Guide

4 Enable the profiles in the velocity inlet boundary conditions

The names of the profiles defined in tabulated data list can now be accessed in theboundary condition panel

Define minusrarrBoundary Conditions

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(a) Select velocity-inlet-1 under Zone in the Boundary Conditions panel and click theSet button

(b) Select tab data vel1 in the Velocity Magnitude drop-down list

The data profile name will now appear to the right of the input line indicatingthat the profile will be used to provide the value of the velocity magnitude ratherthan the input value

(c) Select tab data temp1 in the Temperature drop-down list

(d) Repeat the above two steps for velocity-inlet-2 using the appropriate data profilenames tab data vel2 and tab data temp2

5 Set the solution parameters

Solve minusrarr Controls minusrarrSolution

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(a) Retain the default setting of PISO for Pressure-Velocity Coupling

(b) Set the Relaxation Factor for Pressure Turbulence Kinetic Energy and TurbulenceDissipation Rate to 08

6 Set up two surface monitors for the unsteady solution

(a) Open the Surface Monitors panel

Solve minusrarr Monitors minusrarrSurface

(b) Change the number of Surface Monitors to 4

i Activate Plot Print and Write for monitor-3

ii Change the update mode under Every to Iteration

iii Click on the Define button and define properties as shown in the panel

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This monitor will check the mass flow rate through the pressure outlet foreach iteration Ideally this value should level off to a constant value priorto the next time step

iv Activate Plot Print and Write for monitor-4

v Change the update mode under Every to Time Step

vi Click on the Define button and define properties as shown in the panel

vii Click OK in the Surface Monitors panel

This monitor checks the average static temperature at the pressure outlet for eachtime step

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7 Save case file and data files for the unsteady calculation (at specified time step inter-vals) using the Autosave option

File minusrarr Write minusrarrAutosave

(a) Set Autosave Data File Frequency to 10

(b) Specify the Filename as manifoldgz

By default the filenames will have an integer appended to them indicating the timestep This gives them unique names

8 Save the case file

File minusrarr Write minusrarrCase

(a) Enter manifold-unstcasgz under Case File and click OK

9 Set up the calculation parameters for the unsteady solution

Solve minusrarrIterate

Select a time step based on the period of unsteadiness of the inlet conditions Fromthe plots presented earlier it is observed that the period of the pulsating flow is 008seconds The time step size should be small enough to resolve the features of the un-steadiness For this tutorial you will use a time step size of 0001 seconds resulting in80 steps per period This value is sufficiently small such that the number of iterationsrequired to achieve convergence per each time step is on the order of 20 to 30

(a) Enter 0001 sec for the Time Step Size

(b) Enter 240 for the Number of Time Steps

This will provide enough time steps to simulate three full cycles Ideally thenumber of time steps should be large enough to allow the periodic nature of theflow to be evident

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