imp fluent tutorial

8/23/2019 Imp Fluent Tutorial

1/48

Tutorial 4. Modeling Unsteady

Compressible Flow

Introduction: In this tutorial, FLUENTs coupled implicit solver is usedto predict the time-dependent flow through a two-dimensional noz-zle. As an initial condition for the transient problem, a steady-statesolution is generated to provide the initial values for the mass flowrate at the nozzle exit.

In this tutorial you will learn how to:

Calculate a steady-state solution (using the coupled implicitsolver) as an initial condition for a transient flow prediction

Define an unsteady boundary condition using a user-definedfunction (UDF)

Calculate a transient solution using the second-order implicitunsteady formulation and the coupled implicit solver

Create an animation of the unsteady flow using FLUENTsunsteady solution animation feature

Prerequisites: This tutorial assumes that you are familiar with themenu structure in FLUENT and that you have solved or read Tu-torial 1. Some steps in the setup and solution procedure will notbe shown explicitly.

Problem Description: The geometry to be considered in this tutorialis shown in Figure 4.1. Flow through a simple nozzle is simulatedas a 2D planar model. The nozzle has an inlet height of 0.2 m, andthe nozzle contours have a sinusoidal shape that produces a 10%reduction in flow area. Due to symmetry, only half of the nozzle is

modeled.

c Fluent Inc. November 27, 2001 4-1


2/48

Modeling Unstead Compressible Flow

p = 0.9 atminlet p = 0.7369 atmexit

0.2 m

plane of symmetry

p (t )exit

Figure 4.1: Problem Specification

Preparation

1. Copy the files nozzle/nozzle.msh and nozzle/pexit.c from theFLUENT documentation CD to your working directory (as de-scribed in Tutorial 1).

2. Start the 2D version of FLUENT.

4-2 c Fluent Inc. November 27, 2001


3/48


Step 1: Grid

1. Read in the mesh file nozzle.msh.

File Read Case...

2. Check the grid.

Grid Check

FLUENT will perform various checks on the mesh and will reportthe progress in the console window. Pay particular attention to thereported minimum volume. Make sure this is a positive number.

3. Display the grid.

Display Grid...

To make the view more realistic, you will need to mirror it acrossthe centerline.



4/48


4. Mirror the view across the centerline.Display Views...

(a) Select symmetry under Mirror Planes.

(b) Click Apply.

The grid for the nozzle is shown in Figure 4.2.



5/48


GridFLUENT 6.0 (2d, segregated, lam)

Jan 31, 2001

Figure 4.2: 2D Nozzle Mesh Display



6/48


Step 2: Units

1. For convenience, define new units for pressure.

The pressure for this problem is specified in atm, which is not thedefault unit. You will need to redefine the pressure unit as atm.

Define Units...

(a) Select pressure under Quantities, and atm under Units.

Hint: Use the scroll bar to access pressure, which is not ini-tially visible in the list.

(b) Close the panel.



7/48


Step 3: Models

1. Select the coupled implicit solver.

The coupled implicit solver is the solver of choice for compressible,transonic flows.

Define Models Solver...

Note: Initially, you will solve for the steady flow through the noz-zle. Later, after obtaining the steady flow as a starting point,you will revisit this panel to enable an unsteady calculation.



8/48


2. Enable the Spalart-Allmaras turbulence model.Define Models Viscous...

The Spalart-Allmaras model is a relatively simple one-equation modelthat solves a modeled transport equation for the kinematic eddy(turbulent) viscosity. This embodies a class of one-equation mod-els in which it is not necessary to calculate a length scale related tothe local shear layer thickness. The Spalart-Allmaras model was de-signed specifically for aerospace applications involving wall-boundedflows and has been shown to give good results for boundary layerssubjected to adverse pressure gradients.



9/48


Step 4: Materials

1. Set the properties for air, the default fluid material.

Define Materials...

(a) Select the ideal-gas law to compute Density.

Note: FLUENT will automatically enable solution of the en-ergy equation when the ideal gas law is used. You do notneed to visit the Energy panel to turn it on.



10/48


(b) Retain the default values for all other properties.

! Dont forget to click the Change/Create button to save yourchange.

Step 5: Operating Conditions

1. Set the operating pressure to 0 atm.

Define Operating Conditions...

Here, the operating pressure is set to zero and boundary conditioninputs for pressure will be defined in terms of absolute pressures.Boundary condition inputs should always be relative to the valueused for operating pressure.



11/48


Step 6: Boundary ConditionsDefine Boundary Conditions...

1. Set the conditions for the nozzle inlet (inlet).

(a) Set the Gauge Total Pressure to 0.9 atm.

(b) Set the Supersonic/Initial Gauge Pressure to 0.7369 atm.

The inlet static pressure estimate is the mean pressure at thenozzle exit. This value will be used during the solution initial-ization phase to provide a guess for the nozzle velocity.

(c) In the Turbulence Specification Method drop-down list, selectTurbulent Viscosity Ratio.

(d) Set the Turbulent Viscosity Ratio to 1.

For low to moderate inlet turbulence, a viscosity ratio of 1 isrecommended.



12/48


2. Set the conditions for the nozzle exit (outlet).

(a) Set the Gauge Pressure to 0.7369 atm.

(b) In the Turbulence Specification Method drop-down list, selectTurbulent Viscosity Ratio.

(c) Accept the default value of10 for Backflow Turbulent ViscosityRatio.

If substantial backflow occurs at the outlet, you may need to adjustthe backflow values to levels close to the actual exit conditions.



13/48


Step 7: Solution: Steady Flow

1. Initialize the solution.

Solve Initialize Initialize...

(a) Select inlet in the Compute From drop-down list.

(b) Click Init, and Close the panel.



14/48


2. Set the solution parameters.Solve Controls Solution...

(a) Under Discretization, select Second Order Upwind forTurbulent Viscosity.

Second-order discretization provides optimum accuracy.



15/48


3. Enable the plotting of residuals.Solve Monitors Residual...

(a) Under Options, select Plot.

(b) Click OK.



16/48


4. Enable the plotting of mass flow rate at the flow exit.Solve Monitors Surface...

(a) Increase the number of Surface Monitors to 1.

(b) Turn on the Plot and Write options for monitor-1.

Note: When the Write option is selected in the Surface Mon-itors panel, the mass flow rate history will be written toa file. If you do not select the write option, the historyinformation will be lost when you exit FLUENT.

(c) Click on Define... to specify the surface monitor parametersin the Define Surface Monitor panel.



17/48


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

ii. Select outlet in the Surfaces list.

iii. In the File Name field, enter the name noz ss.out.

iv. Click on OK to define the monitor.

(d) Click on OK in the Surface Monitors panel to enable the mon-itor.



18/48


5. Save the case file (noz ss.cas).

File Write Case...

6. Start the calculation by requesting 800 iterations.

Solve Iterate...

The solution will converge after about 600 iterations. However, theresidual history plot is only one indication of solution convergence.Notice that the mass flow rate has not reached a constant value.To remedy this, you will reduce the convergence criterion for the

continuity equation and iterate until the mass flow rate reaches aconstant value.



19/48


7. Reduce the convergence criterion for the continuity equation.Solve Monitors Residual...

(a) Set the Convergence Criterion for continuity to 1e-04.

(b) Click OK.

Note: To obtain better convergence of the mass flow rate,only the convergence tolerance for the continuity equationis adjusted. In general, the convergence behavior of thecontinuity equation is a good indicator of the overall con-vergence of the solution.

8. Request 500 more iterations.

Solve Iterate...



20/48


After about 400 iterations (1000 total), the mass flow rate has lev-eled off and the solution has converged. The mass flow rate historyis shown in Figure 4.3.

Convergence history of Static Pressure on outletFLUENT 6.0 (2d, coupled imp, S-A)

Jun 20, 2001

Iteration

(kg/s)RateFlow

Mass

10009008007006005004003002001000

-14.2500

-14.5000

-14.7500

-15.0000

-15.2500

-15.5000

-15.7500

-16.0000

-16.2500

-16.5000

-16.7500

-17.0000

Figure 4.3: Mass Flow Rate History

9. Save the data file (noz ss.dat).File Write Data...



21/48


10. Check the mass flux balance.Report Fluxes...

! Although the mass flow rate history indicates that the solutionis converged, you should also check the mass fluxes throughthe domain to ensure that mass is being conserved.

(a) Keep the default Mass Flow Rate option.

(b) Select inlet and outlet in the Boundaries list.

(c) Click Compute.

! The net mass imbalance should be a small fraction (say, 0.5%)of the total flux through the system. If a significant imbalanceoccurs, you should decrease your residual tolerances by at least

an order of magnitude and continue iterating.



22/48


11. Display the steady-flow velocity vectors (Figure 4.4).

Display Vectors...

(a) Select arrow in the Style drop-down list.

(b) Change the Scale to 10.

(c) Click Display.

The steady flow prediction shows the expected form, with peak ve-

locity of about 326 m/s through the nozzle.



23/48


velocity Colored By Velocity Magnitude (m/s) Jan 31, 2001FLUENT 6.0 (2d, coupled imp, S-A)

3.26e+02

1.15e+00

3.37e+01

6.62e+01

9.87e+01

1.31e+02

1.64e+02

1.96e+02

2.29e+02

2.61e+02

2.94e+02

Figure 4.4: Velocity Vectors (Steady Flow)



24/48


12. Display the steady flow contours of static pressure (Figure 4.5).

Display Contours...

(a) Under Options, select Filled.

(b) Click Display.

The steady flow prediction shows the expected pressure distribution,with low pressure near the nozzle throat.



25/48


Contours of Static Pressure (atm) Jan 31, 2001FLUENT 6.0 (2d, coupled imp, S-A)

7.85e-01

4.49e-01

4.83e-01

5.16e-01

5.50e-01

5.84e-01

6.17e-01

6.51e-01

6.84e-01

7.18e-01

7.52e-01

Figure 4.5: Contours of Static Pressure (Steady Flow)



26/48


Step 8: Enable Time Dependence and SetUnsteady Conditions

In this step you will define a transient flow by specifying an unsteadypressure condition for the nozzle.

1. Enable a time-dependent flow calculation.

Define Models Solver...

(a) Under Time, select Unsteady.

(b) Under Unsteady Formulation, select 2nd-Order Implicit.

Implicit (dual) time-stepping allows you to set the physical timestep used for the transient flow prediction (whileFLUENT continuesto determine the time step used for inner iterations based on a

Courant condition). Here, second-order implicit time-stepping isenabled: this provides higher accuracy in time than the first-orderoption.



27/48


2. Define the unsteady condition for the nozzle exit (outlet).The pressure at the outlet is defined as a wave-shaped profile, andis described by the following equation:

pexit(t) = 0.12sin(t) + pexit (4.1)

where

= circular frequency of unsteady pressure (rad/s)pexit = mean exit pressure (atm)

In this case, = 1748.8 rad/s, and pexit = 0.7369 atm.

A user-defined function (pexit.c) has been written to define theequation (Equation 4.1) required for the pressure profile.

Note: To input the value of Equation 4.1 in the correct units, thefunction pexit.c has been multiplied by a factor of 101325to convert from the chosen pressure unit (atm) to the SI unitrequired by FLUENT (Pa). This will not affect the displayedresults. See the separate UDF Manual for details about user-defined functions.



28/48


(a) Read in the user-defined function.

Define User-Defined Functions Interpreted...

i. Enter pexit.c as the Source File Name.

ii. Click Compile.

The user-defined function has already been defined, but itneeds to be compiled within FLUENT before it can be used

in the solver.iii. Close the Interpreted UDFs panel.



29/48


(b) Set the unsteady boundary conditions at the exit.

i. Select udf unsteady pressure (the user-defined function) inthe Gauge Pressure drop-down list.



30/48


Step 9: Solution: Unsteady Flow

1. Set the time step parameters.

The selection of the time step is critical for accurate time-dependentflow predictions. Using a time step of 7.1857 105 seconds, 50time steps are required for one pressure cycle. The pressure cyclebegins and ends with the initial pressure at the nozzle exit.

Solve Iterate...

(a) Set the Time Step Size to 7.1857e-05 s.(b) Set the Number of Time Steps to 300.

(c) Set the Max Iterations per Time Step to 30.



31/48


(d) Click Apply.

2. Modify the plotting of the mass flow rate at the nozzle exit.

Because each time step requires 30 iterations, a smoother plot willbe generated by plotting at every time step.

Solve Monitors Surface...

(a) For monitor-1, select Time Step in the drop-down list underEvery.

(b) Click Define... to modify the surface monitor parameters.

i. In the Define Surface Monitor panel, change the File Nameto noz uns.out.

ii. In the X Axis drop-down list, select Time Step.

iii. Click OK.

(c) Click OK in the Surface Monitors panel.

3. Save the transient solution case file (noz uns.cas).

File Write Case...



32/48


4. Start the transient calculation.Solve Iterate...

! Calculation of 300 time steps will require significant CPU re-sources. Instead of calculating, you can read the data file savedafter the iterations have been completed:

noz uns.dat

(The data file is available in the same directory where youfound the mesh and UDF files.)

By requesting 300 time steps, you are asking FLUENT to compute

six pressure cycles.

Convergence history of Static Pressure on outlet (Time=2.1557e-02)FLUENT 6.0 (2d, coupled imp, S-A, unsteady)

Jun 20, 2001

Time Step

(kg/s)RateFlow

Mass

300250200150100500

-9.0000

-10.0000

-11.0000

-12.0000

-13.0000

-14.0000

-15.0000

-16.0000

-17.0000

-18.0000

Figure 4.6: Mass Flow Rate History (Unsteady Flow)

5. Save the transient solution data file (noz uns.dat).

File Write Data...



33/48


Step 10: Saving and Postprocessing Time-Depen-dent Data Sets

The solution has reached a time-periodic state. To study how the flowchanges within a single pressure cycle, you will now continue the solu-tion for 50 more time steps. You will use FLUENTs solution animationfeature to save contour plots of pressure and Mach number at each timestep, and the autosave feature to save case and data files every 10 timesteps. After the calculation is complete, you will use the solution ani-mation playback feature to view the animated pressure and Mach numberplots over time.

1. Request saving of case and data files every 10 time steps.

File Write Autosave...

(a) Set the Autosave Case File Frequency and Autosave Data FileFrequency to 10.

(b) In the Filename field, enter noz anim.

(c) Click OK.

When FLUENT saves a file, it will append the time step valueto the file name prefix (noz anim). The standard extensions

(.cas and .dat) will also be appended. This will yield filenames of the formnoz anim0340.cas andnoz anim0340.dat,where 0340 is the time step number.



34/48


Optionally, you can add the extension .gz to the end of thefile name (e.g., noz anim.gz), which will instruct FLUENT tosave the case and data files in compressed format, yielding filenames of the form noz anim0340.cas.gz.

2. Create animation sequences for the nozzle pressure and Mach num-ber contour plots.

Solve Animate Define...

(a) Increase the number of Animation Sequences to 2.

(b) Under Name, enter pressure for the first sequence andmach-number for the second sequence.

(c) In the When drop-down lists, select Time Step.

With the default value of 1 for Every, this instructs FLUENTto update the animation sequence at every time step.



35/48


3. Define the animation sequence for pressure.

(a) Click Define... on the line for pressure to set the parametersfor the pressure sequence.

The Animation Sequence panel will open.

(b) Under Storage Type, select Memory.

The Memory option is acceptable for a small 2D case such asthis. For larger 2D or 3D cases, saving animation files withthe Disk option is preferable to avoid using too much of yourmachines memory.

(c) Increase the Window number to 2 and click Set.

Graphics window number 2 will open.

(d) Under Display Type, select Contours.

The Contours panel will open.



36/48


i. In the Contours panel, keep the default selections of Pres-

sure... and Static Pressure.ii. Make sure that Filled is selected under Options, and des-

elect Auto Range.

iii. Enter 0.25 under Min and 1.00 under Max.

This will set a fixed range for the contour plot and subse-quent animation.

iv. Click Display.

Figure 4.7 shows the contours of static pressure in the nozzleafter 300 time steps.

(e) Click OK in the Animation Sequence panel.



37/48


Contours of Static Pressure (atm) (Time=2.1629e-02)FLUENT 6.0 (2d, coupled imp, S-A, unsteady)

Feb 08, 2001

2.50e-01

1.00e+00

9.25e-01

8.50e-01

7.75e-01

7.00e-01

6.25e-01

5.50e-01

4.75e-01

4.00e-01

3.25e-01

Figure 4.7: Pressure Contours at t = 0.0216 s



38/48


4. Define the animation sequence for Mach number.

(a) In the Solution Animation panel, click Define... on the linefor mach-number to set the parameters for the Mach numbersequence.

(b) Under Storage Type in the Animation Sequence panel, selectMemory.

(c) Increase the Window number to 3 and click Set.

Graphics window number 3 will open.

(d) Under Display Type, select Contours.

i. In the Contours panel, select Velocity... and Mach Number.

ii. Make sure that Filled is selected under Options, and des-elect Auto Range.

iii. Enter 0.00 under Min and 1.30 under Max.

iv. Click Display.

Figure 4.8 shows the Mach number contours in the nozzle after300 time steps.

(e) Click OK in the Animation Sequence panel.

(f) Click OK in the Solution Animation panel.



39/48


Contours of Mach Number (Time=2.1629e-02)FLUENT 6.0 (2d, coupled imp, S-A, unsteady)

Feb 08, 2001

0.00e+00

1.30e+00

1.17e+00

1.04e+00

9.10e-01

7.80e-01

6.50e-01

5.20e-01

3.90e-01

2.60e-01

1.30e-01

Figure 4.8: Mach Number Contours at t = 0.0216 s



40/48


5. Continue the calculation by requesting 50 time steps.Requesting 50 time steps will march the solution through an ad-ditional 0.0036 seconds, or roughly one pressure cycle. With theautosave and animation features active (as defined above), the case,data, and animation files will be saved approximately every 0.0007seconds.

Solve Iterate...

When the calculation finishes, you will have five pairs of case anddata files and there will be 50 pairs of contour plots stored in mem-ory. In the next few steps, you will play back the animation se-quences and examine the results at several time steps after readingin pairs of newly-saved case and data files.



41/48


6. Change the display options to include double buffering.Double buffering will allow for a smoother transition between theframes of the animations.

Display Options...

(a) Under Rendering Options, select Double Buffering.

(b) Click Apply.



42/48


7. Play back the animation of the pressure contours.Solve Animate Playback...

(a) Under Sequences, select pressure.

The playback control buttons now become active.

(b) Keep the default settings in the rest of the panel and clickthe play button (the second from the right in the group ofbuttons under Playback).

Examples of pressure contours at t = 0.0227 s and t = 0.0239 s areshown in Figures 4.9 and 4.10.

8. Repeat steps 6 and 7, selecting the appropriate active window andsequence name for the Mach number contours.

Examples of Mach number contours at t = 0.0227 s and t =0.0239 s are shown in Figures 4.11 and 4.12.



43/48



Feb 08, 2001

2.50e-01

1.00e+00

9.25e-01

8.50e-01

7.75e-01

7.00e-01

6.25e-01

5.50e-01

4.75e-01

4.00e-01

3.25e-01



Feb 08, 2001

2.50e-01

1.00e+00

9.25e-01

8.50e-01

7.75e-01

7.00e-01

6.25e-01

5.50e-01

4.75e-01

4.00e-01

3.25e-01




44/48



Feb 08, 2001

0.00e+00

1.30e+00

1.17e+00

1.04e+00

9.10e-01

7.80e-01

6.50e-01

5.20e-01

3.90e-01

2.60e-01

1.30e-01



Feb 08, 2001

0.00e+00

1.30e+00

1.17e+00

1.04e+00

9.10e-01

7.80e-01

6.50e-01

5.20e-01

3.90e-01

2.60e-01

1.30e-01




45/48


Extra: FLUENT gives you the option of exporting an animationas an MPEG file or as a series of files in any of the hardcopyformats available in the Graphics Hardcopy panel (includingTIFF and PostScript).

To save an MPEG file, select MPEG from the Write/RecordFormat drop-down list in the Playback panel and then click theWrite button. The MPEG file will be saved in your workingdirectory. You can view the MPEG movie using an MPEGplayer (e.g., Windows Media Player or another MPEG movieplayer).

To save a series of TIFF, PostScript, or other hardcopy files,select Hardcopy Frames in theWrite/Record Format drop-downlist in the Playback panel. Click on the Hardcopy Options...button to open the Graphics Hardcopy panel and set the appro-priate parameters for saving the hardcopy files. Click Applyin the Graphics Hardcopy panel to save your modified settings.In the Playback panel, click the Write button. FLUENT willreplay the animation, saving each frame to a separate file inyour working directory.

If you want to view the solution animation in a later FLUENTsession, you can select Animation Frames as the Write/RecordFormat and click Write.

! Since the solution animation was stored in memory, it will belost if you exitFLUENT without saving it to one of the formatsdescribed above. Note that only the animation-frame formatcan be read back into the Playback panel for display in a laterFLUENT session.



46/48


9. Display velocity vectors after 30 time steps (Figure 4.13).

(a) Read case and data files for the 30th time step (noz anim0330.casand noz anim0330.dat) into FLUENT.

File Read Case & Data...

(b) Plot vectors at t = 0.0238 s.

Display Vectors...



47/48


i. Select arrow in the Style drop-down list.ii. Change the Scale to 10.

iii. Click Display.

The unsteady flow prediction shows the expected form, withpeak velocity of about 213 m/s through the nozzle at t = 0.0238seconds.

10. Repeat step 9 using case and data files saved for other time stepsof interest.

Velocity Vectors Colored By Velocity Magnitude (m/s) (Time=2.3785e-02) Jun 21, 2001FLUENT 6.0 (2d, coupled imp, S-A, unsteady)

2.13e+02

7.46e-01

2.20e+01

4.33e+01

6.45e+01

8.58e+01

1.07e+02

1.28e+02

1.50e+02

1.71e+02

1.92e+02

Figure 4.13: Velocity Vectors at t = 0.0238 s



48/48


Summary: In this tutorial, you modeled the transient flow of air througha nozzle. You learned how to generate a steady-state solution asan initial condition for the unsteady case, and how to set solutionparameters for implicit time-stepping.

You also learned how to manage the file saving and graphical post-processing for time-dependent flows, using file autosaving to au-tomatically save solution information as the transient calculationproceeds.

Finally, you learned how to use FLUENTs solution animation toolto create animations of transient data, and how to view the ani-mations using the playback feature.

imp fluent tutorial

Documents