Transient Analysis

Tutorial 3. Transient Analysis

Introduction: This tutorial demonstrates how to set up and solve a transient problem.The goal is to determine the time variation of the flow and temperature distributionswithin the cabinet.

In this tutorial, you will learn how to:

Define a transient problem Specify time-dependent parameters for objects Group and copy modeling objects Examine the results of a transient simulation, including animating results over

time

Prerequisites: This tutorial assumes that you are familiar with the menu structure inIcepak and that you have solved or read Tutorial 1. Some steps in the setup andsolution procedure will not be shown explicitly.

Problem Description: The model consists of a natural-convection-cooled heat sinkcomposed of a base plate, four heat sources, and nine tapered fins, as shown inFigure 3.1. The sources are attached to the back of the base plate and each isturned on for 10 seconds, one after the other.

c Fluent Inc. October 23, 2007 3-1

Transient Analysis

Figure 3.1: Problem Specification

Step 1: Open and Define a New Project

1. Start Icepak, as described in Chapter 1 of the Users Guide.

When Icepak starts, the New/existing panel will open automatically.

2. Click New in the New/existing panel to start a new Icepak project.

The New project panel will appear.

3. Specify a name for your project.

(a) In the Project text box, enter the name transient.

(b) Click Create.

3-2 c Fluent Inc. October 23, 2007

Transient Analysis

Icepak will create a default cabinet with the dimensions 1 m 1 m 1 m, anddisplay the cabinet in the graphics window.

You can rotate the cabinet around a central point using the left mouse button, oryou can translate it to any point on the screen using the middle mouse button. Youcan zoom into and out from the cabinet using the right mouse button. To restorethe cabinet to its default orientation, select Home position from the Orient menu.

4. Specify a transient simulation that includes gravity and turbulence effects.

Problem setup Basic parameters

(a) Click the General setup tab in the Basic parameters panel.

i. Under Flow regime, select Turbulent and keep the default Zero equationturbulence model.


Transient Analysis

ii. Turn on the Gravity vector option and keep the default settings.

(b) Click the Transient setup tab in the Basic parameters panel.

i. For the Time variation, select Transient and set the Start and End times to0 and 40 seconds, respectively.

ii. Specify the time step and how often to save the solution data.

A. Click Edit parameters next to the Transient option.

Icepak will open the Transient parameters panel.

B. Set the Time step increment to 1.

C. Set the Solution save interval to 5.

D. Click Accept to save the new time parameters.

iii. Under Initial conditions, set the initial Y velocity to 0.1 m/s.

(c) Click Accept to save the new settings.


Transient Analysis

Step 2: Build the ModelTo build the model, you will first resize the cabinet to its proper size. Then you will createthe objects in the cabinet: a base plate, heat sources, and fin.

1. Resize the default cabinet.

Model Cabinet(a) In the Cabinet panel, click the Geometry tab.

(b) Enter the following coordinates:

xS 0.05 xE 0.35yS 0.1 yE 0.55zS 0.05 zE 0.25

(c) Click Update to resize the cabinet and then click Done to close the panel.

(d) In the Orient menu, select Scale to fit to scale the view of the cabinet to fit thegraphics window.

2. Create the base plate.

(a) Click the button to create a new plate, and then click the to openthe Plates panel.

For the base plate, you will need to change the size and set thermal properties,without changing the default X-Y plane.

(b) Enter the name baseplate in the Name field.

(c) Click the Geometry tab.

(d) Enter the following coordinates for the plate:

xS 0.1 xE 0.3yS 0.2 yE 0.4zS 0.12 zE

(e) Click the Properties tab.

(f) Under Thermal model, select Conducting thick.

(g) Set the Thickness to 0.01 m.

(h) Select Al-Duralumin in the Solid material drop-down list.

Hint: Click the button located next to the Solid material text field to openthe drop-down list. Al-Duralumin is located in the Metals/Alloys category,so you will need to scroll down through the drop-down list to find it.

(i) Click Done to modify the plate and close the Plates panel.


Transient Analysis

3. Create the first tapered fin.

Each fin is physically identical to the others, except with respect to its location. Tocreate the set of nine fins, you will build a single solid block as a template, createtwo copies, each with a specified offset in the x direction, group the three blocks,and then make two copies of the group, offset in the y direction.

(a) Click the button to create a new block, and then click the buttonto open the Blocks panel.

Icepak will create a new solid prism block in the center of the cabinet. You willneed to change the size and shape of the block.

(b) Enter the name fin in the Name field and press the key on thekeyboard.

(c) Click the Geometry tab.

(d) In the Shape drop-down list, select Cylinder.

(e) In the Plane drop-down list, select X-Y.

(f) Turn on the Nonuniform radius option.

(g) Enter the following coordinates for the center point:

xC 0.15yC 0.25zC 0.13

(h) Set the Height to 0.06.

(i) Enter 0.02 for the first (bottom) external radius (Radius), and 0 for the first(bottom) internal radius (Int Radius).

(j) Enter 0.012 for the second (top) external radius (Radius 2), and 0 for thesecond (top) internal radius (Int Radius 2).

(k) Click the Properties tab.

(l) For the Block type, keep the default selection of Solid.

(m) Under Thermal specification, select Al-Duralumin in the Solid material drop-down list.

(n) Click Done to modify the block and close the Blocks panel.


Transient Analysis

4. Copy the first block (fin) to create the second and third blocks (fin.1 and fin.2),each offset by 0.05 m in the x direction.

(a) In the Model manager window, select the fin item under the Model node.

(b) Click the button.

Alternatively, you can right click the fin item in the Model manager window,and select Copy object.

The Copy block fin panel will open.

(c) Enter 2 as the Number of copies.

(d) Turn on the Translate option and specify an X offset of 0.05.

(e) Click Apply.

Icepak will make two copies of the original block, each offset by 0.05 m in thex direction from the previous one.

5. Create the remaining six blocks.

(a) Create a group consisting of the three blocks.

EditCurrent groupCreatei. Keep the default group name.

ii. Click Done.

The item group.1 will be added under the Groups node in the Model man-ager window.

iii. In the Model manager window, click the plus sign next to Groups.

iv. Right-click the group.1 item and select AddName/pattern in the result-ing pull down menu.

Icepak will open a Query dialog box asking you for a text pattern.

v. Enter fin* in the Pattern for objects to add text field of the Query dialogbox.

vi. Click Done.

fin, fin.1, and fin.2 will be added to the group in the Model manager win-dow.


Transient Analysis

(b) Make two copies of this group.

i. Select group.1 under the Groups node in the Model manager window.

ii. Right-click group.1 and select Copy group in the resulting pull down menu.

The Copy group group.1 panel will open.

iii. Enter 2 for Number of copies.

iv. Turn on the Translate option.

v. Enter a Y offset of 0.05 m.

vi. Click Apply.

The display will be updated to show all nine blocks.

6. Create the first heat source.

The heat sources are identical, but will turn on and off at different times during thesimulation. For this reason, you will set up each one independently, rather thanmaking one and copying it.

(a) Click the button to create a new source, and then click the buttonto open the Sources panel.

Icepak will create a prism source in the center of the cabinet. You will need tochange the geometry, orientation, and size of the source, and specify its heatsource parameters.

(b) Click the Geometry tab.

(c) Enter the following coordinates:

xS 0.12 xE 0.18yS 0.22 yE 0.28zS 0.12 zE

(d) Specify the total heat (power) as an exponential function of time:

Q = Q0 + 100e0.025t (3.1)

Here, Q0 is the power output at t = 0 (specified as the Total heat), whichis equal to zero, indicating that the source is turned off at the start of the


Transient Analysis

simulation. The values 100 and 0.025 are specified as the coefficient b and theexponent a, respectively. Note that the units for b are those specified for Totalheat in the Sources panel.

i. Click the Properties tab.

ii. Under Heat source parameters, keep the Total heat of 0 W.

iii. Turn on the Transient option and click Edit.

The Transient power panel will open.

iv. Under Time, enter 0 for tS and 10 for tE.

This indicates that the source will turn on at the start of the simulation,and then turn off after 10 seconds.

v. Select Exponential and set a to 0.025 and b to 100.

vi. Click Done to modify the settings and close the Transient power panel.

(e) Click Update in the Sources panel to update the source definition.


Transient Analysis

7. Create the second heat source.

(a) Click New in the Sources panel and specify the following for the second source(source.2):

Location:xS 0.22 xE 0.28yS 0.22 yE 0.28zS 0.12 zE

Heat source parameters: Total heat = 0 W, Transient Transient power parameters: tS=10, tE=20, Exponential, a=0.025, b=100

! Remember to click Done in the Transient power panel and then click Up-date in the Sources panel when you have completed the specification.

8. Repeat to create the third and fourth sources (source.3 and source.4) with thefollowing specifications:

source.3: Location:

xS 0.12 xE 0.18yS 0.32 yE 0.38zS 0.12 zE

Heat source parameters: Total heat = 0 W, Transient

Transient power parameters: tS=20, tE=30, Exponential, a=0.025, b=100

source.4: Location:

xS 0.22 xE 0.28yS 0.32 yE 0.38zS 0.12 zE

Heat source parameters: Total heat = 0 W, Transient

Transient power parameters: tS=30, tE=40, Exponential, a=0.025, b=100


Transient Analysis

9. Create the first opening in the cabinet.

(a) Click the button to create a new opening, and then click the buttonto open the Openings panel.

You will need to change the orientation and size of the opening.

(b) Click the Geometry tab.

(c) In the Plane drop-down list, select X-Z.

(d) Enter the following coordinates:

xS 0.05 xE 0.35yS 0.1 yE zS 0.05 zE 0.25

(e) Click Update to modify the opening.

10. Create the second opening.

(a) Click New in the Openings panel and specify the following for the secondopening:

Plane: X-Z Location:

xS 0.05 xE 0.35yS 0.55 yE zS 0.05 zE 0.25

(b) Click Done to modify the opening and close the panel.

The completed model will look like Figure 3.2,which is shown in the Isometric view(available in the Orient menu).

Extra: Object names are displayed in Icepak by default. To hide object names from

the model view for ease of viewing, click the button.

11. Check the model to be sure that there are no problems (e.g., objects that are tooclose together to allow for proper mesh generation).

ModelCheck modelNote that you can also click the button to check the model.

Icepak should report in the Message window that 0 problems were found.


Transient Analysis

X

Y

Z

Figure 3.2: Completed Model for the Transient Simulation

12. Check the definition of the modeling objects to ensure that you specified themproperly.

ViewSummary (HTML)The HTML version of the summary will be displayed in your web browser. Thesummary displays a list of all the objects in the model and all the parameters thathave been set for each object. You can view the detailed version of the summary byclicking the appropriate object names or property specifications. If you notice anyincorrect specifications, you can return to the appropriate modeling object panel andchange the settings in the same way that you originally entered them.


Transient Analysis

Step 3: Generate a MeshFor this model, you will generate the mesh in just one step. The resulting mesh will besufficiently fine near object faces to resolve the flow physics properly.

1. Generate the mesh for the model.

ModelGenerate mesh(a) In the Mesh control panel, set the Max X size to 0.02, the Max Y size to 0.02,

and the Max Z size to 0.02.

(b) Turn on the Init height specification and set it to 0.005.

(c) Change the Min elems in gap from 3 to 2.

(d) Click Generate mesh.

2. Examine the mesh on all objects in the model.

(a) Click the Display tab in the Mesh control panel.

The panel will be updated to show the mesh display tools.

(b) Select Surface and select All objects.

(c) Turn on the Display mesh option.

The mesh display on all object faces is shown in Figure 3.3.

3. Examine the mesh on cross-sections of the model.

(a) Turn off the Display mesh and Surface options.

(b) Turn on the Cut plane option.

(c) In the Set position drop-down list, select Z plane through center.

(d) Display the mesh on the x-y plane.

i. Turn on the Display mesh option.

The mesh display plane is through the middle of the model.

ii. Use the slider bar under Plane location to move the plane cut through themodel.

(e) Display the mesh on the y-z and x-z planes.

i. Turn off the Display mesh option.

ii. In the Set position drop-down list, select X plane through center or Y planethrough center.

These settings will result in a mesh display on a y-z or x-z plane, respec-tively.


Transient Analysis

X

Y

Z

Figure 3.3: Mesh on All Object Faces

iii. Turn on the Display mesh option and use the slider bar under Plane locationto move the plane cut through the model.

4. Turn off the mesh display.

(a) Deselect the Display mesh option.

(b) Click Close to close the Mesh control panel.


Transient Analysis

Step 4: Check the Flow RegimeBefore starting the solver, you will first review estimates of the Rayleigh and Prandtlnumbers to check that the proper flow regime is being modeled.

1. Check the values of the Rayleigh and Prandtl numbers.

Solution settings Basic settings(a) Click Reset in the Basic settings panel.

(b) Check the values printed to the Message window.

The Rayleigh and Prandtl numbers are approximately 3.6107 and 0.7, respec-tively, so the flow is turbulent. Since you have already enabled turbulence, nochanges are required.

(c) Click Accept to accept the new solver settings.

Step 5: Save the Model to a Project FileIcepak will save the model for you automatically before it starts the calculation, but it is agood idea to save the model (including the mesh) yourself as well. If you exit Icepak beforeyou start the calculation, you will be able to open the project you saved and continue youranalysis in a future Icepak session. (If you start the calculation in the current Icepaksession, Icepak will simply overwrite your project file when it saves the model.)

FileSave project


Transient Analysis

Step 6: Calculate a Solution

1. Increase the maximum number of iterations per time step.

Solution settings Basic settings(a) Set the Iterations/timestep to 50.

(b) Click Accept.

2. Start the calculation.

SolveRun solution(a) Keep the default settings in the Solve panel.

(b) Click Start solution to start the solver.

The solution will converge after about 670 iterations. Note, however, that the exactnumber of iterations required for convergence may vary on different computers.


Transient Analysis

Step 7: Examine the ResultsSince the simulation is transient, you can generate an animation that shows the flow andtemperature field changing over time.

1. Display contours of temperature on the four sources, animated over time.

PostObject face(a) In the Object drop-down list, select all of the sources.

Hint: To select all of the sources in the tree listing, hold down the key and select each source item: source.1, source.2, source.3, and source.4.

This will result in a display on all of the sources.

(b) Turn on the Show contours option and click Parameters.

The Object face contours panel will open.

(c) In the Object face contours panel, keep the default selection of Temperature inthe Contours of drop-down list.

(d) Keep all defaults and click Done to save the settings and close the panel.

(e) Click Create in the Object face panel to update the graphics display.

(f) Change the orientation of the model to the home position.

OrientHome position(g) Animate the temperature contours over time.

PostTransient settingsThe Post-processing time panel will open.

i. In the Post-processing time panel, select the Time value option.

ii. Click Animate.

The Transient animation panel will open.

iii. In the Transient animation panel, keep all default values and click Animate.


Transient Analysis

The animation will begin, and the display will show the temperature contoursfor the solution at each time. You will see each source get hotter as it is turnedon. Figure 3.4 shows the temperature contours for the sources at t = 20.

X

Y

Z

Time = 20.0

20.0322

21.0451

22.0580

23.0709

24.0838

25.0967

26.1096

27.1225

28.1354

Temperature C

Figure 3.4: Temperature Contours on the Sources at t = 20

To stop the animation, click the red Interrupt button in the upper right handcorner of the Icepak interface,

(h) Click Done to close the Transient animation panel.

(i) Click Done to close the Post-processing time panel.

(j) In the Object face panel, turn off the Active option and click Done.


Transient Analysis

2. Display velocity vectors on a plane cut through the center of the cabinet.

PostPlane cut(a) In the Set position drop-down list, select Z plane through center.

(b) Turn on the Show vectors option.

(c) Click Create.

(d) Animate the velocity vectors over time.

PostTransient settingsi. Select Time value in the Post-processing time panel.

ii. Click Animate to display the Transient animation panel.

iii. Keep the default settings in the Transient animation panel and click Ani-mate to start the animation.

The animation will begin, and the display will show the velocity vectors for thesolution at each time. Air flows from one opening to the other (and passesthrough the fins of the heat sink); you can also see that the velocity distribu-tion changes with time from the initial velocity profile to that governed by thegeometry and other parameters of the system.

Figure 3.5 shows the velocity vectors at t = 20.

To stop the animation, click the red Interrupt button in the upper right handcorner of the Icepak interface,

(e) Click Done to close the Transient animation panel.

(f) Click Done to close the Post-processing time panel.

(g) In the Plane cut panel, turn off the Active option and click Done.


Transient Analysis

X

Y

Z

Time = 20.0

0.000000

0.00754239

0.0150848

0.0226272

0.0301696

0.0377119

0.0452543

0.0527967

0.0603391

Velocity m/s

Figure 3.5: Velocity Vectors at t = 20

Summary: In this tutorial, you set up and solved a transient model and used the ani-mation technique to examine the results over time.


3 Transient Analysis

Documents

Transient Analysis