Exercise: Setting Materials and Environments in Simulation CFD

In CFD simulations, start simple and build in complexity. With that in mind, use a Simulation CFD exercise to define domain materials and explore natural convection on air flow direction and temperature distribution using both constant and variable air properties.

Learning Objectives

  • Gain experience working with multiple materials and material types, including creating custom device material.
  • Understand the impact of editing the material environment for air to account for natural convection.

To demonstrate the use of materials, the office space from Exercise 1 will be enhanced with more internal details, including furniture, a computer and an occupant.  This is in accordance with the best practice of “start simple and build in complexity.”  The impact of natural convection on air flow direction and temperature distribution will be observed by first simulating the office with constant air properties and then again with variable air properties.

Assumptions

The simulation exercise will have several simplifying assumptions to reduce overall complexity and expedite completion of this tutorial:

  • Radiation and solar radiation transmitted through windows will be ignored.
  • All external surfaces are adiabatic (i.e. very well insulated) EXCEPT for the 3 windows.

Simulation Parameters

  • Total flow rate:158 cfm (~4 air exchanges per hour)
  • Summer conditions:
    • Room inlet air temperature: 65F
    • Outside ambient air temperature: 90F
  • Occupant dissipates 60 Watts of thermal energy
  • Computer dissipates 100 Watts with a fan pushing air at 60 CFM


Item Location Description
1 Air Inlets 79 cfm each @ 65F
2 Air Outlet P = 0 psig
3 Windows Triple-pane, steel frame
4 Desk Wood
5 Occupant

60 Watts

6 Computer 100 Watts @ 60cfm

NOTE:  Because the primary focus of this tutorial is materials, many of these assumptions and parameters have been pre-defined.

Simulation Process

1. Download the file CFD-Materials_Exercise2.cfz and save it to a local hard drive folder.

2. Load the file.

a. Start the Simulation CFD interface and click Open in the upper left ribbon, then navigate to and open the CFD-Materials_Exercise2.cfz share file.

NOTE: The *.cfz share file is the compressed file version of a design study folder.  More information on share files can be found here.

3. Assign the fluid volumes.

a. Select the Materials button in the ribbon. 

b. Click on the large air volume, the 2 inlet volumes and the outlet volume to select them. 

c. Click on Edit from the ribbon.

d. Click next to Name and select Air from the dropdown and then hit Apply.

4. Assign a solid material to the desk.

a. Hide the main air domain (right click on it - then select “Hide”). 

b. Click on the desk then click the pencil and paper icon in the tooltip, to bring up the Edit dialog (or right click and select Edit). 

c. Click next to Type and select Solid from the drop down menu.

d. Click next to Name, select Wood (Soft) and then hit Apply.

5. Assign a solid material to the occupant.

a. Click on the volume representing the occupant and assign Human as the solid material.

6. Assign a solid material to the computer housing.

a. Click on the U-shaped volume under the desk that represents the computer housing and assign ABS (Molded) as the solid material.

The 60 cfm fan is custom and will be created by copying an existing fan material from the database, editing the settings, and then saving it to a local custom database.

7. Create Fan Material and apply to computer.

a. Select the block inside the U-shaped volume for selection.

b. Bring up the Edit menu using prefered method from above.

c. Change the Type to Internal Fan/Pump.

d. Next to Material in the dialog box hit the Edit… to bring up the Material Editor.

e. In the Material section in the upper left hand corner change the Name: to Computer.

f.  Use the dropdown menu next to Save to database: and change it to Local.  If this fan is used frequently, a better option would be to save it to My Materials or another custom database.

g. In the Flow section to the right change the Variation method to Constant using the dropdown menu.

h. Change the units to ft3/min using the dropdown menu.

i. Type 60 into the value input box to represent the flow of the computer fan.  A typical computer fan specification can be reviewed here.

j. Select Apply (you will see the value update in the properties section).

k. Click on the Rotational Speed button in the Properties section in the lower left.

l. Enter 0 into the value input box in the Rotational speed section (recall that this is a best practice for fans that are not cylindrical in shape).

m. Select Apply.


Material editor dialog for the computer fan material.

n. Click OK to close the material editor box, only have making certain that the correct settings have been applied.

o. Next to Flow Direction click on the “…” button. Then select the global Y button. 

p. Hit Apply


When assigning an internal fan material, it is important to remember to specify the correct direction for the flow of the fan.

TIP:  If the large air volume was not assigned as a fluid material before the fan, an error message will appear.  The best practice is assign fluids first, then solids followed by devices.

NOTE:  A heat exchanger device material would also be a valid option in this case to represent the computer.  More information on heat exchangers is located here.

Since the focus of this exercise is on materials, other settings such as boundary conditions, meshing and solver settings have already been defined, including:

  • A total heat generation boundary condition of 60W is assigned to the occupant.
  • A total heat generation boundary condition of 100W is assigned to the fan volume.

Please feel free to review these settings; more information on boundary conditions can be found here:

The scenario will be now be cloned and edited to investigate the impact of natural convection.  By running two separate scenarios, the results from each simulation can be directly compared later.

8. Clone the Scenario.

a. Right Click on the FixedAir scenario node in the Design Study Bar to the left and select Clone.

b. Type in VariableAir for the new scenario name.

9. Change the material environment reference to allow the air density to vary as a function of temperature for natural convection.

a. Right Click on Air in the Design Study Bar and select Edit material environment reference…

b. Select the Variable radio button in the upper right corner.

c. Hit OK.

10.  Solve the VariableAir Simulation.

a. Click on Solve in the Ribbon.

b. Go to the Physics tab and set Gravity Components to the -Z direction (0,0,-1).  This lets the solver know which way warmer air will rise up (+Z) and colder air will fall down (-Z).

c. Click on the Solve button.

While the VariableAir scenario is in the process of solving, the FixedAir scenario will be placed in the run queue.  Multiple simulations can be set up and solved in sequence (or in parallel for runs sent to the cloud solver) as described here:

11.  Solve the FixedAir simulation.

a. Double Click in the FixedAir scenario node in the Design Study Bar to activate the scenario.

b. Click on Solve in the Ribbon

c. Click on the Solve button.

NOTE:  The speed of solving will depend on your hardware resources; anticipate up to 2 hours or more for both simulations to complete solving.

The results from the two completed runs will now be directly compared using the Decision Center. For more information on summary images and the Decision Center, please click on the following links:

A results plane has been previously set up and the view settings were saved.  More on view settings can be found here.

12.  Apply the previously saved view settings.

a. While in Results mode, click on the View tab at the upper left of the interface and click on Apply View.

b. Select the file cut.xvs and then click the Open button.

13.  Create a Summary image to compare in the Decision Center.

a. Click on the Results tab

b. Click on the Summary Image icon at the left of the Ribbon.

14.  Compare results using the Decision Center.

NOTE:  Decision Center items cannot be updated for actively solving simulations.

a. Once both simulations have finished solving, click the Decision Center icon in the Ribbon; the Decision Center will appear at the bottom left corner of the interface.

b. Right Click on Image 01 in the Decision Center and select Update image to refresh the results.

c. In the Output bar in the bottom center of the interface, you will see 2 thumbnail images for the Image 01 summary image appear.  Design1:FixedAir is already in the main graphics window.  Click on the Design1: VariableAir thumbnail image and while holding the left mouse button down, drag it up to the main graphics window and release the button.  The slider bar will now become active in the Output Bar.  Move the slider back and forth and compare the results between each run.


The results plane slices through an inlet duct, the computer and the occupant.  The FixedAir scenario (left) has different air flow patterns than the VariableAir scenario (right) in the vicinity of the occupant.

Summary

In this application, natural convection had a clear impact on the flow characteristics and overall temperature distribution in the space.  In the FixedAir scenario, the air properties were left as default Fixed, where the density could not vary.  In the VariableAir scenario, the density was allowed to vary and the effects of natural convection were accounted for.

Air velocities from natural convection effects are relatively weak; this is why the flow patterns did not change near the cold air inlet duct or the computer.  The addition of a ceiling fan to this space would further decrease the influence of natural convection.  Natural convection attempts to stratify the air temperatures (cold near the floor and hot near the ceiling).  A ceiling fan would mix and redistribute the air for more uniform temperatures and increased energy savings.

AttachmentSize
Package icon cfd-materials_exercise2.zip3.42 MB