Exercise: Understanding Simulation CFD Results
- Gain experience assessing the validity of results.
- Compare designs side by side based on simulation results.
- Determine the effects of air exchange rates on temperature gradients and energy consumption.
The number of air changes a space experiences per hour (rate at which all air in the space is replenished) is a design variable impacting performance characteristics such as energy consumption, air flow fields and temperature gradients.
Industry standards, experience, and hand calculations may provide an acceptable range of air change rates based on usage, ranging from a minimum of 4 per hour to 60 per hour for a kitchen space. Those ranges are limited by their inability to capture the dynamics associated with air flow and temperature distribution for each unique space. This is why simulation is used; it accounts for complicated physics that improve a designer’s prediction of operating characteristics.
- Summer day with an ambient temperature of 90F
- HVAC supply of 65F
- Total flow rate:62 cfm (~5 ACH) & 121 cfm (~10 ACH)
- Summer conditions
- Room Inlet Temperature: 65F
- Ambient Air Temperature: 90F
- Surrounding environment (neighboring spaces): 75F
|1||Supply||1.5 m/s @ 55F|
|2||Return||P = 0 psig|
|4||Front external air surface (in front of desk)||Multi-Pane Window||6|
|5||All external air surfaces except front window||Walls||Walls|
The starting point of this exercise is a last results share file which contains ready to interpret results along with a populated decision center. Please review the help system for information about setting up Decision Center items (e.g. Summary Planes, Summary Parts, XY Plots, and Summary Images).
1. Download the CFD-Exercise 6_LR.cfz share file to a local hard drive folder..
2. Load support share file
a. Start the Simulation CFD interface and click Open in the upper left ribbon, then navigate to and open the CFD-Exercise 6_LR.cfz share file.
NOTE: Design 1 and Design 2 of the Design Study Bar (left of interface) contain 5 ACH and 10 ACH scenarios, respectively. The supply and return surface areas were modified such that their velocities would be the same between designs, considering the difference in flow rate (e.g. double the surface area for double the flow).
Interrogate analysis validity by reviewing critical values, performing hand calculations, and confirming values in the Summary File.
3. Toggle Decision Center ON
a. Go to the Results tab and click on the Decision Center icon to toggle the Decision Center ON or OFF.
b. Ensure Decision Center is ON (visible in the bottom left hand of interface).
4. Review Summary Plane
A Summary Plane concisely displays analysis output for the plane across multiple design scenarios.
a. Select the “Summary outlet” node in the Summary Planes item of the Decision Center.
b. The Output Bar will now display the Summary Planes tab
i. A Summary Plane was previously defined at the outlet to extract Mass Flow and Temperature values at the return.
NOTE: Changing Results Quantities and Units is done by selecting the respective pull down menus.
The return temperature for the 5 ACH scenario is nearly 2F (~1C) higher than the 10 ACH scenario. The Mass Flow and Temperature of this Summary Plane at the return can be used in a simple hand calculation to determine the amount of energy in the system.
3. Calculate the amount of energy in the system using Summary Plane and known data.
a. Use the following equation described by many textbooks to calculate the energy based on flow and thermal parameters (SI units will be used):
|Energy = MassFlowRate * SpecificHeatCapacityofAir * TemperatureDifferential|
|SpecificHeatCapacityofAir = 1004 J/kgK|
|Inlet temperature = 18.33C|
b. For the 5 ACH scenario:
|Energy = .035 kg/s* 1004 J/kgK * (23.74C - 18.33C)|
|Note that a Celsius temperature differential is the same as a Kelvin temperature differential and used directly here, A Fahrenheit differential IS NOT the same.|
c. For the 10 ACH scenario:
|Energy = .068 kg/s* 1004 J/kgK * (22.67C - 18.33C)|
This data indicates a difference of approximately 110 Watts between scenarios. If all the thermal boundary conditions are the same between analyses, why is there a difference in the energy between these scenarios?
6. Review Summary File Energy Balance
NOTE: Accessing the Summary File is done in the results of the active design scenario and not from the Decision Center.
a. Enter global results for the 5 ACH design scenario
i. 5 ACH of Design 1 should already be active (highlighted in Blue) in the Design Study Bar.
If it is not active (highlighted in blue as shown):
a. Double clicking “Design 1” will expand and collapse that node. Expand the node to expose 5 ACH.
b. RMB the 5 ACH node and select Activate.
b. Select the Results Tab.
c. Select the Global icon in the Results Tasks section.
d. Select the Summary File button in the Review section found to the right of the Global icon.
e. Scroll down to nearly the bottom and look for "Fluid Energy Balance Information:”
Callout 1 from above is approximately equaled to the 190 Watts calculated in step 4. This verifies that the fluid has exchanged the expected amount of energy from the wall and sources in the analysis. If this value was off by more than 10-15% the simulation may require additional iterations or a refined mesh.
Callout 2 plus Callout 3 should be approximately equalled to Callout 1. Here their sum is 204 Watts which is reasonably close to the value of Callout 1. Callout 2 displays the “Heat Transferred from Wall to Fluid” corresponding to the film coefficient heat flux here. Callout 3 confirms the 60 Watts applied to the occupant.
Other details of the Summary File can be found here.
f. Close the Summary dialog (bottom right of dialog).
g. Review the Summary File for 10 ACH (repeat steps 6a - 6f for 10 ACH scenario).
Reviewing the Summary File in conjunction with the Summary Plane located at the return verifies the amount of energy picked up by the fluid at it moves through the system.
These sanity checks reveal part of the answer to the previous question of why a difference in the energy exists between these scenarios. The difference comes from the amount of energy transferred at the walls between the scenarios. The Heat Transfer from Wall to Fluid between both files indicates an approximate difference of 100 Watts, which is roughly the difference calculated in step 4. Recall the window heat flux being different from previous exercises because of air velocity and temperature differences between the fluid and wall.
NOTE: This is a steady state analysis, the system would have to operate under these conditions for 10 hour to recognize a difference of .1 kwh of energy (100 W = .1KW => times 10 hrs).
Now that the fluid energy balance has been verified, additional data will be extracted to understand the temperature differences between the volumes in the analysis.
7. Review Summary Parts
a. Select the Summary Parts node of the Decision Center.
This activates the Summary Parts tab of the Output Bar.
Summary Parts offer a quick comparison of various results quantities between designs and scenarios. Here we see that the average temperature difference of components in the analysis between the design scenarios is ~3-5F.
NOTE: Even though more energy is transferred through the walls of the 10 ACH scenario the additional air flow is able carry it out of the system, reducing the temperature rise.
Extracting Summary Part and Summary Plane data has helped to quantify the difference between scenarios as roughly 100 Watts of energy and about 2-5 F between components (including return temperature).
The differences will now be depicted using the powerful results visualization tools of Simulation CFD to reveal further insight regarding the flow and thermal operating characteristics of the system.
8. Visualize flow and thermal results simultaneously.
Summary Images in the Design Review Center of the Decision Center will be used along with the manipulation of interface Viewports to display flow and thermal results for each scenario simultaneously.
a. Select “xsecs_ts” in the Decision Center
The xsects_ts tab of the Design Review Center Output Bar will now become active and the first scenario with results has been loaded into the graphics window
NOTE: Each Design Review Center Summary Image will have its own respective tab
b. Modify Viewports to display multiple views
i. Select the Viewports pulldown of the View tab
ii. Select the four equally sized view option highlighted above.
TIP: All new viewports are a copy of the active view when viewports are modified.
c. Ensure Link Views is pushed down (to the right of the Viewports pull down)
i. This allows all four (4) views to be oriented simultaneously.
d. Reorient the model to view it from the right side (relative to the current view)
ii. Select the right arrow of the View Cube in any viewport
TIP: The ACTIVE viewport will have an Autodesk Simulation CFD icon in the bottom right corner of the view port as shown above.
e. Position multiple views
Multiple views will be configured to quickly identify flow and thermal fields across both designs. The final view built here will resemble the following:
i. Select and hold the Design 2::10 ACH icon in the Design Review Center (DRC) to drag it to the top right view port.
ii. Select the “xsecs_vs” tab of the DRC (NOT in the Decision Center)
NOTE: Selecting the Decision Center item would replace the viewports view
iii. Select and drag the first icon (Design 1::5 ACH) into the bottom left viewport
iv. Select and drag the second icon (Design 2::10 ACH) into the bottom right viewport
v. Select the large Output Bar button to minimize that portion of the interface.
vi. Change the velocity scale
1. RMB the velocity legend scale from any viewport (Units and Option should be the only available selections)
2. Select Options
3. Set the Min field to 0 then hit enter
4. Set the Max field to .375 then hit enter
5. Close the dialog (bottom right hand button)
The screen should now display velocity and temperature across both design scenarios from the first image of this step “e”.
This extremely powerful usage of Viewports and the Design Review Center of the Decision Center helps visualize the flow and thermal performance across multiple design scenarios.
The higher velocities in the 10 ACH scenario occurring around the occupant and along walls now become evident with the legend scale change.
As discussed in previous modules, higher velocities along the walls will increase rates of heat transfer and the total heat flux entering the 10 ACH design. This additional air flow is also responsible for lower temperatures in the 10 ACH scenario.
TIP:Modifying legend scales can help exaggerate gradients in the system.
Any views created while dissecting a scenario can be designated as a Summary Image and accessed in the Design Review Center of the Decision Center to compare between designs and scenarios.
A previously created ISO Surface Summary Image will now be compared between the designs to visualize the flow differences in 3D vs. the current 2D planes.
9. View an ISO surface side by side
a. Change the Viewports to two (2) vertically split views.
i. Select Viewports pull down of the View tab
ii. Select the second option for two vertically split model views
b. Select the “iso_vp2” summary image of the DRC node of the Decision Center
This will change the views to use the selected summary image. This image is an ISO Surface displaying all velocities of .2m/s. This will display the volume of air traveling at this specific velocity. Larger surfaces correspond to more air at that speed.
c. Select and hold the second icon (Design 2::10 ACH) of the DRC “iso_vp2” tab and drag it to the right viewport
This view isolates all of the .2m/s velocity air between the designs and visually depicts a larger volume of air, at 0.2 m/s, moving along the walls in the 10 ACH design. This is consistent with the Summary File, hand calculations, and Plane views describing an increased amount of energy transferring through the walls due to higher velocity air movement.
10. Review the vertical XY Plot Data
The final Decision Center item used to interrogate the results in this exercise will be the comparison of an XY Plot. The XY Plot data is extracted along the vertical line shown below for each model.
a. Select “vertical” from the XY Plot Data node of the Decision Center
An XY Plot with data extracted along the vertical line above will be displayed.
b. Change the vertical axis to Fahrenheit
i. RMB the vertical axis legend
ii. Select Units
iii. Select Fahrenheit
This plot extracts data from the air and through the human for both design scenarios. The higher human temperatures are found from .5 meter to nearly 1.5 meter of the horizontal axis. The higher 5 ACH temperatures are evident and are again interpreted as the 3-5F difference found with the previous results visualization and data extraction methods.
Reviewing results begins with some fundamental sanity checks followed by the use of data extraction and results visualization tools that provide insight on operating characteristics difficult to capture in the real world.
In this exercise a 10 ACH scenario would allow an additional ~100 Watts to enter the space but provide a ~3-5 F reduction in air and occupant temperatures. This reduction in temperatures may be unwarranted as the 5 ACH temperatures were not unreasonable. The 10 ACH design would also require an air handler operating at a higher cooling load and air flow volume, consuming more energy than the 5 ACH scenario.