Simulation CFD Solver Settings for AEC Applications
The Solve dialog is the command center for the simulation; it controls how and where the simulation runs along with what data is to be output. Detailed information on accessing and navigating the Solve dialog can be found here:
The Solve dialog is comprised of 3 tabs, each of which have a variety of input fields and buttons, which are detailed here:
- Online Help: Solve Dialog - Control
- Online Help: Solve Dialog - Physics
- Online Help: Solve Dialog - Adaptation
- Adaptation is an optional meshing control and will not be expanded on further in this section.
Step 1 - Determine the Physics
Simulation CFD has the capability of solving a wide variety of fluid flow and heat transfer applications, including those found in AEC. The number of options and settings in the Solve dialog required to address all of these situations can at first appear intimidating. The first step to knowing which settings are required is to determine the physics of the problem being solved.
Despite their diversity, most common AEC simulations can be sorted into 4 primary categories of fluid flow physics. Before evaluating the decision tree below, review the following information:
- Flow Only simulation does not consider any temperature changes. This is the default solver setting for every new simulation.
- Forced Convection
- Natural Convection
- Mixed Convection is the most computationally intensive since it has to consider both forced and natural convection at the same time.
To provide some context, the following table lists some common AEC situations and their respective category according to the decision tree above.
|Case||AEC Application||Physics Category|
|1||Constant temperature flow through an insulated duct where heat transfer is NOT considered.||Flow Only|
|2||Variable temperature flow through a non-insulated duct where heat transfer IS to be considered.||Forced Convection|
|3||External, uniform temperature air flow around a factory building with no heat sources or hot exhaust stacks.||Flow Only|
|4||External air flow around a factory building with smoke stacks emitting hot exhaust.||Forced or Mixed Convection|
|5||Internal, sealed office lobby with occupants. HVAC system is OFF and there are no floor or ceiling fans.||Natural Convection|
|6||Internal, sealed office lobby with occupants. HVAC system is ON and there are no floor or ceiling fans.||Mixed Convection|
|7||Internal, sealed office lobby with occupants. HVAC system is ON and there is a ceiling fan.||Forced or Mixed Convection|
Note that in Cases 4 and 7 in the table above, there is a choice of forced and mixed convection. The buoyancy effects from natural convection are very weak and easily dominated by other flow sources. The velocity of these sources, along with geometric considerations such as the size of the air space, will play a role in determining whether it is forced or mixed convection. In Case 4, wind speeds of 1 mph would have much less impact than if they were set to 30 mph. In Case 7, the location and diameter of the fan, along with its RPM setting (i.e. lowest speed or highest speed) will all be influencing factors.
To determine if mixed convection is necessary, the same process that was used in the previous exercise ***link to Exercise section from Materials module*** should be used:
1. Set up and run as a Forced Convection model
2. Clone scenario
3. Set up and run as a Mixed Convection mode
4. Compare results
a. If results change noticeably, problem is Mixed Convection.
If results do not change noticeably, problem is Forced Convection.
Many AEC simulations, particularly those with large air volumes, will typically need to be run as mixed convection to account for the effects of both forced and natural convection behavior.
Step 2 - Apply Preferred Settings
Preferred settings for AEC applications are listed in the table below according to their simulation category. These settings should always be considered as good initial starting points; further adjustment may be needed in certain cases.
To use this table effectively, review the following links to learn more about these specific solver settings:
- Advection schemes
- Turbulence models
- The ideal turbulence model provides the best compromise between speed, stability, and accuracy. The k-epsilon model is the default for good reason and should be considered as the starting point for every AEC application.
- Auto Forced Convection
|Forced Convection||Natural Convection||Mixed Convection|
|Auto Forced Convection||n/a||ON or OFF||OFF||OFF|
|Advection Scheme||1||2 or 5||2 or 5||2 or 5|
|Turbulence Model||k-epsilon||k-epsilon||k-epsilon / Mixing Length||
Low Re k-epsilon
TIP: For natural and mixed convection, do not forget to set the gravity vector in the Physics tab of the Solve dialog. This controls which direction warmer air will rise and colder air will fall.
For some AEC applications, additional settings to consider include:
- Radiation may need to be considered for AEC applications that have very hot components (e.g., gas turbine test facility) or are subjected to solar heating
The Results quantities button in the Control tab provides access to results visualization options. Some of these results are not output by default since they are not widely used. For some AEC applications, additional results that may be useful, include: