Devices for Efficient CFD Simulation
To better understand the potential impact of devices, consider a ceiling fan. A fan designer trying to determine blade design and tilt angle for an optimal throw pattern would need to simulate the spinning blades. Simulation CFD can achieve this with the Motion module, but it can be challenging and adds complexity.
By contrast, the typical AEC designer is more concerned with system-level performance and not the subtle details of flow characteristics in the immediate vicinity of the fan blades. Rather than complicating the model with spinning fan blades, a better option would be to substitute a simplified representation of the fan characteristics which are obtained from the ceiling fan vendor. Devices have been developed, and continue to be developed, to handle these types of simulation obstacles.
An internal fan/pump device (a fan moves a gas such as air; a pump moves a fluid such as water) adds momentum to the fluid that surrounds it in the system. There are many uses for this device in AEC applications, including:
- Ceiling and floor fans
- Server racks in a datacenter
- Air handlers
- Pumps moving water through a piping system
Internal fan devices can be defined as having a constant flow rate or with a fan curve (flow rate vs. pressure) obtained from the fan specification sheet provided by the fan vendor. The process of assigning an internal fan device and input options are described in further detail here.
Note that internal fans are not restricted to cylindrical geometry and can be rectangular in shape to represent the flow through a desktop computer or a server rack. For the server rack, the individual fans for each rack unit can be lumped together to represent the total flow going through the rack. More information on the characterization of server racks, can be found here.
TIP: If the fan device is not cylindrical in shape, specify a 0 rpm rotational speed to avoid calculation errors when the solver attempts to spin the flow near sharp corners.
The heat exchanger device is similar to an internal fan in that it uses a simple geometric shape to represent the complex internal features (e.g. radiator fins, coolant flow) of air conditioning units or heater coils. Unlike a fan, the heat exchanger has inputs for both the flow and heat transfer characteristics; a fan requires the definition of a separate thermal boundary condition.
More information about assigning heat exchangers can be found in the following locations:
- Online Help: Heat Exchanger Materials
- Component Characterization: Heat Exchangers
- Data Center Components
Resistances are used to represent components which restrict the flow of fluid (i.e, flow impedance). For AEC applications, these components can include filters, diffusers, grates and perforated tiles.
For example, consider a common household furnace filter. The tiny fiber structure of the filter makes it essentially impossible to explicitly model even in CAD let alone CFD. However, the filter plays a role in the HVAC system since it resists the passage of fluid and influences the ultimate flow rate of the furnace air handler (as the filter gets clogged, resistance increases and the air flow rate drops).
The resistance enables the filter to be simulated with a simple volume that has built in mathematical representations of the component characteristics, including:
- Free-area ratio
- For a perforated plate, this is the surface area of the region with the holes divided by the surface area with no holes.
- Head Capacity Curve
- Plot of flow rate versus pressure drop across the device.
- Typically provided by the filter manufacturer
More information on using resistances is available at the following:
In addition to smaller components such as filters, resistances also have some larger scale applications. For example, a group of occupants in a venue will partially obstruct air flow and can be represented with a block assigned as a resistance, as detailed here.
As a best practice, a resistance assigned to a volume should typically have several mesh elements through the thickness (e.g. the direction of primary air flow) to resolve an accurate pressure drop across the component. For relatively thick resistances, such as some HEPA filters that may be inches thick, this requirement does not typically pose a problem.
However, for very thin filters or perforated sheet metal parts, maintaining 2 or 3 elements through the thickness results in very small element sizes and a much higher element count.
Surface resistances were developed to address the challenge of simulating thin components that restrict flow.
The process for assigning a surface resistance is similar to that of volumes except that surfaces, not volumes, must be selected. When CAD is launched or imported into Simulation CFD, surfaces are automatically created between volumes which have mating surfaces. Surfaces can also be created in the CAD program prior to launching into CFD.
TIP: Surfaces can also be assigned as solids to represent thin plate and baffles which totally obstruct flow, as shown here.
Additional details on using surfaces to model thin resistances and solids can be found here: