Devices for Efficient CFD Simulation

Devices are special materials that represent the performance characteristics of complex AEC components during CFD simulations. The strategic use of devices enables the simulation to become more practical without sacrificing the overall fidelity of the results.

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.


The ceiling fan geometry (left) can be represented by a simple cylinder (right) in CAD.  The cylinder is then assigned as an internal fan device where the characteristics (flow rate, rpm) of the fan are input.  This dramatically reduces the complexity of the overall simulation.

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.

Internal Fans

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:

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.


The detailed ceiling and floor fan geometry (left) are simulated with internal fan devices (right) to reduce complexity while still accounting for the influence of these components.  The solid cylinders representing the fans must be created in CAD prior to launching to CFD.

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.

Heat Exchangers

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:


A heat exchanger device replaces complex fin geometry (left) with a simple solid block (right) while still simulating the transfer of thermal energy with the fluid moving past it. Substitution of the heat exchanger details with the block must be done in CAD prior to launching to CFD.

Resistances

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 furnace filter (left) can be represented in CFD as a simple volume assigned as a resistance.

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.

Surface Resistances

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. 


A volume resistance (left) requires several elements across the thickness to calculate the pressure drop across the filter.  A surface resistance (right) calculates the same pressure drop with 60% less mesh in this case.  The black arrows indicate primary flow direction.  Higher pressures are to the left (upstream of the flow direction).

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.


This return grille geometry (left,) is highly detailed and should be omitted from the simulation model (middle).  A surface resistance can be assigned to the imprint surface (highlighted to the right), shared between the main fluid volume and the outlet extension, to account for the flow restriction of the grille.

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: