Computational fluid dynamics (CFD) in IDA ICE

The CFD extension enables detailed simulation of airflows and temperature distributions within buildings. It provides unique insights into indoor climate performance that go far beyond traditional zone-based methods.

Using Computational Fluid Dynamics (CFD), the extension resolves 3D air movement, temperature gradients, draught rate and age of air inside rooms and spaces. This makes it possible to evaluate comfort, ventilation efficiency, and the impact of different design choices with high accuracy.

By seamlessly integrating with IDA ICE, CFD simulations can be launched directly from building models, ensuring consistent boundary conditions and results that reflect the actual indoor environment. The coupling allows for simultaneous assessment of HVAC strategies, occupant comfort, and energy performance.

The CFD simulations are realized by interfacing the OpenFOAM CFD engine, but calculation setup, execution and results visualization are all easily done from within IDA ICE. (The OpenFOAM library is developed for and runs in a Linux environment. Microsoft Windows supports such an environment by virtue of the "Windows Subsystem for Linux", WSL2).

IDA ICE CFD workflow

The CFD workflow in IDA ICE consist of modeling the geometry of the calculation domain, defining thermal and airflow inputs, and performing dynamic simulations to obtain boundary values. It then proceeds with meshing, running the CFD solver, and post-processing. The post-processing phase includes data visualization and extraction of computational results.

Geometry modelling

The geometry of the outer zone surfaces is created in the conventional IDA ICE. All geometries inside the zone (occupants, furniture, beams, etc.) are created within the conventional IDA ICE but needs to be assigned to a CFD role. Objects can be a constrained surface on a zone surface or a 3D-geometry inside the zone.

Note: Always simplify as much as possible. Complex geometries and narrow margins in how geometries are connected might cause errors. Simplified geometry also speeds up the simulation time.

Thermal and airflow input

All objects included in the CFD model are assigned thermal properties. All supply air devices and similar objects are given user-defined volume flows. For all 3D objects that emit heat, the thermal energy is released into the immediate volume adjacent to the object's surface. In other words, the heat output from e.g. occupants is not emitted directly from their surface but instead from the adjacent volume.

Note: IDA ICE CFD will later in the work process perform CFD calculation of a chosen instance in the dynamic simulation.

Boundary condition input

Boundary conditions in CFD is values that define “the edges of the simulation”. I.e. temperature of walls, air flow, emmited heat from occupants etc. The boundary conditions that normally are difficult to determine and time-consuming to enter, are taken automatically from the results of an IDA ICE dynamic simulation. The user only needs to select one of the completed simulations and the time instant from which the boundary conditions are to be adopted.

Mesh

The CFD domain is discretized (partitioned) into a large number of computational cells (often millions). To make it easy to use, the CFD extension does not expose any detailed mesh controls to the user. Only the base cell size and to what extend the cells should be refined at the domain's boundaries need to be defined.

Note: In the animation tab, the mesh is visible and can be inspected. A good tip is to start with a coarse mesh to verify your boundary conditions.

Note: Each cell holds a single value, such as temperature. Increasing the number of cells enhances accuracy but also extends the simulation time.

Post-processing

CFD simulation results can be examined in the Animation tab, much like other IDA ICE outputs. However, some differences apply. For instance, CFD results are static and cannot be animated. The visualization allows users to explore the distribution of temperature, air velocity magnitude and direction (vectors), draught rate, and air age.

The CFD simulation results can be viewed in the Animation tab, similar to how other IDA ICE results are viewed. However, some differences apply. For instance, CFD results are static and cannot be animated. The visualization allows users to explore the distribution of temperature, air velocity magnitude and direction (vectors), draught rate, and age of air.

Note: A measurement volume can be inserted to retrieve specific data for that volume.

Note: A report is automatically generated with key inputs.