CFD modelling in ODS Studio is handled through the OpenFOAM software.
Learning the full intricacies of CFD is a rather complex and specialised field of study that goes beyond the scope of this guide. However some important points to note are as follows.
There are fundamentally two types of solutions that one can try to achieve in CFD. These are: transient and steady-state solutions. Steady-state is the type most commonly practised (traditionally) in the building industry and can be thought of like a time-lapse photo of the flow field. Steady state solutions return a single result and are suitable for cases where the flow field is expected to be fairly “steady” or, in other words, not expected to change through time very much. The SIMPLE algorithm (Semi-Implicit Method for Pressure-Linked Equations) is redominantly used for achieving steady-state results. The alternative to steady-state solutions are transient solutions where results are returned at different points in time (like the frames of a video). Transient solutions obviously require much more storage memory (hard-drive space) to store all of the results at various times. This solution method is important to capture “unsteady” dynamics of the flow-field, that is, aspects of the flow field that are expected to change throughout time such as the creation and transport of eddies in the wake of a building or the unsteady dynamics of buoyant plumes. The PISO algorithm (Pressure Implicit with Splitting of Operators) is predominantly used for achieving transient results.
The use of “incompressible” solvers is not a bad assumption in many cases. Considering the low flow velocity of many building-related flows then it is easy for one to imagine that flow does not compress much as it moves around or through a building.
The “mesh” of a CFD model is akin to pixel-resolution on a graphical render. However the mesh generally has a three-dimensional quality to it so that mesh “cells” can be thought of as
voxels as opposed to pixels. Achieving a high-quality mesh is a big part of CFD modelling and can be a specialised discipline unto itself. The quality of the mesh determines the accuracy of the solution, or sometimes, if a solution can be achieved at all! Without going into exhaustive detail here, the important things to remember are that the mesh should be finer (higher resolution) at points in the flow field where “interesting” stuff is expected to happen. “Interesting” in this case means where velocity (or thermal) gradients are expected to be high. This is, for example, near walls, particularly sharp corners or near objects of different temperature. For objects of different temperature it is often a good idea to add “layers” of very fine mesh-cells near the surface of the object to resolve the thermal dynamics near the surface of an object. This technique will be covered in a later tutorial.