In fact, virtual simulations do not require a physical prototype to perform their experiments. They only need a CAD model of the object that can be easily modified in case the test does not meet a prescribed requirement. Thus it is clear that the first step to undertake when carrying out a CFD/FEA simulation is 3D CAD modelling.
Characteristics of a 3D CAD model
A 3D CAD model is useful for a FEA or CFD analysis as they have some features that are different from the majority of CAD one may encounter. In fact, during the simulation process, the focus is on local object features, not the object as a whole. Some simple rules can be outlined in order to produce a correct model to be used in CFD and FEA analysis. Following these guidelines will allow the user to improve their simulation accuracy, and they are so straightforward that they can be adopted by both experienced and novice users.
Usually, 3D CAD models are created as assemblies containing tens if not hundreds of different parts. It is extremely useful for example for creating the ‘Bill of Materials’ of an object or when evaluating sizes, interference and mass properties. From the simulation point of view, the best thing to do is to simplify the model. Parts inside assemblies are typically reduced as much as possible, only significant features of the component are kept, those that actually influence its mechanic or fluid dynamic behaviour. As a rule of thumb, the number of parts in an assembly should be around 10. Example of how to reduce the number of assembly subparts. The first simplification is obtained removing bolts and nuts, then all assembly subparts are merged together and finally all holes are closed.
Each subpart itself has to be simplified, removing all features that are not relevant for the given analysis. In the example shown in the video below, the variable of interest is the turbulence generated by the motor shape and not the heat exchange taking place on its surface. So, it is possible to model it as a simple cylinder.
In FEA and CFD analysis it is fundamental to keep in mind what is the physical phenomena of interest and the final goal. This allows the user to choose which features are important and which can be removed to simplify the simulation.
When dealing with CFD analysis one has also to remember that the simulation involves the so called ‘fluid volume’, the space surrounding the component, and not the component itself. So, what is modelled is the ‘negative’ counterpart of what one usually models. The following video shows how to do that for a fan geometry.
In CFD analysis the model describes the fluid volume surrounding the component. Usually it is the ‘negative‘ counterpart of what one usually draws.
The Cloud for engineers and designers
The modern IT world, with its smart instruments, allows to exploit cloud computing technologies for both CAD modelling and CFD & FEA simulations.
The main advantages for users of these technologies are flexibility and accessibility, support (available directly inside their workspace) and a significant cost reduction (thanks to solutions as the pay-per-use approach). This is the reason for the growing success of tools like OnShape, a CAD modeller directly available in your browser that allows users to create assemblies and parts following guidelines listed in this post, and CONSELF, an FEA/CFD simulation environment perfectly integrated with the OnShape CAD platform, that makes the first steps in the simulation world more straightforward for novice users.
Ruggero Poletto has a PhD in Computational Fluid Dynamics and is co-founder of CONSELF