The workflow for modeling fluid volumes in CFD is similar to FEA in many ways.   You start by thinking about what you want to study and what the expected results are, build a model, apply boundary conditions, create an analysis mesh, simulate it, and interpret the results. 

Using well modeled geometry and having an understanding of the expected simulation results helps the process of achieving a successful analysis. By taking special care with modeling and your simulation setup at the beginning of the analysis process it is possible to save a great deal of time in the long run.

Estimating Results Before Analysis

Before running the simulation, and trusting the simulation results, you should have an intuitive as well as numerical estimate of the results.  These estimations may come from previous experience, or be based on previous models or industry practice.  Having an idea of what the results should be allows you to “sanity check” the simulation outputs. This is important because the validity of CFD results depends on valid user inputs into the software. If you see unexpected results, you can more easily identify potential errors in the simulation set-up.

Modeling 

Internal Flows (like Pipes)

To create the most useful CAD model for your CFD analysis, it is important to first know whether you are interested in modeling an internal or external flow.  For internal flows that are bounded by solid objects like pipes, you can simply model the object normally in 3D CAD software, such as Autodesk Inventor.  

CFD visualization of internal flows

You then import this CAD geometry into your CFD analysis software, such as Autodesk Simulation CFD and use tools to simplify the model (see more on Autodesk WikiHelp).  In the software you can select the hollow interior space created by the object and use it as your fluid domain.  

External Flows (like Cars)

For external flows you may use two methods. 

 If the flow moves around an object that is not physically contained (a ball or airplane) you may again import the model into Simulation CFD.  This time, you will create a bounding box around the object that defines the extent of the fluid flowing around the object.  This is a virtual wind tunnel.  

Alternatively, you may create the space around the object as a solid within your CAD software; often by making a large box around the original object as a new body.  You can then import these bodies to Simulation CFD and set the external material as whatever fluid you’re analyzing and the solid object to whatever properties it needs.  This may be necessary when you cannot size the bounding box correctly using the methods built into Simulation CFD.

CFD of external flow around a racecar

Sizing the flow domain

One of the most common errors facing a new user of CFD is to use an incorrectly sized flow domain.  This is most easy to see and understand with external flows, such as simulating the flow around a car.  If the flow domain is not large enough there will be an artificial constriction of the flow that can greatly affect results.  Similarly, if the flow has not fully stabilized by the outlet it can yield faulty results.  Additionally, if the inlet is too close to the object drag will not be predicted correctly, especially in incompressible flows.

Level of model detail

For a good CFD analysis, the model needs to be detailed enough to reflect reality, but not so detailed that it takes an inordinate amount of time to run. 

Fluid flow can be very sensitive to small details and over simplifying the model can miss some of those details.  However, under-simplification will cause meshes that take excessively long to run without necessarily increasing the accuracy of the results.  Therefore, it is important to omit those details that will not affect the flow, while including those details that will affect the flow.

An over-detailed model and one that would be better for analysis.  The interferences, fasteners, small gaps and the extremely small fillets were removed.