The modeling aspects described in this section are by no means complete for all of the flow simulations that can be imagined. This discussion is intended only to convey some of the common sources of variability among CFD model results.

A rigorous CFD analysis involves investigating all of the items described in this section. However, it is acknowledged that in practice it is necessary to rely on engineering judgment to reach a balance between producing results in a reasonable amount of time and performing a suitable level of quality assurance on the model. You should always consider the desired level of accuracy of the CFD model when deciding how much effort needs to be invested in quality assurance efforts.

An important observation to be made here is that this CFD solution may not be the true physical solution to the backward facing step problem. Over the course of this study, an attempt has been made to minimize the uncertainties by careful optimization of the problem setup with reference to each of the aspects discussed in the beginning of the section. It is meaningful now to go back to that discussion and recall that although these uncertainties can be minimized it is nearly impossible to eliminate them completely. At this stage, through optimization of all the critical aspects of your model, the uncertainties associated with these modeling aspects have been minimized. The solution at this stage can be deemed as the best possible CFD solution with respect to the aspects of the model discussed in this section. The difference observed between this numerical solution and the true physical solution can be attributed to the following factors:
  • Mathematical modeling: The mathematical models which are used to represent the physical processes are not an exact representation of the physical phenomena. Even the best available models are based on certain approximations and simplifications.
  • Input errors: These are the errors due to uncertainties of certain inputs such as the boundary condition specification. For example, even when the boundary conditions are specified as equivalent to the available experimental values there might be some interpolation errors when these values are projected on to the CFD domain.
  • Uncertainties in experimental results and measurements: This discussion focused on a few guidelines to setup the CFD simulation model for a given problem. However, one key aspect that cannot be overlooked while comparing CFD results with experimental results of a problem is the uncertainties present in the experiment. An experiment, just like CFD analysis, is a controlled simulation of the problem. There are various sources of errors and uncertainties in the experiments such as uncertainties in the setup and environmental conditions in which the experiment is conducted, uncertanties in the measurement instruments used in the experiment and uncertainties in the post-processing equipment.

Due diligence is required to carry out an experiment with application of refined experimental methods to achieve a reliable set of results. A similar diligence, when applied to carrying out a CFD simulation, will help achieve a reliable set of results from CFD. An analysis completed with the above mentioned guidelines supporting the procedure will provide a result which for most purposes can be used as a substitute of the true value of the solution.