## 6.2. Definition of Errors in CFD Simulations

CFD simulations have the following potential sources for errors or uncertainties:

- Numerical Errors

  Numerical errors result from the differences between the exact equations and the discretized equations solved by the CFD code. For consistent discretization schemes, these errors can be reduced by an increased spatial grid density and/or by smaller timesteps.

- Modeling Errors

  Modeling errors result from the necessity to describe flow phenomena such as turbulence, combustion, and multi-phase flows by empirical models. For turbulent flows, the necessity for using empirical models derives from the excessive computational effort to solve the exact equations[1] with a Direct Numerical Simulation (DNS) approach. Turbulence models are therefore required to bridge the gap between the real flow and the statistically averaged equations. Other examples are combustion models and models for interpenetrating continua, for example, two-fluid models for two-phase flows.

- User Errors

  User errors result from incorrect use of CFD software and are usually a result of insufficient expertise by the CFD user. Errors can be reduced or avoided by additional training and experience in combination with high-quality project management and by provision and use of Best Practice Guidelines and associated checklists.

- Application Uncertainties

  Application uncertainties are related to insufficient information to define a CFD simulation. A typical example is insufficient information on the boundary conditions.

- Software Errors.

  Software errors are the result of an inconsistency between the documented equations and the actual implementation in the CFD software. They are usually a result of programming errors.

A more detailed definition of the different errors follows.

[1] The Navier-Stokes equations for single-phase, Newtonian fluids

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