NPARC Alliance CFD V&V Web Site V&V Home       Archive       Tutorial

Glossary of Verification and Validation Terms

The following is a glossary for verification and validation. The objective is to clarify meanings and indicate how some of the terms have specific definitions that may be different than common usage of those terms. For example, verification and validation share the same meaning in most dictionaries and even in common usage among technical personnel; however, here we are concerned with attaching to them specific technical definitions that are associated with different aspects of CFD. Most of the definitions come directly from AIAA G-077-1998, "Guide for the Verification and Validation of Computational Fluid Dynamics Simulations". Other definitions and discussions come from other literature cited in the bibliography. The terms are listed alphabetically.


Calibration

Calibration in the context of CFD simulations is defined as
The process of adjusting numerical or physical modeling parameters in the computational model for the purpose of improving agreement with experimental data. (AIAA G-077-1998)

Calibration has more to do with improving agreement of computational results with experimental results rather than assessing error and uncertainty. Calibration arises from uncertainty in modeling complex physical processes. It adjusts unmeasured or poorly characterized experimental parameters. One use of calibration may be the adjustment of emperical constants found within a turbulence model. While calibration improves agreement for a class of problems, it may make the code less general.

Certification.

Certification of a code encompases verification and validation, but also includes such things as documentation, quality assurance, and version control. Certification is concerned with managerial and programmatic concerns on the use of the CFD code for design activities. Certification is intended to put a "seal" of credibility on a production code for use in a design project. Certification can mean that a certain standard has been satisfied (i.e. ISO, NASA). NASA Glenn has a center level procedure LeR-P2.10.2 entitled "Software Product Assurance" which offers some high level guidance on verification and validation of software.

Code.

A code is a set of computer instructions and data inputs and definitions. This is the CFD code (WIND, NPARC, OVERFLOW, USM3D, ...) which may include ancillary codes and documentation as part of the software package. A code typically has three stages related to the level of validation completed: research, pilot, and production. The production code has been fully validated for the intended design applications, including a system-level validation (full inlet, inlet-aircraft integration).

Credibility.

An improvement of credibility is considered to be the same as confidence building or providing quality to the customer.

Error.

Error is defined as
A recoqnizable deficiency in any phase or activity of modeling and simulation that is not due to lack of knowledge. (AIAA G-077-1998)

Further discussion of error can be found at the page entitled Uncertainty and Error in CFD Simulations.

Grid Convergence.

Grid convergence indicates that as the grid spacing is reduced, the computed simulation results approach the continum result. Here "grid spacing" can refer to both spatial spacing, as well as, time step for the case of unsteady, time-accurate simulations. Further discussion can be found on the page entitled Examining Spatial (Grid) Convergence.

Iterative Convergence.

Iterative convergence indicates that as the discrete equations are iterated, the computed simulation results approach a fixed value. Further discussion can be found on the page entitled Examining Iterative Convergence.

Model.

A model is defined as:
A representation of a physical system or process intended to enhance our ability to understand, predict, or control its behavior. (AIAA G-077-1998)

A conceptual model for CFD consists of the observations, mathematical modeling data, and mathematical (partial differential) equations that describe the physical system. It also includes initial and boundary conditions.

The computational model is the computer program or code that implements the conceptual model. This may be finite-difference, finite-volume, finite-element, or other type of discretization. It includes the algorithms and iterative strategies. Parameters for the computational model include the number of grid points, algorithm inputs, and similar parameters.

Modeling.

Modeling is defined as
The process of construction or modification of a model. (AIAA G-077-1998)

Prediction

. Prediction is defined as
Use of a CFD model to foretell the state of a physical system under conditions for which the CFD model has not been validated. (AIAA G-077-1998)

Prediction is going beyond the validation database and performing simulations of untested systems. This of course, is a tremendous power of CFD and of use in design studies. However, we would like to be able to estimate the accuracy of the predictions. Unfortunately, the verification and validation processes can not formally provide these estimates. The best one can do is point to the verification results and the most similar validation case.

Robustness

. Robustness defines the ability of the numerical method to provide a solution despite variabilities in the initial solution and control parameters. This incorporates issues of fault tolerance. Generally, robustness is achieved at the expense of accuracy.

Simulation.

A simulation is defined as
The exercise or use of a model. (That is, a model is used in a simulation). (AIAA G-077-1998)

For a CFD analysis the application or run of the CFD code is a simulation.

Uncertainty

. Uncertainty is defined as
"A potential deficiency in any phase or activity of the modeling process that is due to the lack of knowledge." (AIAA G-077-1998)

Further discussion of uncertainty can be found at the page entitled Uncertainty and Error in CFD Simulations.

Validation

. Validation is defined as
The process of determining the degree to which a model is an accurate representation of the real world from the perspective of the intended uses of the model. (AIAA G-077-1998)

Further discussion of validation can be found at the page entitled Validation Assessment.

Verification.

Verification is defined as:
The process of determining that a model implementation accurately represents the developer's conceptual description of the model and the solution to the model. (AIAA G-077-1998)

Further discussion of verification can be found at the page entitled Verification Assessment.


Last Updated: Thursday, 17-Jul-2008 08:47:08 EDT