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## Overview of CFD Verification and Validation## IntroductionThis page presents an overview of the process of the verification and validation of computational fluid dynamics (CFD) simulations. The overall objective is to demonstrate the accuracy of CFD codes so that they may be used with confidence for aerodynamic simulation and that the results be considered credible for decision making in design. One should first understand the distinctions between a code, simulation, and model. The formal definitions of these terms are defined on the page entitled Glossary of Verification and Validation Terms. Essentially, one implements a model into a computer code and then uses the code to perform a CFD simulation which yield values used in the engineering analysis. Verification and validation examines the errors in the code and simulation results. Credibility is obtained by demonstrating acceptable levels of uncertainty and error. A discussion of the uncertainties and errors in CFD simulations is provided on the page entitled Uncertainty and Error in CFD Simulations. The levels of uncertainties and errors are determined through verification assessment and validation assessment. Verification assessment determines if the programming and computational implementation of the conceptual model is correct. It examines the mathematics in the models through comparison to exact analytical results. Verification assessment examines for computer programming errors. Validation assessment determines if the computational simulation agrees with physical reality. It examines the science in the models through comparison to experimental results. There is professional disagreement on exact procedures for verification and validation of CFD simulations. CFD is maturing, but still an emerging technology. CFD is a complex technology involving strongly coupled non-linear partial differential equations which attempt to computationally model theoretical and experimental models in a discrete domain of complex geometric shape. A detailed assessment of errors and uncertainties has to concern itself with the three roots of CFD: theory, experiment, and computation. Further, the application of CFD is rapidly expanding with the growth in computational resources. In this work, we primarily follow the verification and validation guidelines established by the AIAA [AIAA-G-077-1998]. Note that this is a guide - no standards yet exist for CFD simulation verification and validation. Other ideas from other researchers in this discipline will also be included. Their papers are referenced in the bibliography. Notable among them is the book on verification and validation published by Roache. Verification and validation are on-going activities due to the complex nature of the CFD codes and expanding range of possible applications. Some basic verfication should be done prior to release of a code and basic validation studies should be performed on classes of flow features prior to use of the code for similar flows. However, as the code continues to develop, verification and validation should continue. ## Use of CFD ResultsThe level of accuracy required from a CFD analysis depends on the desired use of the results. A conceptual design effort may be content with general shock structure information, whereas a detailed design may require accurate determination of the pressure recovery. Each quantity to be determined generally has its own accuracy requirement. Levels of credibility may vary according to the information required. The application of CFD for design and analysis may be catagorized
into three levels according to increased
levels of required accuracy: 1) P for the design
change can be expressed as:
where
The ## Flow CharacteristicsIn applying CFD for flows typical of aerospace systems, we must first understand the characteristics of the flow. We must understand the reality upon which we will validate the CFD code and processes. The flow is characterized primarily by the Mach number. We are interested in analyzing flows spanning the Mach number range from Mach 0 (static conditions) to 25 (access to space). The flow is characterized by high Reynolds numbers which result in regions of laminar flow transitioning to turbulent flow. Flows along the body and inlet surfaces create boundary layers. Adverse pressure gradient may be present for internal flows. At transonic, supersonic, and hypersonic speeds, shock waves are present. Under these conditions, the boundary layer may separate. At hypersonic Mach numbers, real gas effects may become important. This requires use of chemistry models for calorically and thermally perfect gases, equilibrium air, and chemically reaction of gas mixtures. Often the geometry of the system is complex, which has to be physically modeled. Unsteady flow may become important. ## Physical ModelsThere are several physical models that are commonly used within CFD codes: Last Updated: Thursday, 17-Jul-2008 09:00:21 EDT [an error occurred while processing this directive] |