Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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Background Bryan Richards retires in September, 2003 following forty years of research and teaching in aerodynamics. His career has involved both experimental work at Imperial College and Von Karman Institute and CFD at University of Glasgow. The symposium is dedicated to the important topic of how to use CFD and experiments towards the goal of improved understanding of aerodynamics. It is intended to bring together international experts in these fields to look forward to new ways of integrating the two disciplines, and in the process celebrate the varied contributions of Bryan Richards to aerodynamics. Scientific Rationale CFD practitioners and experimentalists have a common goal of understanding aerodynamics. It is therefore surprising that the disciplines often only interact for the validation of CFD. This provides a very limited form of integration but often there is no interaction between the experimentalist and the CFD practitioners. This situation is unsatisfactory from many points of view including (a) the need to have an appreciation of the flow before deciding what should be measured, (b) the desirability of having checks in place on the experimental measurements as they are taken, (c) the difficulty in making certain important measurements, (d) the need to assess the influence of the experimental techniques on the measurements, (e) the ability of CFD to provide detailed flow information and sensitivity at a reasonable cost for some cases, (f) the large cost of CFD calculations for other cases, and (g)the lack of credibility for the CFD results for some flow categories. It could be argued that the process of aerodynamic investigation would be significantly enhanced if the integration of CFD and experiments was much stronger. In particular, the design and reliability of experiments could be significantly enhanced by CFD, the scope of experimental measurements extended through CFD and the credibility of the simulation results enhanced by the availability of suitable measurements from experiments. This sort of closer integration is however rare. The aim of the symposium is to bring together leading researchers from both fields to initiate more careful consideration of these issues and to stimulate new ways of approaching aerodynamic studies. Local Organisers Ken Badcock Department of Aerospace Engineering University of Glasgow Glasgow G12 8QQ United Kingdom Phone: +44(0)141 3304106 Fax: +44(0)141 3305560 gnaa36@aero.gla.ac.uk http://www.gla.ac.uk/Research/CFD George Barakos, Department of Aerospace Engineering University of Glasgow Glasgow G12 8QQ United Kingdom Phone: +44(0)141 3304106 Fax: +44(0)141 3305560 gbarakos@aero.gla.ac.uk http://www.gla.ac.uk/Research/CFD Scientific Committee Prof. Ning Qin Department of Mechanical Eng. University of Sheffield UK Prof. Daniel Favier LABM University of Marseilles France Prof. Richard Hillier Department of Aeronautics Imperial College UK
Experimentalist’s requirements for a safe methodology in CFD code validation Jean Délery ONERA – Centre de Meudon, France, delery@onera.fr Keywords: experimental techniques, code validation, database Abstract In spite of the spectacular progress in CFD there is still a strong need to validate the computer codes by comparison with experiments. The first validation step is the assessment of the code numerical safety and the physical models accuracy. This validation step requires carefully made building block experiments. To be calculable, such experiments must satisfy conditions such as the precise definition of the test set-up geometry, the absence of uncontrolled parasitic effects, a complete information on the flow conditions and indication on the uncertainty margins. Under these conditions, the experiment can be put into a database which will be precious to help in the development of reliable and accurate codes. The paper provides also an overview of modern measurement techniques allowing a precise and thorough description of complex separated flows. Recommendation for the constitution of experimental databases are provided as a conclusion. Introduction Methods for verifying the capability of a code to solve given equations have been the object of close examinations. Identification and elimination of various types of errors and use of precision criteria, methods for convergence testing, rules for establishing grid convergence, are all required when one has to assess the quality of the numerical tool. The various stages of the general process of verification that will give to the code a confidence label permitting to use it for testing theoretical models have been summed up by Roache with references to many authors 1 . The ERCOFTAC association has issued Best Practice Guidelines giving strict recommendations to asses code quality for industrial computational fluid dynamics 2 . The code verification process constitutes by itself a complex program often partially carried out but that should be completely satisfied in the ideal cases. A second step is devoted to the validation of models aimed at predicting flows that cannot be presentable as clearly identified solutions of well known mathematical problems. At this stage, comparison with experimental data is mandatory. In the past, predictive methods were validated by comparison of the computed results with some measured global quantities, such as forces and moments, and with wall properties, namely the pressure and the heat-transfer for hypersonic applications. The skin friction was more rarely available, this quantity being difficult to measure (even now) in compressible flows. However, the flow prediction landscape has completely changed over the past 40 years with the advent of numerical methods solving the Navier-Stokes equations or approaches such as DSMC to predict rarefied flows. However, in their present state the CFD codes are still far from being free of critics, since many difficulties persist both on the numerical and physical sides. There is thus a strong need to validate CFD codes, more particularly from the physical point of view before their routine use for design purposes 3 . A comparison restricted to the wall properties is in general insufficient to validate the most advanced predictive methods. In particular, information on the Mach number, temperature, density fields is essential to elucidate the cause of discrepancies affecting, for example, the wall quantities distribution. Such a requirement is still more demanding in hypersonic flows where one has to represent complex thermochemical processes and/or strongly interacting and shock-separated turbulent flows. In this case, information on turbulence quantities is also needed, which is a formidable challenge in high Mach number flows! The prediction of shock/shock interferences which can have destructive effects on a nearby structure necessitates an accurate prediction of the complex structures resulting from shock intersection. The problem of code validation is crucial in threedimensional applications where the Navier-Stokes approach becomes mandatory. Due to the complexity of such flows, it is clear that the consideration of the surface pressure alone is inadequate, this information giving a very partial view of the flow (in threedimensional flows, it is no longer possible to infer separation from an inspection of the wall pressure distributions). 1
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Figure 11: Vortex Shedding cycle fo
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Figure 13: Comparison with experime
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