A Joint European Initiative to Develop Hybrid Grid Based CFD-Technology for Inviscid and Viscous Flow Computations Applicable to Geometrically Complex Aircraft Configurations

A joint European initiative to develop hybrid grid based CFD-technology with a short problem-turnaround time applicable to complex aircraft configurations has been undertaken. In the context of the Brite-Euram fourth framework programme two projects have been executed, namely FASTFLO I (1996-1998) and FASTFLO II (1998-2000). The objective of this publishable summary report is to provide an overview of the approach and main achievements in these projects. The practical outcome of the joint development has been CFD technology with inviscid (FASTFLO I) and viscous (FASTFLO II) flow modelling for complex aircraft configurations. The resulting CFD technology is based on the hybrid grid approach (Ref 7); this approach combines prismatic grid generation in a layer near aerodynamic surfaces with tetrahedral grid generation in the remainder of the flow domain. It has been concluded that the hybrid grid based CFD technology (both viscous, and inviscid flow models) meets the following two industrial requirements: 1. The CFD problem-tumaround time is within the order of a week for geometrically complex aircraft configurations 2. The CFD technology is able to provide a sufficient accuracy of the aerodynamic entities (like pressures, lift, drag and moments). Through the judicious introduction of hybrid grid generation techniques (that allow a higher level of automation in comparison with the more commonly used classical multi-block grid generation techniques) and the introduction of enhanced physical modelling in the CFD system a large spectrum of fluid flow problems for complex aircraft configurations can now be analysed within a problem tumaround time of the order of a week. Three examples of industrially relevant aerodynamic problems with complex geometries are: Viscous flow analysis to assess the maximum lift capability of high-lift configurations at wind tunnel and flight conditions (at a high Reynolds number) with engines running and flaps and slats deployed. Viscous flow analysis to determine stability and control derivatives of real fighter aircraft including engine inlet and exhaust jet with different down-loadings, e.g. pylon(s), pod(s), store(s) and ftiel tank(s) at high-loaded flow conditions. Viscous flow analysis of civil transport configurations for engine-airframe integration studies to minimise engine/airfi-ame interference for new Ultra-High Bypass Ratio engines. Viscous flow modelling studies for engine inlets and outlets. The introduction of highly automated grid generation algorithms has enabled viscous flow studies of a wider class of aerodynamic problems that which can not be tackled by standard CFD technology because time constraints are not met by the more commonly used multi-block grid generation technology. Economic implications of enabling hybrid grid based CFD-technology in the aerodynamic design process are manifold. Due to the realisation of a short-turnaround time the following cost savings have been realised: Enhanced modelling capability to engineer aerodynamic solutions resulting in a minimisation of business risks and costs associated with the introductions of new aircraft models. Large reduction in labour costs due to application of highly automated grid generation. Reduction of costs due to reduction of the number of intermediate models in an aerodynamic design cycle (first-time-right) In addition an increase in acceptance level of hybrid grid CFD technology in European aerospace industries (SAAB and EADS-M) can be observed. It is observed that a high interest at the FASTFLO partners is observed to exploit the results of the FASTFLO 1 and II projects. A ftirther exploitation of the hybrid grid based CFD technology is foreseen in civil transport projects, military aircraft projects, and in European R&D projects, e.g. Eurolift, Taurus and Hirett. The FASTFLO projects are an important activity towards the development of an industrial production code. However, the accurate and efficient calculation of viscous high Reynolds number flows is still a subject of current international workshops in general to evaluate multi-block structured, overset, and hybrid grids. An example of such an international workshop is the AIAA CFD Drag Prediction Workshop (Ref 10). Another outcome of the research conducted in the FASTFLO projects is that standardisation efforts (such as the CGNS-initiative as an emerging ISO standard) are crucially needed in Europe in order to reduce cost associated to exchange, preparation and modification of geometrical information (as needed for wind tunnel modelling and CFD geometry definition) for application of CFD-technology.