Application of computational aerodynamics to airplane design

OMPUTATIONAL aerodynamics has been defined as the engineering discipline which deals with the simulation of flowfields about aircraft through the numerical solution of the equations of fluid motion using digital computers This simulation process is referred to as numerical flow simulation; another way of simulating flow about airplanes is the wind tunnel Wind tunnel technology is almost as old as aeronautics; wind tunnel testing became the principal tool of aerodynamic design and development very early in the history of airplane engineering Of course in addition to the ex perimental approach analytical methods based on aerodynamic theory always have been used to obtain valuable design information; but in order to reduce the computational effort to a humanly tolerable level, many simplifying assumptions had to be introduced These simplifications not only had to do with the mathematical modeling of the physical problem e g inviscid incompressible flow but they also involved the geometry of the configuration and flow condition e g the lifting line concept to represent a wing These simplications did not make possible the calculation of many aerodynamic characteristics and posed a high degree of uncertainty about the accuracy and validity of the numerical results Therefore these results could be used only as design guidelines and large emphasis had to be placed on wind tunnel testing The advent of the digital computer and the impressive growth of its capabilities and cost effectiveness—as illustrated in Fig 1—now have made it possible to eliminate many of the simplifications that had been introduced in analytical methods This has increased the amount of in formation that can be extracted from a calculation and at the same time provides more accuracy and a greater range of validity for the results of numerical flow simulation This has turned computational aerodynamics into a major tool for airplane design This paper surveys the use of computational aerodynamics as a design tool Much development work in computational aerodynamics is of a fundamentally scientific nature con cerned with what type of numerical flow simulation is feasible within the boundaries of our knowledge of flow physics and numerical algorithms This paper deals with the engineering aspects of numerical flow simulation i e. what elements are practical in an environment where schedule and budget constraints predominate and where multidisciplinary in teraction is the way of life and particularly with the factors that affect its effectiveness in such an environment Even though computer system characteristics do fundamentally affect the effectiveness of computational aerodynamics they are not the subject of this paper The survey presented here is of necessity, selective and consequently incomplete The main intent of this paper is to highlight those elements of computational aerodynamics that are important for airplane application what form they have taken and to indicate what future developments are considered necessary to increase the general utility of computational aerodynamics Three dimensionality geometrical complexity and computational efficiency are stressed as overdriving considerations for design application First some general characteristics and needs of airplane design projects are discussed; within this context particular attention is given to the meaning of effectiveness and the identification of the computer code characteristics that affect it Next the code elements that are part of a typical numerical flow simulation are identified; the nature of these elements how they impact code effectiveness and how they are affected by considerations of effectiveness are addressed subsequently The author has drawn freely from developments at the Lockheed California Company to illustrate some points; for this he is indebted to many of his colleagues at Lockheed