The Role of Cfd in Preliminary Aerospace Design

This paper discusses the role that computational fluid dynamics plays in the design of aircraft. An overview of the design process is provided, covering some of the typical decisions that a design team addresses within a multi-disciplinary environment. On a very regular basis trade-offs between disciplines have to be made where a set of conflicting requirements exists. Within an aircraft development project, we focus on the aerodynamic design problem and review how this process has been advanced, first with the improving capabilities of traditional computational fluid dynamics analyses, and then with aerodynamic optimizations based on these increasingly accurate methods. 1 Background The past 25 years have seen a revolution in the entire engineering design process as computational simulation has come to play an increasingly dominant role. Most notably, computer aided design (CAD) methods have essentially replaced the drawing board as the basic tool for the definition and control of the configuration. Computer visualization techniques enable the designer to verify that no interferences exist between different parts in the layout, and greatly facilitate decisions on the routing of electrical wiring and hydraulic piping. Similarly, structural analysis is now almost entirely carried out by computational methods, typically finite element methods. Commercially available software systems have been progressively developed and augmented with new features, and can treat the full range of requirements for aeronautical structures, including the analysis of stressed skin into the nonlinear range. Also, they are very carefully validated against a comprehensive suite of test cases before each new release. Hence, engineers place complete confidence in their results. Accordingly, the structural design is routinely committed on the basis of computational analysis, while structural testing is limited to the role of verification that the design truly meets its specified requirements of ultimate strength and fatigue life. Computational simulation of fluid flow has not yet reached the same level of maturity. While commercial software for the simulation of fluid flow is offered by numerous vendors, aircraft companies continue to make substantial investments in the in-house development of their own methods. At the same time there are major ongoing efforts to develop the science of computational fluid dynamics (CFD) in government research agencies and Universities. This reflects the fact that fluid flow is generally more complex and harder to predict than the behavior of structures. The complexity and range of phenomena that characterize fluid flow is well illustrated in Van Dyke’s Album of Fluid Motion [1]. The concept of a numerical wind tunnel, which might eventually allow computers “to supplant wind tunnels in the aerodynamic design and testing process”, was already a topic of discussion in the 1970-1980. In their celebrated paper of 1975, Chapman, Mark and Pirtle [2] listed three main objective of computational aerodynamics: 1. To provide flow simulations that are either impractical or impossible to obtain in wind tunnels or other ground based 1 Copyright c © 2003 by A. Jameson experimental test facilities. 2. To lower the time and cost required to obtain aerodynamic flow simulations necessary for the design of new aerospace vehicles. 3. Eventually, to provide more accurate simulations of flight aerodynamics than wind tunnels can. There have been major advances towards these goals. Despite these, CFD is still not being exploited as effectively as one would like in the design process. This is partially due to the long set-up times and high costs, both human and computational, associated with complex flow simulations. This paper examines ways to exploit computational simulation more effectively in the overall design process, with the primary focus on aerodynamic design, while recognizing that this should be part of an integrated multi-disciplinary process. The emphasis is on the second of the original goals for a numerical wind tunnel, but also on taking computational methods a step further, to use them not only to predict the aerodynamic properties, but also to find superior designs. The key idea is to embed both analysis and optimization methods in the design process, drawing from both CFD and control theory. Design trade-offs and the design process itself are surveyed in the next sections. Section 4 discusses the state of the art for CFD in applied aerodynamics, in particular the accuracy of drag prediction. Sections 5-8 examine the way in which optimization techniques can be integrated with CFD. The paper concludes with case studies which apply aerodynamic shape optimization methods within the design process. 2 Aerodynamic Design Tradeoffs Focusing on the design of long range transport aircraft, a good first estimate of performance is provided by the Breguet range equation: Range = VL D 1 SFC log W0 +Wf W0 . (1) Here V is the speed, L/D is the lift to drag ratio, SFC is the specific fuel consumption of the engines, W0 is the loading weight(empty weight + payload+ fuel resourced), and W f is the weight of fuel burnt. Equation (1) already displays the multi-disciplinary nature of design. A light weight structure is needed to reduce W0. The specific fuel consumption is mainly the province of the engine manufacturers, and in fact the largest advances in the last 30 years have been in engine efficiency. The aerodynamic designer should try to maximize VL/D, but must consider the impact of shape modifications on structure weight. The drag coefficient can be split into an approximately fixed component CD0 , and the induced drag due to lift as CD = CD0 + C2 L πεAR (2) where AR is the aspect ratio, and ε is an efficiency factor close to unity. CD0 includes contributions such as friction and form drag. It can be seen from this equation that L/D is maximized by flying at a lift coefficient such that the two terms are equal, so that the induced drag is half the total drag. Moreover, the actual drag due to lift Dv = 2L2 περV 2b2 varies inversely with the square of the span b. Thus there is a direct conflict between reducing the drag by increasing the span and reducing the structure weight by decreasing it. It also follows from equation (1) that one should try to maximize V L/D. This means that the cruising speed V should be increased until it approaches the speed of sound C, at which point the formation of shock waves causes the onset of ”dragrise” . Typically the lift to drag ratio will drop from around 19 at a Mach number V/C in the neighborhood of 0.85, to the order of 4 at Mach 1. Thus the optimum cruise speed will be in the transonic regime, when shock waves are beginning to form , but remain weak enough only to incur a small drag penalty. The designer can reduce shock drag and delay the onset of drag-rise by increasing the sweep back of the wing or reducing its thickness. Increasing the sweepback increases the structure weight, and may incur problems with stability and control. Decreasing the thickness both reduces the fuel volume (since the wing is used as the main fuel tank), and increases the structure weight, because for a given stress level in the skin and a given skin thickness, the bending moment that can be supported is directly proportional to the depth of the wing. In the absence of winglets, the optimum span load distribution is elliptic, giving an efficiency factor ε = 1. When, however, the structure weight is taken into account, it is better to shift the load distribution inboard in order to reduce the root bending moment. It may also be necessary to limit the section lift coefficient in the outboard part of the wing in order to delay the onset of buffet, which may occur when the lift coefficient is increased to make a turn at a high Mach number. 3 Design Process The design process can generally be divided into three phases: conceptual design, preliminary design, and final detailed design, as illustrated in Figure 1. 2 The conceptual design stage defines the mission in the light of anticipated market requirements, and determines a general preliminary configuration capable of performing this mission, together with first estimates of size, weight and performance. A conceptual design requires a staff of 15-30 people. Over a period of 1-2 years, the initial business case is developed. The costs of this phase are the range of 6-12 million dollars, and there is minimal airline involvement In the preliminary design stage the aerodynamic shape and structural skeleton progress to the point where detailed performance estimates can be made and guaranteed to potential customers, who can then, in turn, formally sign binding contracts for the purchase of a certain number of aircraft. At this stage the development costs are still fairly moderate. A staff of 100-300 people is generally employed for up to 2 years, at a cost of 60120 million dollars. Initial aerodynamic performance is explored through wind tunnel tests. In the final design stage the structure must be defined in complete detail, together with complete systems, including the flight deck, control systems (involving major software development for fly-by-wire systems), avionics, electrical and hydraulic systems, landing gear, weapon systems for military aircraft, and cabin layout for commercial aircraft. Major costs are incurred at this stage, during which it is also necessary to prepare a detailed manufacturing plan, together with appropriate facilities and tooling. A staff of thousands of people define every part of the aircraft. Wind Tunnel validation of the final design is carried out. Significant development costs are incurred over a 3 year period, plus an additional year of Flight Testing and Structural Qualification Testing for Certification. Total costs are in the range of 3-12 billion dollars. Thus, the final design would normally be carried out only if sufficient orders have been received to indicate a reasonably high probability of recovering a significant fraction of the investment. For a commercial aircraft there are extensive discussions with airlines. In the development of commercial aircraft, aerodynamic design plays a leading role during the preliminary design stage, during which the definition of the external aerodynamic shape is typically finalized. The aerodynamic lines of the Boeing 777 were frozen, for example, when initial orders were accepted before the initiation of the detailed design of the structure. Figure 2 illustrates the way in which the aerodynamic design process is embedded in the overall preliminary design. The starting point is an initial CAD definition resulting from the conceptual design. The inner loop of aerodynamic analysis is contained in an outer multi-disciplinary loop, which is in turn contained in a major design cycle involving wind tunnel testing. In recent Boeing practice, three major design cycles, each requiring about 4-6 months, have been used to finalize the wing design. Improvements in CFD which would allow the elimination of a major cycle would significantly shorten the overall design process and therefore reduce costs. Conceptual Design