An Overview of Computational Fluid Dynamics for Application to Advanced Propulsion Systems

Abstract

Validation of computational fluid dynamics (CFD) codes appropriate for subsonic through hypersonic applications requires careful consideration of the physical processes encountered in these flight regimes and detailed comparisons with quality experimental datasets that simulate these processes. The primary objective of this study is to overview the state of the practice of current CFD tools used for advanced propulsion system design and analysis. In addition, this paper provides an overview of the validation requirements of CFD codes applicable to advanced propulsion systems, and identifies the enabling technology requirements in the development of the next generation of CFD tools. Several major propulsion systems such as rockets, engines, plumes, scramjets, and gas turbine and pulse detonation engines are discussed. The physical processes that dominate combustion phenomenology are described for various engine types. An assessment of the state of the art of numerical approximations used to simulate the flow-field effects of these phenomena is provided. Computational issues related to numerical algorithms and the implementation of physical models are also addressed. During the last several years, most U.S. industries have witnessed major changes in their business environment. These changes include significantly curtailed government spending, reduced corporate product development budgets, and the transition from a national to a global economy. In view of these changes, U.S. industries have been forced to critically review their basic engineering and manufacturing processes and identify potential cost-saving initiatives to maintain their business share and broaden their market base. The traditional design and analysis practices rely heavily on costly full-scale prototype development and testing. Introduction of high-fidelity design and analysis tools such as computational fluid dynamics (CFD) early in the product development cycle was identified as one way to alleviate the testing costs and develop products better, faster, and cheaper. In the design of advanced propulsion systems, computational modeling plays a major role in defining the required performance over the flight envelope, as well as in testing the sensitivity of the design to the various modes of operation (e.g., afterburner, rocket, ramjet, and scramjet). Computational modeling techniques, complemented with select ground and flight testing, are expected to be the engineering approach of choice in the development of new Air Force and NASA space propulsion programs. Therefore, increased emphasis is placed on developing and applying CFD models to simulate the flow-field environments and performance of advanced propulsion systems. This places a premium on the development of the next generation of computational tools so that this can be used effectively and reliably in a design environment by non-CFD specialists. Experience gained from the use of current research-oriented CFD models is essential to guide the successful development of engineering application tools. Since the new approaches will rely less on testing and more on CFD results, a careful strategy is needed to ensure that the models are appropriate for the phenomena being simulated and that they are applied in a timely and efficient manner. An approach to assessing the CFD model involves the methodology described as follows. Currently, available CFD codes are validated by careful comparison with measurements. The validation determines the range of validity of the model and identifies improvements needed in the physical approximations and numerical algorithms.