This work provides a historical perspective of the developments of propulsion system inlet/engine integration. The flow physics of inlet distortion are discussed, along with the stability effects on the engine compression systems. Throughout the document, both commercial and military applications are considered. Developments in analyses from the original works characterizing inlet distortion, the use of distortion descriptors and early modeling concepts, and the modeling and simulation required today are reviewed. The special requirements and testing techniques necessary to adequately describe inlet distortion and the effects on the propulsion system are summarized. The aspects of pressure distortion, temperature distortion, and swirl are addressed and the physics of interaction described. The compression system response to different types of distortion is discussed with the interaction with various compression system components and designs explained. Axial and centrifugal compressors are covered, as the resulting physics of the inlet distortion effects on each has a different characteristic. An overview of the necessary flow physics to consider in the integration of compression systems in both low and high-Mach number applications is presented.

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Modeling compression systems using parallel compressor theory has been used for the analysis of compression system operability since the 1960s. Parallel compressor models have been traditionally designed and used for the analysis of circumferential distortion effects as a means to evaluate the impact of various inlet flow field disturbances on compressor operation. This paper (the first of two) provides a review of the parallel compressor concept and discusses extensions to the original theory. These extensions include the incorporation of dynamic response, application to complex distortions, and the application to inlet swirl. Understanding these effects and the application of parallel compressor theory extensions is required to produce analytical models and computer simulations that can be used to enhance the development testing and the understanding of the response of gas turbine compression systems. Once a computer simulation has been constructed for a particular test article, it can be exercised and results compared against test results where distortion-generator devices (such as distortion screens) have been used, generally with favorable accuracy. The usefulness of the extended parallel compressor model is derived from its ease of use, simplicity, and ability for quick turn-around of results. It is often more desirable to have an analysis capability that is easy and quick to use than to have one that is extremely accurate, especially when understanding basic physics is of primary concern during a test operation. Extreme accuracy may require large amounts of computer resources and take days or weeks to compute a single performance point. While this may be acceptable for design, the limitations of high-fidelity simulations make them impractical to use due to the time constraints imposed by the pace of testing. Applying a timely analysis capability, using a parallel compressor simulation can provide a new physical understanding of the effects of complex distortion during the testing process when comparing the analytical and test results. This concept is presented in two companion papers: the first paper, Part 1, concentrates on the parallel compressor concepts, theory and limitations of the methodology while the second paper, Part 2 [1] presents applications of the approximate methods developed and compares results with experimental data.

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