The adoption of very high bypass ratios is an effective strategy to improve the performance of turbofans for civil aviation. Very high-bypass ratio turbofans are characterized by large-diameter fans and large intake diameters, whereas the overall length of the engine and its installation have to be contained to prevent excessive weight growth. As a result, the main components of the low-pressure compression system (LPC) are closer to each other in modern and future engines than they were in less recent designs, if distances are measured in terms of engine diameter. As the axial decay of potential distortions takes place over distances comparable to their tangential wavelength, modern and future engines also display stronger interactions between the intake, the fan, the OGVs and the pylons, causing performance and integrity issues that need to be addressed at the design stage.

The direct computation of whole LPC systems through CFD is expensive on account of the large extent of the domain and the wide range of wavelengths/frequencies that need to be resolved. This makes CFD unsuitable for bypass design. However, as the interaction between intake, fan, OGV and bypass takes place mainly through flow potential over long wavelengths, drastic simplifications can be introduced. Correspondingly, considerable computational savings are possible by resorting to semi-analytical models.

In this Part I of a two-part paper, a model for the intake/fan/OGV bypass interaction problem based on potential flow is presented. The model can describe the potential component of the LPC flow field, as well as vorticity distortions transmitted through the fan and those generated by the fan as a result of the non-uniform work done around the circumference. The method is validated against CFD results on a configuration typical of modern machines for civil aviation service and is demonstrated as a useful preliminary design tool.

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