Flow distortion computational investigation in a serpentine diffuser

Abstract

Unmanned Aerial Vehicles are currently receiving great attention and interest by the air forces because they are able to operate in highly-threatened missions without risk to life and reduced through-life costs. The serpentine-diffuser seems to be promising in order to provide the low-observability capabilities required by these aeroplanes without lack of compactness. The present project intends to provide further insight on the complex flow phenomena occurring within the serpentine diffusers, focusing especially on the total-pressure and swirl distortion issues. The geometry under investigation was previously studied experimentally at the Swedish Defence Research Agency, and the test results will be used to validate the computations. The mass-flow rate impact on steady-state inlet-distortion will be assessed using Reynolds Averaged Navier-Stokes approaches. Furthermore, the unsteady nature of the distortion will be investigated using the more computationally-expensive Delayed Detached Eddy Simulation approach. The analysis will consist of statistical, spectral and Proper Orthogonal Decomposition analyses. The results presented in this project will show that the swirl-distortion is mostly independent on the mass-flow rate, while the total-pressure distortion intensity increases markedly as the mass-flow increases. The dynamic distortion will be found to be of major importance since the instantaneous peak values encountered are well above the time-averaged data. The spectral analysis shows that the majority of high-energy frequency-bands are shared by both total-pressure and swirl-angle fluctuations at the AIP. Finally, the POD offers the possibility to shed some light on the highly-turbulent separated-flow behaviour encountered at the top region of the AIP, by the identification of the most-energetic characteristic oscillations. Moreover, it shows that the radial total-pressure distortion is more dependent on the more-energetic flow structures while the circumferential component is importantly affected by the smaller flow features.