Assessment of the Delayed Detached-Eddy Simulation Method for Predicting Off-Design Operating Conditions of an Axial Compressor

Abstract

Reliable, highly accurate prediction of off-design operating conditions using numerical methods is essential for overcoming the challenges of achieving greener aviation. To assess their capabilities, the high-speed TU Darmstadt axial compressor stage is considered at near stall conditions. Steady Reynolds-averaged Navier-Stokes (RANS) methods fail to capture the unsteady flow physics. Hence, the results show large deviations from experimental reference data. This was investigated by a comprehensive pre-study involving the variation of the mesh density, turbulence model extensions and operating point control mechanisms. Improvements were seen when using unsteady methods such as unsteady RANS (U-RANS) or the Delayed Detached-Eddy Simulation (DDES). Both methods show much closer agreement with the experiment while DDES outperforms U-RANS. These improvements were found to be due to the fact that DDES resolved both the deterministic and stochastic scales of the unsteady flow field, especially in separated flow regions. Further, a thorough assessment of two mechanisms to control the unsteady operating point near stall, namely the mass flow controller and a converging-diverging nozzle, was done. The mass flow controller performed best in terms of operability, predictive accuracy and reduced requirements on a priori knowledge from pre-cursor RANS simulations. The DDES method showed its potential and represents a powerful approach to investigating unsteady off-design operating conditions with high accuracy (below 1% average relative error in radial profiles of the total pressure and temperature ratio, as well as the circumferential flow angle).