Semi-Inverse Design Applied to an Eight Stage Transonic Axial Flow Compressor

Abstract

Semi-Inverse Design is a class of Computational Fluid Dynamics (CFD) procedures for calculating aerofoil geometry from prescriptions of pressure loading and thickness distribution; when describing an aerofoil in three dimensions a stacking axis is also needed. All sixteen blade rows of an eight stage transonic axial flow compressor have been simultaneously designed in a single inverse 3D CFD calculation. The tip Mach number of the first rotor was just over sonic with a peak value in excess of 1.25. Apart from the substitution of geometric by aerodynamic aerofoil boundary conditions, the CFD model is the same as that used for direct analysis and includes rotor and stator clearance gaps as well as stator shroud leakage flows. This has profound implications for 3D CFD in the context of the total design process at a system level. The technique allows 3D CFD to behave like a design point through flow in the sense that we can now ask the question of 3D CFD - “what is the performance of a turbo-machine that has this design intent?” as opposed to “does this set of aerofoils meet the design intent and if they do what is its performance? And if they do not how should they be changed?” Inverse design provides a means of conveying design intent up through the “fidelity levels” from 1-D and 2-D, through Low Fidelity 3D CFD to High Fidelity CFD. Thus the desired loading distribution may be determined cheaply using 2D blade to blade analysis, either by iterative direct analysis (the current approach) or using a semi-inverse technique in 2D. As an illustration of this, a 1-D tool will be presented which produces estimates of aerofoil shape and surface velocities in real time to enable the designer to manipulate the loading distributions which are then used in Inverse 3D CFD to realize the final geometry.