Like all profiles surrounded by flow, turbomachinery blades are also subject to steady and unsteady aerodynamic forces. In addition, due to the varying arrangement of rotors and stators in the compressor and in the turbine itself, unsteady forces greatly influence turbomachines, as these forces tend to exhibit significantly higher frequencies compared to those in aircraft wings (above 100 Hz for low modular forms to the part far above 10 kHz).
The development of modern compressors with Blisk rotors using complex blade geometries has, in recent years, increased the demands on CFD methods to calculate the unsteady flow around these profile forms reliably and within reasonable periods of time. Specifically, verification of the flutter resistance needs an adequate calculation of the motion-induced air forces for a variety of cases (parameter space, including operating points, modes, phase difference angles). For this purpose, a standard method of calculation has been developed based on linearised Navier-Stokes equations, which is widely used by the European engine industry and research institutes. Validation of the procedure is imperative for the further development of CFD programmes and to safeguard the calculated unsteady aerodynamics for such 3D compressor blades.
Validation of linearTRACE
Within the framework of the Aerolight project, the unsteady pressure distributions (see figure above) and the aerodynamic damping determined therefrom, which were calculated with linearTRACE, were compared with results from the linearised Euler program Lin3D, developed and used by MTU. The comparison with this data was necessary to validate the flutter calculations for the design of the Aerolight compressor rotor with the – at the time new – linearised version of the CFD programme TRACE. Because no other validation data was available back then, the results were calculated with Lin3D, even though the data did not represent the complete physics, which was implemented in the linearTRACE programme (Navier-Stokes vs. Euler).