Forced response numerical investigation of a transonic compressor rotor

Abstract: The present thesis discusses the forced response of a rotor’s bladed-disk when excited by the periodic force produced by a wake generator, or Inlet Guide Vane (IGV),with 8 equally-spaced blades. These components are part of an axial transonic compressor rig, moreover the Transonic Compressor Darmstadt 1 (TCD1), located at the Technical University of Darmstadt. The investigation is within the ARIAS (Advanced Research Into Aeromechanical Solutions) project, that aims to improve the predictive capabilities of the design methods used in the aircraft propulsion engine’s industry to study aerodynamically induced blade vibrations. The methodology comprised a set of numerical analyses that were conducted using the software Ansys, addressing both the structural and the aerodynamics sides of the aeroelastic problem, in a co-dependent way. At first, steady-state CFD simulations were conducted to study the mesh convergence and to analyse the operation of the compressor at N80 speed, obtaining its compressor map and its peak efficiency operation point. From these, the aerodynamic static force being applied on theblade was obtained, allowing to determine the pre-stresses of the bladed-disk, orblisk. With a modal analysis, the natural modes of the rotor blisk at N80 speed were obtained and the resonance crossing M2 EO8 was identified, plotting the Campbell and ZZENF diagrams. From the modal displacements of the critical mode, a blade flutter analysis was conducted in order to compute the aerodynamic damping ratio of the rotor blisk. With a transient CFD analysis, the periodic forcing being applied onthe rotor blisk that arises from the IGV wake pattern was determined and exported as Fourier coefficients. Finally, a harmonic simulation was carried out to analyse the forced response of the blisk, introducing both the aerodynamic damping value and the unsteady forcing mapped onto the blade. As a result, the frequency response of both the blade maximum alternating deflections and equivalent stresses was obtained, as well as the respective spatial contour plots at the obtained resonance frequency. Some of the results of the conducted analyses were investigated in order to analyse the aerodynamic phenomena occurring, where it was possible to identify vortex shedding, leading shock, tip-clearance and horse-shoe vortex.  Afterwards, the numerical results were compared to the experimental data optained from rig tests conducted at the TCD1 by partners of the ARIAS projects. The numerical model management to predict well the compressor map, thus the steady-state of the flow, as well as the resonance frequency. The aerodynamic damping value was over- predicted when compared to the entire data set. When it came to the harmonic analysis results, the numerical model under-predicted by one order of magnitude the maximum deflection, and the equivalent stress is, as well, far below the experimental results. These results led to conclude that the model is under-predicting the aerodynamix unsteady forcing that results from the wake pattern generated by the IGV and excites the rotor blades.