Experimental and Numerical Analysis of Cavity/Mean-Flow Interaction in Low Pressure Axial Flow Turbines

The increasing performance of modern aeroengines led the research towards the optimization of machine components not deeply analyzed in the past.In this context,the mechanisms driving the interaction process between the secondary flows evolving at the hub of low-pressure turbines with the rotor-stator cavity systems have been poorly investigated in the literature.In this work,an experimental and numerical analysis of the interaction between the endwall near wall flow and the leakage flow of a real cavity system is presented.The experimental results were carried out in the annular low-pressure axial flow turbine of the University of Genova.Experimental blade loading and pressure distributions into the cavity,as well as the measured total pressure loss coefficient,have been used for a proper validation of CFD results.Both steady and unsteady calculations were carried out through the commercial solver Numeca.Particularly,several numerical approaches have been tested into this work:RANS,Non Linear Harmonic(NLH),and URANS.The most promising CFD techniques have been firstly identified by comparison with experimental results and then systematically employed to extend the analysis of secondary flow-cavity flow interaction to positions and quantities not available from the experiments.Losses characterizing the mean flow-cavity flow interaction process will be shown to cover a great amount of the overall stage losses and should be properly accounted for the design of future optimized cavity configurations.

Experimental Analysis of the Aerodynamic Performance of an Innovative Low Pressure Turbine Rotor

In the present work the aerodynamic performances of an innovative rotor blade row have been experimentally investigated. Measurements have been carried out in a large scale low speed single stage cold flow facility at a Reynolds number typical of aeroengine cruise, under nominal and off-design conditions. The time-mean blade aerodynamic loadings have been measured at three radial positions along the blade height through a pressure transducer installed inside the hollow shaft, by delivering the signal to the stationary frame with a slip ring. The time mean aerodynamic flow fields upstream and downstream of the rotor have been measured by means of a five-hole probe to investigate the losses associated with the rotor. The investigations in the single stage research turbine allow the reproduction of both wake-boundary layer interaction as well as vortex-vortex interaction. The detail of the present results clearly highlights the strong dissipative effects induced by the blade tip vortex and by the momentum defect as well as the turbulence production, which is generated during the migration of the stator wake in the rotor passage. Phase-locked hot-wire investigations have been also performed to analyze the time-varying flow during the wake passing period. In particular the interaction between stator and rotor structures has been investigated also under off-design conditions to further explain the mechanisms contributing to the loss generation for the different conditions.

Large Eddy Simulation of the By-pass Transition Process under Different Inlet Turbulence Conditions

The transition process of the boundary layer developing over a flat plate with elevated inlet Free Stream Turbulence Intensity(FSTI)has been studied by means of Large Eddy Simulation(LES).To this purpose,four cases with different inflow disturbances have been tested varying the magnitude and the length scale of turbulence.LES has been performed by using the finite-volume ANSYS Fluent code.The computational domain,which was constituted by a rectangular domain with a zero thickness plate,was based on an ERCOFTAC test case in order to provide a validation with a well-known set of data by comparing the boundary layer integral parameters and mean and fluctuating streamwise velocity profiles.The four cases were discussed within the paper by looking at classical statistical properties as well as advanced post-processing tools.It was shown that the decrease in the free stream turbulence level postpones the transition location,whereas the variation of the integral length scale has a very low influence on the distribution of the time-mean flow properties.Proper Orthogonal Decomposition(POD)has been applied to the instantaneous LES flow fields in order to provide a statistical representation of the structures responsible for transition and their response to free-stream turbulence intensity and length scale.The presence of vortical filaments parallel to the wall,typically referred as boundary layer streaks,is clearly identified;their characteristic dimensions and how they change as a function of FSTI properties were analyzed within the paper.