Transition of the state of the boundary layer from laminar
to turbulent plays an important role in the aerodynamic loss generation
on turbine airfoils. An accurate simulation of the transition
process and of the state of the boundary layer is therefore
crucial for prediction of the aerodynamic efficiency of components
in rotating machines. A lot of the research in the past years
dealt with the transition over laminar separation bubbles, especially
concerning flows in low pressure turbines (LPT) of air jet
engines. Nevertheless, by-pass transition is also frequent in turbomachines
at higher Reynolds numbers as well as for properly
designed profiles. Compared with transition over a laminar separation
bubble, a by-pass transition is experimentally much more
difficult to detect with standard measurement techniques. In such
cases it becomes necessary to use more sophisticated techniques,
such as hot film anemometry, hot wires or Preston probes in order
to obtain accurate information on the state of the boundary
layer.
The study presented is carried out using a linear cascade
with a LPT blade profile with strong front loading and gentle
flow deceleration at the rear suction side of the blade. Measurements
were performed at the High-Speed Cascade Wind Tunnel
of the Institute of Jet Propulsion at engine relevant Mach and
Reynolds numbers. Emphasis is put on the evaluation of the different
transition processes at mid-span and its influence on profile
losses. The data post processing was adapted for compressible
flows, which allows a more accurate determination of the
transition area as well as qualitatively better distributions of the
wall shear stress. Finally, comparisons with simulations, using
computational fluid dynamics (CFD) tools, are performed and
fields for improvement of the turbulence and transition models
are identified.
Keywords: transition, turbine, hot-film anemometry
«Transition of the state of the boundary layer from laminar
to turbulent plays an important role in the aerodynamic loss generation
on turbine airfoils. An accurate simulation of the transition
process and of the state of the boundary layer is therefore
crucial for prediction of the aerodynamic efficiency of components
in rotating machines. A lot of the research in the past years
dealt with the transition over laminar separation bubbles, especially
concerning flows in low pressure t...
»