The overall efficiency of a jet engine and hence the
specific fuel consumption is strongly determined by the
aerodynamic efficiency of the Low Pressure Turbine
(LPT). The LPT is therefore a main target for
aerodynamic improvements. Due to the fact that most
of the LPT profile losses (about 2/3) are caused in the
suction surface boundary layer [3, 21] and these are
strongly linked to the state of the boundary layer, it is of
utmost importance to know if and, if the case, where
transition occurs. Especially on LPT airfoils with
typically low Reynolds numbers this is a crucial point
of interest in order to improve the profile loss
predictions. Precise knowledge of the transition process
is also crucial for computational fluid dynamics (CFD)
validation. Only with well calibrated transition models
confident loss predictions are to be expected.
The area of transition on profiles with boundary layer
separation is usually obvious by measuring the pressure
distribution. Thereby, the localization of transition can
be imprecise due to the fact that the separation and the
reattachment point do not always coincide with
transition start and end for different bubble types
[7, 14]. However, with increasing turbulence or less
deceleration transition with separation turns to bypass
transition [14]. Without separation bubble, the accurate
detection of transition from the pressure distribution
turns out to be difficult or even impossible. Hence
analyses with more sophisticated measurement
techniques have to be performed to determine onset and
end of transition. Basically two measurement methods
are convenient to examine the transition process:
Today, commonly hot films are used to obtain a semiquantitative
quasi wall shear stress. In addition RMS
and skewness of the signal can be used to localise
transition onset and end [2, 8, 10, 15, 25]. With a
Preston tube, the total pressure close to the surface can
be measured and, with the local static pressure on the
surface, the dynamic pressure close to the wall can be
determined [11, 12, 15]. Both methods are well
established to detect boundary layer transition.
However, conducting the data with a Preston probe is
much less effort than with hot films.
The data presented here are the results of experiments
which were conducted to detect boundary layer
transition on a profile without separation, designed for
low Reynolds numbers. The data is also used to
compare and validate the methods against each other
and to investigate the physical limitations.
This paper describes and compares these two
experimental methods to characterise the transition
process on the suction surface of a low pressure turbine
cascade showing no evidences of boundary layer
separation in the pressure distribution. For that purpose
a typical range of operation points is examined to
compare the methods against each other.
«The overall efficiency of a jet engine and hence the
specific fuel consumption is strongly determined by the
aerodynamic efficiency of the Low Pressure Turbine
(LPT). The LPT is therefore a main target for
aerodynamic improvements. Due to the fact that most
of the LPT profile losses (about 2/3) are caused in the
suction surface boundary layer [3, 21] and these are
strongly linked to the state of the boundary layer, it is of
utmost importance to know if and, if the case, where
t...
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