Validation of turbine noise prediction tools with acoustic - MTU Aero ...

As can be seen, the microphones have been equally spaced in the axial direction and have been arranged
in two lines to reduce the axial extent. Based on the assumption that the mean aerodynamic and aeroacoustic
field are time-invariant for the duration of one test campaign, this setup using a traversable duct section
permits the recording of the full circumference with a drastically reduced number of sensors. Finally, at the
downstream end of the acoustic test section, the flow is redirected again into the vertical direction and leaves
the laboratory through an exhaust stack at ambient conditions.
Concerning the aerodynamic design, the LPT stage has been designed to represent the last stage of an
LPT of a modern commercial aero engine at approximately half scale. With respect to the characteristic
small expansion of the flow path at the rear stages of a typical engine, the cylindrical annular duct cross
section motivated by acoustic reasons can be justified. Although designed for an aero design point (ADP), the
rig has been operated at the three noise certification operating points (OP) approach, cutback, and sideline
which are related to the ADP by the following factors (compare Table 2) to represent realistic operating
conditions.
Table 2. Rig operating points in relation to ADP [Ref. 2]
parameter ADP Sideline Cutback Approach
total pressure ratio [%] 100 97 85 73
mass flow [%] 100 95 75 50
shaft speed [%] 100 97 91 71
shaft power [%] 100 87 48 16
II.B. Aerodynamic and acoustic measurements
The aerodynamic conditions within the test rig have been determined at several axial positions as indicated
in the sketch in figure 4. In the inlet plane (plane 0) to the turbomachinery stage, the inflow is assumed to be
circumferentially and radially uniform, and the total pressure has been recorded by a Kiel probe at 5 radial
locations. Together with the total temperature, this value has been used to normalize the measurements in
the downstream planes to cancel slight variations in the operating conditions (mainly due to variations of
the inflow characteristics). Behind the IGV, in plane A, five-hole probe measurements have been performed
covering 4 sectors of 24 ◦ azimuthal extension on the circumference. Within these sectors, 21 radial and 17
axial positions have been recorded starting approximately 4 mm from the duct walls (due to the dimensions
of the probe).
Figure 4. Aerodynamic measurement planes in STTF rig
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American Institute of Aeronautics and Astronautics