Investigation of the Suction Side Boundary Layer Development on Low Pressure Turbine Airfoils With and Without Separation Using a Preston Probe

Abstract

Characterizing the transition process of airfoils can be very challenging and requires often extensive measurement methods. Frequently at low Reynolds numbers the suction side separation often occurs close to the trailing edge so that asserting reattachment of the flow to form a closed separation bubble from the profile pressure distributions becomes uncertain. In the current work the suction side transition process is investigated more precisely with a convenient method to determine the dynamic pressure close to the suction surface using a Preston probe (flattened Pitot tube). Therefore four low pressure turbine airfoils, which show different characteristics of the transition process in the static pressure distribution have been investigated at the High-Speed Cascade Wind Tunnel at the Universität der Bundeswehr München at constant Mach number and under a wide range of Reynolds numbers (40 000 to 400 000). It is shown that this method is appropriate to determine transition start and end as well as the separation and reattachment point of a separated flow as long as the probe height is small enough compared to the boundary layer thickness. The measurement results are compared to profile pressure distributions and hot-wire boundary layer profiles. Also the influence of periodic unsteady inflow conditions on the dynamic pressure near the wall is revealed in the time average. Limitations due to the probe geometry are discussed and a method to estimate the influence of the probe geometry on the measured dynamic pressure coefficient is suggested.

Active Boundary Layer Control on a Highly Loaded Turbine Exit Case Profile

“…Due to vortex structures in the bubble as well as the distance between the stream line through the Pitot probe head and the suction side surface, the actually measured dynamic pressure ratios are normally slightly bigger than zero especially when the separation bubbles are thin. This was also confirmed by Stotz et al [14] who compared constant temperature anemometry (CTA) boundary layer measurements to this measurement technique. In the current measurements, due to the comparison with the Mach number distribution, a pressure ratio below q Pitot /q ∞ = 0.05 is regarded as close enough to zero to represent a separation zone.…”

“…The inner probe head height is 0.1 mm. More information concerning this measurement technique can be found in Stotz et al [14] and Brachmanski et al [15]. It was used for measurements along the suction side surface in order to investigate the boundary layer development as well as for total pressure measurements perpendicular to the suction surface at 98% of the axial chord length as shown in Figure 3b.…”

High Frequency Boundary Layer Actuation by Fluidic Oscillators at High Speed Test Conditions

“…It reveals as well if and where the transition takes place. Decreasing values can either represent a laminar or turbulent boundary layer, whereas a strong increase is an indicator for transition, compare Stotz et al [22]. According to Fig.…”

“…More details on the Preston probe head shape and size, the procedure to determine the boundary layer condition (laminar, turbulent or separated), and the determination of the transition point are given in [15,22]. Furthermore, the total pressure loss coefficient calculated from the Preston probe data is used to compare different actuator operating points.…”

Active Boundary Layer Control on a highly loaded Turbine Exit Case profile

“…More information concerning this measurement technique can be found in Stotz et al (2014) and Brachmanski et al (2014). It was used for measurements along the suction side surface in order to investigate the boundary layer development as well as for total pressure measurements perpendicular to the suction surface at 98% of the axial chord length as shown in Fig.…”