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Separated flow transition can be seen occurring (between the wakes) on the low speed profile and BR710 profile. The last sensor on the BR710 profile was located at 94%s, whereas the last sensors on the other profiles were at around 96%s. If separated flow transition occurs late on the blade, then the sensors on BR715 profile may not be far enough back along the surface to measure this. The data from NGV2 of the BR715UHL turbine shows very similar features to NGV2 of the BR715 turbine and so has therefore not been shown. Figure 16 A comparison of low speed and high speed measurements of ensemble quasi wall shear stress from various profiles.

Figure 16 Separated flow transition can be seen occurring (between the wakes) on the low speed profile and BR710 profile. The last sensor on the BR710 profile was located at 94%s, whereas the last sensors on the other profiles were at around 96%s. If separated flow transition occurs late on the blade, then the sensors on BR715 profile may not be far enough back along the surface to measure this. The data from NGV2 of the BR715UHL turbine shows very similar features to NGV2 of the BR715 turbine and so has therefore not been shown. Figure 16 A comparison of low speed and high speed measurements of ensemble quasi wall shear stress from various profiles.

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This paper is organised so that it clearlty shows the development of both the understanding of wake-boundary layer interactions and the development of high and then ultra high lift LP turbine blades. After an introduction to the subject, the experimental methods are then presented. Hot film results are then presented from LP turbines that represent conventional, high lift and finally ultra high lift designs. Each turbine design represents a step in the evolution of our understanding of the aerodymanics of these turbines and the resutls show the increasing complexity of the flows.
This paper is organised so that it clearlty shows the development of both the understanding of wake-boundary layer interactions and the development of high and then ultra high lift LP turbine blades. After an introduction to the subject, the experimental methods are then presented. Hot film results are then presented from LP turbines that represent conventional, high lift and finally ultra high lift designs. Each turbine design represents a step in the evolution of our understanding of the aerodymanics of these turbines and the resutls show the increasing complexity of the flows.
Figure 2 A schematic diagram of wake induced transition and separated flow transition. Wakes shed from upstream blade rows travel over downstream blades disturb their boundary layers, see Hodson [4] and Ladwig and Fottner [5]. As a boundary layer becomes more receptive to disturbances, the high turbulence in the wakes eventually causes the formation of turbulent spots through the mechanism of bypass transition. Schauber and Klebanoff [6] showed that these spots travel down stream in a characteristic manner. An ST diagram is a contour plot of, for example shear stress, with time on the ‘y’ axis and surface distance on the ‘x’ axis, see Figure 2. The different celerities of the leading and trailing edges of the turbulent spots mean that the regions of high shear seen in ST diagrams diverge in time. This is because the leading and trailing edge velocities of turbulent spots are reported to travel at approximately 90% and 50% of the local free stream velocity for zero pressure gradient flow. Behind the spot is a calmed region of flow that has a full velocity profile, which is shown in Figure 2 as region ‘C’. This profile (which is almost linear out to the freestream, Cumpsty et al [7]) is very stable to disturbances and will not so easily separate in an adverse pressure gradient, as a laminar profile would. The value of the trailing edge velocity is highly dependent on the local pressure gradient, but the leading edge velocity is almost unaffected by it. The ends of calmed regions that trail turbulent spots are variously reported to travel at approximately 30% of the local free stream velocity, see Gostelow et al [8]. The effect of variable pressure gradient on the velocity of the calmed region has not been documented.
Figure 2 A schematic diagram of wake induced transition and separated flow transition. Wakes shed from upstream blade rows travel over downstream blades disturb their boundary layers, see Hodson [4] and Ladwig and Fottner [5]. As a boundary layer becomes more receptive to disturbances, the high turbulence in the wakes eventually causes the formation of turbulent spots through the mechanism of bypass transition. Schauber and Klebanoff [6] showed that these spots travel down stream in a characteristic manner. An ST diagram is a contour plot of, for example shear stress, with time on the ‘y’ axis and surface distance on the ‘x’ axis, see Figure 2. The different celerities of the leading and trailing edges of the turbulent spots mean that the regions of high shear seen in ST diagrams diverge in time. This is because the leading and trailing edge velocities of turbulent spots are reported to travel at approximately 90% and 50% of the local free stream velocity for zero pressure gradient flow. Behind the spot is a calmed region of flow that has a full velocity profile, which is shown in Figure 2 as region ‘C’. This profile (which is almost linear out to the freestream, Cumpsty et al [7]) is very stable to disturbances and will not so easily separate in an adverse pressure gradient, as a laminar profile would. The value of the trailing edge velocity is highly dependent on the local pressure gradient, but the leading edge velocity is almost unaffected by it. The ends of calmed regions that trail turbulent spots are variously reported to travel at approximately 30% of the local free stream velocity, see Gostelow et al [8]. The effect of variable pressure gradient on the velocity of the calmed region has not been documented.
Lift coefficient vs. Mach number for a variety of blade profiles, including the high lift profiles of the BR715 and BR710 demonstrator. The BR715UHL turbine achieves even high lift coefficients. After Haselbach et al (2001). Cobley et al [11] describes the many parts of the design process leading to the development of the LP turbines in the BR700 series of engines. They also describe some of the design methodologies used to develop the high lift profiles. Harvey et al [12] showed that the efficiency of these LP turbines is approximately the same as previous generations. In particular the BR715 actually achieved an efficiency above previous generation LP turbines as well as 20% reduction in blade numbers. Figure 3 (taken from Haselbach et al [13]) shows the increased lift obtained for the three profiles shown here as compared to current Rolls-Royce designs. These levels of performance were obtained without a loss penalty.
Lift coefficient vs. Mach number for a variety of blade profiles, including the high lift profiles of the BR715 and BR710 demonstrator. The BR715UHL turbine achieves even high lift coefficients. After Haselbach et al (2001). Cobley et al [11] describes the many parts of the design process leading to the development of the LP turbines in the BR700 series of engines. They also describe some of the design methodologies used to develop the high lift profiles. Harvey et al [12] showed that the efficiency of these LP turbines is approximately the same as previous generations. In particular the BR715 actually achieved an efficiency above previous generation LP turbines as well as 20% reduction in blade numbers. Figure 3 (taken from Haselbach et al [13]) shows the increased lift obtained for the three profiles shown here as compared to current Rolls-Royce designs. These levels of performance were obtained without a loss penalty.
gure 4 NGV2 of the BR715UHL LP turbine showing the hot film array of sensors on the right hand aerofoil. The measurement of wall shear stress using hot films was developed by Bellhouse and Schultz [14] and is now a well-established technique. It is possible to calibrate hot films, but this is a difficult and time consuming process see Hodson [15] and Davies and O'Donnell [16]. Also, errors of 20% or more arise when hot films calibrated in a laminar flow are used to measure a turbulent flow. The hot film sensors were used on the aft part of suction surfaces and were therefore likely to see laminar, turbulent, calmed and separated flow at different positions of the upstream rotor, see Halstead et al [17] or Banieghbal et al [18]. Calibration for all these conditions would be extremely difficult and quite probably impossible. It is for the above reasons that the hot film sensors are used in an uncalibrated form, which yields a pseudo shear stress. When used in an uncalibrated manner the absolute level of the shear stress is unknown, but it is the relative magnitude of the shear stress level from sensor to sensor that is obtained. With careful analysis, this relative or ‘quasi’ shear stress is all that is required to obtain information regarding the state of the boundary layers.
gure 4 NGV2 of the BR715UHL LP turbine showing the hot film array of sensors on the right hand aerofoil. The measurement of wall shear stress using hot films was developed by Bellhouse and Schultz [14] and is now a well-established technique. It is possible to calibrate hot films, but this is a difficult and time consuming process see Hodson [15] and Davies and O'Donnell [16]. Also, errors of 20% or more arise when hot films calibrated in a laminar flow are used to measure a turbulent flow. The hot film sensors were used on the aft part of suction surfaces and were therefore likely to see laminar, turbulent, calmed and separated flow at different positions of the upstream rotor, see Halstead et al [17] or Banieghbal et al [18]. Calibration for all these conditions would be extremely difficult and quite probably impossible. It is for the above reasons that the hot film sensors are used in an uncalibrated form, which yields a pseudo shear stress. When used in an uncalibrated manner the absolute level of the shear stress is unknown, but it is the relative magnitude of the shear stress level from sensor to sensor that is obtained. With careful analysis, this relative or ‘quasi’ shear stress is all that is required to obtain information regarding the state of the boundary layers.
Figure 5 Time history traces of quasi wall shear stress for the cruise condition of the BR710 LP turbine.
Figure 5 Time history traces of quasi wall shear stress for the cruise condition of the BR710 LP turbine.
Figure 8 ST diagrams of (a) non-dimensional ensemble mean quasi wall shear stress, (b) non-dimensional ensemble RMS and (c) ensemble skew. Data is taken from NGV2 of the BR715 LP turbine.
Figure 8 ST diagrams of (a) non-dimensional ensemble mean quasi wall shear stress, (b) non-dimensional ensemble RMS and (c) ensemble skew. Data is taken from NGV2 of the BR715 LP turbine.
The raw signals from the hot film sensors of the NGV2 of the BR715 LP turbine are presented in Figure 7. The signal from the sensor at 78%s is characteristic of a disturbed laminar (and mostly separated) boundary layer. Occasionally, a passing rotor wake causes transition to occur via the formation of a turbulent spot, but only in about | in 5 wake passings, at this Reynolds number and surface position. The sensors further downstream indicate that more turbulent spots have formed as the boundary layer becomes more receptive to the wake disturbances. Figure 7 Raw Hot film data from NGV2 of the BR715 LP turbine
The raw signals from the hot film sensors of the NGV2 of the BR715 LP turbine are presented in Figure 7. The signal from the sensor at 78%s is characteristic of a disturbed laminar (and mostly separated) boundary layer. Occasionally, a passing rotor wake causes transition to occur via the formation of a turbulent spot, but only in about | in 5 wake passings, at this Reynolds number and surface position. The sensors further downstream indicate that more turbulent spots have formed as the boundary layer becomes more receptive to the wake disturbances. Figure 7 Raw Hot film data from NGV2 of the BR715 LP turbine
Figure 9 shows the variation in time mean quasi wall shear stress variation over the surface of NGV3 of the BR715 LP turbine at the cruise condition. The shear stress starts relatively high and drops as the boundary layer thickens despite the flow acceleration. After the minimum at 75%s, the shear stress increases due to the boundary layer undergoing transition. This increases the amount of turbulent flow at each sensor location and therefore the mean level of quasi wall shear stress. Computational predictions showed that time mean position of boundary layer separation occurs at approximately 65%s.
Figure 9 shows the variation in time mean quasi wall shear stress variation over the surface of NGV3 of the BR715 LP turbine at the cruise condition. The shear stress starts relatively high and drops as the boundary layer thickens despite the flow acceleration. After the minimum at 75%s, the shear stress increases due to the boundary layer undergoing transition. This increases the amount of turbulent flow at each sensor location and therefore the mean level of quasi wall shear stress. Computational predictions showed that time mean position of boundary layer separation occurs at approximately 65%s.
This beating in this data is similar to that measured by Ardnt [21]. In that investigation Arndt showed that rotor 1 and rotor 2 interactions gave rise to amplitude modulations of the strength of the wakes that entered NGV3. Regions marked ‘H1’ reduce in strength as they reach the start of regions ‘H3’ because they are older turbulent boundary layer. New turbulent boundary layer is formed at ‘H2’, which has a higher quasi wall shear stress. Figure 10 ST diagrams of non-dimensional ensemble mean quasi wall shear stress from NGV3 of the BR715 LP turbine. Cruise conditions
This beating in this data is similar to that measured by Ardnt [21]. In that investigation Arndt showed that rotor 1 and rotor 2 interactions gave rise to amplitude modulations of the strength of the wakes that entered NGV3. Regions marked ‘H1’ reduce in strength as they reach the start of regions ‘H3’ because they are older turbulent boundary layer. New turbulent boundary layer is formed at ‘H2’, which has a higher quasi wall shear stress. Figure 10 ST diagrams of non-dimensional ensemble mean quasi wall shear stress from NGV3 of the BR715 LP turbine. Cruise conditions
Figure 12. Predicted (solid line) and measured (circles) static pressures on NGV3 of the BR715U LP turbine. Cruise conditions. After Haselbach et al [13]. The final hot film measurements are taken from the BR715U LP turbine. These show similar results as from the 3" stage NGV in the BR715 turbine but it should be remembered that blades on this LP turbine generate 12% more lift than the BR715 turbine.
Figure 12. Predicted (solid line) and measured (circles) static pressures on NGV3 of the BR715U LP turbine. Cruise conditions. After Haselbach et al [13]. The final hot film measurements are taken from the BR715U LP turbine. These show similar results as from the 3" stage NGV in the BR715 turbine but it should be remembered that blades on this LP turbine generate 12% more lift than the BR715 turbine.
probe traversed upstream of the inlet of NGV3 from an MTU LP turbine and observed a similar beating of the inlet turbulence field. Figure 11 Ensemble mean data for one revolution of rotor 2 of the BR715 LP turbine.
probe traversed upstream of the inlet of NGV3 from an MTU LP turbine and observed a similar beating of the inlet turbulence field. Figure 11 Ensemble mean data for one revolution of rotor 2 of the BR715 LP turbine.
Figure 14 Ensemble mean skew for to sensors showing the change in periodicity of the data as the flow travels down stream. The blade numbers for this turbine are different to the previous one and no beating could be seen in the data. Again, this is not to say that beating does not occur, but it could be that it is disguised by the separated flow transition, which seems to be the dominant transition mechanism.
Figure 14 Ensemble mean skew for to sensors showing the change in periodicity of the data as the flow travels down stream. The blade numbers for this turbine are different to the previous one and no beating could be seen in the data. Again, this is not to say that beating does not occur, but it could be that it is disguised by the separated flow transition, which seems to be the dominant transition mechanism.
‘igure 15ST diagrams of (a) non-dimensional ensemble mean quasi wall shear stress, (b) non-dimensional ensemble RMS and (c) ensemble skew. Data is taken from NGV3 of the BR715U LP turbine.
‘igure 15ST diagrams of (a) non-dimensional ensemble mean quasi wall shear stress, (b) non-dimensional ensemble RMS and (c) ensemble skew. Data is taken from NGV3 of the BR715U LP turbine.
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