The influence of free-stream turbulence on wake dispersion and boundary layer transition processes has been studied in a 1.5-stage axial compressor. An inlet grid was used to produce turbulence characteristics typical of an embedded stage in a multistage machine. The grid turbulence strongly enhanced the dispersion of inlet guide vane (IGV) wakes. This modified the interaction of IGV and rotor wakes, leading to a significant decrease in periodic unsteadiness experienced by the downstream stator. These observations have important implications for the prediction of clocking effects in multistage machines. Boundary layer transition characteristics on the outlet stator were studied with a surface hot-film array. Observations with grid turbulence were compared with those for the natural low turbulence inflow to the machine. The transition behavior under low turbulence inflow conditions with the stator blade element immersed in the dispersed IGV wakes closely resembled the behavior with elevated grid turbulence. It is concluded that with appropriate alignment, the blade element behavior in a 1.5-stage axial machine can reliably indicate the blade element behavior of an embedded row in a multistage machine.

Data from a surface hot-film array on the outlet stator of a 1.5-stage axial compressor are analyzed to look for direct evidence of natural transition phenomena. An algorithm is developed to identify instability waves within the Tollmien–Schlichting (T–S) frequency range. The algorithm is combined with a turbulent intermittency detection routine to produce space-time diagrams showing the probability of instability wave occurrence prior to regions of turbulent flow. The paper compares these plots for a range of blade loading, with free-stream conditions corresponding to the maximum and minimum inflow disturbance periodicity produced by inlet guide vane clocking. Extensive regions of amplifying instability waves are identified in nearly all cases. The implications for transition prediction in decelerating flow regions on axial turbomachine blades are discussed.

The interaction between wakes of an adjacent rotor-stator or stator-rotor blade row pair in an axial turbomachine is known to produce regular spatial variations in both the time-mean and unsteady flow fields in a frame relative to the upstream member of the pair. This paper examines the influence of such changes in the free-stream disturbance field on the viscous losses of a following blade row. Hot-wire measurements are carried out downstream of the outlet stator in a 1.5-stage axial compressor having equal blade numbers in the inlet guide vane (IGV) and stator rows. Clocking of the IGV row is used to vary the disturbance field experienced by the stator blades: the influence on stator wake properties is evaluated. The magnitude of periodic fluctuations in ensemble-average stator wake thickness is significantly influenced by IGV wake-rotor wake interaction effects. The changes in time-mean stator losses appear marginal.

The influence of free-stream turbulence on wake dispersion and boundary layer transition processes has been studied in a 1.5-stage axial compressor. An inlet grid was used to produce turbulence characteristics typical of an embedded stage in a multistage machine. The grid turbulence strongly enhanced the dispersion of inlet guide vane (IGV) wakes. This modified the interaction of IGV and rotor wakes, leading to a significant decrease in periodic unsteadiness experienced by the downstream stator. These observations have important implications for the prediction of clocking effects in multistage machines. Boundary layer transition characteristics on the outlet stator were studied with a surface hot-film array. Observations with grid turbulence were compared with those for the natural low turbulence inflow to the machine. The transition behavior under low turbulence inflow conditions with the stator blade element immersed in the dispersed IGV wakes closely resembled the behavior with elevated grid turbulence. It is concluded that with appropriate alignment, the blade element behavior in a 1.5-stage axial machine can reliably indicate the blade element behavior of an embedded row in a multistage machine.

Data from a surface hot-film array on the outlet stator of a 1.5 stage axial compressor are analyzed to look for direct evidence of natural transition phenomena. An algorithm is developed to identify instability waves within the Tollmien Schlichting (T-S) frequency range. The algorithm is combined with a turbulent intermittency detection routine to produce space∼time diagrams showing the probability of instability wave occurrence prior to regions of turbulent flow. The paper compares these plots for a range of blade loading, with free-stream conditions corresponding to the maximum and minimum inflow disturbance periodicity produced by inlet guide vane clocking. Extensive regions of amplifying instability waves are identified in nearly all cases. The implications for transition prediction in decelerating flow regions on axial turbomachine blades are discussed.