Studies on a Blade Leading Edge Separation Bubble Affected by Periodic Wakes: Its Transitional Behavior and Boundary Layer Loss Reduction

Abstract

This study deals with extensive hot-wire probe measurements of wake-affected separation bubble on the leading edge of a test model. The purpose of the study is to investigate time-resolved response of the separation bubble to incoming wake passing. Another focus is placed on the wake effect on aerodynamic loss generated in the separated boundary layer, seeking any relationship between the suppression of the separation bubble on a cascade airfoil and aerodynamic gain due to the clocking in turbomachines. The test model has a semicircular leading edge and two flat-plates. Incoming wakes are generated by circular cylinders which are horizontally fixed in the wake generator. Several types of wake generating cylinders are used in order to change wake properties. The hot-wire measurements have revealed the time-resolved responses of the separated boundary layer to the wake passage. Effects of calmed regions just behind the moving wakes are also identified. INTRODUCTIONA number of studies have been made on unsteady flow effects upon separated boundary layers or separation bubbles over on the suction side of turbine blades. , examined wake-separation interaction phenomena, using linear cascades and wake generators. The purpose of those studies was mainly for seeking how and to what extent blade loading can be increased without any severe separation on blade suction surface by taking advantage of wakes passing over the surface. Lou and Hourmouziadis [5] documented transitional behaviors of separation bubbles on a flat plate subjected to main flow fluctuations, where a pressure gradient over the flat plate was imposed using a contoured wall. A similar study was made by Volino and Hultgren [6], focusing on the effect of free-stream turbulence upon the separation bubble.In contrast, fewer studies have been made on the investigation of periodic wakes affecting leading edge separation bubble on a compressor blade, while several studies have been made on steady-state separation bubbles on the blade leading edge [7], [8], [9]. Paxson and Mayle [10] measured wake-affected velocity profiles of the boundary layer on the leading edge of a blunt test model. Funazaki et al. [11], followed by Funazaki and Kato [12], using a test mode similar to that of Paxaon and Mayle, examined the interaction between a stationary wake from an upstream bar and separated boundary layer on the test model with semi-circular leading edge. They changed the vertical po-

Studies on Effects of Periodic Wake Passing upon a Blade Leading Edge Separation Bubble (Suppresion of Separation Bubble Due to Wake Passing)

“…As mentioned earlier, test facility used in this study was almost the same as that in the previous study (8) . Figure 1 shows the experimental setup, including the detail of the wake generator.…”

“…Special attention is paid to the emergence of wake-induced turbulence spots followed by calmed region. These flow events were already reported by Funazaki and Kato (8) with less quantitative discussion, though. Time-accurate numerical simulation is also made using a RANS (Reynolds-Averaged Navier-Stokes equations) solver with Wilcox k-ω turbulence model to provide an idea on how the incoming periodic wakes interact with the separation bubble.…”

“…Similarly, a simplified flat-plate model experiment was made by Ottavy et al (9) , followed by Chun and Sung (10) . The present study is an extended version of the previous study by Funazaki and Kato (8) using almost the same test facility, aiming at the clarification about how and to what extent the periodic wake passing suppresses the leading edge separation bubble occurring near the leading edge of the test model under various flow conditions, such as wake-passing Strouhal number, inlet Reynolds number and incident of the model. Special attention is paid to the emergence of wake-induced turbulence spots followed by calmed region.…”

“…Flow visualizations and aerodynamic loss measurements were made in their study, showing that the shear layer was found to be slightly affected by the wake passing or increased free-stream turbulence. Funazaki and Kato (8) , using a simple scaled leading edge model of a compressor blade and the moving bar mechanism, executed detailed measurements of the separated boundary layer on the test model affected by the periodic wake passing. Similarly, a simplified flat-plate model experiment was made by Ottavy et al (9) , followed by Chun and Sung (10) .…”