EXPERIMENTAL INVESTIGATION OF WAKE-INDUCED TRANSITION IN A HIGHLY LOADED LINEAR COMPRESSOR CASCADE

Lothar Hilgenfeld and Michael Pfitzner

Institut fuer Strahlantriebe, Universitaet der Bundeswehr Muenchen Werner-Heisenberg-Weg 39, 85579 Neubiberg, Germany Lothar. Hilgenfeld@unibw-muenchen. de

Abstract The effects of wake passing on boundary layer transition in a highly loaded linear compressor cascade were investigated using different measurement techniques, such as fast-response Kulite sensors, hot-film array and hot-wire measurements. The experiments were performed in a high speed cascade wind tunnel with an in­let Mach number of 0.67 and Reynolds number 450000. Cylindrical bars moving parallel to the cascade inlet plane simulate the wakes of an upstream blade row. These basic investigations of unsteady transition also contribute to a database for numerical code validation at Mach and Reynolds numbers representative for real turbomachinery conditions.

Keywords: unsteady compressor fbw, wake passing, boundary layer transition

1. Introduction

The periodic disturbances caused by wake shedding of upstream blade rows and their downstream migration are a major source of unsteadiness in turboma­chines. This inherently unsteady ft>w plays a significant role in the loss gen­eration process of axial turbomachinery blades. The boundary layer transition on the profile from laminar to turbulent is the main source of loss generation and significantly affected by the unsteady behavior of the ft>w.

An overview of the fundamental transition modes relevant in turbomachines was given by Mayle (1991), who pointed out that the free stream turbulence, the pressure gradient and the strength of the wake are the major parameters in the multimode transition process dominated by wake-passing effects. An increase in wake passing frequency results in a higher freestream turbulence level (Schobeiri et al. 1995). Over the past years many experimental investiga­tions of the boundary layer in turbomachines were performed. Detailed basic

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K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 357-368. © 2006 Springer. Printed in the Netherlands.

research work on the boundary layer development on compressor blades were carried out by Halstead et al. (1997) and Walker et al. (1999). They reveal the fundamental effects regarding to wake passing like early onset of transition in the wake-induced path, suppression of a laminar separation bubble and the existence of a calmed region. The calmed region partially suppress laminar separation due to its higher shear stress level and delays the onset of transition in regions where under steady inflow conditions the ft>w is fully turbulent.

Most of the boundary layer investigations were conducted in low speed wind tunnel fbws or low speed research compressors (e. g. Mailach and Vogeler 2003). Only few investigations are available for compressible, high-speed compressor ft>w, see for example Teusch et al. (2000).

Considering these unsteady effects in current aerodynamic design methods, reliable transition and turbulence models in unsteady CFD codes are necessary, which consider the effects of wake passing. Nevertheless numerical code val­idation has still to be performed based on experimental test cases. Hence, one objective of the present investigation is to provide a detailed unsteady database for numerical code validation. The present work contributes to this objective as part of a joint research effort on unsteady fbws in turbomachines. An overview of the complete project and its scope is given by Hourmouziadis (2000). For this reason experimental investigations focusing on wake-induced transition have been performed in a highly loaded linear compressor cascade at Mach and Reynolds numbers representative for real turbomachinery conditions.