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Unsteady boundary-layer transition in low-pressure turbines

  • JOHN D. COULL (a1) and HOWARD P. HODSON (a1)
Abstract

This paper examines the transition process in a boundary layer similar to that present over the suction surfaces of aero-engine low-pressure (LP) turbine blades. This transition process is of significant practical interest since the behaviour of this boundary layer largely determines the overall efficiency of the LP turbine. Modern ‘high-lift’ blade designs typically feature a closed laminar separation bubble on the aft portion of the suction surface. The size of this bubble and hence the inefficiency it generates is controlled by the transition between laminar and turbulent flow in the boundary layer and separated shear layer. The transition process is complicated by the inherent unsteadiness of the multi-stage machine: the wakes shed by one blade row convect through the downstream blade passages, periodically disturbing the boundary layers. As a consequence, the transition to turbulence is multi-modal by nature, being promoted by periodic and turbulent fluctuations in the free stream and the inherent instabilities of the boundary layer. Despite many studies examining the flow behaviour, the detailed physics of the unsteady transition phenomena are not yet fully understood. The boundary-layer transition process has been studied experimentally on a flat plate. The opposing test-section wall was curved to impose a streamwise pressure distribution typical of modern high-lift LP turbines over the flat plate. The presence of an upstream blade row has been simulated by a set of moving bars, which shed wakes across the test section inlet. Further upstream, a grid has been installed to elevate the free-stream turbulence to a level believed to be representative of multi-stage LP turbines. Extensive particle imaging velocimetry (PIV) measurements have been performed on the flat-plate boundary layer to examine the flow behaviour. In the absence of the incoming bar wakes, the grid-generated free-stream turbulence induces relatively weak Klebanoff streaks in the boundary layer which are evident as streamwise streaks of low-velocity fluid. Transition is promoted by the streaks and by the inherent inflectional (Kelvin–Helmholtz (KH)) instability of the separation bubble. In unsteady flow, the incoming bar wakes generate stronger Klebanoff streaks as they pass over the leading edge, which convect downstream at a fraction of the free-stream velocity and spread in the streamwise direction. The region of amplified streaks convects in a similar manner to a classical turbulent spot: the leading and trailing edges travel at around 88% and 50% of the free-stream velocity, respectively. The strongest disturbances travel at around 70% of the free-stream velocity. The wakes induce a second type of disturbance as they pass over the separation bubble, in the form of short-span KH structures. Both the streaks and the KH structures contribute to the early wake-induced transition. The KH structures are similar to those observed in the simulation of separated flow transition with high free-stream turbulence by McAuliffe & Yaras (ASME J. Turbomach., vol. 132, no. 1, 2010, 011004), who observed that these structures originated from localised instabilities of the shear layer induced by Klebanoff streaks. In the current measurements, KH structures are frequently observed directly under the path of the wake. The wake-amplified Klebanoff streaks cannot affect the generation of these structures since they do not arrive at the bubble until later in the wake cycle. Rather, the KH structures arise from an interaction between the flow disturbances in the wake and localised instabilities in the shear layer, which are caused by the weak Klebanoff streaks induced by the grid turbulence. The breakdown of the KH structures to small-scale turbulence occurs a short time after the wake has passed over the bubble, and is largely driven by the arrival of the wake-amplified Klebanoff streaks from the leading edge. During this process, the re-attachment location moves rapidly upstream. The minimum length of the bubble occurs when the strongest wake-amplified Klebanoff streaks arrive from the leading edge; these structures travel at around 70% of the free-stream velocity. The bubble remains shorter than its steady-flow length until the trailing edge of the wake-amplified Klebanoff streaks, travelling at 50% of the free-stream velocity, convect past. After this time, the reattachment location moves aft on the surface as a consequence of a calmed flow region which follows behind the wake-induced turbulence.

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Corresponding author
Email address for correspondence: jdc38@cam.ac.uk
References
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Alfredsson, P. H. & Matsubara, M. 1996 Streaky structures in transition. In Transitional Boundary Layers in Aeronautics (ed. Henkes, R. & van Ingen, J.), pp. 374386, Elsevier.
Asai, M., Minagawa, M. & Nishioka, M. 2002 The instability and breakdown of a near-wall low-speed streak. J. Fluid Mech. 445, 289314.
Cai, C. & Harrington, P. B. 1998 Different discrete wavelet transforms applied to denoising analytical data. J. Chem. Inf. Comput. Sci. 38 (6), 11611170.
Coull, J. D. & Hodson, H. P. 2010 Predicting the profile loss of high lift low pressure turbines. ASME Paper GT2010-22675.
Coull, J. D., Thomas, R. L. & Hodson, H. P. 2010 Velocity distributions for low pressure turbines. Trans. ASME, J. Turbomach. 132 (4), 041006.
Diwan, S. S. & Ramesh, O. N. 2009 On the inflectional instability of a laminar separation bubble. J. Fluid Mech. 629, 263298.
Durbin, P. A., Zaki, T. A. & Liu, T. 2009 Interaction of discrete and continuous boundary layer modes to cause transition. Intl J. Heat Fluid Flow 30, 403410.
Ginoux, J. 1965 Streamwise vortices in laminar flow. In Proc. AGARDoGRAPH '97, Part II, Paris.
Halstead, D. E. 1996 Boundary layer development in multi-stage low pressure turbines PhD thesis, Iowa State University.
Hodson, H. P. 1983 The detection of boundary layer transition and separation in high speed turbine cascades In Proc. Seventh Symp. Measurement Techniques for Transonic and Supersonic Flow, Aachen, Germany, 2123 September.
Hughes, J. D. & Walker, G. W. 2001 Natural transition phenomena on an axial compressor blade. J. Turbomach. 123 (2), 392401.
Inger, G. R. 1975 Three dimensional disturbances in reattaching separated flows. In Proc. AGARD Conf., no. 168, Gottingen.
Jacobs, R. G. & Durbin, P. A. 1998 Shear sheltering and the continuous spectrum of the Orr–Sommerfeld equation. Phys. Fluids 10 (8), 10.1063/1.869716.
Jacobs, R. G. & Durbin, P. A. 2001 Simulations of bypass Transition. J. Fluid Mech. 428, 185212.
Lewalle, J., Ashpis, D. E. & Sohn, K.-H. 1997 Demonstration of wavelet techniques in the spectral analysis of bypass transition data. NASA Technical Publication 3555.
Marxen, O., Lang, M., Rist, U., Levin, O. & Henningson, D. S. 2009 Mechanisms for spatial steady three-dimensional disturbance growth in a non-parallel and separating boundary layer. J. Fluid Mech. 634, 165189.
Matsubara, M., & Alfredsson, P. H. 2001 Disturbance growth in boundary layers subjected to free-stream turbulence. J. Fluid Mech. 430, 149168.
McAuliffe, B. R. & Yaras, M. I. 2010 Transition mechanisms in separation bubbles under low and high freestream turbulence. Trans. ASME, J. Turbomach. 132 (1), 011004.
Opoka, M. M. & Hodson, H. P. 2008 Experimental investigation of unsteady transition processes on high-lift T106A turbine blades. J. Propulsion Power 24 (3), 424432.
Orth, U. 1993 Unsteady boundary-layer transition in flow periodically disturbed by wakes. Trans. ASME, J. Turbomach. 115 (4), 707713.
Pfeil, H., Herbst, R. & Schröder, T. 1982 Investigations of the laminar-turbulent transition of boundary layers disturbed by wakes ASME Paper 82-GT-0406.
Roshko, A. & Thomke, G. J. 1966 Observations of turbulent reattachment behind an axi-symmetric downstream facing step in supersonic flow. AIAA J. 4, 975980.
Schlichting, H. 1979 Boundary Layer Theory. 7th edn. McGraw-Hill
Schubauer, G. B. & Klebanoff, P. S. 1955 Contributions on the mechanics of boundary layer transition. NACA TN 3489 and NACA rep1289.
Solomon, W. J., Walker, G. J. & Hughes, J. D. 1999 Periodic transition on an axial compressor stator: incidence and clocking effects: Part II – Transition onset predictions. Trans. ASME, J.Turbomach. 121 (3), 408415
Stanislas, M., Okamoto, K., Kähler, C. J. & Westerweel, J. 2005 Main results of the Second International PIV Challenge. Exp. Fluids 39, 170191. (see also p. 35 and p. 37 of the Minutes of the Second International PIV Challenge: http://www.univ-lille1.fr/pivnet/sig32/min_chal_2003/minutes_challenge03_3.pdf)
Stieger, R. D. & Hodson, H. P. 2004 The transition mechanism of highly loaded low-pressure turbine blades. Trans. ASME, J. Turbomach. 126 (4), 536543.
Stieger, R. D. & Hodson, H. P. 2005 The unsteady development of a turbulent bar wake through a downstream low-pressure turbine cascade. Trans. ASME, J. Turbomach. 127 (2), 288394.
Watmuff, J. H. 1999 Evolution of a wave packet into vortex loops in a laminar separation bubble. J. Fluid Mech. 397, 119169.
Wheeler, A. P. S., Sofia, A. & Miller, R. J. 2007 The effect of leading-edge geometry on wake interactions in compressors. ASME Paper GT2007-27802.
White, F. M. 1999 Viscous Fluid Flow 2nd edn. McGraw-Hill.
Wissink, J. G., Rodi, W. & Hodson, H. P. 2006 The influence of disturbances carried by periodically incoming wakes on the separating flow around a turbine blade. Intl J. Heat Fluid Flow 27 (4).
Wu, X., Jacobs, R. G., Hunt, J. C. R. & Durbin, P. A. 1999 Simulation of boundary layer transition induced by periodically passing wakes. J. Fluid Mech. 398, 109153.
Yarusevych, S., Kawall, J. G. & Sullivan, P. E. 2008 Separated-shear-layer development on an airfoil at low reynolds numbers AIAA J. 46 (12), 30603069.
Zhang, X. F. & Hodson, H. P. 2007 Effects of Reynolds number and freestream turbulence intensity on the unsteady boundary layer development on an ultra-high-lift airfoil. ASME Paper GT2007-27274.
Zhang, X. F., Hodson, H. P. & Harvey, N. W. 2005 Unsteady boundary layer studies on ultra-high-lift low pressure turbines Proc. IMechE, Part A: J. Power and Energy 219, 451460.
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