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Research Papers

Boundary Layer Transition on the High Lift T106A Low-Pressure Turbine Blade With an Oscillating Downstream Pressure Field

[+] Author and Article Information
Maciej M. Opoka1

Whittle Laboratory, University of Cambridge, Cambridge, UKMaciej.Opoka@rolls-royce.com

Richard L. Thomas, Howard P. Hodson

Whittle Laboratory, University of Cambridge, Cambridge, UK

1

Currently at Rolls Royce Deutschland, Dahlewitz, Germany.

J. Turbomach 130(2), 021009 (Feb 29, 2008) (10 pages) doi:10.1115/1.2751142 History: Received June 27, 2006; Revised January 15, 2007; Published February 29, 2008

This paper presents the results of an experimental study of the interaction between the suction surface boundary layer of a cascade of low-pressure (LP) turbine blades and a fluctuating downstream potential field. A linear cascade equipped with a set of T106 LP turbine blades was subjected to a periodic variation of the downstream pressure field by means of a moving bar system at low-speed conditions. Measurements were taken in the suction surface boundary layer using 2D laser Doppler anemometry, flush-mounted unsteady pressure transducers and surface shear stress sensors. The Reynolds number, based on the chord and exit conditions, was 1.6×105. The measurements revealed that the magnitudes of the suction surface pressure variations induced by the oscillating downstream pressure field, just downstream of the suction peak, were approximately equal to those measured in earlier studies involving upstream wakes. These pressure field oscillations induced a periodic variation of the transition onset location in the boundary layer. Two turbulence levels were investigated. At a low level of inlet freestream turbulence of 0.5%, a separation bubble formed on the rear part of the suction surface. Unsteady measurements of the surface pressure revealed the presence of high-frequency oscillations occurring near the start of the pressure recovery region. The amplitude of these fluctuations was of the order of 7–8% of exit dynamic pressure, and inspection of the velocity field revealed the presence of Kelvin-Helmholtz-type shear layer vortices in the separated free shear layer. The frequency of these shear layer vortices was approximately one order-of-magnitude greater than the frequency of the downstream passing bars. At a higher inlet freestream turbulence level of 4.0%, which is more representative of real engine environments, separation was prevented by an earlier onset of transition. Oscillations were still observed in suction surface shear stress measurements at a frequency matching the period of the downstream bar, indicating a continued influence on the boundary layer from the oscillating pressure field. However, the shear layer vortices seen in the lower turbulence intensity case were not so clearly observed, and the maximum amplitude of suction surface pressure fluctuations was reduced.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

T106A LP turbine bar-passing rig

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Figure 2

T106A cascade, with downstream bars shown at time t∕τ0=0.0

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Figure 3

Steady-state surface pressure coefficient Cp2is, Re2is=1.6×105, Tu1=0.5% (black) and 4.0% (grey)

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Figure 4

Steady flow suction surface boundary layer momentum thickness (θ) and shape factor (H12), Re2is=1.6×105, Tu1=0.5% (black), and 4.0% (grey)

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Figure 5

Ensemble-averaged suction surface pressure coefficient, Re2is=1.6×105, Fred=0.46, ϕ=0.83, Tu1=0.5% (black), and 4.0% (grey)

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Figure 6

Ensemble-averaged suction surface pressure coefficient, Re2is=1.6×105, Fred=0.46, ϕ=0.83, and Tu1=0.5%

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Figure 7

Ensemble-averaged nondimensional velocity at the edge of the boundary layer Re2is=1.6×105, Fred=0.46, ϕ=0.83 and Tu=0.5%

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Figure 8

Traces of normalized ensemble-averaged surface quasi-wall shear stress, Re2is=1.6×105, Fred=0.46, ϕ=0.83, and Tu1=0.5%

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Figure 9

Streamlines calculated from ensemble-averaged 2D LDA data, Re2is=1.6×105, Fred=0.46, ϕ=0.83, and Tu1=0.5%

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Figure 10

Ensemble-averaged boundary layer shape factor, Re2is=1.6×105, Fred=0.46, ϕ=0.83, and Tu1=0.5%

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Figure 11

Ensemble-averaged boundary layer momentum thickness, Re2is=1.6×105, Fred=0.46, ϕ=0.83, and Tu1=0.5%

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Figure 12

Comparison between traces of normalized raw quasiwall shear stress for Tu1=0.5% (black) and Tu1=4.0% (grey), illustrating effect of a downstream potential field at Re2is=1.6×105 for six surface locations s∕S0=0.63, 0.75, 0.82, and 0.96

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Figure 13

Ensemble-averaged boundary layer momentum thickness measured with LDA at s∕S0=0.96, Re2is=1.6×105, Fred=0.46, ϕ=0.83, Tu1=0.5%, and Tu1=4.0%

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