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Research Papers

Exploitation of Subharmonics for Separated Shear Layer Control on a High-Lift Low-Pressure Turbine Using Acoustic Forcing

[+] Author and Article Information
Chiara Bernardini

Visiting Researcher
Department of Energy Engineering,
University of Florence,
Via di S. Marta,
Florence 3, Italy
e-mail: bernardini.3@osu.edu

Stuart I. Benton

e-mail: benton.53@osu.edu

Jen-Ping Chen

Associate Professor
e-mail: chen.1210@osu.edu

Jeffrey P. Bons

Professor
e-mail: bons.2@osu.edu
Department of Mechanical and
Aerospace Engineering,
The Ohio State University,
2300 West Case Rd.,
Columbus, OH 43235

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 9, 2013; final manuscript received August 8, 2013; published online October 24, 2013. Editor: Ronald Bunker.

J. Turbomach 136(5), 051018 (Oct 24, 2013) (9 pages) Paper No: TURBO-13-1144; doi: 10.1115/1.4025586 History: Received July 09, 2013; Revised August 08, 2013

The mechanism of separation control by sound excitation is investigated on the aft-loaded low-pressure turbine (LPT) blade profile, the L1A, which experiences a large boundary layer separation at low Reynolds numbers. Previous work by the authors has shown that on a laminar separation bubble such as that experienced by the front-loaded L2F profile, sound excitation control has its best performance at the most unstable frequency of the shear layer due to the exploitation of the linear instability mechanism. The different loading distribution on the L1A increases the distance of the separated shear layer from the wall and the exploitation of the same linear mechanism is no longer effective in these conditions. However, significant control authority is found in the range of the first subharmonic of the natural unstable frequency. The amplitude of forced excitation required for significant wake loss reduction is higher than that needed when exploiting linear instability, but unlike the latter case, no threshold amplitude is found. The fluid-dynamics mechanisms under these conditions are investigated by particle image velocimetry (PIV) measurements. Phase-locked PIV data gives insight into the growth and development of structures as they are shed from the shear layer and merge to lock into the excited frequency. Unlike near-wall laminar separation sound control, it is found that when such large separated shear layers occur, sound excitation at subharmonics of the fundamental frequency is still effective with high-Tu levels.

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References

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Figures

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Fig. 1

Schematic of test section

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Fig. 2

Suction surface acceleration parameter K from inviscid calculations for L1A and L2F blade profiles

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Fig. 3

Uncontrolled PIV time-averaged normalized velocity magnitude contours superposed on normalized Reynolds stress isocontours (four levels from 0.01 to 0.04). Top to bottom: low-, med-, high-Tu. Left column: L1A. Right column: L2F.

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Fig. 4

PSD in the uncontrolled separating shear layer; left: L1A; right: L2F

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Fig. 5

Integrated wake loss coefficient normalized by the uncontrolled case versus forcing frequency; top: L1A; bottom: L2F. Amplitude held constant at Δu/Uin = 2.7% for L1A and Δu/Uin = 0.5% for L2F.

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Fig. 6

PIV time-averaged normalized velocity magnitude contours superposed on normalized Reynolds stress isocontours (four levels from 0.01 to 0.04); left: control at ff = 50 Hz; right: control at ff = 120 Hz. Med-Tu. L1A.

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Fig. 7

Ensemble-averaged swirl strength contours from phase-locked PIV data; left: control at ff = 50 Hz; right: control at ff = 120 Hz. L1A.

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Fig. 8

Instantaneous streamlines superposed on normalized spanwise vorticity contours from phase-locked PIV data; left: t/T = 0.0; right: t/T = 0.2; control at ff = 50 Hz. L1A.

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Fig. 9

Evolution of spectral components at ff and 2ff for ff = fn/2 and momentum thickness versus downstream distance (adaptation from Ref. [24])

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Fig. 10

PSD in the controlled separating shear layer (75% < Cx < 80%) for three ff at med-Tu. L1A.

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Fig. 11

Integrated wake loss coefficient normalized by the uncontrolled case versus forcing amplitude; top: L1A; control at ff = 50 Hz; bottom: L2F; control at ff = 110 Hz

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Fig. 12

Left: time-averaged normalized velocity magnitude superposed on normalized Reynolds stress isocontours (four levels from 0.01 to 0.04). Right: ensemble averaged normalized spanwise vorticity at phase t/T = 0.0. Control at ff = 50 Hz, med-Tu, at four forcing amplitudes. L1A.

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Fig. 13

PSD in the cascade passage at three Tu levels

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Fig. 14

Time-averaged normalized velocity magnitude superposed on normalized Reynolds stress isocontours (four levels from 0.01 to 0.04); left: control at ff = 50 Hz; right: control at ff = 120 Hz at three Tu levels. L1A.

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Fig. 15

Ensemble averaged isolevels of Sw = 200, phase t/T = 0.0; top: control at ff = 50 Hz; bottom: control at ff = 120 Hz. L1A.

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