Research Papers

The Effect of Acoustic Excitation on Boundary Layer Separation of a Highly Loaded LPT Blade

[+] Author and Article Information
Chiara Bernardini

Visiting Researcher
Department of Energy Engineering,
University of Florence,
Via di S. Marta, 3,
50121 Florence, Italy
e-mail: bernardini.3@osu.edu

Stuart I. Benton

e-mail: benton.53@osu.edu

Jeffrey P. Bons

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

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the Journal of Turbomachinery. Manuscript received July 2, 2012; final manuscript received August 30, 2012; published online June 24, 2013. Editor: David Wisler.

J. Turbomach 135(5), 051001 (Jun 24, 2013) (9 pages) Paper No: TURBO-12-1113; doi: 10.1115/1.4007834 History: Received July 02, 2012; Revised August 30, 2012

An experimental investigation of the effect of acoustic excitation on the boundary layer development of a highly loaded low-pressure turbine blade at low-Reynolds number is investigated. The aim of this work is to study the effect of excitation at select frequencies on separation which could give indications about active flow control exploitation. The front-loaded L2F blade is tested in a low-speed linear cascade. The uncontrolled flow presents a separation bubble on the suction surface at Reynolds numbers below 40,000. For these conditions, the instability of the shear layer is documented using hot-wire anemometry. A loudspeaker upstream of the cascade is directed towards the passage inlet section. A parametric study on the effect of amplitude and frequency is carried out. The effect of the excitation frequency is observed to delay separation for a range of frequencies. However, the control authority of sound is found to be most effective at the fundamental frequency of the shear layer. The amplitude of perturbation is significant in the outcome of control until a threshold value is reached. PIV measurements allow a deeper understanding of the mechanisms leading to the reduction of separation. Data has been acquired with a low inlet turbulence level (<1%) in order to provide a cleaner environment which magnifies the effects of the excitation frequency, and with an increased turbulence intensity level of 3% which is representative of more typical engine values. Integrated wake loss values are also presented to evaluate the effect on blade performance.

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Fig. 1

Schematic of test section and speaker positioning

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Fig. 2

PIV window reference system and hot-film positions

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Fig. 3

Amplitude of velocity fluctuation at excitation frequency

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Fig. 4

PIV data for Re = 20,000, low- (upper) and high- (bottom) Tu. Left column: time averaged velocity magnitude, right column: Reynolds shear stress.

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Fig. 5

PIV data for Re = 40,000, low- (upper) and high- (bottom) Tu. Left column: time averaged velocity magnitude, right column: Reynolds shear stress.

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Fig. 6

Premultiplied spectra at Re = 20,000 at the wall distance indicated by the dashed lines (upper: low-Tu, lower: high-Tu) on the velocity profile inset measured at this streamwise location

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Fig. 7

Pitchwise distribution of wake loss coefficient, Re = 20,000, low-Tu

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Fig. 8

Integrated wake loss coefficient normalized by the uncontrolled case

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Fig. 9

PIV data for Re = 20,000, control at f = 110 Hz, low-Tu. Left: time averaged velocity magnitude; right: Reynolds shear stress.

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Fig. 10

Instantaneous PIV images at four instants: streamlines superposed on normalized spanwise vorticity contours (ωzCx/Ue). Left column: Re = 20,000, low-Tu, uncontrolled; right column Re = 20,000, low-Tu, control at f = 110 Hz.

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Fig. 11

Hot-film raw velocity signal at boundary layer edge at axial position 2, Re = 20,000, low-Tu. Top: uncontrolled case, bottom: control at f = 110 Hz.

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Fig. 12

Premultiplied PSD at boundary layer edge at axial position 2, Re = 20,000. Low- (left) and high- (right) Tu. Uncontrolled and control at f = 110 Hz.

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Fig. 13

Instantaneous PIV images at four instants: streamlines superposed on normalized spanwise vorticity contours (ωzCx/Ue). Left column: Re = 20,000, high-Tu, uncontrolled; right column: Re = 20,000, high-Tu, control at f = 110 Hz.

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Fig. 14

Pitchwise distribution of wake loss coefficient, Re = 10,000, low- and high-Tu

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Fig. 15

Range of most effective frequencies and relative normalized integrated wake losses for each Re

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Fig. 16

Premultiplied PSD at boundary layer edge, axial position 2, Re = 10,000, 30,000, and 50,000 with low-Tu

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Fig. 17

Amplitude effect on integrated wake loss coefficient normalized by the uncontrolled case, Re = 20,000 and f = 110 Hz



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