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Research Papers

Forcing of Separation Bubbles by Main Flow Unsteadiness or Pulsed Vortex Generating Jets—A Comparison

[+] Author and Article Information
Christoph Lyko

e-mail: christoph.lyko@ilr.tu-berlin.de

Jerrit Dähnert

e-mail: jerrit.daehnert@rolls-royce.com

Dieter Peitsch

e-mail: dieter.peitsch@ilr.tu-berlin.de
Department of Aeronautics and Astronautics,
Technical University of Berlin,
Marchstr. 12-14,
Berlin 10587, Germany

1Present address: Rolls-Royce Deutschland Ltd & Co KG, Eschenweg 11, Blankenfelde-Mahlow 15827, Germany.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 2, 2013; final manuscript received July 8, 2013; published online October 23, 2013. Editor: Ronald Bunker.

J. Turbomach 136(5), 051016 (Oct 23, 2013) (12 pages) Paper No: TURBO-13-1125; doi: 10.1115/1.4025214 History: Received July 02, 2013; Revised July 08, 2013

Low pressure turbines typically operate in the low Reynolds number regime. Depending on the loading of the blade, they may exhibit detached flow with associated reattachment in the rear part of the suction surface. Additionally, the flow is highly time-dependent due to the sequence of rotating and stationary blade rows. The work presented in this paper covers experimental efforts taken to investigate this type of flow in detail. Typical low pressure turbine flow conditions have been chosen as baseline for the experimental work. A pressure distribution has been created on a flat plate by means of a contoured upper wall in a low speed wind tunnel. The distribution matches the one of the Pak-B airfoil. Unsteadiness is then superimposed in two ways: A specific unsteadiness was created by using a rotating flap (RF) downstream of the test section. This results in almost sinusoidal periodic unsteady flow across the plate, simulating the interaction between stator and rotor of a turbine stage. Furthermore, pulsed blowing by vortex generating jets (VGJ) upstream of the suction peak was used to influence the transition process and development of the separation bubble. Measurements have been performed with hot-wire anemometry. Experimental results are presented to compare both forcing mechanisms. In sinusoidal unsteady main flow, the transition occurs naturally by the breakdown of the shear layer instability, which is affected by periodic changes in the overall Reynolds number and thus pressure gradient. In opposition, active flow control (AFC) by VGJ triggers the transition process by impulse and vorticity injection into the boundary layer, while maintaining a constant Reynolds number. The flow fields are compared using phase averaged data of velocity und turbulence intensity as well as boundary layer parameters, namely shape factor and momentum thickness Reynolds number. Finally, a model to describe the time mean intermittency distribution is refined to fit the data.

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Figures

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Fig. 1

Wind tunnel for periodic unsteady flow

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Fig. 4

Inflow conditions for test case RF. Amplitude at the top panel, and energy spectrum at the bottom panel.

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Fig. 5

Blowing velocity VB compared to a reference signal

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Fig. 6

Wall normal maximum turbulent intermittency ⟨γ⟩max. RF on the left side, AFC on the right side.

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Fig. 7

Normalized velocity timetrace (u/U¯∞) for two selected periods at position x/Lss = 1 and z/Lss = 0.02 with corresponding intermittency function (γ) at the top panel and phase averaged intermittency (⟨γ⟩) for the complete timetrace at the bottom panel. The position of the timetrace is marked with black crosses in Fig. 11.

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Fig. 8

Wall normal maximum turbulence intensity ⟨u'⟩max/U¯∞. RF on the left side, AFC on the right side.

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Fig. 9

Velocity and turbulence intensity profiles for the RF test case at x/Lss = 1. Phases are: t/T=10/8, t/T=11/8, and t/T=12/8.

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Fig. 10

Normalized velocity timetrace (u/U¯∞) for two selected periods at position x/Lss = 0.55 and z/Lss = 0.005 with corresponding intermittency function (γ)

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Fig. 11

Contour plots for the test case with RF. Normalized ensemble averaged mean velocity on the left side and ensemble averaged turbulent intensity on the right side for eight phases. The black crosses refer to Fig. 7.

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Fig. 12

Contour plots for the test case with AFC. Normalized ensemble averaged mean velocity on the left side and ensemble averaged turbulent intensity on the right side for eight phases.

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Fig. 13

Velocity and turbulence intensity profiles for the AFC test case at x/Lss = 1. Phases are: t/T=12/8, t/T=5/8, and t/T=6/8.

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Fig. 14

Pressure coefficient cp for eight phases. Time averaged values are black dotted. The solid black line is the inviscid solution. RF on the left side, AFC on the right side.

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Fig. 15

Shape factor ⟨H12⟩ for two periods. RF on the left side, AFC on the right side.

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Fig. 16

Momentum thickness Reynolds number ⟨Re2⟩ for two periods. RF on the left side, AFC on the right side.

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Fig. 17

Time averaged shape factor H¯12. Comparison of unsteady to steady (ST) results. The black line shows the actuator position.

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Fig. 18

Time averaged momentum thickness Reynolds number R¯e2. Comparison of unsteady to steady (ST) results. The black line shows the actuator position.

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Fig. 19

Intermittency function (Eq. (5)) compared to the measured data for the RF test case

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Fig. 20

Intermittency function (Eq. (5)) compared to the measured data for the AFC test case

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