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Research Papers

Transient Performance of Separated Flows: Characterization and Active Flow Control

[+] Author and Article Information
J. Saavedra

Mechanical Engineering,
Purdue University,
West Lafayette 47907, IN;
von Karman Institute for Fluid Mechanics,
Rhode-Saint-Genèse B-1640, Belgium
e-mail: jorsaaga@gmail.com

G. Paniagua

Mechanical Engineering,
Purdue University,
West Lafayette 47907, IN;
von Karman Institute for Fluid Mechanics,
Rhode-Saint-Genèse B-1640, Belgium
e-mail: gpaniagua@me.com

Manuscript received June 22, 2018; final manuscript received June 22, 2018; published online September 7, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(1), 011002 (Sep 07, 2018) (11 pages) Paper No: GTP-18-1311; doi: 10.1115/1.4040685 History: Received June 22, 2018; Revised June 22, 2018

The aerothermal performance of the low-pressure turbine in unmanned aerial vehicles is significantly abated at high altitude, due to boundary layer separation. Different flow control strategies have been proposed to prevent boundary layer separation, such as dielectric barrier discharges (DBD) and synthetic jets. However, the optimization of the control approach requires a better characterization of the separated regions at transient conditions. The present investigation analyzes the behavior of separated flows, reporting the inception and separation length, allowing the development of efficient flow control methods under nontemporally uniform inlet conditions. The development of separated flows was investigated with numerical simulations including Unsteady Reynolds average Navier–Stokes (URANS) and large Eddy simulations (LES). The present research was performed on a wall-mounted hump, which imposes a pressure gradient representative of the suction side of low pressure turbines. Through sudden flow accelerations, we looked into the dynamic response of the shear layer detachment as it is modulated by the mean flow evolution. Similarly, we studied the behavior of the recirculation bubble under periodic disturbances imposed at various frequencies ranging from 10 to 500 Hz, at which the Reynolds number oscillates between 40,000 and 180,000. As a first step into the flow control, we added a slot to allow flow injection and ingestion upstream of the separation inception. Exploring the behavior of the separated region at different conditions, we defined the envelope for its periodic actuation. We found that by matching the actuator frequency with the frequency response of the separated region, the performance of the actuation is boosted.

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Figures

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Fig. 1

Domain generation for flow separation dynamic scales analysis: (a) geometry, (b) slope of the wall mounted hump, and (c) curvature of the wall mounted hump evolution

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Fig. 2

Two-dimensional domain for numerical analysis of flow separation dynamics

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Fig. 3

Mesh sensitivity analysis: (a) grid convergence index, (b) momentum boundary layer profiles at various axial locations: 0.15, 0.3, and 0.45 m, and (c) wall shear stress evolution along the plate

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Fig. 4

Validation of the numerical methodology

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Fig. 5

LES numerical domain

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Fig. 6

(a) inlet boundary condition for sudden flow acceleration, (b) inlet boundary condition for periodic flow perturbation, (c) massflow through the domain, and (d) drag coefficient during the sudden flow acceleration

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Fig. 7

Free stream, wall fluxes and separated region evolution during the sudden flow acceleration

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Fig. 8

Axial velocity contours at various instances during the blowdown compared to steady evaluations along the profile

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Fig. 9

Mean flow, separated region and wall fluxes evolution at various excitation frequencies for complete inlet fluctuation period

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Fig. 10

Axial velocity profiles at various instances during the period at x = 0.25 m

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Fig. 11

Integral drag along the plate for various excitation frequencies: (a) drag signal after periodic convergence and (b) fast Fourier transform of the drag signal for 8 periods

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Fig. 12

(a) Energy spectra on the domain at three given points and (b) axial velocity contour at t/p = 0.75 with q-criterion display

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Fig. 13:

Large Eddy simulation versus 2D SST transitional results: (a) bubble length, (b) separation inception, and (c) heat transfer coefficient at x = 0.25 m during the periodic oscillation

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Fig. 14

Domain for injection analysis

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Fig. 15

Flow separation control through slot pressure

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Fig. 16

Slot pressure fluctuation effect on the separated flow region

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Fig. 17

Momentum profiles at x = 0.25 m for various injection frequencies

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Fig. 18

Thermal boundary layer profiles at x = 0.25 m for various injection frequencies

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Fig. 19

Injection pressure amplitude effect on the separated flow region and thermal isolation

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Fig. 20

Injection pressure amplitude effect on the separated flow region and thermal insulation

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