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Research Papers: Forced Convection

Interactions of Separation Bubble With Oncoming Wakes by Large-Eddy Simulation

[+] Author and Article Information
S. Sarkar

Professor
Mem. ASME
Department of Mechanical Engineering,
Indian Institute of Technology Kanpur,
Kanpur, Uttar Pradesh 208016, India
e-mail: subra@iitk.ac.in

Harish Babu

Department of Mechanical Engineering,
Indian Institute of Technology Kanpur,
Kanpur, Uttar Pradesh 208016, India
e-mail: harishb@iitk.ac.in

Jasim Sadique

Department of Mechanical Engineering,
Indian Institute of Technology Kanpur,
Kanpur, Uttar Pradesh 208016, India
e-mail: jasimsadique@gmail.com

1Corresponding author.

Manuscript received August 14, 2014; final manuscript received August 18, 2015; published online October 21, 2015. Assoc. Editor: Jim A. Liburdy.

J. Heat Transfer 138(2), 021703 (Oct 21, 2015) (12 pages) Paper No: HT-14-1534; doi: 10.1115/1.4031645 History: Received August 14, 2014; Revised August 18, 2015

The unsteady flow physics and heat transfer characteristics due to interactions of periodic passing wakes with a separated boundary layer are studied using large-eddy simulation (LES). A series of airfoils of constant thickness with rounded leading edge are employed to obtain the separated boundary layer. Wake data extracted from precursor LES of flow past a cylinder are used to replicate a moving bar that generates wakes in front of a cascade (in this case, an infinite row of the model airfoils). This setup is a simplified representation of the rotor–stator interaction in turbomachinery. With a uniform inlet, the laminar boundary layer separates near the leading edge, undergoes transition due to amplification of disturbances, becomes turbulent, and finally reattaches forming a separation bubble. In the presence of oncoming wakes, the characteristics of the separated boundary layer have changed and the impinging wakes are found to be the mechanism affecting the reattachment. Phase-averaged results illustrate the periodic behavior of both flow and heat transfer. Large undulations in the phase-averaged skin friction and Nusselt number distributions can be attributed to the excitation of the boundary layer by convective wakes forming coherent vortices, which are being shed and convect downstream. Further, the transition of the separated boundary layer during the wake-induced path is governed by a mechanism that involves the convection of these vortices followed by increased fluctuations, where viscous effect is substantial.

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Figures

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Fig. 1

A schematic of the wake-generating cylinders sweeping at a speed Vcyl ahead of the flat plate cascade

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Fig. 2

Variation of time-averaged skin friction coefficient (Cf) for different grid levels

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Fig. 3

Profiles of time-averaged (a) streamwise velocity U¯ and (b) rms velocity fluctuations u′ at indicated streamwise locations for different grid levels (refer to Fig. 2 for legend)

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Fig. 4

Variation of time-averaged bubble length with turbulence intensity

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Fig. 5

Profiles of (a) mean streamwise velocity U¯, (b) rms streamwise velocity fluctuation u', (c) mean temperature θ¯, (d) rms temperature fluctuation θ′, (e) streamwise turbulent heat flux −u′θ′¯, and (f) wall-normal heat flux v′θ′¯ computed at seven streamwise locations measured from the blend point at x/l = 0.22, 0.44, 0.66, 1.09, 1.27, 1.64, and 2.55 (LES results with uniform inlet are denoted by firm line, wake inlet for the top surface by dashed–dotted, wake inlet for the bottom surface by dots, and the experiment with uniform inlet by circle)

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Fig. 6

Time-averaged coefficient of friction (Cf) along with Stanton number (Sn)

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Fig. 7

Profiles of (a) maximum rms values of velocity fluctuations and (b) maximum rms values of temperature fluctuations superimposed with streamwise velocity fluctuations for flow with passing wakes

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Fig. 8

Contours of instantaneous (a) streamwise velocity and (b) temperature at t/tp = 0.3 in the xy plane at the midspan and the xz plane at 0.04D from both top and bottom walls

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Fig. 9

Contours of instantaneous (a) streamwise velocity and (b) temperature at t/tp = 0.6 in the xy plane at the midspan and the xz plane at 0.04D from both top and bottom walls

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Fig. 10

Spectra of (a) streamwise velocity and (b) temperature fluctuation at eight streamwise locations x/l = 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 0.9, and 1.2 and at y/l = 0.04 from the wall for the upper surface

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Fig. 11

Contours of phase-averaged vorticity illustrating wake boundary layer interactions

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Fig. 12

Profiles at phase-averaged (a) streamwise velocity 〈U〉, (b) rms streamwise velocity fluctuation 〈u′〉, (c) temperature 〈θ〉, (d) rms temperature fluctuation 〈θ′〉, (e) streamwise turbulent heat flux 〈−u′θ′〉, and (f) wall-normal heat flux 〈v′θ′〉

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Fig. 13

Phase-averaged (a) Cf and (b) Nu distribution at the top surface superimposed with time-averaged distribution

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