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Large-eddy simulation of mixing in a recirculating shear flow

Published online by Cambridge University Press:  08 March 2010

GEORGIOS MATHEOU ,
ARISTIDES M. BONANOS ,
CARLOS PANTANO  and
PAUL E. DIMOTAKIS
GEORGIOS MATHEOU*
Affiliation:
Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125, USA
ARISTIDES M. BONANOS
Affiliation:
Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125, USA
CARLOS PANTANO
Affiliation:
Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
PAUL E. DIMOTAKIS
Affiliation:
Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125, USA
*
†Email address for correspondence: matheou@caltech.edu
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Abstract

The flow field and mixing in an expansion-ramp geometry is studied using large-eddy simulation (LES) with subgrid scale (SGS) modelling. The expansion-ramp geometry was developed to investigate enhanced mixing and flameholding characteristics while maintaining low total-pressure losses. Passive mixing was considered without taking into account the effects of chemical reactions and heat release, an approximation that is adequate for experiments conducted in parallel. The primary objective of the current work is to validate the LES–SGS closure in the case of passive turbulent mixing in a complex configuration and, if successful, to rely on numerical simulation results for flow details unavailable via experiment. Total (resolved-scale plus subgrid contribution) probability density functions (p.d.f.s) of the mixture fraction are estimated using a presumed beta-distribution model for the subgrid field. Flow and mixing statistics are in good agreement with the experimental measurements, indicating that the mixing on a molecular scale is correctly predicted by the LES–SGS model. Finally, statistics are shown to be resolution-independent by computing the flow for three resolutions, at twice and four times the resolution of the coarsest simulation.

JFM classification

Turbulent Flows: Turbulence simulation Mixing: Turbulent mixing
Type
Papers
Information
Journal of Fluid Mechanics , Volume 646 , 10 March 2010 , pp. 375 - 414
Copyright
Copyright © Cambridge University Press 2010

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References

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Matheou et al. supplementary movie

Movie 1. Mixture fraction iso-surfaces for Case A1, lowerst resolution (top stream Mach number is 0.35 and bottom to top stream mass-injection ratio is 0.09). Three values of the mixture fraction are plotted: 0.8, 0.5, and 0.2, corresponding to red, green, and blue iso-surfaces, respectively. In this lowerst-resolution LES, the recirculation-region length is overpredicted.

Download Matheou et al. supplementary movie(Video)
Video 10.4 MB

Matheou et al. supplementary movie

Movie 2. Mixture fraction iso-surfaces for Case A2, medium resolution (top stream Mach number is 0.35 and bottom to top stream mass-injection ratio is 0.09). Three values of the mixture fraction are plotted: 0.8, 0.5, and 0.2, corresponding to red, green, and blue iso-surfaces, respectively. The increase in resolution compared to Case A1 (Movie 1) captures the flow features more accurately. Spanwise-organised structrures can be observed in the primary shear layer. Near the bottom wall, in the recirculation region, the flow is moving upstream forming a secondary mixing layer at the base of the ramp (x ~ 0.1). The primary shear layer reattachment location and recirculating flow unsteady characteristics are also visible.

Download Matheou et al. supplementary movie(Video)
Video 11.2 MB

Matheou et al. supplementary movie

Movie 3. Mixture fraction iso-surfaces for Case A3, highest resolution (top stream Mach number is 0.35 and bottom to top stream mass-injection ratio is 0.09). Three values of the mixture fraction are plotted: 0.8, 0.5, and 0.2, corresponding to red, green, and blue iso-surfaces, respectively. The finer grid allows the resolution of smaller spatial scales. Howerver, the flow features are almost identical as the medium resolution case (Movie 2).

Download Matheou et al. supplementary movie(Video)
Video 9.5 MB