Coherent Structures in a Non-equilibrium Large-Velocity-Defect Turbulent Boundary Layer

Abstract

The characteristics of the coherent structures in a strongly decelerated large-velocity-defect boundary layer are analysed by direct numerical simulation. The simulated boundary layer starts as a zero-pressure-gradient boundary layer, decelerates under a strong adverse pressure gradient, and separates near the end of the domain, in the form of a very thin separation bubble. The Reynolds number at separation is R e 𝜃 =3912 and the shape factor H=3.43. The three-dimensional spatial correlations of (u, u) and (u, v) are investigated and compared to those of a zero-pressure-gradient boundary layer and another strongly decelerated boundary layer. These velocity pairs lose coherence in the streamwise and spanwise directions as the velocity defect increases. In the outer region, the shape of the correlations suggest that large-scale u structures are less streamwise elongated and more inclined with respect to the wall in large-defect boundary layers. The three-dimensional properties of sweeps and ejections are characterized for the first time in both the zero-pressure-gradient and adverse-pressure-gradient boundary layers, following the method of Lozano-Durán et al. (J. Fluid Mech. 694, 100–130, [2012]). Although longer sweeps and ejections are found in the zero-pressure-gradient boundary layer, with ejections reaching streamwise lengths of 5 boundary layer thicknesses, the sweeps and ejections tend to be bigger in the adverse-pressure-gradient boundary layer. Moreover, small near-wall sweeps and ejections are much less numerous in the large-defect boundary layer. Large sweeps and ejections that reach the wall region (wall-attached) are also less numerous, less streamwise elongated and they occupy less space than in the zero-pressure-gradient boundary layer.

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Acknowledgments

Funded in part by the Multiflow program of the European Research Council. AGG and YM were supported in part respectively by ITU-BAP and NSERC of Canada. YM thanks TUBITAK (2221 Program) for support during the collaboration stay in Turkey. The computations were made possible by generous grants of computer time from Barcelona supercomputing center and from the national center for high performance computing of Turkey. The authors would like to thank Prof. Jiménez for organizing the First and Second Multiflow Summer Workshops, Juan Sillero and Prof. Jiménez for providing their ZPG TBL data and the programs for the two-point correlations, and Adrián Lozano-Durán and Prof. Jiménez for the processing programs for the Q structures analysis.

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Correspondence to Yvan Maciel.

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Maciel, Y., Simens, M.P. & Gungor, A.G. Coherent Structures in a Non-equilibrium Large-Velocity-Defect Turbulent Boundary Layer. Flow Turbulence Combust 98, 1–20 (2017). https://doi.org/10.1007/s10494-016-9737-2

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Keywords

  • Turbulence simulation
  • Turbulent boundary layers
  • Wall-bounded turbulence
  • Adverse pressure gradient
  • Coherent structures
  • Two-point correlations
  • Quadrant analysis