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 Re 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 threedimensional 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., vol. 694, 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.

Adverse-Pressure-Gradient Effects on Turbulent Boundary Layers: Statistics and Flow-Field Organization

“…The mode organization is however affected by both the pressure gradient and the Reynolds number, and in agreement with Ref. [23] sweeps/ejections are moved farther from the wall. The contribution of the modes to the Reynolds stresses and turbulence production is analyzed by reconstructing these quantities with different numbers of modes.…”

“…This finding confirms the claim in Ref. [23] according to which, in APG TBLs, wall-attached large sweeps and ejections are less numerous than in ZPG TBLs.

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“…[13] reports that the spectral distribution of energy in the wake region of APG TBLs is similar to that of the ZPG TBLs; nevertheless, the three-dimensional spatial correlations reported in Ref. [23] show that large-scale structures in the outer region of large-defect boundary layers are shorter in the streamwise direction and more inclined with respect to the wall.…”

Pressure-Gradient Turbulent Boundary Layers Developing Around a Wing Section

“…These effects are also connected with the modification of the large-scale motions from the APG, since these structures are usually wall-attached eddies which leave their footprint all the way down to the wall, affecting the momentum transfer across the whole boundary layer. This is also related to the recent findings by Maciel et al [38], who claim that the u structures tend to be shorter and more inclined with respect to the wall for increasing APGs, compared with the ones found in ZPG boundary layers.…”

Revisiting History Effects in Adverse-Pressure-Gradient Turbulent Boundary Layers

“…Numerical simulations have been used to study the characteristics of PG TBLs, starting from the early work by Spalart and Watmuff [ 16 ], followed by the simulations by Skote et al [ 17 ], and more recently by Lee and Sung [ 18 ] and Gungor et al [ 19 ]. The effect of pressure gradient on the coherent structures in TBLs has been studied, among others, by Marquillie et al [ 20 ] (who focused on near-wall streaks) and by Maciel et al [ 21 ] (who analyzed two-point correlations of the streamwise velocity).…”