Secondary flows in turbulent boundary layers over longitudinal surface roughness

Hwang, Hyun Tae; Lee, Jae Hwa

doi:10.1103/physrevfluids.3.014608

Cited by 62 publications

(151 citation statements)

References 25 publications

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“…The picture in figure 3 above is consistent with that, showing a good match between theory and results for δ/ℓ k > 2. This is also in agreement with results from Vanderwel & Ganapathisubramani (2015) and Hwang & Lee (2018) that show that the excitation of secondary motions is most effective around a spanwise roughness spacing of S/δ = O(1), but decreases drastically when S is decreased. The current analytical model may shed some further light on the characterization of roughness length scales in rough boundary layers.…”

Section: Discussionsupporting

confidence: 91%

On the decay of dispersive motions in the outer region of rough-wall boundary layers

2019

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In rough-wall boundary layers, wall-parallel non-homogeneous mean-flow solutions exist that lead to so-called dispersive velocity components and dispersive stresses. They play a significant role in the mean-flow momentum balance near the wall, but typically disappear in the outer layer. A theoretical framework is presented to study the decay of dispersive motions in the outer layer. To this end, the problem is formulated in Fourier space, and a set of governing ordinary differential equations per mode in wavenumber space is derived by linearizing the Reynolds-averaged Navier-Stokes equations around a constant background velocity. With further simplifications, analytically tractable solutions are found consisting of linear combinations of exp(−kz) and exp(−Kz), with z the wall distance, k the magnitude of the horizontal wavevector k, and where K(k, Re) is a function of k and the Reynolds number Re. Moreover, for k → ∞ or k1 → 0, K → k is found, in which case solutions consist of a linear combination of exp(−kz) and z exp(−kz), and are Reynolds number independent. These analytical relations are verified in the limit of k1 = 0 using the rough boundary layer experiments by Vanderwel and Ganapathisubramani (J. Fluid Mech. 774, R2, 2015) and are in good agreement for ℓ k /δ ≤ 0.5, with δ the boundary-layer thickness and ℓ k = 2π/k.

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Section: Discussionsupporting

confidence: 91%

On the decay of dispersive motions in the outer region of rough-wall boundary layers

2019

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“…while the distribution for h = 0 is in good agreement with the studies of ridge-type roughness(Hwang & Lee 2018;Vanderwel et al 2019). The Reynolds stress v w at different elevation of the smooth stripes h. Brown solid lines mark the isolines for the streamwise mean velocity distribution with isolevels corresponding toFigure 4.…”

supporting

confidence: 87%

“…This flow topology is similar to the secondary flows over ridge-type roughness (e.g. Hwang & Lee (2018)).…”

Section: Global Flow Propertiessupporting

confidence: 64%

“…The turbulence property that is found to be related to this switch is the v w Reynolds stress component. This quantity, which is related to the transport of turbulent kinetic energy (Hwang & Lee 2018) and whose spatial gradients occur in the mean momentum budget for v and w (Stroh et al 2016), switches sign in agreement with the rotational direction of the secondary motion. This sign switch is related to the relative roughness height through the different deflections that spanwise velocity fluctuations experience for protruding or recessed roughness.…”

Section: Discussionmentioning

confidence: 84%

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Rearrangement of secondary flow over spanwise heterogeneous roughness

et al. 2020

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Turbulent flow over a surface with streamwise-elongated rough and smooth stripes is studied by means of direct numerical simulation (DNS) in a periodic plane open channel with fully resolved roughness. The goal is to understand how the mean height of roughness affects the characteristics of the secondary flow formed above a spanwise-heterogeneous rough surface. To this end, while the statistical properties of roughness texture as well as the width and spacing of the rough stripes are kept constant, the elevation of the smooth stripes is systematically varied in different simulation cases. Utilizing this variation three configurations representing protruding, recessed and an intermediate type of roughness are analysed. In all cases secondary flows are present and the skin friction coefficients calculated for all the heterogeneous rough surfaces are meaningfully larger than what would result from the area-weighted average of those of homogeneous smooth and rough surfaces. This drag increase appears to be linked to the strength of the secondary flow. The rotational direction of the secondary motion is shown to depend on the relative surface elevation. The present results suggest that this rearrangement of the secondary flow is linked to the spatial distribution of the spanwise-wall-normal Reynolds stress component which carries opposing signs for protruding and recessed roughness.

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“…Using direct numerical simulation (DNS), they found that the width of the free-slip area (note that the effect of free-slip area on the flow field is similar with the low-roughness regions for the flows over rough walls) might be a relevant scaling parameter for the secondary flow topology while the influence of the no-slip spacing is weaker, which is opposite to the conclusion drawn by Willingham et al [17]. Recently, Hwang et al [23] systematically studied the two parameters of the pitch (P) and width (S) for roughness elements in turbulent boundary layer over longitudinal surface roughness using DNS. Their analysis showed that the size of the secondary flow is mostly determined by the value of P−S, i.e., the valley width.…”

Section: Introductionmentioning

confidence: 83%

Flow separation control over a rounded ramp with spanwise alternating wall actuation

Moulinec

Emerson

2019

Physics of Fluids

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An implicit large-eddy simulation is carried out to study turbulent boundary-layer separation from a backward-facing rounded ramp with active wall actuation control. This method, called spanwise alternating distributed strips control, is imposed onto the flat plate surface upstream of a rounded ramp by alternatively applying out-of-phase control and in-phase control to the wall-normal velocity component in the spanwise direction. As a result, the local turbulence intensity is alternatively suppressed and enhanced, leading to the creation of vertical shear-layers, which is responsible for the presence of large-scale streamwise vortices. These vortices exert a predominant influence on the suppression of the flow separation. The interaction between the large-scale vortices and the downstream recirculation zone and free shear-layer is studied by examining flow statistics. It is found that in comparison with the non-controlled case the flow separation is delayed, the reattachment point is shifted upstream, and the length of the mean recirculation zone is reduced up to 8.49%. The optimal control case is achieved with narrow in-phase control strips. An in-depth analysis shows that the delay of the flow separation is attributed to the activation of the near-wall turbulence by the in-phase control strips and the improvement of the reattachment location is mainly due to the large-scale streamwise vortices, which enhance the momentum transport between the main flow and separated region.

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Secondary flows in turbulent boundary layers over longitudinal surface roughness

Cited by 62 publications

References 25 publications

On the decay of dispersive motions in the outer region of rough-wall boundary layers

On the decay of dispersive motions in the outer region of rough-wall boundary layers

Rearrangement of secondary flow over spanwise heterogeneous roughness

Flow separation control over a rounded ramp with spanwise alternating wall actuation

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