Unsteady and Transitional Flows Behind Roughness Elements

Ergin, F. Gökhan; White, Edward B.

doi:10.2514/1.17459

Cited by 123 publications

(126 citation statements)

References 29 publications

(45 reference statements)

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“…The value of Re h has an important effect on the streak amplitude, length scales and perturbation growth rate (Choudhari & Fischer 2005;Ergin & White 2006;Denissen & White 2008). The former factors can modulate the route towards transition.…”

Section: Introductionmentioning

confidence: 99%

On Reynolds number dependence of micro-ramp-induced transition

Schrijer

Scarano

2018

J. Fluid Mech.

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The variation of transitional flow features past a micro-ramp is investigated when the Reynolds number is decreased approaching the critical regime. Experiments are conducted in the incompressible flow spanning from supercritical to subcritical roughness-height-based Reynolds number (Re h = 1170, 730, 460 and 320) with tomographic particle image velocimetry. The effect of Re h on three-dimensional flow behaviour is analysed in a domain encompassing 73 ramp heights in the streamwise direction. Above the critical Re h , the primary vortex pair and induced central low-speed region in the mean flow field are active over longer range when decreasing Re h . In the instantaneous flow, at Re h < 1000, the hairpin vortices induced by Kelvin-Helmholtz (K-H) instability progress gradually from close to the micro-ramp into the region where the overall shear layer is destabilized, indicating the correlation between the K-H instability and the onset of transition. The breakdown of K-H vortices as observed at Re h = 1170, does not occur at lower Re h . Decreasing Re h , the secondary vortex structures make their first appearance significantly downstream, postponing the formation of sideward disturbances, which destabilize the local shear layer by ejection events. Two major types of eigenmodes with symmetric and asymmetric spatial distribution of velocity fluctuations in the near wake are clearly identified by proper orthogonal decomposition. The symmetric and asymmetric modes correspond to the presence of vortex shedding and a sinuous wiggling motion respectively. It is found that Re h is the key factor determining the importance of the symmetric mode. At Re h = 1170, the disturbance energy of the symmetric mode decays before the onset of transition, suggesting that it is relatively insignificant in the process. However, decreasing Re h to 730 and 460, the symmetric mode produces continuous growth of high level disturbance energy, leading to transition.

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Section: Introductionmentioning

confidence: 99%

On Reynolds number dependence of micro-ramp-induced transition

Schrijer

Scarano

2018

J. Fluid Mech.

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“…Our interest here is in studying the transient growth of the wakes behind a linear array of roughness element, which usually occurs in a region where their spanwise length scale is of the order of the local boundary layer thickness (e.g., Ergin and White, 2006) and (as will be shown below) the flow is governed by the BRE. We, therefore, consider an incompressible flat plate boundary layer that is perturbed by a spanwise periodic linear array of roughness elements at some downstream location, say (Note that we have omitted the star superscript on U  even though it denotes a dimensional quantity.)…”

Section: Formulation and Scalingmentioning

confidence: 99%

“…The solution for the roughness wake flow (which exhibits transient growth) is now governed by the nonlinear form of the BRE at the lowest order of approximation, and can, therefore, provide an appropriate base flow for studying the secondary instability and the eventual breakdown into turbulence that was noted by Ergin & White (2006). But, the second order term in the BRE solution, which is O(R 1/8 ) relative to the zeroth order solution and formally corresponds to the linear solution given in GSDC 2, turns out to be much larger than the leading order nonlinear contribution at the start of the BRE region (at least at the finite Reynolds numbers relevant to the experiments).…”

Section: Introductionmentioning

confidence: 99%

“…It is, therefore, important to understand the physical mechanisms related to potential disturbance growth in the wake flow behind the surface roughness. Ergin and White (2006) investigated the steady and unsteady disturbances generated by a spanwise array of cylindrical roughness elements in the context of transient algebraic growth. They found that the steady disturbance energy decreases rapidly just behind the roughness element with transient growth occurring further downstream.…”

Section: Introductionmentioning

confidence: 99%

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Nonlinear wakes behind a row of elongated roughness elements

et al. 2016

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This paper is concerned with the high Reynolds number flow over a spanwise periodic array of roughness elements with inter element spacing of the order of the local boundary layer thickness. While earlier work by Goldstein, Sescu, Duck and Choudhari (2010) and Goldstein, Sescu, Duck and Choudhari (2011) was mainly concerned with smaller roughness heights that produced relatively weak distortions of the downstream flow, the focus here is on extending the analysis to larger roughness heights and streamwise elongated planform shapes that together produce a qualitatively different, nonlinear behavior of the downstream wakes. The roughness scale flow now has a novel triple deck structure that is somewhat different from related studies that have previously appeared in the literature. The resulting flow is formally nonlinear in the intermediate wake region, where the streamwise distance is large compared to the roughness dimensions but small compared to the downstream distance from the leading edge, as well as in the far wake region where the streamwise length scale is of the order of the downstream distance from the leading edge. In contrast, the flow perturbations in both of these wake regions were strictly linear in the earlier work by Goldstein et al (2010Goldstein et al ( , 2011. This is an important difference because the nonlinear wake flow in the present case provides an appropriate basic state for studying the secondary instability and eventual breakdown into turbulence. _______________________________________________________________

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“…This showed that, the roughness elements had no effect on the rate of transition. Ergin & White (2006) carried out an experimental study in a flat plate boundary layer downstream of a spanwise array of cylindrical roughness elements at both subcritical and supercritical values of Re k . They observed rapid transition only for Re k =334 because of the sufficiently large fluctuation growth, and they stated that the growth of unsteady disturbance increased with the increasing Re k .…”

mentioning

confidence: 99%