Tomographic-PIV measurement of the flow around a zigzag boundary layer trip

Elsinga, Gerrit E.; Westerweel, J.

doi:10.1007/s00348-011-1153-8

Cited by 50 publications

(30 citation statements)

References 37 publications

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“…At intermediate α, the Reynolds shear stress distributions become approximately constant starting at values beyond 0.02 and converging to values close to 0.005. Very similar values and trends were found in the region behind a zigzag roughness strip (Elsinga and Westerweel, 2012) and the order of magnitude corresponds with turbulent flow conditions over a flat plate (Klebanoff, 1955;Erm and Joubert, 1991;Ducros et al, 1996;de Graaff and Eaton, 2000). On the smooth surface the Reynolds shear stress values and distribution found are comparable to those found for separation bubbles on low Reynolds number airfoils (Yuan et al, Fig.…”

Section: Discussionsupporting

confidence: 76%

Feather roughness reduces flow separation during low Reynolds number glides of swifts

Bokhorst

Kat

Elsinga

et al. 2015

Journal of Experimental Biology

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Swifts are aerodynamically sophisticated birds with a small arm and large hand wing that provides them with exquisite control over their glide performance. However, their hand wings have a seemingly unsophisticated surface roughness that is poised to disturb flow. This roughness of about 2% chord length is formed by the valleys and ridges of overlapping primary feathers with thick protruding rachides, which make the wing stiffer. An earlier flow study of laminar-turbulent boundary layer transition over prepared swift wings suggested that swifts can attain laminar flow at a low angle of attack. In contrast, aerodynamic design theory suggests that airfoils must be extremely smooth to attain such laminar flow. In hummingbirds, which have similarly rough wings, flow measurements on a 3D printed model suggest that the flow separates at the leading edge and becomes turbulent well above the rachis bumps in a detached shear layer. The aerodynamic function of wing roughness in small birds is, therefore, not fully understood. Here, we performed particle image velocimetry and force measurements to compare smooth versus rough 3D-printed models of the swift hand wing. The high-resolution boundary layer measurements show that the flow over rough wings is indeed laminar at a low angle of attack and a low Reynolds number, but becomes turbulent at higher values. In contrast, the boundary layer over the smooth wing forms open laminar separation bubbles that extend beyond the trailing edge. The boundary layer dynamics of the smooth surface varies non-linearly as a function of angle of attack and Reynolds number, whereas the rough surface boasts more consistent turbulent boundary layer dynamics. Comparison of the corresponding drag values, lift values and glide ratios suggests, however, that glide performance is equivalent. The increased structural performance, boundary layer robustness and equivalent aerodynamic performance of rough wings might have provided small ( proto) birds with an evolutionary window to high glide performance.

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Section: Discussionsupporting

confidence: 76%

Feather roughness reduces flow separation during low Reynolds number glides of swifts

Bokhorst

Kat

Elsinga

et al. 2015

Journal of Experimental Biology

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“…The rotating direction of the leg-buffer structures is coherent with that of the time-averaged secondary vortex pair discussed in the mean flow organization ( § 3). It has been conjectured that the secondary vortex pair emerges as an artefact of temporal averaging of leg-buffer vortices and does not occur in the instantaneous flow organization (Elsinga & Westerweel 2012). The further intensity increase of the leg-buffer vortices gives rise to a second unique large leg-shaped vortex structure outwards (figure 7b, green dash-dot line).…”

Section: Instantaneous Flow Organizationmentioning

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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“…The hairpin generation mechanisms were supported from experiment by flow visualization (Smith et al 1991) and from DNS (Zhou et al 1999), and gained further credibility by observations of hairpin packets in the boundary layer transition region (e.g. Wu 2010; Elsinga & Westerweel 2012). Kim, Sung & Adrian (2008) later confirmed that hairpins can auto-generate new hairpins within a fully turbulent environment, but still they had to forcefully introduce a very strong initial hairpin vortex structure into their turbulent channel flow.…”

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confidence: 90%

Experimental observation of hairpin auto-generation events in a turbulent boundary layer

Jodai

Elsinga

2016

J. Fluid Mech.

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Time-resolved tomographic particle image velocimetry experiments show that new hairpin vortices are generated within a fully developed and unperturbed turbulent boundary layer. The measurements are taken at a Reynolds number based on the momentum thickness of 2038, and cover the near-wall region below y + = 140, where y + is the wall-normal distance in wall units. Instantaneous visualizations of the flow reveal near-wall low-speed streaks with associated quasi-streamwise vortices, retrograde inverted arch vortices, hairpin vortices and hairpin packets. The hairpin heads are observed as close to the wall as y + = 30. Examples of hairpin packet evolution reveal the development of new hairpin vortices, which are created upstream and close to the wall in a manner consistent with the auto-generation model (Zhou et al., J. Fluid Mech., vol. 387, 1999, pp. 353-396). The development of the new hairpin appears to be initiated by an approaching sweep event, which perturbs the shear layer associated with the initial packet. The shear layer rolls up, thereby forming the new hairpin head. The head subsequently connects to existing streamwise vortices and develops into a hairpin. The time scale associated with the hairpin auto-generation is 20-30 wall units of time. This demonstrates that hairpins can be created over short distances within a developed turbulent boundary layer, implying that they are not simply remnants of the laminar-to-turbulent transition process far upstream.

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Tomographic-PIV measurement of the flow around a zigzag boundary layer trip

Cited by 50 publications

References 37 publications

Feather roughness reduces flow separation during low Reynolds number glides of swifts

Feather roughness reduces flow separation during low Reynolds number glides of swifts

On Reynolds number dependence of micro-ramp-induced transition

Experimental observation of hairpin auto-generation events in a turbulent boundary layer

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