Distributed Roughness Effects on Transitional and Turbulent Boundary Layers

Vadlamani, Nagabhushana Rao; Tucker, Paul G.; Durbin, Paul A.

doi:10.1007/s10494-017-9864-4

Cited by 61 publications

(20 citation statements)

References 47 publications

(90 reference statements)

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“…Compared with a smooth surface, a significant upward shift of the logarithmic region in the surfaces of sample B and sample G is observed over two bulk Reynold numbers Re b . This is in agreement with the experimental result of Toussaint et al , The offset distance is denoted by −Δ U + . The detailed surface properties are summarized in Table ; when −Δ U + is negative, the velocity distribution shifts downward, and the drag is greater than the corresponding smooth wall.…”

Section: Resultssupporting

confidence: 92%

Experimental Investigations of the Turbulent Boundary Layer for Biomimetic Protrusive Surfaces Inspired by Pufferfish Skin: Effects of Spinal Density and Diameter

et al. 2021

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Pufferfish is known for its extension of tiny spinecovered skin that appears to increase skin drag and may act as turbulisors, reducing overall drag while serving a protective function. Therefore, the present study addresses a neglected aspect of how spines affect the turbulent boundary layer (TBL) for drag reduction in the pufferfish skin. Particle image velocimetry (PIV) was utilized to investigate the TBL structure on the biomimetic spine-covered protrusion samples inspired by the back skin of the pufferfish. The comparison samples of two sparse "ktype" arrangements (hexagon and staggered) for three types of rough element sizes with roughness heights k + = 5.5−6.5 (nearly hydraulically smooth) and smooth case in bulk Reynolds numbers (Re b = 37,129 and 44,554) were tested. The results of turbulence statistics of these samples indicate that both the sample (type hexagon) for large rough density (λ = 0.0215) with small roughness elements and the sample (type staggered) for small rough density (λ = 0.0148) with large roughness elements have a drag reduction rate of 5−11%. These two kinds of bionic surfaces have a similar morphology to that seen in the distribution of pufferfish spines and probably serve a similar hydrodynamic function. Vortex identification shows that the spines in the front section for large density with small rough elements stabilize the TBL and generate many small-scale vortices and the dense spines with large rough elements at the back section have the effect of separating the vortices. The retrograde vortex generated by them is beneficial to increasing the driving force of the pufferfish. In addition, these two rough surfaces may effectively delay the separation of the TBL. These results will provide a preliminary research foundation for the development of a more practical prototype of the bionic drag-reducing surfaces and strengthen the theoretical investigation concerning drag reduction exploration.

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Section: Resultssupporting

confidence: 92%

Experimental Investigations of the Turbulent Boundary Layer for Biomimetic Protrusive Surfaces Inspired by Pufferfish Skin: Effects of Spinal Density and Diameter

et al. 2021

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“…The effect of introducing a wide range of length scales and broadband disturbances inside a boundary layer is not sufficiently well studied and consequently, the mechanism of transition caused by a distributed roughness is not yet clearly understood (Durbin (2017)). Previous studies have used numerical simulations (Drews et al (2011), von Deyn et al (2020), Vadlamani et al (2018) and hotwire measurements (Kuester and White (2015), Anand and Diwan (2020)) to characterize some aspects of the transition process associated with different types of distributed roughness. Drews et al (2011), using direct numerical simulations, demonstrated the presence of streamwise vorticity and spanwise non-uniformity for the flow downstream of a localized patch (in spanwise direction) of random distributed roughness.…”

Section: Introductionmentioning

confidence: 99%

“…von Deyn et al (2020) and Vadlamani et al (2018) showed that the distributed roughness causes steady streaks, which develop secondary instability and become unsteady further downstream, ultimately breaking down into a turbulent flow. Note that von Deyn et al ( 2020) modelled the distributed roughness as a collection of hemispherical roughness elements (of the same height) ran-domly distributed throughout the computational domain, whereas Vadlamani et al (2018) used sinusoidal roughness elements of the same height distributed in a regular pattern throughout the transition region. Kuester and White (2015) carried out experiments on a localised patch of random distributed roughness (fabricated using rapid prototyping) and reported transient growth of disturbances downstream of it.…”

Section: Introductionmentioning

confidence: 99%

Characterization of streak development for boundary layer transition caused by isolated and distributed roughness

Joseph

Kumar

Diwan

2022

Preprint

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In this work, we characterize the early stages of transition in a flat-plate boundary layer caused by two types of surface roughness: distributed roughness (a strip of 24-grit emery) and isolated roughness (a cylindrical element). Towards this, we carry out extensive particle image velocimetry (PIV) measurements. The distributed roughness consists of a range of grit heights and introduces a broad range of disturbances downstream of it. In the pre-transitional region, both isolated and distributed roughness cause steady streaks, which for the latter case show a lack of spanwise symmetry. This is in contrast to the freestream turbulence (FST), which introduces unsteady streaks in the pre-transitional boundary layer. We propose tilting of spanwise vortices downstream of the distributed roughness to be the mechanism for generation of streamwise vortices, which are likely precursors to the onset of transition. For both the roughness configurations, the steady streaks develop instability resulting in localized streak breakdowns. The wall-normal PIV visualizations show streak instability features qualitatively similar to those for the FST-induced transition. For the distributed roughness, both the outer instability of lifted-up streaks and the inner instability due to streak interaction are present, whereas for the isolated roughness, the inner instability is dominant while the outer instability is much weaker. Conditional sampling of fluctuating velocity shows an asymmetry in the positive and negative fluctuations after the onset of transition, in a manner similar to that observed for the FST-induced transition. These observations suggest that the wall-normal distribution of unsteady velocity fluctuations in the early stages of transition is qualitatively similar irrespective of the source of disturbance (roughness/FST) causing transition. We expect that this commonality of features among different types of transition will be helpful in modelling flow over aerodynamic surfaces such as turbine blades and aircraft wings.

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“…The most unstable mode is associated with the sinuous or varicose type of deformations of low-speed velocity streak, which grows approximately 30 times faster than the scenario in the absence of roughness. The importance of the separated shear layer from the roughness element is also reported in Vadlamani, Tucker & Durbin (2018), where the effects of distributed roughness in subsonic transitional boundary-layer flow are studied. They reported that the sinuous secondary instability of the low-speed streak destabilises the detached shear layer above the streaks, promoting transition to turbulence.…”

Section: Roughness-induced Instabilitymentioning

confidence: 93%

Transition induced by wall-normal vibration in flow around a flat plate with roughness

et al. 2022

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The effects of wall-normal vibration on bypass transition in a boundary-layer flow over a flat plate with roughness elements in the form of circular cylinders are investigated using direct numerical simulations (DNS), linear Floquet analyses and dynamic mode decompositions (DMD). The vibration of the plate strengthens the streamwise vorticity, consequently enhancing the velocity streaks and reducing the critical Reynolds number for transition. A map is constructed to identify the coupling effect of the vibration amplitude and Reynolds number on transition. Among all investigated combinations of height and diameter of roughness, the critical Reynolds numbers at different vibration amplitude $A$ can be unified by a scaling function of $(1-10A)$ . Two instability modes are identified in the vibration-induced transition process: a wake mode immediately downstream of the roughness, related to the inviscid Kelvin–Helmholtz instability of the wall-normal shear in the wake; and a streak mode, in response to the spanwise shear of the streaky flow, occurring further downstream. The first one results in the generation of central hairpin vortices which then feed the second one. Further development of the streak instability leads to two arrays of hairpin vortices. Results from linear Floquet analyses and DMD further confirm the two modes of instability observed in DNS. A quantitative study suggests that the amplification of vibration-induced disturbance by the base shear dominates the production of streamwise vorticity and subsequently the hairpin vortices.

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Distributed Roughness Effects on Transitional and Turbulent Boundary Layers

Cited by 61 publications

References 47 publications

Experimental Investigations of the Turbulent Boundary Layer for Biomimetic Protrusive Surfaces Inspired by Pufferfish Skin: Effects of Spinal Density and Diameter

Experimental Investigations of the Turbulent Boundary Layer for Biomimetic Protrusive Surfaces Inspired by Pufferfish Skin: Effects of Spinal Density and Diameter

Characterization of streak development for boundary layer transition caused by isolated and distributed roughness

Transition induced by wall-normal vibration in flow around a flat plate with roughness

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