Dynamics of low-speed streak evolution and interaction in laminar boundary layer

Deng, Si-Chao; Pan, Chong; Wang, Jinjun

doi:10.1007/s10409-014-0075-0

Cited by 10 publications

(6 citation statements)

References 28 publications

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“…Kumar et al (2015) characterized the flow past an isolated, highaspect-ratio cylindrical roughness and demonstrated varicose breakdown of the steady streaks. There also exist studies that have investigated transition caused by an array of roughness elements; see Duriez et al (2009), Ye et al (2019) and Deng et al (2014) among others.…”

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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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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“…The trend observed here certainly gives credence to this idea; the streamwise increase in streak spacing demonstrates that streaks are aggregating and growing in scale. As we move farther downstream, we can expect the growth, liftoff, and decay of streaks to modify wall heat fluxes and contribute to a turbulent transition, behaviors which have been observed in similar flows [8,[21][22][23]. A plot of the mean streak width vs. streamwise location for each of the four tested scenarios can be seen in Fig.…”

Section: Results Of Analysismentioning

confidence: 76%

Investigating coherent streaks in wildfires via heated plates in crosswind

Miller

Finney

McAllister

et al. 2017

Fire Safety Journal

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“…no leading edge separation exists, and the zero-pressure-gradient condition is well maintained (Pan et al. 2008; He, Wang & Pan 2013; Deng, Pan & Wang 2014). The orientations of the Cartesian coordinate system defined herein correspond to the streamwise direction, the spanwise direction and the wall-normal direction of the flat plate, respectively.…”

Section: Experimental Set-up and Measurement Techniquementioning

confidence: 99%

Tomographic PIV investigation on near-wake structures of a hemisphere immersed in a laminar boundary layer

Tu,

Wang,

Gao

et al. 2023

J. Fluid Mech.

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The near wake of a hemisphere immersed in a laminar boundary layer is studied utilizing time-resolved tomographic particle image velocimetry (TPIV). Focus is placed on the three-dimensional vortical structures and the formation details of hairpin vortices before the onset of transition. The three-dimensional instantaneous pressure field of the hemisphere wake is reconstructed for better understanding the flow mechanism. Experiments are carried out with Reynolds number $Re_{r}=1370$ , based on the hemisphere radius $R$ . Features of periodicity of the near wake are analysed using proper orthogonal decomposition and Fourier transformation. The velocity fluctuation in the wall-normal direction is shown to be crucial to the formation of hairpin vortices in the near wake. By investigating the transport of mass and vorticity, and the correlation between pressure and hairpin vortex strength, the formation mechanism is revealed clearly. Specifically, the main hairpin vortices (MHVs) are formed within the reaction of outer high-speed flow and near-wall flow. The formation of the head portion is followed by the leg portion formation. The shedding of the MHVs is highly correlated with the pressure, as well as the pressure gradient in the wall-normal direction. For the side hairpin vortices (SHVs), the leg portion is formed first, followed by the generation of the head portion thanks to induction of the re-oriented standing vortices. The generation of the SHVs can be regarded as the downstream bridging of the standing vortices, similar to the generation of hairpin vortices due to the connection of streamwise vortices in turbulent boundary layers.

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Dynamics of low-speed streak evolution and interaction in laminar boundary layer

Cited by 10 publications

References 28 publications

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

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

Investigating coherent streaks in wildfires via heated plates in crosswind

Tomographic PIV investigation on near-wake structures of a hemisphere immersed in a laminar boundary layer

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