Aerofoil wake-induced transition characteristics on a flat-plate boundary layer

Veerasamy, Dhamotharan; Atkin, Chris; Ponnusami, Sathiskumar A.

doi:10.1017/jfm.2021.452

Cited by 7 publications

(9 citation statements)

References 36 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The slight downwards shift of the profile at the most advanced chordwise positions is due to the flow finishing the transition process. These observations made in figures 18 and 19( fj, non-modal) implies the co-existence of both modal and non-modal instability mechanisms, confirming the observations in DNS (Hosseinverdi & Fasel 2019) and experimental investigations (Istvan & Yarusevych 2018;Verdoya et al 2021) on LSBs subjected to FST, along with the experimental results of Veerasamy et al (2021) for an attached boundary layer developing over a flat plate. In contrast, in configurations where Tu < 1 % (refer to figure 19(a-e), modal), wall-normal disturbance profiles do not agree with theoretical predictions and do not exhibit the same behaviour as for configurations with Tu > 1 %, with the maxima of the peaks being in the range y/δ 1 = 0.3-0.5, implying that there is no formation of streaks and that only a modal transition mechanism is present.…”

Section: Co-existence Of a Modal And A Non-modal Instabilitiessupporting

confidence: 82%

“…2021) on LSBs subjected to FST, along with the experimental results of Veerasamy et al. (2021) for an attached boundary layer developing over a flat plate. In contrast, in configurations where (refer to figure 19( a – e ), modal), wall-normal disturbance profiles do not agree with theoretical predictions and do not exhibit the same behaviour as for configurations with , with the maxima of the peaks being in the range , implying that there is no formation of streaks and that only a modal transition mechanism is present.…”

Section: Resultsmentioning

confidence: 83%

“…2005), and is larger than what is observed for pure modal transition (, Arnal & Julien 1978). The co-existence of modal and non-modal instabilities in attached boundary layers has been found to have similar effects on the maximum of the peak (Veerasamy, Atkin & Ponnusami 2021). Moreover, increasing decreases the rate at which the fluctuations diminish into the free stream.…”

Section: Resultsmentioning

confidence: 89%

“…The eigenfunction exhibits two distinct peaks at approximately , corresponding roughly to the inflection point and , which is indicative of a viscous modal instability (Veerasamy et al. 2021). Rist & Maucher (2002) showed that Tollmien–Schlichting waves were more likely to emerge in LSBs with small wall-normal distances, and a KH instability in LSBs with higher wall-normal distances.…”

Section: Resultsmentioning

confidence: 99%

“…The eigenfunction of the most amplified frequency predicted by LST is presented in figure 18(a), and shows acceptable agreement with the experiment for the filtered fluctuating streamwise velocity profile in the wall-normal direction for the most amplified frequency band. The eigenfunction exhibits two distinct peaks at approximately y/δ 1 = 1, corresponding roughly to the inflection point and y/δ 1 = 0.3, which is indicative of a viscous modal instability (Veerasamy et al 2021). Rist & Maucher (2002) showed that Tollmien-Schlichting waves were more likely to emerge in LSBs with small wall-normal distances, and a KH instability in LSBs with higher wall-normal distances.…”

Section: Co-existence Of a Modal And A Non-modal Instabilitiesmentioning

confidence: 97%

See 4 more Smart Citations

Disturbance growth in a laminar separation bubble subjected to free-stream turbulence

et al. 2023

View full text Add to dashboard Cite

Experiments were conducted to study the transition and flow development in a laminar separation bubble (LSB) formed on an aerofoil. The effects of a wide range of free-stream turbulence intensity ( $0.15\,\%< Tu<6.26\,\%$ ) and streamwise integral length scale ( $4.6\ {\rm mm}<\varLambda _{u}<17.2\ {\rm mm}$ ) are considered. The co-existence of modal instability due to the LSB and non-modal instability caused by streaks generated by free-stream turbulence is observed. The flow field is measured using hot-wire anemometry, which showed that the presence of streaks in the boundary layer modifies the mean-flow topology of the bubble. These changes in the mean flow field result in the modification of the convective disturbance growth, where an increase in turbulence intensity is found to dampen the growth of the modal instability. For a relatively fixed level of $Tu$ , the variation of $\varLambda _{u}$ has modest effects. However, a slight advancement of the nonlinear growth of disturbances and eventual breakdown with the decrease in $\varLambda _{u}$ is observed. The data show that the streamwise growth of the disturbance energy is exponential for the lowest levels of free-stream turbulence and gradually becomes algebraic as the level of free-stream turbulence increases. Once a critical turbulence intensity is reached, there is enough energy in the boundary layer to suppress the laminar separation bubble, resulting in the non-modal instability taking over the transition process. Linear stability analysis is conducted in the fore position of the LSB. It accurately models incipient disturbance growth, unstable frequencies and eigenfunctions for configurations subjected to turbulence intensity levels up to 3 %, showing that the mean-flow modification due to the non-modal instability dampens the modal instability.

show abstract

Section: Co-existence Of a Modal And A Non-modal Instabilitiessupporting

confidence: 82%

Section: Resultsmentioning

confidence: 83%

Section: Resultsmentioning

confidence: 89%

Section: Resultsmentioning

confidence: 99%

Section: Co-existence Of a Modal And A Non-modal Instabilitiesmentioning

confidence: 97%

See 3 more Smart Citations

Disturbance growth in a laminar separation bubble subjected to free-stream turbulence

et al. 2023

View full text Add to dashboard Cite

show abstract

Influence of roughness-generated streaks on laminar separation bubbles

Jaroslawski,

Forte,

Vermeersch

et al. 2024

Exp Fluids

View full text Add to dashboard Cite

This experimental investigation studies the impact of streaks on two-dimensional laminar separation bubbles forming over an aerofoil. Streaks are introduced into the boundary layer using cylindrical roughness elements, and the resulting mean and unsteady flow fields are measured using hot-wire anemometry. The observed streaks generated by roughness exhibit analogous behavior to those generated by freestream turbulence, significantly altering the mean-flow characteristics of the bubble, including reductions in its length, height, and the introduction of spanwise velocity gradients. These mean flow modifications have a damping effect on convective disturbance growth. The experiments suggest the coexistence of modal instability due to the laminar separation bubble and non-modal instability due to streaks. We perform linear stability analysis at the fore position of the laminar separation bubble and find that by incorporating a spanwise wavenumber into the calculations, the experimental findings are reasonably modeled, indicating an oblique modal instability. To investigate the combined effect of roughness and the presence of freestream turbulence, we increase the turbulence level from the baseline in the presence of a roughness forcing configuration. We find that increasing the turbulence intensity leads to an enhancement of non-modal instability, accompanied by distinctive chordwise disturbance growth compared to lower freestream turbulence intensity levels. Moreover, increasing the turbulence intensity level in the presence of roughness not only bypasses the modal instability observed without the addition of turbulence but also suppresses laminar separation.

show abstract

Statistical characterization of free-stream turbulence induced transition under variable Reynolds number, free-stream turbulence, and pressure gradient

et al. 2021

View full text Add to dashboard Cite

In this work, the free-stream turbulence (FST) induced transition of a flat plate boundary layer is studied using particle image velocimetry (PIV) under variable Reynolds number (Re), FST intensity, and adverse pressure gradient (APG). Overall, 10 different flow conditions were tested concerning the variation of these parameters. The streak spacing and the probability density function (PDF) of turbulent spot nucleation are computed for all cases. The streak spacing is shown to be constant in the transition region once scaled with the turbulent displacement and momentum thickness, with resulting values of around 3 and 5, respectively. Nucleation events are shown to occur near the position where the dimensionless streak spacing reaches such constant values. The streamwise position where most turbulent spots are formed is strongly influenced by the FST intensity level. Additionally, the PDF of spot nucleation becomes narrower with increase in the APG, while FST has the opposite effect. A common distribution of all the PDFs is provided as a function of a similarity variable accounting for the streak spacing, the shape factor of the boundary layer, and the FST intensity.

show abstract

Aerofoil wake-induced transition characteristics on a flat-plate boundary layer

Abstract: Abstract

Cited by 7 publications

References 36 publications

Disturbance growth in a laminar separation bubble subjected to free-stream turbulence

Disturbance growth in a laminar separation bubble subjected to free-stream turbulence

Influence of roughness-generated streaks on laminar separation bubbles

Statistical characterization of free-stream turbulence induced transition under variable Reynolds number, free-stream turbulence, and pressure gradient

Contact Info

Product

Resources

About