Effects of free-stream turbulence intensity on transition in a laminar separation bubble formed over an airfoil

Istvan, Mark S.; Yarusevych, Serhiy

doi:10.1007/s00348-018-2511-6

Cited by 80 publications

(75 citation statements)

References 54 publications

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“…For the highest free-stream turbulence intensity case (2%), the transition mechanism is dominated by K-modes. A similar conclusion was drawn by Istvan and Yarusevych [31] from their experimental study on transition in a laminar separation bubble formed over the suction side of an airfoil under four freestream turbulence intensity levels (0.06%, 0.32%, 0.51%, 1.99%). They found that under low freestream turbulence intensity spanwise rolls associate with the K-H instability are the main contributors to the overall energy of velocity fluctuations.…”

Section: Discussionsupporting

confidence: 84%

“…Nevertheless they identified that the streamwise streaky structures formed upstream of separation also play an important role in the transition process. In the present study under 2.9% free-stream turbulence which is higher than those in the two previous studies [30,31], no single distinct frequency peak can be found from the PSD of streamwise velocity fluctuations at eight streamwise locations downstream from around the mean separation location as shown in Figure 12 below (spectra of v' and w' do not show any distinct peaks either).…”

Section: Instability and Transition In The Separated Shear Layercontrasting

confidence: 71%

“…They found that the transition was due to the interaction between the K-H instability and the streaky structures formed upstream of separation. In an experimental study by Istvan and Yarusevych [31] on the effects of free-stream turbulence on transition in a laminar separation bubble it was shown that for the highest free-stream turbulence intensity case (1.99%) in their study the inviscid shear layer instability (K-H instability) is still at work. Nevertheless they identified that the streamwise streaky structures formed upstream of separation also play an important role in the transition process.…”

Section: Instability and Transition In The Separated Shear Layermentioning

confidence: 99%

“…Bazel and Fasel [30] demonstrated that at elevated free-stream turbulence levels up to 2.5% streaky structures were observed in the laminar boundary layer upstream of the separation bubble and transition was due to both the primary inviscid shear-layer instability (K-H instability) and the streamwise streaks caused by the free-stream turbulence. Istvan and Yarusevych [31] performed experiments to investigate the effects of free-stream turbulence on transition in a laminar separation bubble formed over the suction side of a NACA 0018 airfoil at a Reynolds number of 80,000. Four cases under different free-stream turbulence intensity levels (0.06%, 0.32%, 0.51%, 1.99%) were considered and their results show that the shear layer rolls up into vortices which are shedded further downstream.…”

Section: Introductionmentioning

confidence: 99%

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Separated boundary layer transition under pressure gradient in the presence of free-stream turbulence

Li,

Yang

2019

Physics of Fluids

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Large-eddy simulation (LES) has been carried out to investigate the transition process of a separated boundary layer on a flat plate. A streamwise pressure distribution is imposed to mimic the suction surface of a low-pressure turbine (LPT) blade and the free-stream turbulence intensity at the plate leading edge is 2.9%. A dynamic sub-grid scale model is employed in the study and the current LES results compare well with available experimental data and previous LES results. The transition process has been thoroughly analysed and streamwise streaky structures, known as the Klebanoff Streaks have been observed much further upstream of the separation. However, transition occurs in the separated shear layer and is caused by two mechanisms: streamwise streaks and the inviscid K-H instability. Analysis suggests that streamwise streaks paly a dominant role in the transition process as those streaks severely disrupt and break up the K-H rolls once they are formed, leading to significant three-dimensional (3D) motions very rapidly. It is also demonstrated in the present study that the usual secondary instability stage under low free-stream turbulence intensity where coherent two-dimensional (2D) spanwise rolls get distorted gradually and eventually broken up into 3D structures has been bypassed.

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Section: Discussionsupporting

confidence: 84%

Section: Instability and Transition In The Separated Shear Layercontrasting

confidence: 71%

Section: Instability and Transition In The Separated Shear Layermentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Separated boundary layer transition under pressure gradient in the presence of free-stream turbulence

Li,

Yang

2019

Physics of Fluids

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“…It should be noted that instead of free-stream turbulence, the separation bubble and transition process both decide the influence mechanism. These two questions have captured researchers' attention for decades [34][35][36][37][38]. Hatman and Wang [35] distinguished the separation bubble as three modes, shown in Figure 12.…”

Section: Deformations Influence Of Ellipticalmentioning

confidence: 99%

The Analysis of Leading Edge Deformations on Turbomachine Blades

Liu

2019

Energies

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With recent advancements in the development of material and manufacturing technology, the leading edge geometry of turbomachine blades has attracted widespread attention. “Sharp” leading edges always have a better aerodynamic performance, though it is prone to deformations easily. Thus, flat plates and real compressor cascades with different leading edge deformations were investigated to study the influence, which is applicable for thin blades at low speeds. Different boundary layer characteristics, including the velocity profile, transition process, and loss, are compared. The results show that there are several kinds of contradictory influence mechanisms and that the final phenomenon is closely related to the condition of the original boundary layer. In low turbulence, with large and laminar separation, the deformations can suppress separation and decrease loss. In high turbulence, with short and transitional separation, deformations can promote the transition process and increase the loss. The sensitivities of different the original leading edge shapes are also compared. This indicates that a good design always has a better robustness at low turbulence values, while it is worse at high turbulence values. The cascade experiment and simulation show that the deformation influence is similar to flat plates and that it is enlarged near the hub, which affects the corner separation.

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Low Aspect Ratio Wing Under Large-Scale Turbulent Inflow Conditions at Low Reynolds Numbers

Herbst

Hain

Kähler

2019

Notes on Numerical Fluid Mechanics and Multidisciplinary Design

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Effects of free-stream turbulence intensity on transition in a laminar separation bubble formed over an airfoil

Cited by 80 publications

References 54 publications

Separated boundary layer transition under pressure gradient in the presence of free-stream turbulence

Separated boundary layer transition under pressure gradient in the presence of free-stream turbulence

The Analysis of Leading Edge Deformations on Turbomachine Blades

Low Aspect Ratio Wing Under Large-Scale Turbulent Inflow Conditions at Low Reynolds Numbers

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