Stabilisation of subcritical bypass transition in the leading-edge boundary layer by suction

John, Michael O.; Obrist, Dominik; Kleiser, Leonhard

doi:10.1080/14685248.2014.933226

Cited by 10 publications

(7 citation statements)

References 22 publications

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“…The streamwise baseflow velocity component w B (x, y) increases by approximately 0.1W ∞ near the wall where the amplified modes have significant amplitude, leading to increased phase speeds c r of all modes. Furthermore, the growth rates c i of most of the discrete modes decrease with increasing suction (John et al 2014b), analogously to the linear primary stability of the SHBL (Hall et al 1984). …”

Section: Stabilization By Suctionmentioning

confidence: 91%

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Secondary instability and subcritical transition of the leading-edge boundary layer

2016

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The leading-edge boundary layer (LEBL) in the front part of swept airplane wings is prone to three-dimensional subcritical instability, which may lead to bypass transition. The resulting increase of airplane drag and fuel consumption implies a negative environmental impact. In the present paper, we present a temporal biglobal secondary stability analysis (SSA) and direct numerical simulations (DNS) of this flow to investigate a subcritical transition mechanism. The LEBL is modelled by the swept Hiemenz boundary layer (SHBL), with and without wall suction. We introduce a pair of steady, counter-rotating, streamwise vortices next to the attachment line as a generic primary disturbance. This generates a high-speed streak, which evolves slowly in the streamwise direction. The SSA predicts that this flow is unstable to secondary, time-dependent perturbations. We report the upper branch of the secondary neutral curve and describe numerous eigenmodes located inside the shear layers surrounding the primary high-speed streak and the vortices. We find secondary flow instability at Reynolds numbers as low as Re ≈ 175, i.e. far below the linear critical Reynolds number Re crit ≈ 583 of the SHBL. This secondary modal instability is confirmed by our three-dimensional DNS. Furthermore, these simulations show that the modes may grow until nonlinear processes lead to breakdown to turbulent flow for Reynolds numbers above Re tr ≈ 250. The three-dimensional mode shapes, growth rates, and the frequency dependence of the secondary eigenmodes found by SSA and the DNS results are in close agreement with each other. The transition Reynolds number Re tr ≈ 250 at zero suction and its increase with wall suction closely coincide with experimental and numerical results from the literature. We conclude that the secondary instability and the transition scenario presented in this paper may serve as a possible explanation for the well-known subcritical transition observed in the leading-edge boundary layer.

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Section: Stabilization By Suctionmentioning

confidence: 91%

“…The implementation of the disturbances and the boundary conditions follow Obrist et al (2012) and John et al (2014b). The computational domain has inflow planes at z = 0 and y = L y and one (optionally permeable) wall at y = 0.…”

Section: Numerical Simulation Methodsmentioning

confidence: 99%

Secondary instability and subcritical transition of the leading-edge boundary layer

2016

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“…In addition, the transformation presented herein from one classical base flow to another might allow extension of the nonlinear stability results known from the ASBL to the SHBL. An example of such concepts is the secondary streak-instability ) of the SHBL, which is stabilized by increasing suction (John, Obrist & Kleiser 2014).…”

Section: Discussionmentioning

confidence: 98%

A class of exact Navier–Stokes solutions for homogeneous flat-plate boundary layers and their linear stability

2014

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We introduce a new boundary layer formalism on the basis of which a class of exact solutions to the Navier-Stokes equations is derived. These solutions describe laminar boundary layer flows past a flat plate under the assumption of one homogeneous direction, such as the classical swept Hiemenz boundary layer (SHBL), the asymptotic suction boundary layer (ASBL) and the oblique impingement boundary layer. The linear stability of these new solutions is investigated, uncovering new results for the SHBL and the ASBL. Previously, each of these flows had been described with its own formalism and coordinate system, such that the solutions could not be transformed into each other. Using a new compound formalism, we are able to show that the ASBL is the physical limit of the SHBL with wall suction when the chordwise velocity component vanishes while the homogeneous sweep velocity is maintained. A corresponding non-dimensionalization is proposed, which allows conversion of the new Reynolds number definition to the classical ones. Linear stability analysis for the new class of solutions reveals a compound neutral surface which contains the classical neutral curves of the SHBL and the ASBL. It is shown that the linearly most unstable Görtler-Hämmerlin modes of the SHBL smoothly transform into Tollmien-Schlichting modes as the chordwise velocity vanishes. These results are useful for transition prediction of the attachment-line instability, especially concerning the use of suction to stabilize boundary layers of swept-wing aircraft.

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“…The equations (2.1a,b) are solved using a sixth-order finite-difference scheme in space on a staggered Cartesian grid, and a third-order explicit low-storage Runge-Kutta scheme in time (Henniger et al 2010b;Zolfaghari & Obrist 2021). The solver has been exhaustively validated and used to study various transitional flows (Henniger, Kleiser & Meiburg 2010a;Obrist, Henniger & Kleiser 2012;John, Obrist & Kleiser 2014. For integrating complex surfaces (e.g.…”

Section: Direct Numerical Simulationmentioning

confidence: 99%

Sensitivity and downstream influence of the impinging leading-edge vortex instability in a bileaflet mechanical heart valve

et al. 2022

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Bileaflet mechanical heart valves (BMHV) create unphysiological turbulent flow. Such turbulent flow involves multiple instability mechanisms interacting with one another in a confined geometry. For instance, an impinging leading-edge vortex (ILEV) instability creates disturbances at the leading edge of the valve leaflets, while potentially promoting turbulence downstream of the BMHV (Zolfaghari and Obrist, Phys. Rev. Fluids, vol. 4, 2019). In this article, we use adjoint-based methods to study the structural sensitivity of the ILEV instability in the BMHV, and to quantify the role of this instability in the maximum disturbance energy growth in the wake of the BMHV. We first present a direct numerical simulation to show the effect of the ILEV instability on the turbulent flow in the wake of the valve. Second, we perform a modal linear stability analysis on a two-dimensional subdomain attached to the leading edge of one leaflet. We investigate the sensitivity of the global modes associated with this flow using their adjoints, and then show a passive control scenario using a local feedback source. This results in a partial improvement in the flow oscillations downstream of the leaflets. We finally present a non-modal approach to identify the optimal initial conditions for achieving maximum energy growth at arbitrary locations. We show that, for sufficiently large times, the optimal initial condition for highest energy growth in the wake points at the leading edge, which includes the ILEV instability. Our study illustrates that an improved leading-edge shape can effectively reduce turbulence in the wake of the BMHV.

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Stabilisation of subcritical bypass transition in the leading-edge boundary layer by suction

Cited by 10 publications

References 22 publications

Secondary instability and subcritical transition of the leading-edge boundary layer

Secondary instability and subcritical transition of the leading-edge boundary layer

A class of exact Navier–Stokes solutions for homogeneous flat-plate boundary layers and their linear stability

Sensitivity and downstream influence of the impinging leading-edge vortex instability in a bileaflet mechanical heart valve

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