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Control of a separating boundary layer with travelling waves on the wall
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0002-4346-4732
2011 (English)Report (Other academic)
Abstract [en]

We perform numerical simulations of control of a separating laminar boundary layer by means of blowing and suction at the wall in the form of traveling waves. Separation is imposed by prescribing accelerating and decelerating free-stream velocity for the flow over a flat plate. We find that downstream traveling waves already at very low amplitudes are able to eliminate the separation and induce a turbulent but attached boundary layer flow. Upstream traveling waves of relatively higher amplitudes only slightly reduce separation while keeping the flow laminar. The amplitude of the blowing/suction needed to achieve such significant effects are considerably smaller than those previously considered for drag reduction and transition delay in plane geometries.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , 14 p.
National Category
Other Materials Engineering
URN: urn:nbn:se:kth:diva-33797OAI: diva2:417770
QC 20110518Available from: 2011-05-18 Created: 2011-05-18 Last updated: 2011-05-18Bibliographically approved
In thesis
1. Optimisation and control of shear flows
Open this publication in new window or tab >>Optimisation and control of shear flows
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Transition to turbulence and flow control are studied by means of numerical simulations for different simple shear flows. Linear and non-linear optimisation methods using the Lagrange multiplier technique are employed.

In the linear framework as objective function the standard disturbance kinetic energy is chosen and the constraints involve the linearised Navier–Stokes equations. We consider both the optimal initial condition leading to the largest disturbance energy growth at finite times and the optimal time-periodic forcing leading to the largest asymptotic response for the case of the flat plate boundary layer excluding the leading edge. The optimal disturbances for spanwise wavelengths of the order of the boundary layer thickness are streamwise vortices exploiting the lift-up mechanism to create streaks. For long spanwise wavelengths it is the Orr mechanism combined with the amplification of oblique wave packets that is responsible for the disturbance growth. Also linear optimal disturbances are computed around a leading edge and the effect of the geometry is considered. It is found that two-dimentional disturbances originating upstream, relative to the leading edge of the plate are inefficient at generating a viable disturbance, while three dimentional disturbances are more amplified.

In the non-linear framework a new approach using ideas from non-equilibrium thermodynamics is developed. We determine the initial condition on the laminar/turbulent boundary closest to the laminar state. Starting from the general evolution criterion of non-equilibrium systems we propose a method to optimise the route to the statistically steady turbulent state, i.e. the state characterised by the largest entropy production. This is the first time information from the fully turbulent state is included in the optimisation procedure. The method is applied to plane Couette flow. We show that the optimal initial condition is localised in space for realistic flow domains, while the disturbance visits bent streaks before breakdown.

Feedback control is applied to the bypass-transition scenario with high levels of free-stream turbulence. The flow is the flat-plate boundary layer. In this scenario low frequency perturbations enter the boundary layer and streamwise elongated disturbances emerge due to non-modal growth. The so-called streaky structures are growing in amplitude until they reach high enough energy levels and break down into turbulent spots via their secondary instability. When control is applied in the form of wall blowing and suction, the growth of the streaks is delayed, which implies a delay of the whole transition process. Additionally, a comparison with experimental work is performed demonstrating a remarkable agreement in the disturbance attenuation once the differences between the numerical and experimental setup are reduced.

Open-loop control with wall travelling waves by means of blowing and suction is applied to a separating boundary layer. For downstream travelling waves we obtain a mitigation of the separation of the boundary layer while for upstream travelling waves a significant delay in the transition location accompanied by a modest reduction of the separated region.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. ix, 37 p.
Trita-MEK, ISSN 0348-467X ; 2011:04
shear flows, flow control, optimal disturbances, Lagrange method, transition to turbulence, non-linear dynamics
National Category
Mechanical Engineering
urn:nbn:se:kth:diva-33771 (URN)978-91-7415-987-5 (ISBN)
Public defence
2011-05-27, D3, Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Swedish Research CouncilSwedish e‐Science Research Center
QC 20110518Available from: 2011-05-18 Created: 2011-05-17 Last updated: 2012-05-24Bibliographically approved

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