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Transition delay in boundary-layer flows via reactive control
KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0003-0010-489X
2016 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Fördröjning av laminärt-turbulent omslag i gränsskiktströmning genom reaktiv kontroll (Swedish)
Abstract [en]

Transition delay in boundary-layer flows is achieved via reactive control of flow instabilities, i.e. Tollmien-Schlichting (TS) waves. Adaptive and model-based control techniques are investigated by means of direct numerical simulations (DNS) and experiments. The action of actuators localised in the wall region is prescribed based on localised measurement of the disturbance field; in particular, plasma actuators and surface hot-wire sensors are considered.

Performances and limitations of this control approach are evaluated both for two-dimensional (2D) and three-dimensional (3D) disturbance scenarios. The focus is on the robustness properties of the investigated control techniques; it is highlighted that static model-based control, such as the linear-quadratic- Gaussian (LQG) regulator, is very sensitive to model-inaccuracies. The reason for this behaviour is found in the feed-forward nature of the adopted sensor/actuator scheme; hence, a second, downstream sensor is introduced and actively used to recover robustness via an adaptive filtered-x least-mean-squares (fxLMS) algorithm.

Furthermore, the model of the flow required by the control algorithm is reduced to a time delay. This technique, called delayed-x least-mean-squares (dxLMS) algorithm, allows taking a step towards a self-tuning controller; by introducing a third sensor it is possible to compute on-line the suitable time-delay model with no previous knowledge of the controlled system. This self-tuning approach is successfully tested by in-flight experiments on a motor-glider.

Lastly, the transition delay capabilities of the investigated control con- figuration are confirmed in a complex disturbance environment. The flow is perturbed with random localised disturbances inside the boundary layer and the laminar-to-turbulence transition is delayed via a multi-input-multi-output (MIMO) version of the fxLMS algorithm. A positive theoretical net-energy- saving is observed for disturbance amplitudes up to 2% of the free-stream velocity at the actuation location, reaching values around 1000 times the input power for the lower disturbance amplitudes that have been investigated. 

Abstract [sv]

I den här avhandlingen har reglertekniska metoder tillämpats för att försena omslaget från ett laminärt till ett turbulent gränsskikt genom att dämpa tillväxten av små instabiliteter, så kallade Tollmien-Schlichting vågor. Adaptiva och modellbaserade metoder för reglering av strömning har undersökts med hjälp av numeriska beräkningar av Navier-Stokes ekvationer, vindtunnelexperiment och även genom direkt tillämpning på flygplan. Plasmaaktuatorer och varmtrådsgivare vidhäftade på ytan av plattan eller vingen har använts i experimenten och modellerats i beräkningarna.

Prestanda och begränsningar av den valda kontrollstrategin har utvärderats för både tvådimensionella och tredimensionella gränsskiktsinstabiliteter. Fokus har varit på metodernas robusthet, där vi visar att statiska metoder som linjär-kvadratiska regulatorer (LQG) är mycket känsliga för avvikelser från den nominella modellen. Detta beror främst på att regulatorer agerar i förkompenseringsläge (”feed-foward”) på grund av strömningens karaktär och placeringen av givare och aktuatorer. För att minska känsligheten mot avvikelser och därmed öka robustheten har en givare införts nedströms och en adaptiv fXLMS algoritm (filtered-x least-mean-squares) har tillämpats.                 

Vidare har modelleringen av fXLMS-algoritmen förenklats genom att ersätta överföringsfunktionen mellan aktuatorer och givare med en lämplig tidsfördröjning.  Denna  metod som kallas för dxLMS (delayed-x least-mean-squares) kräver att ytterligare en givare införs långt uppströms för att kunna uppskatta hastigheten på de propagerande instabilitetsvågorna. Denna teknik har tillämpats framgångsrikt för reglering av gränsskiktet på vingen av ett segelflygplan.

Slutligen har de reglertekniska metoderna testas för komplexa slumpmässiga tredimensionella störningar som genererats uppströms lokalt i gränsskiktet. Vi visar att en signifikant försening av laminärt-turbulentomslag äger rum med hjälp av en fXLMS algoritm. En analys av energibudgeten visar att för ideala aktuatorer och givare kan den sparade energiåtgången på grund av minskad väggfriktion vara upp till 1000 gånger större än den energi som använts för reglering.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 200 p.
Series
TRITA-MEK, ISSN 0348-467X ; 2016:10
Keyword [en]
flow control, drag reduction, net energy saving, adaptive control, model-based control, optimal control, flat-plate boundary layer, laminar-to- turbulent transition, plasma actuator, direct numerical simulation, in-flight experiments
Keyword [sv]
strömningsstyrning, friktionsreduktion, netto energibesparing, adaptiv styrning, modellbaserad styrning, optimal kontroll, gränsskikt öve en plan platta, laminärt till turbulent omslag, plasma aktuator, DNS, flyg prov
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-187173ISBN: 978-91-7729-030-8 (print)OAI: oai:DiVA.org:kth-187173DiVA: diva2:929110
Public defence
2016-06-13, Kollegiesalen, Brinellvägen 8, Stockholm, 10:30 (English)
Opponent
Supervisors
Funder
Swedish Research Council, 2012- 4246
Available from: 2016-05-17 Created: 2016-05-17 Last updated: 2016-05-17Bibliographically approved
List of papers
1. Adaptive and model-based control theory applied to convectively unstable flows
Open this publication in new window or tab >>Adaptive and model-based control theory applied to convectively unstable flows
2014 (English)In: Applied Mechanics Review, ISSN 0003-6900, E-ISSN 1088-8535, Vol. 66, no 6, 060801- p.Article, review/survey (Refereed) Published
Abstract [en]

Research on active control for the delay of laminar-turbulent transition in boundary layers has made a significant progress in the last two decades, but the employed strategies have been many and dispersed. Using one framework, we review model-based techniques, such as linear-quadratic regulators, and model-free adaptive methods, such as least-mean square filters. The former are supported by a elegant and powerful theoretical basis, whereas the latter may provide a more practical approach in the presence of complex disturbance envi- ronments, that are difficult to model. We compare the methods with a particu- lar focus on efficiency, practicability and robustness to uncertainties. Each step is exemplified on the one-dimensional linearized Kuramoto-Sivashinsky equa- tion, that shows many similarities with the initial linear stages of the transition process of the flow over a flat plate. Also, the source code for the examples are provided. 

Keyword
Flow control, Control theory, Optimal control, Adaptive control, Boundary-layer flow, Fluid dynamics
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-154050 (URN)10.1115/1.4027483 (DOI)000346238700001 ()2-s2.0-84902832513 (Scopus ID)
Funder
Swedish Research Council, 2012- 4246
Note

QC 20141015

MATLAB scrips available at www.mech.kth.se/~nicolo/ks

Available from: 2014-10-13 Created: 2014-10-13 Last updated: 2017-12-05Bibliographically approved
2. On the role of adaptivity for robust laminar flow control
Open this publication in new window or tab >>On the role of adaptivity for robust laminar flow control
Show others...
2015 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 767, R1-R12 p.Article in journal (Refereed) Published
Abstract [en]

In boundary layer flows, one may reduce skin friction drag by delaying the onset of laminar-to-turbulent transition via the attenuation of small-amplitude Tollmien Schlichting (TS) waves In this work, we use numerical simulations and experiments to compare the robustness of adaptive and model-based techniques for reducing the growth of two-dimensional TS disturbances. In numerical simulations, the optimal linear quadratic Gaussian (LQG) regulator shows the best performance under the conditions it was designed for However, it is found that the performance deteriorates linearly with the drift of the Reynolds number from its nominal value. As a result, an order-of-magnitude loss of performance is observed when applying the computation-based I.QG controller in wind-tunnel experiments In contrast, it is shown that the adaptive filtered-X least-mean-squares (FXLMS) algorithm is able to maintain an essentially constant performance for significant deviations of the nominal values of the disturbance amplitude and Reynolds number.

Keyword
boundary layer control, flow control, instability control
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-154351 (URN)10.1017/jfm.2015.45 (DOI)000349688900001 ()2-s2.0-84949117226 (Scopus ID)
Funder
Swedish Research Council Formas, VR-2012-4246, VR-2010-3910
Note

Updated from Manuscript to Article. QC 20150488

Available from: 2014-10-20 Created: 2014-10-20 Last updated: 2017-12-05Bibliographically approved
3. In-flight active-wave-cancelation via delayed-x-LMS control algorithm in a laminar boundary layer
Open this publication in new window or tab >>In-flight active-wave-cancelation via delayed-x-LMS control algorithm in a laminar boundary layer
Show others...
(English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114Article in journal (Refereed) Submitted
Abstract [en]

This manuscript demonstrates the first successful application of the delayed-x-LMS (dxLMS) control algorithm for TS-wave cancelation. Active wave cancelation of two-dimensional broad-band Tollmien-Schlichting (TS) disturbances is per- formed with a single DBD plasma actuator. The experiments are conducted in flight on the pressure side of a laminar flow wing glove, mounted on a manned glider. The stability properties of the controller are investigated in detail with

experimental flight data, DNS and stability analysis of the boundary layer. Finally, a model-free approach for dxLMS operation is introduced to operate the controller as a “black box” system, which automatically adjusts the controller settings based on a group speed measurement of the disturbance wave packets. The modified dxLMS control algorithm allows to operate the controller without a model and an adaption based on varying conditions that may occur during flight in atmosphere. 

Keyword
active flow control, active wave cancelation, fxLMS, dxLMS, in-flight measurements, DNS, DBD plasma actuator
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-187171 (URN)
Funder
Swedish Research Council, 2012-4246
Note

QC 20160523

Available from: 2016-05-17 Created: 2016-05-17 Last updated: 2017-11-30Bibliographically approved
4. Centralised Versus Decentralised Active Control of Boundary Layer Instabilities
Open this publication in new window or tab >>Centralised Versus Decentralised Active Control of Boundary Layer Instabilities
2014 (English)In: Flow Turbulence and Combustion, ISSN 1386-6184, E-ISSN 1573-1987, Vol. 93, no 4, 537-553 p.Article in journal (Refereed) Published
Abstract [en]

We use linear control theory to construct an output feedback controller for the attenuation of small-amplitude three-dimensional Tollmien-Schlichting (TS) wavepackets in a flat-plate boundary layer. A three-dimensional viscous, incompressible flow developing on a zero-pressure gradient boundary layer in a low Reynolds number environment is analyzed using direct numerical simulations. In this configuration, we distribute evenly in the spanwise direction up to 72 localised objects near the wall (18 disturbances sources, 18 actuators, 18 estimation sensors and 18 objective sensors). In a fully three-dimensional configuration, the interconnection between inputs and outputs becomes quickly unfeasible when the number of actuators and sensors increases in the spanwise direction. The objective of this work is to understand how an efficient controller may be designed by connecting only a subset of the actuators to sensors, thereby reducing the complexity of the controller, without comprising the efficiency. If n and m are the number of sensor-actuator pairs for the whole system and for a single control unit, respectively, then in a decentralised strategy, the number of interconnections deceases mn compared to a centralized strategy, which has n (2) interconnections. We find that using a semi-decentralized approach - where small control units consisting of 3 estimation sensors connected to 3 actuators are replicated 6 times along the spanwise direction - results only in a 11 % reduction of control performance. We explain how "wide" in the spanwise direction a control unit should be for a satisfactory control performance. Moreover, the control unit should be designed to account for the perturbations that are coming from the lateral sides (crosstalk) of the estimation sensors. We have also found that the influence of crosstalk is not as essential as the spreading effect.

Keyword
Flow control, Boundary layer flows, Hydrodynamic stability, Control theory, Model reduction
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-145661 (URN)10.1007/s10494-014-9552-6 (DOI)000345076500001 ()2-s2.0-84920259408 (Scopus ID)
Note

QC 20141215 Updated from manuscript to article in journal.

Available from: 2014-05-26 Created: 2014-05-26 Last updated: 2017-12-05Bibliographically approved
5. Energy efficiency and performance limitations of linear adaptive control for transition delay
Open this publication in new window or tab >>Energy efficiency and performance limitations of linear adaptive control for transition delay
(English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) Submitted
Abstract [en]

Reactive-control techniques have been longly investigated to control local flow instabilities in boundary-layer flows – Tollmien-Schlichting waves – that would eventually cause laminar-to-turbulence transition. Several studies have been published about the control of two-dimensional (2D) disturbances supposing a transition delay. In this study, a three-dimensional (3D) disturbance environment is considered in a 2D zero-pressure-gradient boundary-layer flow. A control- law based on a multi-input-multi-output (MIMO) filtered-x least-mean-squares (fxLMS) adaptive algorithm is introduced and its performances are analysed for increasing disturbance amplitude. Transition delay is achieved by the investigated control set-up; moreover, an energy budget is conducted in order to asses the net energy saving capabilities of the investigated control approach. Ideal as well as real actuators models are considered, focusing in particular on dielectric-barrier-discharge (DBD) plasma actuators. To our knowledge, this is the first time that drag-reduction and energy-saving capabilities are studied for reactive transition-delay techniques. 

Keyword
fxLMS, plasma actuator, drag reduction
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-187172 (URN)
Funder
Swedish Research Council, 2012-4246
Note

QC 20160518

Available from: 2016-05-17 Created: 2016-05-17 Last updated: 2017-11-30Bibliographically approved

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