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On the relevance of Reynolds stresses in resolvent analyses of turbulent wall-bounded flows
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
Univ Paris Saclay, LIMSI, UPR 3251 CNRS, F-91400 Orsay, France..
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-7864-3071
Cent Nantes, LHEEA, UMR 6598, CNRS, F-44300 Nantes, France..
2019 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 867, p. 969-984, article id PII S0022112019001964Article in journal (Refereed) Published
Abstract [en]

The ability of linear stochastic response analysis to estimate coherent motions is investigated in turbulent channel flow at the friction Reynolds number Re-r = 1007. The analysis is performed for spatial scales characteristic of buffer-layer and large-scale motions by separating the contributions of different temporal frequencies. Good agreement between the measured spatio-temporal power spectral densities and those estimated by means of the resolvent is found when the effect of turbulent Reynolds stresses, modelled with an eddy-viscosity associate with the turbulent mean flow, is included in the resolvent operator. The agreement is further improved when the flat forcing power spectrum (white noise) is replaced with a power spectrum matching the measures. Such a good agreement is not observed when the eddy-viscosity terms are not included in the resolvent operator. In this case, the estimation based on the resolvent is unable to select the right peak frequency and wall-normal location of buffer-layer motions. Similar results are found when comparing truncated expansions of measured streamwise velocity power spectral densities based on a spectral proper orthogonal decomposition to those obtained with optimal resolvent modes.

Place, publisher, year, edition, pages
CAMBRIDGE UNIV PRESS , 2019. Vol. 867, p. 969-984, article id PII S0022112019001964
Keywords [en]
turbulent boundary layers
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-249766DOI: 10.1017/jfm.2019.196ISI: 000463073000001Scopus ID: 2-s2.0-85063881975OAI: oai:DiVA.org:kth-249766DiVA, id: diva2:1307660
Note

QC 20190429

Available from: 2019-04-29 Created: 2019-04-29 Last updated: 2020-05-08Bibliographically approved
In thesis
1. Modelling and control of turbulent and transitional flows
Open this publication in new window or tab >>Modelling and control of turbulent and transitional flows
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The dynamics of fluid motion can accurately be described by the Navier– Stokes equations. Manipulating these equations to reduce their complexity but preserving their main characteristics has always been a key research activity in the field of fluid mechanics. Effort has been made to provide high-fidelity models for wall-bounded turbulent flows or reduced-order models for applications such as drag reduction, lift enhancement, or noise suppression. Model order reduction has also been employed for studying the dynamics of the Navier-Stokes equations. In this PhD thesis, the emphasis is on providing computationally inexpensive methods for industrial applications.

Numerical simulations are performed to tackle model order reduction for flow control of transitional boundary-layer flows for drag reduction. It is assumed that localized wall sensors and actuators are available, and that only the time signals accessible in experiments are provided. Thus, a method to include unknown high-dimensional input disturbances in a reduced-order model of a transitional boundary-layer flow is presented. The method is applied for the design of an optimal controller for drag reduction through delay of transition. Moreover, the role of the actuator is discussed and a comparison between realistic actuators and actuators computed using optimization methods is presented. Here, the emphasis is on the effectiveness of the actuators for the studied flow control cases.

Numerical simulations are also performed to tackle high-fidelity modeling in wall-bounded turbulent flows. The accuracy of the resolvent analysis in predicting the most energetic flow structures in a wall-bounded turbulent flow is quantified for different temporal frequencies. A direct comparison between the predictions from the resolvent analysis and the flow structures identified in DNS data is presented. Moreover, the beneficial effects attained with the inclusion of the Reynolds-stresses via an eddy-viscosity model are clarified for flows with friction Reynolds number up to 1007.

Abstract [sv]

Dynamiken av fluiders rörelse kan väl beskrivas med hjälp av Navier-Stokes ekva- tioner. Att manipulera dessa ekvationer för att minska deras komplexitet utan att förlora väsentliga egenskaper har alltid varit ett viktigt forskningsomr ̊ade inom strömningsmekanik. Mycket forskning har utförts för att utveckla högkvalitativa modeller t.ex. för beskrivning av väggbundna turbulenta flöden samt styrlagar baserad på lågordnings modeller för applikationer som motståndsminskning, förhöjning av lyftkraft eller bullerreducering. Modellreduktion har också använts för att studera strömningsdynamiken som beskrivs av Navier-Stokes ekvationer.

Tonvikten av arbetet presenterat i denna doktorsavhandling ligger på utveckling av beräkningsmässigt snabba och effektiva metoder för industriella tillämpningar. Vi har utfört numeriska simuleringar för att framta reducerade modeller för strömningsstyrning i syfte att minska motst ̊andet i gränsskiktsflöde genom att fördröja laminär-turbulent omslag. I dessa beräkningar har vi antagit att lokaliserad väggmätning och styrning är möjlig och att endast inputdata i form av tidssignaler fr ̊an dessa mättningar är tillgängliga. Vi har tagit fram en metod för att ta hänsyn till det högdimensionella bruset i mätdata i utvecklingen av lågordnings modeller för gränsskiktsflöden under inverkan av hög friströmsturbulens. Metoden har använts för att utforma en optimal styrlag för motst ̊andsminskning i gränsskiktsflöden. Dessutom har vi undersökt aktuatorns roll och jämfört realistiska aktuatorer med de beräknade med hjälp av optimeringsmetoder. Jämförelsen har gjorts med betoning på aktuators effektivitet i de studerade fallen.

Vi har också utfört numeriska simuleringar av väggbundna turbulenta flöden för att utveckla modeller med hög fidelitet av dessa flöden. Genom resultaten av dessa simuleringar har vi undersökt noggrannheten av den så kallade “resolvent” analysen för prediktering av de mest energiska strukturerna i väggbundna turbulenta flöden. En direkt jämförelse mellan strukturer identifierade i simuleringsdata och de predikterade genom resolvent analysen har presenterats. Dessutom har vi visat fördelarna med användning av en “eddy-viscosity” modell för Reynolds-spänningar i resolvent analysen för strömningsfall med friktion Reynoldstal upp till 1007.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. p. 56
Series
TRITA-SCI-FOU ; 2020:08
Keywords
reduced order modeling, flow control, resolvent analysis, transi- tional boundary layer, turbulent boundary layer, drag reduction, high-fidelity wall-turbulence modelling, modellreduktion, strömningskontroll, resolvent analys, laminär- turbulent omslag, turbulent gränsskikt, motståndsminskning, väggturbulens modellering
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-273165 (URN)978-91-7873-489-4 (ISBN)
Public defence
2020-06-04, Live-streaming: https://kth-se.zoom.us/j/62627066334 If you lack computer or computer skills, contact Luca Brandt, brandtl@kth.se, Stockholm, 10:00 (English)
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Supervisors
Note

QC 20200512

Available from: 2020-05-12 Created: 2020-05-08 Last updated: 2020-05-26Bibliographically approved

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