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Analysis of time-resolved PIV measurements of a confined co-flowing jet using POD and Koopman modes
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), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
2011 (English)Report (Other academic)
Abstract [en]

Modal analysis by proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) of experimental data from a fully turbulent flow is presented. The flow case is a turbulent confined jet with co-flow, with Reynolds number based on the jet thickness of Re=10700. Experiments are performed with time-resolved Particle Image Velocimetry (PIV). The jet is created in a square channel with the confinement ratio is 1:5. Statistics of the flow are presented in terms of mean and fluctuating fields. Analysis of spatial spectra and temporal spectra reveal the presence of dominant wavelengths and frequenciesembedded in broad-band turbulent spectrum. Frequencies in the shearlayer migrate from St ≈ 1 near the jet inlet to St < 0.1 at 18 jet thickness downstream.

This flow case provides an interesting and challenging benchmark for testing POD and DMD and discussing their efficacy in describing a fully turbulent case. At first, issues related to convergence and physical interpretation of the modes are discussed, then the results are analyzed and compared. POD analysis reveals the most energetic spatial structures that are related to the flapping of the jet; a low frequency peak (St = 0.02) is found when the associated temporalmode is analyzed. Higher order modes revealed the presence of fasteroscillating shear flow modes combined to a recirculation zone near the inner jet. The flapping of the inner jet is sustained by this region. A good agreement is found between DMD and POD; however, DMD is able to rank the modes by frequencies, isolating structures associated to harmonics of the flow.

Place, publisher, year, edition, pages
2011.
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-29753OAI: oai:DiVA.org:kth-29753DiVA: diva2:397420
Note

QC 20110214

Available from: 2011-02-14 Created: 2011-02-14 Last updated: 2017-08-17Bibliographically approved
In thesis
1. Feedback control and modal structures in transitional shear flows
Open this publication in new window or tab >>Feedback control and modal structures in transitional shear flows
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

 

Two types of shear flows are investigated in this thesis; numerical simulations are performed for the analysis and control of the perturbation arising in a boundary layer over a flat plate, whereas PIV measurements are analysed for the investigation of a confined turbulent jet. Modal structures of the flows are identified: the aim is to understand the flow phenomena and to identify reduced-order models for the feedback control design. The attenuation of three-dimensional wavepackets of streaks and Tollmien-Schlichting (TS) waves in the boundary layer is obtained using feedback control based on arrays of spatially localized sensors and actuators distributed near the rigid wall. In order to tackle the difficulties arising due to the dimension of the discretized Navier-Stokes operator, a reduced-order model is identified, preserving the dynamics between the inputs and the outputs; to this end, approximate balanced truncation is used. Thus, control theory tools can be easily handled using the low-order model. We demonstrate that the energy growth of both TS wavepackets and streak-packets is substantially and efficiently mitigated, using relatively few sensors and actuators. The robustness of the controller is investigated by varying the number of actuators and ensors, the Reynolds number and the pressure gradient. The configuration can be possibly reproduced in experiments, due to the localization of sensing and actuation devices. A complete analysis of a confined turbulent jet is carried out using timeresolved PIV measurements. Proper orthogonal decomposition (POD) modes and Koopman modes are computed and analysed for understanding the main features of the flow. The frequencies related to the dominating mechanisms are identified; the most energetic structures show temporal periodicity.

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. vii, 33 p.
Series
Trita-MEK, ISSN 0348-467X ; 2011:01
Keyword
flow control, flat-plate boundary layer, laminar-turbulent transition
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:kth:diva-29754 (URN)978-91-7415-863-2 (ISBN)
Presentation
2011-02-18, Sal E3, KTH, Osquars backe 14, Stockhkolm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish e‐Science Research Center
Note
QC 20110214Available from: 2011-02-14 Created: 2011-02-14 Last updated: 2012-05-24Bibliographically approved
2. Experimental Studies of Complex Flows through Image-Based Techniques
Open this publication in new window or tab >>Experimental Studies of Complex Flows through Image-Based Techniques
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with the development of experimental techniques for the study of complex flows inspired to a large extent by the papermaking process. In particular one part of this thesis is devoted to the development of laboratory experiments based on index-of-refraction matching and imaging techniques to study the behavior of dilute and concentrated suspension of elongated particles. Another part is aimed at exploring the potential of the synergy between experiments and numerical simulations to access quantities otherwise not-measurable in complex flows. Highspeedimaging experiments have been specifically designed for this purpose.

The first of the Refractive IndexMatching (RIM) experiment was aimed at studying the flow generated during the filtration of a fiber suspension using Particle Image Velocimetry (PIV) and pressure drop measurements. The experiments were performed in a vertical laboratory filtration device. Index of refraction matching of fibers and fluids allowed measurements to be performed in the proximity and, to some extent, in the forming network during filtration. The area over which the forming network induces velocity gradients has been measured and have been found to be independent of the Reynolds number but dependent on the fiber length and the structure of the network. Analysis of the flow scales in the proximity of the network showed that the signature of the mesh used to filter the suspension is never completely suppressed as the network thickness increases. Also, pressure drop measurements over a static fiber network have been performed. A linear dependence of the pressure drop with the basis weight (mass of fibers in the network per unit area) and a non-dimensional filtration resistance independent of filtration velocity and network thickness (if network compressibility is accounted for) was found. These findings can help explain characteristics that are observed on paper sheets and help improvede watering efficiency.

The second RIM experiment was aimed at measuring the interactions of Taylorscale elongated particles with turbulence. RIM particles with embedded tracers and Stereoscopic PIV were combined to simultaneously measure fluid phase and particle velocity. The novelty of this technique is that it allows to measure the three-dimensional angular velocity vector of arbitrarily shaped particles. This technique was applied to study the interaction of neutrally buoyant ellipsoidal particles with stationary homogeneous isotropic turbulence. The results were compared to the case of spherical particles. The main result is that both spherical and ellipsoidal particles provide enhancement of the small scales and reduction of the large scales at volume concentrations as low as 0.1%. However, the reduction of the large scales was much more evident for spherical particles. These results highlight the fact that particle elongation introduces different mechanisms of turbulent modulation as compared to the spherical particles.

The first of the high-speed imaging experiments was to provide a database for test and validation of a CFD-based flow observer for complex flows. For this purpose time resolved measurements of a turbulent confined jet have been performed with high-speed PIV. The measurements have been used both as a feedback signal and as a reference for the evaluation of a CFD-based estimator for complex flows. Furthermore, based on the measurements Kalman filters have been designed and implemented in the observer. The experimental data have also been used to compare two modal decompositions, namely Proper Orthogonal Decomposition and Dynamical Modal Decomposition and evaluate their ability to describe the global behavior of complex flow.

The second of the high-speed imaging experiment was applied to study spreading of a droplet on a solid surface. These experiments have been performed with extremely high time-resolution (140000 fps), over a range of parameters (in terms of droplet viscosity, equilibrium contact angle and droplet size) larger than any other experiment reported in the literature in a single work. By combining the experiments and direct numerical simulations a dissipative mechanisms arising from the contact line movement has been identified and the corresponding macroscopic coefficient has been measured.i

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. viii, 56 p.
Series
Trita-MEK, ISSN 0348-467X ; 2001:03
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-33821 (URN)978-91-7415-988-2 (ISBN)
Public defence
2011-05-19, Sal D1, Lindstedtsvägen 17, KTH, Stockholm, 10:15 (English)
Opponent
Supervisors
Note
QC 20110519Available from: 2011-05-19 Created: 2011-05-19 Last updated: 2011-06-10Bibliographically approved

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