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Turbulence modulation and rotational dynamics of large nearly neutrally buoyant particles in homogeneous isotropic turbulence
KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
200? (English)Report (Other academic)
Abstract [en]

This paper is an experimental investigation of turbulence modulation effects by Taylor-scale size particles in the dilute regime. Experiments are performed on a turbulence tank able to provide Homogeneous Isotropic Turbulence at Reλ ≈ 270. A novel experimental technique capable of simultaneously measuring rotational rates of arbitrarily shaped particles and fluid velocity using standard Stereoscopic Particle Image Velocimetry (Stereo-PIV) and Index-of-Refracion matching is presented here. Particles of the same IoR of water with embedded tracers allowed the measurement of the velocity of the portion of particles in the measurement plane. A novel algorithm based on the assumption of solid body rotation, is then used to extract particle rotation rates. We compare the results from two particle shapes to the single phase measurements: spherical and ellipsoidal particles with aspect ratio 2. It is found that spherical particles provide a 15% turbulence reduction, about five times more than what is provided by ellipsoidal particles at the same volume fraction (φv ≈ 0.1%), and with less particle surface area available. These result suggest that there might be an turbulence production mechanism for ellipsoidal particles that is not present for spheres. This hypothesis is supported by spectral analysis. Pivoting effect is observed for both spherical and ellipsoidal particles, but for the latter, the reduction in the small wavenumber region is less evident. Preliminary results of statistics of rotational rates shows that ellipsoidal particles tend to have an enhanced rotational velocity as compared to spheres.

Place, publisher, year, edition, pages
200?. , 24 p.
National Category
Applied Mechanics
URN: urn:nbn:se:kth:diva-34547OAI: diva2:421973
QC 20110610Available from: 2011-06-10 Created: 2011-06-10 Last updated: 2011-06-10Bibliographically approved
In thesis
1. 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.
Trita-MEK, ISSN 0348-467X ; 2001:03
National Category
Mechanical Engineering
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)
QC 20110519Available from: 2011-05-19 Created: 2011-05-19 Last updated: 2011-06-10Bibliographically approved

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