Browsing by Author "Mac Intyre, Jonatan"

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  • Influence of aspect ratio on vortex formation in X-junctions: Direct numerical simulations and eigenmode decomposition
    (2020-12) Correa, P.; Gomba, J.; Mac Intyre, Jonatan; Ubal, S.; Perazzo, C.A.; Hulin, J.P.; Auradou, H.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We study numerically the appearance and number of axial vortices in the outlets of X-shaped junctions of two perpendicular channels of rectangular sections with facing inlets. We explore the effect of the aspect ratio of the cross section, AR, on the number of vortices created at the center of the junction. Direct numerical simulations (DNSs) performed for different values of the Reynolds number Re and AR demonstrate that vortices with their axis parallel to the outlets, referred to as axial vortices, appear above critical Reynolds numbers Rec. As AR increases from 1 to 11, the number of vortices observed increases from 1 to 4, independently of Re. For AR = 1, the single axial vortex induces an interpenetration of the inlet fluids in the whole section; instead, for larger AR’s for which more vortices appear, the two inlet fluids remain largely segregated in bands, except close to the vortices. The linear stability analysis demonstrates that only one leading eigenmode is unstable for a given set of values of AR and Re. This mode provides a simplified model of the flow field, reproducing its key features such as the number of vortices and their distance. Its determination with this method requires a much smaller computational load than the DNS. This approach is shown to allow one to determine quickly and precisely the critical Reynolds number Rec and the sensitivity function S, which characterizes the influence of variations of the base flow on the unstable one.
  • Interaction between a free-falling sphere and structure dynamics in a heterogeneous thixotropic fluid
    (2023-08-01) Koochi, H.; Mac Intyre, Jonatan; Korhonen, M.; Puisto, A.; Maleki-Jirsaraei, N.; Alava, M. J.
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The impact of thixotropy on the settling behavior of a solid sphere is investigated utilizing a finite element-computational fluid dynamics simulation. Flow behavior is evaluated by coupling the Navier-Stokes equations with the dynamic evolution of an initially heterogeneous fluid's microstructure. Studying the structure dynamics around the settling sphere allows us to identify a variety of irregular and linear settling regimes. Settling regimes are varied by the degree of structuring, the degree of associated heterogeneity, the local morphology of the heterogeneous microstructure, and the stress induced by the sphere. In addition, the settling velocity profile of the relatively light spheres temporarily fluctuates in a case where the settling time of the sphere is long enough to capture the local heterogeneity. Ultimately, we compare the results of the simulation of dropping spheres with those of the numerical simulation of different rheological tests. This illustrates that the competition between kernels of orthokinetic and perikinetic build-up and shear-induced break-down of the microstructure indeed allows an understanding of the connection between the fluids' flow curve and the settling behaviors. Furthermore, settling regimes are characterized based on the rates of build-up and break-down of the microstructure. Moreover, the loss of fore-aft symmetry is observed in the flow field around the settling sphere as a result of a viscosity gradient behind and ahead of the sphere.
  • Local time-dependent microstructure of aging TEMPO nanofibrillated cellulose gel
    (2023-01) Koochi, Hakimeh; Mac Intyre, Jonatan; Viitanen, Leevi; Puisto, Antti; Maleki-Jirsaraei, Nahid; Alava, Mikko
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The aging behavior of TEMPO-CNF suspensions is investigated using rheological measurements and experiments involving the free-falling solid sphere. The properties of CNF suspensions, which are formed of water and cellulose fibers with a large aspect ratio, are fundamentally different from those commonly known as model low-density gels. We characterize their aging utilizing SAOS rheometry following a period of shear rejuvenation, measuring the effect of aging time on the observed stress overshoot, and additionally measure the rheological hysteresis under continuous shear. In addition to such tests, which probe the sample at the bulk level, we study their local aging behavior via the Stokes' experiment. During SAOS, the aging of the material results in a logarithmic growth of the elastic modulus over the first 50 minutes. In the Stokes' experiment, depending on the sphere size, we find a systematic decrease in the sphere velocity with aging times up to 16 days and identify a range of intermittent particle motion. Based on our experimental evidence, we propose that the aging effect within the TEMPO-CNFs occurs due to the restructuring of fibrous elements by Brownian diffusion and that the aging of the system does not develop homogeneously across the whole sample as a consequence of the wide size distribution of CNFs. Finally, we note that this may be one of the primary reasons why the rheological data on even the same batch of CNFs show a large scatter.
  • Machine learning and predicting the time dependent dynamics of local yielding in dry foams
    (2020-06) Viitanen, Leevi; Mac Intyre, Jonatan; Koivisto, Juha; Puisto, Antti; Alava, Mikko
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The yielding of dry foams is enabled by small elementary yield events on the bubble scale, “T1”s. We study the large-scale detection of these in an expanding two-dimensional (2D) flow geometry using artificial intelligence (AI) and nearest neighbor analysis. A good level of accuracy is reached by the AI approach using only a single frame, with the maximum score for vertex centered images highlighting the important role the vertices play in the local yielding of foams. We study the predictability of T1s ahead of time and show that this is possible on a timescale related to the waiting time statistics of T1s in local neighborhoods. The local T1 event predictability development is asymmetric in time, and measures the variation of the local property to yielding and similarly the existence of a relaxation timescale after local yielding.
  • Measuring biofoam anisotropy using optical coherence tomography
    (2022-05-27) Mac Intyre, Jonatan; Raka, Doruntina; Aydin, Mehmet; Viitanen, Leevi; Koivisto, Juha
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Optical coherence tomography enables quick scans of translucent objects in a simple environment. Here, we apply this technique to wood-based biofoam. We measure the geometrical properties of the foam, such as bubble eccentricity and density fluctuations, in addition to characterising the possible orientation of fibres. We find that the wood-based foams are extremely suitable for optical coherence tomography due to their translucent nature and large changes of optical density between air-filled bubbles and solid films. Measurement of bubble eccentricity revealed a reasonably high aspect ratio of 1:2, enabling the orientation of long cellulose fibres if added to the mixture. The results demonstrate an effective method to characterise foamlike metamaterials. Furthermore, focusing on eccentricity enables the adjustment of the foam’s manufacturing method and, in turn, helps to produce anisotropic structures.
  • Multiphase CFD modeling of front propagation in a Hele-Shaw cell featuring a localized constriction
    (2021-08-17) Mac Intyre, Jonatan; Puisto, Antti; Korhonen, Marko; Alava, Mikko; Ortín, Jordi
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We study a liquid-gas front propagation in a modulated Hele-Shaw cell by means of multiphase computational fluid mechanics based on the three-dimensional Navier-Stokes equations. In the simulations an obstacle that partially fills the gap is placed at the center of the cell, and the liquid-gas interface is driven at a constant velocity. We study the morphological differences between imbibition and drainage for a wide range of capillary numbers, and explore how the wetting properties of the constriction affect the amount of liquid that remains trapped in the draining process. We observe increasing remaining volumes with increasing capillary number and decreasing contact angle. The present CFD implementation for a single mesa defect provides insight into a wide number of practical applications.
  • Predicting and following T1 events in dry foams from geometric features
    (2021-07-15) Tainio, Oskar; Viitanen, Leevi; Mac Intyre, Jonatan; Aydin, Mehmet; Koivisto, Juha; Puisto, Antti; Alava, Mikko
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Machine learning techniques have been recently applied in predicting deformation in amorphous materials. In this study, we extract structural features around liquid film vertices from images of flowing 2D foam and apply a multilayer perceptron to predict local yielding. We evaluate their importance in the description of the T1 events and show that a high level of predictability may be achieved using well-chosen combinations of features as the prediction data. The most relevant features are extracted by performing the predictions separately for isolated sets of features, and these findings are verified using principal component analysis. Using this approach, we determine which properties of the images are most important with regard to the physics of the processes. Our findings indicate that film lengths and angles between the liquid films joining at the vertex are the most important features that predict the local yield events. These two features describe 83% of the yield events. As an application, we extract the statistics of event waiting times from the experiment.
  • Predicting effect of fibers on thermal gelation of methylcellulose using Bayesian optimization
    (2022) Miranda Valdez, Isaac; Viitanen, Leevi; Mac Intyre, Jonatan; Puisto, Antti; Koivisto, Juha; Alava, Mikko
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Understanding of the viscoelastic behavior of a polymer is a prerequisite for its thermomechanical processing beyond laboratory scale. Utilizing rheological characterization is a powerful tool to comprehend the complex nature and time-dependent properties of macromolecular materials. Nevertheless, it consumes time as rheometry involves iterating experiments under several conditions to visualize the non-linear behavior of materials under varying conditions. The work hereunder examines the rheology of cellulosic aqueous suspensions prepared using cellulose fibers as the dispersed phase (Refcell and Storacell) and methylcellulose (MC) as the polymeric matrix. Interfacial phenomena between MC and cellulose fibers arise in particle laden systems with supramolecular structures formed by non-covalent interactions. Therefore, this study elucidates the rheological evolution of these interactions as a function of temperature and fiber concentration. This study displays how researchers may reduce the number of rheological experiments and save time utilizing a novel method based on a Bayesian optimization with Gaussian processes.
  • Vibration controlled foam yielding
    (2020-10-21) Rinkinen, Oona; Viitanen, Leevi; Mac Intyre, Jonatan; Koivisto, Juha; Puisto, Antti; Alava, Mikko
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In rheological terms, foams are time independent yield stress fluids, displaying properties of both solid and liquid materials. Here we measure the propagation of a 2D dry foam in a radially symmetric Hele-Shaw cell forcing local yielding. The yield rate is manipulated by mechanical vibration with frequencies from 0 to 150 Hz. The flow speed is then extracted from the video stream and analyzed using digital image correlation software. The data are modeled analytically by a Guzman–Arrhenius type of energy landscape where the local yielding of foam correlates with the number of oscillations, i.e. attempts to cross the energy barrier. The model is confirmed in an auxiliary experiment where the vibrated foam stays in its flowing state at the same small driving pressures, where the flow of the unvibrated foam ceases. We conclude that the yield stress behaviour of foams under an external perturbation can be summarized using a simple energy landscape model. The vibration affects the films causing the stress to occasionally and locally exceed the yield threshold. This, thus, prevents the foam from jamming as in a static configuration even when the global driving is below the yield point of the foam.
  • Viscoelastic coarsening of quasi-2D foam
    (2023-12) Guidolin, Chiara; Mac Intyre, Jonatan; Rio, Emmanuelle; Puisto, Antti; Salonen, Anniina
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Foams are unstable jammed materials. They evolve over timescales comparable to their “time of use", which makes the study of their destabilisation mechanisms crucial for applications. In practice, many foams are made from viscoelastic fluids, which are observed to prolong their lifetimes. Despite their importance, we lack understanding of the coarsening mechanism in such systems. We probe the effect of continuous phase viscoelasticity on foam coarsening with foamed emulsions. We show that bubble size evolution is strongly slowed down and foam structure hugely impacted. The main mechanisms responsible are the absence of continuous phase redistribution and a non-trivial link between foam structure and mechanical properties. These combine to give spatially heterogeneous coarsening. Beyond their importance in the design of foamy materials, the results give a macroscopic vision of phase separation in a viscoelastic medium.