Browsing by Author "Mohtaschemi, Mikael"
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Item Customer segmentation in the Finnish mobile phone business using community detection on weighted graphs(2009) Mohtaschemi, Mikael; Peltomäki, Matti; Informaatio- ja luonnontieteiden tiedekunta; Alava, MikkoItem Detection of application used on a mobile device based on network traffic(2018-06-18) Kiritchenko, Dmitri; Mohtaschemi, Mikael; Sähkötekniikan korkeakoulu; Asokan, NSmartphones have become very popular over the past years, thus being owned by almost every individual, the devices also follow their owners throughout the day thus having access to a lot of information about their users. Additionally various companies provide additional services through applications on mobile devices which makes them highly interested in what people do with their mobile devices, as it allows perfection of these services. To collect usage data, on top of having user consent, a company must be able to actually see what is happening on the device. But in regards to growing concern about user privacy, operating systems on mobile devices isolate applications limiting their access to only a small part of information of what is happening on the device. Options like running surveys exist, but are highly dependent on honesty of the people and expensive. To gain the information about running applications network traffic can be utilized as more and more devices are constantly connected to the internet. On the other hand, as well as application isolation, the network traffic is also being more and more protected. This thesis starts with reviewing previous works to give a picture of what kind of information can be extracted from mobile device and it's network traffic and how it can be used. The main aim of this thesis is to implement a system that detects the used applications and their running times by combining mobile network traffic with application launch times and using machine learning. To assess the detection quality and scalability thoroughly, several tests are performed. The implemented detection system shows good potential as it achieves near perfect results in optimal conditions, yet to provide these conditions in every case, a lot of work has to be done still.Item Dynamic hysteresis in the rheology of complex fluids(American Physical Society (APS), 2015) Puisto, Antti; Mohtaschemi, Mikael; Alava, Mikko J.; Illa, Xavier; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceRecently, rheological hysteresis has been studied systematically in a wide range of complex fluids combining global rheology and time-resolved velocimetry. In this paper we present an analysis of the roles of the three most fundamental mechanisms in simple-yield-stress fluids: structure dynamics, viscoelastic response, and spatial flow heterogeneities, i.e., time-dependent shear bands. Dynamical hysteresis simulations are done analogously to rheological ramp-up and -down experiments on a coupled model which incorporates viscoelasticity and time-dependent structure evolution. Based on experimental data, a coupling between hysteresis measured from the local velocity profiles and that measured from the global flow curve has been suggested. According to the present model, even if transient shear banding appears during the shear ramps, in typical narrow-gap devices, only a small part of the hysteretic response can be attributed to heterogeneous flow. This results in decoupling of the hysteresis measured from the local velocity profiles and the global flow curve, demonstrating that for an arbitrary time-dependent rheological response this proposed coupling can be very weak.Item Modeling Oscillatory Rheology of Viscoelastic and Thixotropic Complex Fluids(2014-11-24) Salmenjoki, Henri; Mohtaschemi, Mikael; Perustieteiden korkeakoulu; Alava, MikkoItem Modelling transient shear banding in Couette geometry(2011) Mohtaschemi, Mikael; Puisto, Antti; Teknillisen fysiikan laitos; Perustieteiden korkeakoulu; School of Science; Alava, MikkoLong-living transient shear banding in a simple yield stress fluid has recently been observed in Couette rheometer. Here a computational model which shows qualitatively similar behavior to the experimental findings is investigated in detail. The model is based on population balance equations which are frequently used to model colloids. An attempt is made to fit the experimental results and to reproduce the experimentally found relationship between the characteristic scalings between the shear rate and stress controlled fluidization times and the Herschel - Bulkley fit to the steady state flow curve. No exact fit could be obtained and the relationship could not be unambiguously reproduced with the model. At low shear rates deviations from the power-law scaling in the fluidization times were obtained. These are most likely due to the geometry induced stress heterogeneity and the stress plateau in the flow curve.Item Models for fibre aggregation and orientation in nanocellulose suspensions(2014-06-27) Korhonen, Marko; Mohtaschemi, Mikael; Perustieteiden korkeakoulu; Alava, MikkoItem Patterns, Entropy, and Predictability of Human Mobility and Life(Public Library of Science (PLoS), 2012) Qin, Shao-Meng; Verkasalo, Hannu; Mohtaschemi, Mikael; Hartonen, Tuomo; Alava, Mikko J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceCellular phones are now offering an ubiquitous means for scientists to observe life: how people act, move and respond to external influences. They can be utilized as measurement devices of individual persons and for groups of people of the social context and the related interactions. The picture of human life that emerges shows complexity, which is manifested in such data in properties of the spatiotemporal tracks of individuals. We extract from smartphone-based data for a set of persons important locations such as “home”, “work” and so forth over fixed length time-slots covering the days in the data-set (see also [1], [2]). This set of typical places is heavy-tailed, a power-law distribution with an exponent close to −1.7. To analyze the regularities and stochastic features present, the days are classified for each person into regular, personal patterns. To this are superimposed fluctuations for each day. This randomness is measured by “life” entropy, computed both before and after finding the clustering so as to subtract the contribution of a number of patterns. The main issue that we then address is how predictable individuals are in their mobility. The patterns and entropy are reflected in the predictability of the mobility of the life both individually and on average. We explore the simple approaches to guess the location from the typical behavior, and of exploiting the transition probabilities with time from location or activity A to B. The patterns allow an enhanced predictability, at least up to a few hours into the future from the current location. Such fixed habits are most clearly visible in the working-day length.Item Spatially resolved rheology modeling of complex fluids(Aalto University, 2015) Mohtaschemi, Mikael; Puisto, Antti, Dr., Aalto University, Department of Applied Physics, Finland; Teknillisen fysiikan laitos; Department of Applied Physics; Complex Systems and Materials; Perustieteiden korkeakoulu; School of Science; Alava, Mikko, Prof., Aalto University, Department of Applied Physics, FinlandMany complex fluids show yield stress behavior. However, the term yield stress has been subject of much controversy. The separation of yield stress fluids into thixotropic and simple ones resolves many of these issues. This division is mainly driven by experimental results and is suspect to active theoretical development. This thesis addresses yield stress fluids and associated phenomena through continuum modeling for fluids with time dependent structure evolution. In addition to homogeneous laminar shear modeling, the emergence of spatial effects in viscometric flow situations is addressed. Therefore the models are coupled to the creeping flow solution (1-D Stokes equation) of a concentric cylinder geometry, which enables comparisons with experimental observations. Further, the results from thixotropic yield stress fluids are applied to the analysis of rheology measurements of nanocellulose suspensions, which have peculiar rheological properties. In particular, shear rate sweeps are simulated utilizing a structural model for thixotropic yield stress fluids. The results indicate that spatial flow heterogeneities have to be taken into account. Additionally wall slip, which is known to play an important role in the flow of complex fluids is addressed through a simple model. The results in this thesis add to the understanding of nanocellulose suspensions and complex fluids in general.Item Start-up inertia as an origin for heterogeneous flow(2017-02-23) Korhonen, Marko; Mohtaschemi, Mikael; Puisto, Antti; Illa Tortos, Xavier; Alava, Mikko J.; Department of Applied Physics; Complex Systems and MaterialsFor quite some time nonmonotonic flow curve was thought to be a requirement for shear banded flows in complex fluids. Thus, in simple yield stress fluids shear banding was considered to be absent. Recent spatially resolved rheological experiments have found simple yield stress fluids to exhibit shear banded flow profiles. One proposed mechanism for the initiation of such transient shear banding process has been a small stress heterogeneity rising from the experimental device geometry. Here, using computational fluid dynamics methods, we show that transient shear banding can be initialized even under homogeneous stress conditions by the fluid start-up inertia, and that such mechanism indeed is present in realistic experimental conditions.Item Transient shear banding in time-dependent fluids(2013-02-22) Illa, Xavier; Puisto, Antti; Lehtinen, Arttu; Mohtaschemi, Mikael; Alava, Mikko J.; University of Barcelona; Department of Applied PhysicsWe study the dynamics of shear-band formation and evolution using a simple rheological model. The description couples the local structure and viscosity to the applied shear stress. We consider in detail the Couette geometry, where the model is solved iteratively with the Navier-Stokes equation to obtain the time evolution of the local velocity and viscosity fields. It is found that the underlying reason for dynamic effects is the nonhomogeneous shear distribution, which is amplified due to a positive feedback between the flow field and the viscosity response of the shear thinning fluid. This offers a simple explanation for the recent observations of transient shear banding in time-dependent fluids. Extensions to more complicated rheological systems are considered.Item Transient shear banding in time-dependent fluids(American Physical Society (APS), 2013) Illa, Xavier; Puisto, Antti; Lehtinen, Arttu; Mohtaschemi, Mikael; Alava, Mikko J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of ScienceWe study the dynamics of shear-band formation and evolution using a simple rheological model. The description couples the local structure and viscosity to the applied shear stress. We consider in detail the Couette geometry, where the model is solved iteratively with the Navier-Stokes equation to obtain the time evolution of the local velocity and viscosity fields. It is found that the underlying reason for dynamic effects is the nonhomogeneous shear distribution, which is amplified due to a positive feedback between the flow field and the viscosity response of the shear thinning fluid. This offers a simple explanation for the recent observations of transient shear banding in time-dependent fluids. Extensions to more complicated rheological systems are considered.