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  • Presentation: 2017-12-05 15:15 Q2, Stockholm
    Petersson, Björn
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Modeling of a Retrodirective Channel With Active Antenna Arrays for Cross-Eye Jamming2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Cross-Eye (CE) jamming is a technique aimed at inducing false target angles in radar measurements. The potential to deceive monopulse radar, a radar type commonly used due to its angular accuracy and resilience against jamming, has captured the interest of Electronic Warfare (EW) system manufacturers. CE jamming may increase the capability of self-defense against radar guided missiles.

    In this thesis, a CE jammer, based on the retrodirective channel design, for use in an EW systems is modeled and analyzed. The focus of the analysis is on the non-reciprocal Active Electronically Scanned Arrays (AESA). An electrical model of the jammer system and the AESA, including variations in the electrical properties, together with three different system correction methods are used. A simulation procedure, using single frequency (CW) signals, is defined and used for estimating total level of reciprocity, which is a primary contributor to jammer performance. Reciprocity error sensitivity for variations in model parameters is simulated to indicate which AESA aspects are essential for performance. Further, the influence of characterization and calibration noise is investigated.

    The usability of the model is demonstrated in different types of simulations, which highlights the differences between the system correction methods. Simulations show that a relatively high level of reciprocity can be achieved when using system correction, if measurement noise and model limitations are disregarded. The expected degradation in reciprocity from differences in electrical properties between the reception and transmission branch of the AESA has been confirmed. The effect of characterization and calibration noise, for realistic levels of noise, seems to be a dominating factor in the total reciprocity error. A significant increase in reciprocity error can likely be expected when non-CW signals are used and when realistic levels of noise and non-linear effects are added.

  • Presentation: 2017-12-11 10:15 E3, Stockholm
    Offermans, Nicolas
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Towards adaptive mesh refinement in Nek50002017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The development of adaptive mesh refinement capabilities in the field of computational fluid dynamics is an essential tool for enabling the simulation of larger and more complex physical problems. While such techniques have been known for a long time, most simulations do not make use of them because of the lack of a robust implementation. In this work, we present recent progresses that have been made to develop adaptive mesh refinement features in Nek5000, a code based on the spectral element method. These developments are driven by the algorithmic challenges posed by future exascale supercomputers. First, we perform the study of the strong scaling of Nek5000 on three petascale machines in order to assess the scalability of the code and identify the current bottlenecks. It is found that strong scaling limit ranges between 5, 000 and 220, 000 degrees of freedom per core depending on the machine and the case. The need for synchronized and low latency communication for efficient computational fluid dynamics simulation is also confirmed. Additionally, we present how Hypre, a library for linear algebra, is used to develop a new and efficient code for performing the setup step required prior to the use of an algebraic multigrid solver for preconditioning the pressure equation in Nek5000. Finally, the main objective of this work is to develop new methods for estimating the error on a numerical solution of the Navier–Stokes equations via the resolution of an adjoint problem. These new estimators are compared to existing ones, which are based on the decay of the spectral coefficients. Then, the estimators are combined with newly implemented capabilities in Nek5000 for automatic grid refinement and adaptive mesh adaptation is carried out. The applications considered so far are steady and two-dimensional, namely the lid-driven cavity at Re = 7, 500 and the flow past a cylinder at Re = 40. The use of adaptive mesh refinement techniques makes mesh generation easier and it is shown that a similar accuracy as with a static mesh can be reached with a significant reduction in the number of degrees of freedom.

  • Presentation: 2017-12-13 14:00 Sal L51, Stockholm
    Yang, Can
    KTH, School of Architecture and the Built Environment (ABE), Urban Planning and Environment, Geoinformatics.
    Discovering Contiguous Sequential Patterns in Network-Constrained Movement2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    A large proportion of movement in urban area is constrained to a road network such as pedestrian, bicycle and vehicle. That movement information is commonly collected by Global Positioning System (GPS) sensor, which has generated large collections of trajectories. A contiguous sequential pattern (CSP) in these trajectories represents a certain number of objects traversing a sequence of spatially contiguous edges in the network, which is an intuitive way to study regularities in network-constrained movement. CSPs are closely related to route choices and traffic flows and can be useful in travel demand modeling and transportation planning. However, the efficient and scalable extraction of CSPs and effective visualization of the heavily overlapping CSPs are remaining challenges.

    To address these challenges, the thesis develops two algorithms and a visual analytics system. Firstly, a fast map matching (FMM) algorithm is designed for matching a noisy trajectory to a sequence of edges traversed by the object with a high performance. Secondly, an algorithm called bidirectional pruning based closed contiguous sequential pattern mining (BP-CCSM) is developed to extract sequential patterns with closeness and contiguity constraint from the map matched trajectories. Finally, a visual analytics system called sequential pattern explorer for trajectories (SPET) is designed for interactive mining and visualization of CSPs in a large collection of trajectories.

    Extensive experiments are performed on a real-world taxi trip GPS dataset to evaluate the algorithms and visual analytics system. The results demonstrate that FMM achieves a superior performance by replacing repeated routing queries with hash table lookups. BP-CCSM considerably outperforms three state-of-the-art algorithms in terms of running time and memory consumption. SPET enables the user to efficiently and conveniently explore spatial and temporal variations of CSPs in network-constrained movement.

    The full text will be freely available from 2017-12-12 13:47
  • Presentation: 2017-12-13 15:15 D3, Stockholm
    Negi, Prabal
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, Centres, SeRC - Swedish e-Science Research Centre.
    Boundary layers over wing sections2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    The understanding of developing boundary layers over wings is an important topic from the perspective of industrial applications. An increased understanding would be consequential not only for achieving higher fuel efficiency but also in the design of aircraft control strategies. With these aims in mind, the current work aims to further the understanding of developing boundary layer over wingsections. The study is performed with two particular perspectives in mind -unsteady aerodynamic effects in a pitching airfoil and turbulent boundary layerstructure in non-equilibrium boundary layers over a stationary airfoil.The boundary layer evolution in unsteady natural laminar flow airfoils undergoing small-amplitude pitch-oscillations is investigated. For high Reynolds numbers the origins of the non-linear unsteady aerodynamic response of laminar airfoils is explained on the basis of quasi-steady assumptions. Temporal nonlinearitiesin aerodynamic forces are shown to be inherently linked to thenon-linearities of static aerodynamic force coefficients and that a simple phaselagconcept can model the observed non-linear unsteady response. On the other hand at lower Reynolds numbers, when there exists an unstable leading edgelaminar separation bubble, the unsteady response is dynamically rich and changes in boundary layer characteristics can be abrupt. Such quasi-steadyphase-lag concepts are no longer appropriate to explain the unsteady flow physics in such a case. For the case of stationary airfoils, flow statistics for flow around an airfoil at two different Reynolds numbers are compared to assess Reynolds number effects in non-equilibrium flows. Pressure gradient effects found to be stronger at low Reynolds numbers, leading to higher energy in the larger structures present  in the outer part of the turbulent boundary layer.

  • Presentation: 2017-12-15 14:30 V3, Stockholm
    Boström, Erik
    KTH, School of Engineering Sciences (SCI), Mechanics, Stability, Transition and Control.
    Boundary Conditions for Spectral Simulations of Atmospheric Boundary Layers2017Licentiate thesis, monograph (Other academic)
    Abstract [en]

    An atmospheric boundary layer (ABL) is generally a very high Reynolds number boundary layer over a fully rough surface that is influenced by different external forces. Numerical simulations of ABLs are typically demanding, particularly due to the high Reynolds numbers. Large eddy simulation (LES) where the grid filtered Navier--Stokes equations are solved together with a turbulence model for the subgrid-scale motions is the most accurate and widely used technique to date for ABLs. However, high Reynolds numbers, filtered equations and rough surfaces do not support the simple no-slip boundary conditions together with a feasible grid resolution. A paramount part for the performance of an ABL LES simulation therefore lies in the quality of approximate wall boundary conditions, so called wall models.

         The vast majority of LES codes used for ABL simulations rely on spatial discretization methods with low order finite difference approximations for the derivatives in the inhomogeneous wall normal direction. Furthermore, the wall boundary conditions are typically chosen in a mesh-dependent, non-local way, relying on the finite differences formulation.

         In this thesis we focus on solving the ABL LES equations with a fully (pseudo) spectral Fourier--Chebyshev code. We present how wall boundary conditions can be formulated through Robin boundary conditions and how to implement these in the normal-velocity normal-vorticity formulation that we solve. A new idea of specifying boundary conditions directly in Fourier space where also the turbulence intensity statistics can be controlled is presented and verified. The present results show that the Robin-type formulation is effective at least in near-equilibrium boundary layers.

         The code and boundary conditions were tested in both low and high Reynolds number (open and full) channel flows of neutral and stable stratification. Results were validated with both low to moderate Reynolds number DNS statistics as well as with the logarithmic law. Our results indicate great potential for both the the new boundary condition formulation and the specific code implementation. Further analysis of more complex flow situations will show whether the Robin-type formulation will give similarly good results.

  • Presentation: 2017-12-18 13:00 E52, Stockholm
    Lupo, Giandomenico
    KTH, School of Engineering Sciences (SCI), Mechanics.
    Detailed simulations of droplet evaporation2017Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Droplet evaporation (and condensation) is one of the most common instancesof multiphase flow with phase change, encountered in nature as well as intechnical and industrial applications. Examples include falling rain drops, fogsand mists, aerosol applications like electronic cigarettes and inhalation drugdelivery, engineering applications like spray combustion, spray wet scrubbing orgas absorption, spray drying, flame spray pyrolysis.Multiphase flow with phase change is a challenging topic due to the inter-twined physical phenomena that govern its dynamics. Numerical simulation isan outstanding tool that enables us to gain insight in the details of the physics,often in cases when experimental studies would be too expensive, impracticalor limited.In the present work we focus on simulation of the evaporation of smalldroplets. We perform simulation of evaporation of a pure and two−componentdroplet, that includes detailed thermodynamics and variable physical andtransport properties. Some of the conclusions drawn include the importance ofenthalpy transport by species diffusion in the thermal budget of the system, andthe identification and characterization of evaporating regimes for an azeotropicdroplet.In the second part we develop a method based on the immersed boundaryconcept for interface resolved numerical simulation of laminar and turbulentflows with a large number of spherical droplets that undergo evaporation orcondensation.

  • Presentation: 2017-12-18 14:00 D3, Stockholm
    Khoshkar, Sara
    KTH, School of Architecture and the Built Environment (ABE), Sustainable development, Environmental science and Engineering, Sustainability Assessment and Management.
    Paving the way for green qualities in urban development-Role of Environmental Assessment2017Licentiate thesis, monograph (Other academic)
    Abstract [en]

    Implementing urban development projects in planning practice while simultaneously providing sufficient green spaces has proven to be challenging. As a result, there is a growing need for practical approaches and tools for the integration of urban green qualities in the on-going densification of cities. Environmental assessment, as a proactive decision aiding tool, can hold an important role in integrating green qualities in urban development plans and projects. However, in recent years environmental assessment has only added moderate value to planning issues regarding green space. Therefore, this thesis was designed to contribute to the knowledge and understanding on the role environmental assessment can have in the integration of green qualities in future urban development plans and projects in efforts to aid practitioners. This aim was achieved through examination of existing urban green space planning practice in the Stockholm region (Paper I) and the practice of environmental assessment in a selection of European countries in relation to the following factors identified to be important for practice: timing, quality control, alternatives, monitoring and public participation (Paper II). The findings from both studies were then analysed to identify possibilities of green space planning within the framework of these factors. A qualitative research was employed for this study including: semi-structured interviews with municipal planners in the Stockholm region and environmental assessment experts for the European Commission, literature review, document analysis and case study analysis. The case studies analyzed in Paper I were located in municipalities of Haninge and Huddinge, suburban areas located to the south of Stockholm. In Paper II, the experts interviewed were environmental assessment experts from the European Commission from: Austria, Denmark, Finland, Greece, Luxembourg, the Netherlands, Norway and Slovenia. Through analysis of the results from both studies, the potential role of environmental assessment in integrating green qualities in urban development plans and projects are explored and discussed in relation to the factors. The potential of environmental assessment to enhance public participation and dialogues amongst actors, or bring forth green space issues within alternatives are a few of the roles discussed. Furthermore, a selection of pathways is suggested for the integration of green qualities in future urban development through the application of environmental assessment. For example, the implementation of developer dialogues in the environmental assessment process and the development of knowledge exchange platforms for sharing experiences in relation to green space planning and environmental assessment.