Swedish Defence Research Agency

Linkoping, Sweden

Swedish Defence Research Agency is a Swedish government agency for defence research that reports to the Ministry of Defence. It has its headquarters in Kista, Stockholm and other locations in Grindsjön south of Stockholm, Linköping and Umeå. The activities of FOI include research, development of technology and analysis, primarily for military defence, but also for civil emergency, security and other civilian applications.In 2011, FOI employed around 950 people. Wikipedia.

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Holm P.D.,Swedish Defence Research Agency
IEEE Transactions on Antennas and Propagation | Year: 2017

Efficient methods for computing ground-wave propagation over a smooth spherical earth with an exponential refraction index are presented. The methods are based on a residue series, which is valid for large distances near and beyond the radio horizon. The previous solution, which also uses a residue series, involves extensive numerical calculations that are time consuming. Here, it is shown that it is possible to reduce the number of numerical calculations considerably. By using approximate expressions, it is possible to remove many of the time-consuming calculations and still get very good results, which are shown in several examples. © 1963-2012 IEEE.

Gustafsson F.,Linköping University | Hendeby G.,Swedish Defence Research Agency
IEEE Transactions on Signal Processing | Year: 2012

The unscented Kalman filter (UKF) has become a popular alternative to the extended Kalman filter (EKF) during the last decade. UKF propagates the so called sigma points by function evaluations using the unscented transformation (UT), and this is at first glance very different from the standard EKF algorithm which is based on a linearized model. The claimed advantages with UKF are that it propagates the first two moments of the posterior distribution and that it does not require gradients of the system model. We point out several less known links between EKF and UKF in terms of two conceptually different implementations of the Kalman filter: the standard one based on the discrete Riccati equation, and one based on a formula on conditional expectations that does not involve an explicit Riccati equation. First, it is shown that the sigma point function evaluations can be used in the classical EKF rather than an explicitly linearized model. Second, a less cited version of the EKF based on a second-order Taylor expansion is shown to be quite closely related to UKF. The different algorithms and results are illustrated with examples inspired by core observation models in target tracking and sensor network applications. © 2011 IEEE.

Schubert J.,Swedish Defence Research Agency
International Journal of Approximate Reasoning | Year: 2012

We construct alternative frames of discernment from input belief functions. We assume that the core of each belief function is a subset of a so far unconstructed frame of discernment. The alternative frames are constructed as different cross products of unions of different cores. With the frames constructed the belief functions are combined for each alternative frame. The appropriateness of each frame is evaluated in two ways: (i) we measure the aggregated uncertainty (an entropy measure) of the combined belief functions for that frame to find if the belief functions are interacting in interesting ways, (ii) we measure the conflict in Dempster's rule when combining the belief functions to make sure they do not exhibit too much internal conflict. A small frame typically yields a small aggregated uncertainty but a large conflict, and vice versa. The most appropriate frame of discernment is that which minimizes a probabilistic sum of the conflict and a normalized aggregated uncertainty of all combined belief functions for that frame of discernment. © 2011 Elsevier Inc. All rights reserved.

Fureby C.,Swedish Defence Research Agency
Flow, Turbulence and Combustion | Year: 2010

In this study, Large Eddy Simulation (LES) has been used to predict the flow, mixing and combustion in both a single burner laboratory gas turbine combustor and in an 18 burner annular combustor, having identical cross sections. The LES results for the single burner laboratory combustor are compared with experimental data for a laboratory model of this combustor, and with other LES predictions, with good agreement. An explicit finite volume based LES model, using the mixed subgrid model together with a partially stirred reactor model for the turbulence chemistry interactions, is used. For the annular combustor, with the swirlers parameterized by jet inflow boundary conditions, we have investigated the influence of the a-priori unknown combustor exit impedance, the influence of the swirler characteristics and the fuel type. The combustion chemistry of methane-air and ndecane- air combustion is modeled by a two-step reaction mechanism, whereas NOx is separately modeled with a one-step mechanism. No experimental data exists for the annular combustor, but these results are compared with the single burner LES and experimental results available. The combustor exit impedance, the swirler-and fuel characteristics all seem to influence the combusting flow through the acoustics of the annular combustor. To examine this in greater detail time-series and eigenmodes of the combustor flow fields are analyzed and comparisons are made also with results from conventional thermoacoustic eigenmode analysis, with reasonable agreement. The flow and pressure distributions in the annular combustor are described in some detail and the mechanisms by which the burners interact are outlined. © 2009 Springer Science + Business Media B.V.

Johansson B.,Swedish Defence Research Agency | Johansson B.,Lund University
Energy | Year: 2013

Energy security is sometimes used to advocate renewable energy systems. Renewable energy systems can improve some aspects of security, but they will not automatically lead to the removal of all types of security problems and new problems will most certainly arise. This paper analyses energy security aspects of renewable energy systems on the basis of a broad typology on energy and security. Renewable energy sources do not suffer from the same long-term resource availability problems as finite fossil resources and their geographical location is less concentrated, but other issues such as dependence on variable flowing resources and competition for scarce land resources will grow in importance. Many security issues related to energy are also dependent on the energy carrier rather than the energy resource and on the existence of effectively functioning institutions and regulations. New interdependencies will appear and will have to be handled within future international and bilateral institutional frameworks. © 2013 The Author.

Johansson B.,Swedish Defence Research Agency | Johansson B.,Lund University
Energy | Year: 2013

A broadened typology describing the interconnection between energy and security is developed in this paper, with the aim of improving understanding of the relationship between energy and security by applying different research and policy perspectives. One approach involves studying energy as an object exposed to security threats, using concepts such as security of supply or security of demand. Another approach involves studying the role of the energy system as the subject in generating or enhancing insecurity. The latter approach includes studying the conflict-generating potential inherent in the economic value of energy, the risk of accidents and antagonistic attacks to energy infrastructure and the security risks related to the negative environmental impact of the energy system. In order to make a comprehensive analysis of the security consequences of proposed energy policies or strategies, all these aspects should be taken into account to varying degrees. The typology proposed here could be a valuable tool for ensuring that all security aspects have been considered. © 2013 Elsevier Ltd.

Ivansson S.M.,Swedish Defence Research Agency
Journal of the Acoustical Society of America | Year: 2014

Global optimization methods can be used to numerically determine optimal design parameters for an object. However, this does not by itself give a good appreciation of other parameter choices that may be almost as good and even preferable from other points of view. In the present paper, Markovchain Monte Carlo methods are used to go beyond the optimal solution and create an ensemble of object models in parameter space that covers a set of favorable models uniformly. In direct analogy with applications to Bayesian inversion with determination of an unknown posterior probability density, projections of the model ensemble onto parameter axes and planes are used to exhibit parameter sensitivities and dependencies. Design of anechoic rubber coatings, with cylinder cavities having axes in a lateral direction, is considered as a particular application. The anechoic effect is evaluated by the efficient layer-multiple-scattering method, which is extended to handle cylinder scatterers of noncircular cross sections and mixed types. As anticipated by computed scattering and absorption cross sections for an isolated cavity, the favorable coatings have oblate cavity cross-section shapes, which is useful to achieve good low-frequency reflection reduction with a thin coating. © 2014 Acoustical Society of America.

Schubert J.,Swedish Defence Research Agency
International Journal of Approximate Reasoning | Year: 2011

In this article we develop a method for conflict management within Dempster-Shafer theory. The idea is that each piece of evidence is discounted in proportion to the degree that it contributes to the conflict. This way the contributors of conflict are managed on a case-by-case basis in relation to the problem they cause. Discounting is performed in a sequence of incremental steps, with conflict updated at each step, until the overall conflict is brought down exactly to a predefined acceptable level. © 2010 Elsevier Inc. All rights reserved.

Nelander A.,Swedish Defence Research Agency
IEEE National Radar Conference - Proceedings | Year: 2010

A number of antenna array concepts are investigated for three-dimensional radar imaging. The concepts are based on switched arrays and wide-band signals to generate a virtual array or co-array to span a sector volume in the wave vector space to get image resolution. The proposed arrays can be used for short range imaging in penetrating radar applications. This approach is quite general and it can also be used in other system concepts. Virtual arrays have mostly been used in radio astronomy and ultrasonic imaging. The related synthetic arrays with antenna motion have been more common in radar systems. © 2010 IEEE.

Fureby C.,Swedish Defence Research Agency
Journal of Engineering for Gas Turbines and Power | Year: 2012

Present-day demands on combustion equipment are increasing the need for improved understanding and prediction of turbulent combustion. Large eddy simulation (LES), in which the large-scale flow is resolved on the grid, leaving only the small-scale flow to be modeled, provides a natural framework for combustion simulations as the transient nature of the flow is resolved. In most situations; however, the flame is thinner than the LES grid, and subgrid modeling is required to handle the turbulence-chemistry interaction. Here we examine the predictive capabilities between LES flamelet models, such as the flamelet progress variable (LES-FPV) model, and LES finite rate chemistry models, such as the thickened flame model (LES-TFM), the eddy dissipation concept (LES-EDC) model, and the partially stirred reactor model (LES-PaSR). The different models are here used to examine a swirl-stabilized premixed flame in a laboratory gas turbine combustor, featuring the triple annular research swirler (TARS), for which high-quality experimental data is available. The comparisons include velocity and temperature profiles as well as combustor dynamics and NO formation. © 2012 American Society of Mechanical Engineers.

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