Nd:YAG Laser Welding of 5083 Aluminium Alloy using Filler Wire

High reflectivity and high thermal conductivity, high vapour pressure of alloyingelements as well as low liquid surface tension and low ionisation potential, make laser welding of aluminium and its alloys a demanding task.Problems that occur during welding are mainly process instabilities of the keyhole and the melt pool, increased plasma formation above the melt pool and loss of alloying elements. These problems lead to unwanted metallurgical defects like hot cracks and porosity in the weld bead andother problems concerning the shape and appearance of the weld bead. In order to minimise the defects and improve the weld quality, the process and beam parameters need to be carefully adjusted along with a consideration concerning the use of filler wire for the welding process. In this work the welding of 3,0 mm thick grade 5083 aluminium alloy plates using a 3,0 kW Nd:YAG laser with grade 5183 filler wire addition is investigated. The plates were welded as butt joints with air gap sizes 0,5 mm, 0,7mm and 1,0 mm. The analysis of the weld beads obtained from the weldedsamples showed that the least imperfections were produced with 0,7 mm air gaps at moderate welding speeds. The analysis also covered the calculation of the melting efficiency and the study of the shape of the weld bead. The melting efficiency was on average around 20 % for the melting process of the welded plates. The weld beads showed the characteristic V-shape of a laser weld and retained this shape during the whole series of experiments.

Theoretical and numerical study of thermomagnetic convection in magnetic fluids

In this thesis, the magnetic field control of convection instabilities and heat and mass transfer processesin magnetic fluids have been investigated by numerical simulations and theoretical considerations. Simulation models based on finite element and finite volume methods have been developed. In addition to standard conservation equations, themagnetic field inside the simulation domain is calculated from Maxwell equations and the necessary terms to take into account for the magnetic body force and magnetic dissipation have been added to the equations governing the fluid motion.Numerical simulations of magnetic fluid convection near the threshold supportedexperimental observations qualitatively. Near the onset of convection the competitive action of thermal and concentration density gradients leads to mostly spatiotemporally chaotic convection with oscillatory and travelling wave regimes, previously observed in binary mixtures and nematic liquid crystals. In many applications of magnetic fluids, the heat and mass transfer processes including the effects of external magnetic fields are of great importance. In addition to magnetic fluids, the concepts and the simulation models used in this study may be applied also to the studies of convective instabilities in ordinary fluids as well as in other binary mixtures and complex fluids.

The Evolution of Securities Lending and Risk Mitigation: A Finnish Market Case Study

Although securities lending is an important function of the financial markets, it has not received that much academic attention. This study examines the evolution of European securities lending and risk management with an emphasis on the development of collateral management, the function responsible for reducing credit risk. The effects of the recent financial instabilities are also considered. The evolution of the Finnish securities lending market is examined in more detail through a case-study. This study can be classified as a constructive qualitative case study. The initial practical knowledge comes from the author's own experience and additional insight and theoretical background is acquired through a literature review. The case study is based on research, semi-structured interviews and a brief analysis of numerical data. The main observation of this study was that securities lending is now recognized as more of an investment management discipline than an operational support function. The recent financial instabilities have resulted in an increased focus on risk and transparency. In securities lending this is directly reflected in collateral management guidelines and procedures. Collateral management has become increasingly technologically developed and automated. Collateral optimization initiatives have been started to make the process more efficient, liquid, and cost effective. Although securities lending is generally an OTC-market with no standard market place, centralized exchange-like models have been introduced. Finnish securities lending has now shifted towards the more common global OTC model. Although the Finnish securities lending industry has developed, and the main laws governing it (tax legislation) have changed, there is still need for development. There are still not many Finnish participants involved and due to legal issues most securities loans are collateralized with cash and not securities (e.g. government bonds).

Studies of Solar Activity with Emphasis on Quasi-Periodic Oscillations

In this thesis mainly long quasi-periodic solar oscillations in various solar atmospheric structures are discussed, based on data obtained at several wavelengths, focussing, however, mainly on radio frequencies. Sunspot (Articles II and III) and quiet Sun area (QSA) (Article I) oscillations are investigated along with quasi-periodic pulsations (QPP) in a flaring event with wide-range radio spectra (Article IV). Various oscillation periods are detected; 3–15, 35–70 and 90 minutes (QSA), 10-60 and 80-130 minutes (in sunspots at various radio frequencies), 3-5, 10-23, 220-240, 340 and 470 minutes (in sunspots at photosphere) and 8-12 and 15-17 seconds (in a solar flare at radio frequencies). Some of the oscillation periods are detected for the first time, while some of them have been confirmed earlier by other research groups. Solar oscillations can provide more information on the nature of various solar structures. This thesis presents the physical mechanisms of some solar structure oscillations. Two different theoretical approaches are chosen; magnetohydrodynamics (MHD) and the shallow sunspot model. These two theories can explain a wide range of solar oscillations from a few seconds up to some hours. Various wave modes in loop structures cause solar oscillations (<45 minutes) both in sunspots and quiet Sun areas. Periods lasting more than 45 minutes in the sunspots (and a fraction of the shorter periods) are related to sunspot oscillations as a whole. Sometimes similar oscillation periods are detected both in sunspot area variations and respectively in magnetic field strength changes. This result supports a concept that these oscillations are related to sunspot oscillations as a whole. In addition, a theory behind QPPs at radio frequencies in solar flares is presented. The thesis also covers solar instrumentation and data sources. Additionally, the data processing methods are presented. As the majority of the investigations in this thesis focus on radio frequencies, also the most typical radio emission mechanisms are presented. The main structures of the Sun, which are related to solar oscillations, are also presented. Two separate projects are included in this thesis. Solar cyclicity is studied using the extensively large solar radio map archieve from Metsähovi Radio Observatory (MRO) at 37 GHz, between 1978 and 2011 (Article V) covering two full solar cycles. Also, some new solar instrumentation (Article VI) was developed during this thesis.

Methodology for wind turbine blade geometry optimization

Nowadays, the upwind three bladed horizontal axis wind turbine is the leading player on the market. It has been found to be the best industrial compromise in the range of different turbine constructions. The current wind industry innovation is conducted in the development of individual turbine components. The blade constitutes 20-25% of the overall turbine budget. Its optimal operation in particular local economic and wind conditions is worth investigating. The blade geometry, namely the chord, twist and airfoil type distributions along the span, responds to the output measures of the blade performance. Therefore, the optimal wind blade geometry can improve the overall turbine performance. The objectives of the dissertation are focused on the development of a methodology and specific tool for the investigation of possible existing wind blade geometry adjustments. The novelty of the methodology presented in the thesis is the multiobjective perspective on wind blade geometry optimization, particularly taking simultaneously into account the local wind conditions and the issue of aerodynamic noise emissions. The presented optimization objective approach has not been investigated previously for the implementation in wind blade design. The possibilities to use different theories for the analysis and search procedures are investigated and sufficient arguments derived for the usage of proposed theories. The tool is used for the test optimization of a particular wind turbine blade. The sensitivity analysis shows the dependence of the outputs on the provided inputs, as well as its relative and absolute divergences and instabilities. The pros and cons of the proposed technique are seen from the practical implementation, which is documented in the results, analysis and conclusion sections.