Dielectric waveguides are crucial devices in the making of integrated-optical circuits. It is very important to analyze this type of waveguides so we can optimize the design for better performance. Analysis of bent waveguides has been a difficult problem in the past. In a bent waveguide, two coordinate systems are needed to fully describe the ongoing complex scattering process in the transition region of the waveguide. It is extremely hard to analyze such problems for methods built on a single coordinate system such as the finite-difference,finite-element methods and the beam propagation method (BPM). In this thesis, we adopt dual mode-field representations (for all the low and higher-order modes), one for the incident and reflected waves and the other for the transmitted waves, to study bending effects. To calculate the wave fields, we apply the analytic continuity principle to allow the waves to analytically extend and join smoothly on the bordering line. By matching the two continuity conditions of both the fields and their normal derivatives we get two matrix equations for the reflection and transmission coefficients. For symmetrical bending waveguide, the task can be further reduced to solving two smaller problems each with even or odd symmetry on the bordering line. As the bent angle increases the governing matrix equation becomes more singular. As a result, all the elements in the matrix are calculated with closed-form formulae to minimize the stability problem. In addition, special numerical methods are used to extend the range of the bending angles that this method can handle. In conclusion, our theory can calculate microwave bending waveguides up to 30 degrees and for dielectric slab waveguide with 15 degree bent angle. With this method we are able to compute small reflection coefficients of about -60dB and less.

An experimental and numerical study of air-air bent exhaust ejectors was carried out. The objective of the study was to determine the effect of a bend on the performance of exhaust ejectors. The ejectors consisted of nozzles, mixing tubes and in some cases entraining diffusers. As part of this study the effect of swirl in the primary flow and the temperature ratio of the primary to the secondary flow were also investigated. The study included testing of round and oblong sectioned ejectors with and without entraining diffusers. The experimental testing was performed on two different wind tunnels capable of blowing air at a maximum mass flow rate of 2.2 kg/s at ambient and elevated temperatures. Flow measurements were made upstream of the nozzle, at the nozzle exit and at the exit of the ejector. Pumping, pressure rise and total efficiency of the ejector were studied with respect to the bend angle, swirl angle and the primary to secondary flow temperature ratio. A commercial CFD code was used to evaluate the effectiveness of commercial CFD using limited resources for designing of such devices. The pumping ratio and pressure rise decrease with the increase in the degree of bend. Swirl up to a certain angle has a useful effect on the performance of a bent ejector. The entraining diffuser enhances the performance of a bent ejector. The CFD models based on commercial solver were able to predict the flow structures and the variation of the performance parameters with the bend and swirl angle. However, generally the CFD models were not able to predict the exact values of the performance parameters. / Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2008-03-31 05:53:54.89 / NSERC DND WR Davis

In cryptography especially in block cipher design, Boolean functions are the basic elements. A cryptographic function should have high nonlinearity as it can be attacked by linear attack. In this thesis the highest possible nonlinear boolean functions in the even dimension, that is bent functions, basic properties and construction methods of bent functions are studied. Also normal bent functions and generalized bent functions are presented.

The primary focus of this dissertation is on the analyses of self-resonant bent antennas. The need for the accurate characterization of such antennas due to their growing importance in present day wireless communications is the motivation for this work. To this end, several self-resonant bent antennas are analyzed which includes an inverted-L antenna (ILA), a meander-line dipole (MLD) antenna, a meander-line bow-tie (MLBT) antenna, a dual meander antenna, and a printed meander antenna. A simple analytical model, based on the induced EMF method, is presented to compute the input impedance of the ILA. First, a sinusoidal distribution of current on the antenna, with zero current at the end is assumed, and then an expression for the input impedance is derived using the near-fields of the antenna. The accuracy of the formulation is verified by comparing the results computed using it with that from NEC [1] computation. Unlike the analytical solutions available in the literature, our proposed solution is not restricted to antennas that are electrically small. In addition the new formulation can be extended to treat other antennas, such as the T-antenna, the folded unipole antenna, and the loop-loaded monopole antenna. The input impedance, radiation pattern, and gain of the MLD and MLBT antennas are computed and correlated with their parameters. Input impedances of both antennas are computed using NEC. Simple analytical models are presented to compute the radiation patterns of the MLD and the MLBT antennas. For each antenna, a sinusoidal distribution of current is assumed and closed-form expressions for the radiation fields are derived. The results computed using the analytical models are verified by comparing them with the results from the NEC computation. Since in each model the radiation pattern of an antenna is expressed in terms of ready to evaluate algebraic expressions, the computation of such pattern is fast and easy. The input impedance and radiation characteristics of a dual meander antenna are computed using NEC. Similarly as before the input impedance, radiation pattern, and gain of this antenna are also correlated with its parameters. The input impedance and radiation pattern of a planar printed meander antenna are investigated using the Finite-Difference Time-Domain (FDTD) technique. The antenna is modeled on a dielectric substrate both in the presence and absence of a metallic ground plane. Characteristics of the antenna are examined as function of dielectric constant, and substrate thickness. New results of input impedance, radiation pattern, and gain are presented which are vital for the design of such antennas. Several novel applications of self-resonant bent antennas are described. First, a wide-band dual meander-sleeve antenna is designed, manufactured, and measured for application in dual frequency vehicular personal communication. The antenna can operate simultaneously in the 824-894 MHz and 1850-1990 MHz bands of the PCS system. Second, an MLBT dipole is introduced as a feed for plane sheet reflectors. Numerical results computed using NEC show that the feed when used in front of a plane sheet reflector, results in superior radiation characteristics than a conventional dipole feed, namely, it reduces the reflector dimension by 46% for the same front to back ratio, beam width and gain. Finally, a compact plane sheet reflector antenna is described that uses an MLBT monopole feed. Since the antenna uses a monopole, a balun is not required. This antenna has a gain and half-power beam width of 8.4 dBi and 94° respectively. / Graduate

The existence of a thermotropic biaxial nematic liquid crystal phase has been a topic of great interest for almost half a century. Of the various mesogenic shapes suggested as being able to form this phase, theory has suggested that the V-shaped or "bent-core" molecule is one of the most promising candidates. In this thesis we use a simple mesogenic model of a bent-core molecule, constructed from a number of repulsive Weeks-Chandler-Andersen potentials that are assembled into a rigid V shape. Using this model we explore the spontaneous phase behaviour that occurs in a wide array of different systems of mesogens, using molecular dynamics simulations and isotropic initial conditions. We study the relationship between molecular bend angle and phase behavior for molecules constructed from 11 potentials. We find that the phase behaviour splits into two regions, above and below a critical bend angle. Molecules wider than this angle exhibit isotropic, uniaxial nematic and smectic A phases. Narrower molecules show no uniaxially aligned phases, and instead have a clustered phase with short-range ordering and no global alignment director. Increasing system size improves the smectic layering in the wider molecules, but does not affect the global alignment of the narrower molecules. Our model is extended to include the effect of the arm length of the molecule by changing the number of potentials from which the mesogens are constructed. As the molecule is reduced in size, the critical bend angle is seen to move slowly towards more linear molecules, reducing the size of the parameter space in which uniaxial nematic alignment is possible. At 5 beads, all mesophases are seen to disappear and systems remain isotropic. We also study the behaviour of binary mixtures of bent-core molecules, both of differing arm lengths and of differing bend angles. For arm length mixtures, molecules are seen to remain mixed in the isotropic and nematic phases, and phase separate on transition to a smectic phase. In addition, uniaxial nematic phases are induced in systems that have no nematic phase of their own in isolation. For mixtures of different bend angles, systems remain fully mixed in the smectic phases for differences of up to 10 degrees, and beyond this the two components begin to separate at the nematic–smectic transition.

Σε αυτή τη Μεταπτυχιακή Διπλωματική Εργασία γίνεται μία προσπάθεια κατασκευής ασφαλέστερων block ciphers (Κεφ.1). Αρχικώς παρουσιάζουμε αναλυτικά το θεωρητικό υπόβαθρο της Γραμμικής Κρυπτανάλυσης (Κεφ.2) και της Διαφορικής Κρυπτανάλυσης (Κεφ.3). Στη συνέχεια, προχωρούμε σε μια εφαρμογή της Γραμμικής και Διαφορικής Κρυπτανάλυσης στον αλγόριθμο CAST-128 (Κεφ.4), που είναι ένας σύγχρονος block cipher αλγόριθμος. Στο Κεφ.5 αναφερόμαστε στις σχεδιαστικές παραμέτρους ενός block cipher (S-box Design, Framework Κρυπτογραφήματος, Key Schedule και Round Function). Η συμβολή μας συνίσταται στην κατασκευή κρυπτογραφικά ασφαλών S-boxes με τη χρήση Bent συναρτήσεων και συναρτήσεων Maiorana και η μελέτη της μη γραμμικότητας του προκύπτοντος κρυπτοσυστήματος (Κεφ.6). Στο τελευταίο κεφάλαιο (Κεφ.7) αναφερόμαστε στις μελλοντικές προεκτάσεις και στο ερευνητικό πλάνο που έχουμε θέσει με αφετηρία αυτή τη Μεταπτυχιακή Διπλωματική Εργασία. Στο Παράρτημα Α παραθέτουμε τον Κώδικα Υλοποίησης της κατασκευής των S-boxes. / In this Master Thesis we make an attempt to construct secure block ciphers (Chap.1). First of all, we present in details the theoretical background of Linear Cryptanalysis (Chap.2) and Differential Cryptanalysis (Chap.3). Then we apply Linear and Differential Cryptanalysis in CAST-128 algorithm (Chap.4), which is a modern block cipher algorithm. In Chap.5 we present the design parameters of a block cipher (S-box Design, Feistel Framework, Key Schedule and Round Function). Our contribution is the construction of cryptographically secure S-boxes using Bent Functions and Maiorana Functions and also we study the non-linearity of the constructed cryptosystem (Chap.6). In the last Chapter (Chap.7) we present our future research plan, based on this Master Thesis. The Source Code of the S-boxes construction can be found in Appendix A.

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<p>Examensarbetet är utfört på avdelningen för Fluida och mekatroniska system vid Linköpings universitet i uppdrag åt Sunfab Hydraulics AB.</p><p>Höga ljudnivåer är en stor nackdel vid användning av hydrauliska system. Ofta är det hydraulmaskiner av deplacementtyp som står för en stor del av ljudemissionerna. Eftersom maskinerna arbetar med stora tryckskillnader uppstår flödespulsationer vid in- och utlopp som fortplantar sig vidare i systemet. Dessa medför påfrestningar på maskinen som kan leda till läckage eller haveri. Flödespulsationer kan delas upp i två delar; kinematiska och kompressionsberoende. De kinematiska pulsationerna beror på det begränsade antalet cylindrar och kolvarnas varierande hastighet. På grund av oljans kompressibilitet uppstår pulsationer när cylindertrycket hastigt växlar mellan trycknivåerna.</p><p>Arbetet har resulterat i en modell i programmet HOPSAN som kan simulera flödesdynamiken i en axialkolvmaskin med fast deplacement av typen bent-axis. I modellen kan bland annat ventilskivans geometri förändras för att studera hur flödespulsationerna påverkas vid olika driftspunkter.</p><p>Simuleringsmodellen har med två-mikrofon metoden validerats för en maskin med deplacement 47.1 cm3/varv. Mätutrustningen som använts är anpassad för pumpar vilket innebär att vid mätningarna på motorlocket kördes maskinen som pump.</p><p>För att underlätta tillverkningen av krypspår borras små hål vid dess spets. Dessa finns implementerade i modellen för att undersöka hur flödespulsationerna påverkas. Simuleringar påvisar ett större läckflöde mellan njurarna vilket resulterar i en liten minskning av den volymetriska verkningsgraden vid låga varvtal. För att bättre kunna använda modellen till utveckling av befintliga maskiner borde Sunfab se över om hålen kan tas bort.</p><p>För- och nackdelar med stängd respektive öppen geometri kan undersökas i modellen. Den senare innebär att ett korsande flöde mellan hög- och lågtryckssidan tillåts. Simuleringar visar att vid låga varvtal är den volymetriska verkningsgraden lägre för motorlock med öppen geometri. Stängd geometri ger bättre startmoment på grund av frånvaron av läckflöde men ger däremot större flödespulsationer.</p><p>Optimering av krypspåren visar att maximala tvärsnittsarean kan justeras lite för att minska amplituden av flödespulsationerna.</p>