Complex Centers of Polynomial Differential Equations

We present some results on the existence and nonexistence of centers for polynomial first order ordinary differential equations with complex coefficients. In particular, we show that binomial differential equations without linear terms do not complex centers. Classes of polynomial differential equations, with more than two terms, are presented that do not have complex centers. We also study the relation between complex centers and the Pugh problem. An algorithm is described to solve the Pugh problem for equations without complex centers. The method of proof involves phase plane analysis of the polar equations an a local study of periodic solutions.Comment: 18 page

Polynomial Differential Equations with Small coefficients

Classes of polynomial differential equations of degree n are considered. An explicit upper bound on the size of the coefficients are given which implies that each equation in the class has exactly n complex periodic solutions. In most of the classes the upper bound can be improved when we consider real periodic solutions. We present a proof to a recent conjecture on the number of periodic solutions. The results are used to give upper bounds for the number of limit cycles of polynomial two-dimensional systems.Comment: 15 page

A Review of Control Techniques Future Trends in Wind Energy Turbine Systems with Doubly Fed Induction Generators (DFIG)

تعتبر طاقة الرياح حاليا واحدة من أكثر مصادر الطاقة الخضراء النظيفة الملاءمة على نطاق واسع في العالم. تم تطوير العديد من مبادئ توربينات الرياح بستخدام  المولدات المختلفة لتحويل طاقة الرياح المتاحة إلى طاقة كهربائية. يعد نظام المولد الحثي ذي التغذية المزدوجة DFIG لتوربينات الرياح ذات السرعة المتغيرة نسبيا (VSWT) هو الأكثر ملاءمة لطاقة توربينات الرياح بسبب فوائده العديدة مقارنة بتوربينات الرياح ذات السرعة الثابتة نسبيا (FSWT). تقدم هذه الورقة مراجعة و مقارنة عن طاقة توربينات الرياح المختلفة وملخصًا قيمًا للعمل الأخير المتعلقة بأنظمة طاقة الرياح المختلفة (WECS) لنمذجة DFIG وأقصى نقطة طاقة MPP وأحدث نظام تحكم للتشغيل. ومن ناحية أخرى تم في الدراسة الحالية تقديم مقارنات ومناقشات بين توربينات الرياح المختلفة لتكون مفيدة للدراسات البحثية.Wind energy is currently widely regarded as one of the most favorable green clean sources of energy. Several wind turbine principles with various generator architectures have been evolved to exchange the available wind energy into electric power. The DFIG partial Variable-Speed Wind Turbine (VSWT) system is most proper for wind turbine energy because of its numerous benefits over Fixed-Speed Wind Turbines (FSWT). This paper introduces a comparative review of the different wind turbine conversion energy and a valuable summary of the recent work in the literature on different Wind Energy Conversion Systems (WECS) of a DFIG modeling, Maximum Power Point (MPP), and the latest control system for operation. On the other side, comparisons and discussions between different wind turbines have been presented in the current study to be beneficial for research studies

Preparation of Cu3SbSe4 Doping with Aluminum and Studying on its Thermoelectrical Properties

في هذا البحث تمت دراسة خصائص الانتقال الكهربائي الحراري والناقلية الكهربائية للمركب AlxSe4 Cu3Sb1-x (x = 0, 0.03, 0.05 and 0.07) ولمدى من درجات الحرارة المطبقة  (298 – 553) Kوقد اظهرت النتائج  ان الزيادة في قيمة ( x = 0) to (x =0.07), زيادة في تركيز الفجوات من( 2.03×1018 الى 2.82 × 1018 cm-3) تبعا لتغير احلال او استبدال Al3+ مكان Sb5+ والذي سوف يؤدي الى نقصان كبير في قيم المقاومية الكهربائية  للمركب  Cu3Sb1-xAlxSe4، وفي نفس الوقت فان الزيادة الحاصلة في تركيز  الفجوات سوف تؤدي الى انتقال من حالة الهبوط الحاد عند (x = 0)الى النزول التدريجي او الجزئي عند(x = 0.05, 0.07) وان حالة الاستقرار ستكون عند x = 0.07))  وان عامل القدرة (PF)لكل النماذج المشوبة Cu3Sb1-xAlxSe4 ظهر هنالك تحسن فيما يخص تركيز الفجوات .وان العامل kL  ووالذي يمثل عامل التوصلية الحرارية للشبيكة يتناقص في حالة الزيادة العالية من التشويب  أي عند النموذج(x =0.07) ،وكنتيجة نهائية فان المؤشر الكبير  لقيمة  ZT = 1.28 والتي تم الحصول عليها للمركب ذو الصيغة Cu3Sb0.97Al0.03Se4   في درجة  458K, كبيرة بقدر خمسة مرات  كما لو في النموذج Cu3SbSe4والذي يمثل المركب بدون تشويبThe electrical transport and thermoelectric properties of Cu3Sb1-xAlxSe4 (x = 0, 0.03, 0.05 &0.07) compounds are investigated in the temperature range of (298 – 553) K. The results indicate that with increasing Al content from ( x = 0) to (x =0.07), hole concentration increases monotonically from (2.03×1018 to 2.82 × 1018 cm-3) due to the substitution of Al3+ for Sb5+, thus leading to a large decrease in the electrical resistivity of Cu3Sb1-xAlxSe4. Meanwhile, the increase in hole concentration leads to a transition from a non-degenerate (x = 0) to a partial degenerate (x = 0.05, 0.07) and then to a degenerate state   (x = 0.07). The power factor (PF) of all the Al-doped Cu3Sb1-xAlxSe4 samples is remarkably improved due to the optimization of hole concentration. Lattice thermal conductivity kL of the heavily doped sample(x = 0.07) is reduced. As a result, a large thermoelectric figure of merit ZT = 1.28 is obtained for Cu3Sb0.97Al0.03Se4 at 458K, which is around 5 times as large as that of the un-doped Cu3SbSe4 sample

Behavior of One – Way Concrete Slabs with Edge Beams Reinforced/Strengthened by CFRP Rods under Uniformly Distributed Load

This research presents an experimental investigation on the flexural behavior of eight one-way concrete slabs with edge beams under uniformly distributed load. The models are reinforced or strengthened using CFRP rods and two control models reinforced by deformed steel bars.The dimensions of one-way slab is 1.05 m width, 1.25 m length and 0.1 m thick., while each edge beam is of length 1.25m and depth 0.2m by width 0.1m. Different reinforcement ratios were used. The models were tested under universal testing machine and supported at corners on four stiff steel columns. The models were tested up to failure to study their flexural behavior including load-deflection curves, crack patterns and mode of failure. Among the conclusions obtained, the models reinforced by CFRP rods can attain flexural strength higher than those reinforced by deformed steel bars of same amount. This increase is about (38-44%)

Effects of Steel Fibers Geometry on the Mechanical Properties of SIFCON Concrete

This research aims to shed light on the effect of steel fiber shape, length, diameter, and aspect ratio on the mechanical properties of slurry infiltration fiber reinforced concrete (SIFCON). This study comprised of casting and testing three groups of SIFCON specimens with 6% fiber volume fraction. The first group was reinforced with micro steel fiber, other reinforced by hook end steel fibers, while the last group of specimens reinforced by mixing two shape of steel fiber as hybrid fiber (3% micro steel fiber +3% hook end steel fiber). Silica fume was used as a partial replacement (10%) by weight of cement. 3.7% super plasticizer was used to make the slurry liquid enough to penetrate through the fiber network, while the w/c ratio kept constant at 0.33. It was found from the results achieved that the compressive strength, static modulus of elasticity, splitting tensile strength and toughness are extremely affected by the geometry of fibers because the network of fibers formed and their density depends on the size and shape of fibers. Where the values of micro steel fibers are far outweighing the values of hooked end fibers. It was also deduced from empiricism results that combining long and short fibers gives excellent results

SYNTHESIS, CHARACTERIZATION, AND ANTIMICROBIAL EVALUATION OF NEW N-PHENYLCINNAMAMIDE DERIVATIVES LINKED TO ASPIRIN AND IBUPROFEN

Objective: The objective of this study was to synthesize phenylcinnamamide (substituted acetanilide chalcone) derivatives linked to aspirin and ibuprofen with potential antibacterial and antifungal activity.Methods: Substituted acetanilide compounds were reacted with different arylaldehydes through Claisen-Schmidt condensation in the presence of KOH. They formed differently substituted acetanilide chalcones (1a-e) which are linked to aspirin and ibuprofen through an ester linkage to form compounds (2a-j) using ethyl chloroformate (ECF) as a catalyst.Results: The synthesized compounds have been characterized by elemental analysis, Fourier transform infrared and1H-nuclear magnetic resonance spectroscopy. An antibacterial evaluation was achieved for Gram-positive bacteria (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli) and antifungal for Candida albicans.Conclusion: Compounds (2a-j) have shown intermediate antimicrobial activity against different strains of microorganisms

Periodic solutions with nonconstant sign in Abel equations of the second kind

The study of periodic solutions with constant sign in the Abel equation of the second kind can be made through the equation of the first kind. This is because the situation is equivalent under the transformation $x\mapsto x^{-1}$, and there are many results available in the literature for the first kind equation. However, the equivalence breaks down when one seeks for solutions with nonconstant sign. This note is devoted to periodic solutions with nonconstant sign in Abel equations of the second kind. Specifically, we obtain sufficient conditions to ensure the existence of a periodic solution that shares the zeros of the leading coefficient of the Abel equation. Uniqueness and stability features of such solutions are also studied.Comment: 10 page

Recommendations for the Classroom Technology and Layout at WPI

The Academic Technology Center (ATC) at WPI replaces the technology in each classroom every five years. The goal of this project, sponsored by the ATC, was to determine how current classroom technology could be improved to enhance the teaching and learning experience. Using interviews and surveys we gathered both qualitative and quantitative information from more than 140 faculty members and 550 students. Based on those findings, we formulated recommendations that include increasing the size of the podium tops and moving the projector screen to maximize board space