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Comparison of different stator topologies for BLDC drives
Mitja Garmut, 2020, magistrsko delo

Opis: The focus of this Master's thesis was to increase the output-power density of a fractional-horsepower BLDC drive. Different stator segmentation topologies were analyzed and evaluated for this purpose. The presented analysis was performed by using various models with different complexity levels, where a Magnetic Equivalent Circuit (MEC) model and a 2D transient Finite Element Method (FEM) model combined with a power-loss model, were applied systematically. Characteristic behavior of the BLDC drive was obtained in this way. The models were validated with measurement results obtained on an experimental test drive system. The influence of the weakening of the magnetic flux density and flux linkage, due to segmentation were analyzed based on the validated models. Furthermore, the increase of the thermal-stable output power and efficiency was rated, due to the consequently higher slot fill factor. Lastly, a detailed iron-loss analysis was performed for different stator topologies. The performed analysis showed that segmentation of the stator can enable a significant increase of the output power of the discussed BLDC drives, where the positive effects of segmentation outweigh the negative ones from the electromagnetic point of view. Segmentation, however, also impacts other domains, such as Mechanical and Thermal, which was out of the scope of this thesis, and will be performed in the future.
Ključne besede: fractional-horsepower BLDC drive, stator segmentation, fill factor increase, thermal-stable output power, Finite Element Method model
Objavljeno: 17.11.2020; Ogledov: 130; Prenosov: 0
.pdf Celotno besedilo (1,69 MB)

Jorge Rodríguez Antolín, 2020, diplomsko delo

Opis: The following work examines the possibility of impregnating chosen model drug into bioaerogels to obtain final formulation with added value. The drug used in this study was esomeprazole, used to treat acid-related diseases. In the first part of the work, bioaerogels were prepared. Polysaccharide aerogels are lightweight biocompatible and biodegradable materials, suitable for applications in pharmaceutical industry. For this purpose, three different cores were prepared: pectin, alginate and their mixture, followed by coating with chitosan layer. The production of the bioaerogels follows a sol-gel synthesis and supercritical drying technique. All samples were characterised, and optimisation was performed based on examined properties. Aerogels having a pectin core and chitosan coating showed the highest surface area and the highest adsorption capacity. In the second part, the impregnation of esomeprazole was performed using two different methods: supercritical impregnation and diffusion via sol-gel synthesis. For supercritical impregnation, supercritical carbon dioxide was used as a solvent for impregnation of the drug. In the diffusion method, the model drug was added during sol-gel synthesis using ethanol as solvent. Finally, complete characterisation of prepared formulation followed by drug release studies was performed. The study showed successful impregnation of esomeprazole using either carbon dioxide or ethanol as a solvent. Bioaerogels proved to be promising as carriers for achieving the optimal release of the chosen drug.
Ključne besede: Bioaerogels, polysaccharides, supercritical impregnation, diffusion method, esomeprazole
Objavljeno: 09.07.2020; Ogledov: 200; Prenosov: 26
.pdf Celotno besedilo (1,28 MB)

Modified coefficient of subgrade reaction to laterally loaded piles
Ayhan Gurbuz, 2018, izvirni znanstveni članek

Opis: An accurate prediction of the load capacity of a laterally loaded pile at a permissible displacement is an important concern at the design stage. In contrast to many sophisticated methods, Broms’ method based on moment equilibrium has been preferred by engineers to predict the load capacities of laterally loaded piles due to both its simplicity and because it is established on a way of hand calculation. However, Broms’ method typically overestimates a pile’s lateral load capacity as it requires a constant coefficient of horizontal subgrade reaction (nh) into analyses, regardless of the magnitude of the pile’s top displacement. In this study, modified coefficients of subgrade reactions(nh*) that are sensitive to the pile’s top displacement in cohesionless soils are first proposed to improve the performance level of Broms’ method for the prediction of the load capacity of a laterally loaded pile as the pile’s top displacement increases. The modified values of nh* are calibrated using 45 independently free-head, single-driven, full-scale pile tests from 23 sites in cohesionless soils. It is demonstrated that Broms’ method with nh* would correctly estimate a pile’s lateral load-deflection behavior with accuracy levels similar to more complicated methods.
Ključne besede: Broms' method, coefficient of horizontal subgrade reaction, p-y curves, strain wedge model
Objavljeno: 11.10.2018; Ogledov: 568; Prenosov: 295
.pdf Celotno besedilo (735,27 KB)
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A simplified method to analyze pile-supported and geosynthetic-reinforced embankments and the influence significance analysis of the design parameters
Liu Feicheng, Zhang Jianjing, Yan Shijie, Cao Licong, 2018, izvirni znanstveni članek

Opis: A simplified method for evaluating a pile-supported embankment reinforced with geosynthetic (PGRS embankment) is proposed in this paper. The method takes into account not only the arching effect, the membrane effect of the deflected geosynthetic, and the subsoil reaction, but also the pile head settlement, which makes the method applicable for floating piles, as well as piles seated on a firm soil layer. The settlement of the subsoil surface is considered to consist of two parts: (a) the settlement of the subsoil surface equals that of the pile cap with no deformation in geosynthetic yet; (b) the subsoil surface subsides along with the geosynthetic deforming, and the deflected geosynthetic being considered as catenary shaped. The formula for the maximum differential settlement between the subsoil surface and the piles is worked out by analyzing the force equilibrium of the geosynthetic and the stress-strain relationship of the geosynthetic at the edge of the pile cap. The comparison of the calculated results with the observed data and the six current analytical methods has been implemented to verify the proposed method. The influence of the tensile stiffness of the geosynthetic, compression modulus of soft soil, soft soil thickness, embankment height, internal friction angle of the embankment fill and the pile spacing on the subsoil reaction, the stress concentration ratio (SCR) and the tension of the geosynthetic are investigated using the proposed method. The influence significance of these factors has been investigated using the evaluation theory of binary variance analysis for the non-repeatability tests, which helps optimize the design of the PGRS embankment.
Ključne besede: pile-supported embankment, geosynthetic, simplified method, influence significance, binary variance analysis
Objavljeno: 11.10.2018; Ogledov: 548; Prenosov: 294
.pdf Celotno besedilo (1,23 MB)
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Effect of particle roundness and morphology on the shear failure mechanism of granular soil under strip footing
Babak Karimi Ghalehjough, Suat Akbulut, Semet Çelik, 2018, izvirni znanstveni članek

Opis: This study investigates the effect of particles roundness and morphology on the shear failure mechanism of soil. A strip footing was modeled under laboratory conditions. Calcareous soil was tested with three roundness classes: angular, rounded and well-rounded shapes with sizes of 0.30 mm to 4.75 mm. These were divided into six different groups at three relative densities of 30%, 50% and 70%. A series of photographs was taken during the tests and analyzed using the particle image velocimetry (PIV) method to understand the soil-deformation mechanism. The results showed that increasing the sample sizes increased the affected area of the soil. At the same time, increasing the relative density caused a punching failure mechanism that went towards the general failure. The shear failure mechanism of the soil changed from general toward punching shear failure with increasing particle roundness. This effect was larger with the smaller materials. Underneath the affected layers of soil, the angular samples were deeper than the rounded and well-rounded samples. The affected depth in the angular soil was approximately 1.5B in the smallest size group. This was more than 3B and near 4B in the largest size group. Both the sides and the underlying soil layers should be considered on angular soils. The area under the footing becomes more important than the side parts after increasing the roundness of the particles.
Ključne besede: particle roundness, morphology of particles, shear failure mechanism, strip footing, PIV method, ultimate bearing capacity
Objavljeno: 11.10.2018; Ogledov: 654; Prenosov: 265
.pdf Celotno besedilo (701,47 KB)
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Numerical study of the dynamic active lateral earth pressure coefficient of cohesive soils
Mehrab Jesmani, Hossein Alirezanejad, Hamed Faghihi Kashani, Mehrad Kamalzare, 2017, izvirni znanstveni članek

Opis: Retaining walls are proposed in many projects, such as bridges, coastal structures, road constructions and wherever lateral support is required for the vertical surface of an excavation. The active lateral pressure coefficient of soil, Ka , is an important parameter for studying the static and dynamic behaviors of these retaining walls. Many studies have evaluated this coefficient in static situations, but in most previous dynamic studies, researchers have worked on the behavior of cohesionless backfill soil or made simplifying assumptions (e.g., pseudo-static status) for cohesive soils as backfill soil. In this study, the size of the active lateral earth pressure coefficient (Ka) was studied in a full dynamic situation (Kae). A retaining wall with cohesive backfill soil is evaluated using the finite-difference method (FDM) and the effects of important soil and loading properties are assessed. The model is based on Mohr-Coulomb failure criteria under seismic loading. The results show that the value of Kae at the top of the wall, where it is highly sensitive to any variation in the soil and loading properties, is greater than one due to the high pressure value induced by the horizontal dynamic acceleration and the presence of tension cracks.
Ključne besede: dynamic active lateral earth pressure coefficient (Kae), cohesive backfill soil, finite difference method (FDM), tension cracks, retaining wall, seismic loading
Objavljeno: 18.06.2018; Ogledov: 623; Prenosov: 43
.pdf Celotno besedilo (3,26 MB)
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Reinforced piled embankment for a high-speed railway over soft soil
Yan Zhuang, Xiaoyan Cui, 2015, izvirni znanstveni članek

Opis: A geosynthetic, reinforced, piled embankment is an effective and economic method to solve the problems of possible bearing failure, unacceptable settlement and slope instability for an embankment built over soft soil; this has led to its widespread use, especially for high-speed railway embankments. Some design methods have been developed to assess the performance of these reinforced structures, which are mainly based on the results from small-scale models and numerical simulations. However, the reliability of these methods needs to be validated under full-scale field tests. This paper presents a numerical and analytical study for a full-scale field test of the Fengyang high-speed railway embankment. The results were analyzed and discussed in terms of the settlement of subsoil, the stressconcentration ratio (SCR), the axial force and the frictional stress of the pile. They showed that the settlement of the subsoil, from both the finite-element method (FEM) and the analytical method, were in good agreement with the measurement, and thus was a reliable parameter to assess the performance of the piled embankment with reasonable accuracy. The SCR was overestimated by the modified Terzaghi method, with a difference of 25%, while it was underestimated by the FEM, with a difference of approximately 20%. It was also shown that the tensile force in the reinforcement could be effectively assessed using the proposed analytical method, while it was overestimated by the FEM with a difference of 44%.
Ključne besede: reinforced piled embankment, high-speed railway, numerical simulation, analytical method
Objavljeno: 15.06.2018; Ogledov: 558; Prenosov: 35
.pdf Celotno besedilo (578,98 KB)
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Numerical simulation of intact rock behaviour via the continuum and Voronoi tesselletion models
Teja Fabjan, Diego Mas Ivars, Vladimir Vukadin, 2015, izvirni znanstveni članek

Opis: The numerical simulation of intact rock microstructure and its influence on macro-scale behaviour has received a lot of attention in the research community in recent years. Generating a grain-like structure with polygonal area contacts is one of the avenues explored for describing the rock’s microstructure. A Voronoi tessellation implemented in the Universal Distinct-Element Code (UDEC) is used to generate models with a polygonal microstructure that represent intact rock. The mechanical behaviour of the Voronoi polygons is defined by micro-properties, which cannot be measured directly in the laboratory. A numerical calibration procedure is needed to produce the macroscopic response of a model that corresponds to the material behaviour measured during a laboratory experiment. In this research, Brazilian, direct tensile, uniaxial compressive and biaxial test models are constructed to simulate the intact rock behaviour under a standard laboratory stress. An extensive series of parametric sensitivity analyses are executed in order to understand the influence of the input micro-properties on every model test behaviour and predict the relation between the micro-properties and the model’s macro response. The results can be treated as general guidelines for a complete and efficient intact rock calibration procedure. In parallel, a continuum-based model using the Mohr-Coulomb constitutive relationship is running as a benchmark. It has been shown that the Voronoi-based models through their microstructure approach better reproduce the Brazilian to direct tensile strength ratio, and show a better representation of the dilation, crack pattern and post-peak behaviour in comparison to continuum models.
Ključne besede: distinct-element method, parametric sensitivity analysis, intact rock, Voronoi tessellation, micromechanical properties, standard laboratory test
Objavljeno: 15.06.2018; Ogledov: 619; Prenosov: 59
.pdf Celotno besedilo (1,57 MB)
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Influence of the virtual strain rate of noncohesive granular media on the discrete element method
Kiichi Suzuki, 2015, izvirni znanstveni članek

Opis: The discrete element method (DEM) is an alternative computational tool for augmenting laboratory experiments because of its advantages in detailing macro- and micro-mechanical information. However, it should be noted that the DEM does not usually consider the convergence for each time step, because of the necessity for a huge calculation time. In that case, it indicates that the uniqueness of the solution is not guaranteed, except in the case of a very small strain rate, even though the behavior looks qualitatively reasonable. At first, the influence of strain rate among numerically imaginary input parameters for a non-cohesive material was investigated for monotonic, biaxial shear tests. Then, new findings were obtained from the DEM simulations. Strain rate has a significant influence on the shear behavior, especially after the peak strength of dense specimens. A quasi-static steady state exists, not a static steady state. The “strong” fabric ratio is closely related to the stress ratio. The maximum slip coordination number occurs around the phase-transformation ratio and the shear band appears around the peak strength.
Ključne besede: discrete element method, DEM, induced anisotropy, quasi-static steady state, strain rate, uniqueness
Objavljeno: 14.06.2018; Ogledov: 403; Prenosov: 40
.pdf Celotno besedilo (690,97 KB)
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Design optimization for symmetrical gravity retaining walls
Erol Sadoğlu, 2014, izvirni znanstveni članek

Opis: The optimization for symmetrical gravity retaining walls of different heights is examined in this study. For this purpose, an optimization problem of continuous functions is developed. The continuous functions are the objective function defined as the cross-sectional area of the wall and the constraint functions derived from external stability and internal stability verifications. The verifications are listed as the overturning, the forward sliding, the bearing capacity, the shears in the stem and the bendings in the stem. The heights of the walls are selected as 2.0, 3.0, and 4.0 m in order to investigate the outline of the optimum cross-section and the effect of the wall height on the outline. Additionally, the physical and mechanical properties of the soil are kept constant in order to compare only the effect of the height on the geometry. The upper and lower bounds of the solution space are specified to be as wide as possible and the minimum dimensions suggested for the gravity retaining walls are not taken into account. A common feature of the optimum cross-sections of walls with different heights is to have a very wide lower part like a wall foundation and a slender stem. However, other than the forward sliding constraint, the bending constraints are active at the optimum values of the variables.
Ključne besede: gravity retaining wall, nonlinear optimization, continuous variables, interior point method
Objavljeno: 14.06.2018; Ogledov: 461; Prenosov: 45
.pdf Celotno besedilo (168,77 KB)
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