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Preprint
A free boundary approach to the Rosensweig instability of ferrofluids
(2017-04-18)
We establish the existence of saddle points for a free boundary problem describing the two-dimensional free surface of a ferrofluid which undergoes normal field instability (also known as Rosensweig instability). The starting point consists in the ferro-hydrostatic equations for the magnetic potentials in the ferrofluid and air, and the function describing their interface. The former constitute the strong form for the Euler-Lagrange equations of a convex-concave functional. We extend this functional in order to ...
Dissertation
Transfer characteristics of white light interferometers and confocal microscopes
(2017-11-23)
Optical profilers present significant advantages in comparison with stylus profilometry and scanning probe microscopy due to their non-contacting technique and their ability to scan the entire measurement field simultaneously. However, optical profilers suffer from different systematical artifacts. The dissertation investigates the transfer characteristics of optical profilometers, namely white light interferometers and confocal microscopes.
In white light interferometry dispersion in the optical system leads to ...
Dissertation
Development, Evaluation and Optimization of Fabrication Processes for Daylight Guiding Systems Based on Micromirror Arrays
(2017-01-11)
In this work, fabrication processes for daylight guiding systems based on micromirror arrays are developed, evaluated and optimized.Two different approaches are used: At first, nanoimprint lithography is used to fabricate large area micromirrors by means of Substrate Conformal Imprint Lithography (SCIL).Secondly,a new lithography technique is developed using a novel bi-layered photomask to fabricate large area micromirror arrays. The experimental results showing a reproducible stable process, high yield, and is ...
Dissertation
Development and assessment of volume resolving velocimetry for turbomachinery test facilities
(Deutsches Zentrum für Luft- und Raumfahrt DLR, 2017-12-21)
The present thesis assesses the applicability of volume resolving velocimetry
for turbo machinery test facilities. Tomographic particle image velocimetry has been implemented in a transonic cascade wind tunnel. Assessment is based on 3D-3C measurements of the passage vortex in an axial compressor cascade at subsonic inflow (Ma1=0.6). Results show broad consistency with stereoscopic PIV with improved depth resolution. A novel Fast Multiplicative Algebraic Reconstruction Technique (FMART) enables signi cantly faster ...
Dissertation
Optical Sensor Array based Nanospectrometer: Fabrication of high resolution 3D NanoImprint master templates and bonding of Fabry-Pérot filter array onto photodetector array
(2017-05-11)
In the field of optical spectroscopy, spectrometers are high resolution optical sensors that have emerged as a platform for a wide range of application fields. Commercially available spectrometers are bulky and expensive. Hence, this limits its integration into modern mobile and wearable devices. To overcome this limitation, miniaturization of spectrometers is an interesting target for a wide range of emerging applications. The most attractive concept of fabricating miniaturized spectrometers is the Fabry-Pérot filter ...
Dissertation
Design and Numerical Simulation of Dielectric Photonic Crystal Devices and Investigation of an Optical Characterization Method
(2017-11-09)
In this research work, dielectric photonic crystal (PhC) structures based on low refractive index slab waveguide are investigated. These two-dimensional slab waveguide PhC structures work on the principle of guided-mode resonance and find their application in optical filters operated in the near-infrared range. The design and numerical simulation of the presented PhCs are carried out using finite-difference time-domain (FDTD) method. The dielectric materials used in the presented structures are zirconium dioxide/niobium ...
Dissertation
Residual-Layer-Free 3D Nanoimprinting using Novel Hybrid Soft Templates
(2017-08-23)
Hybrid 3D soft templates are designed and evaluated for residual layer free 3D imprinting in UV-Substrate Conformal Imprint Lithography (SCIL). 3D imprinting usually results in laterally varying residual layers, which cannot be removed by conventional etching processes in a satisfactory manner. The hybrid templates introduced in this thesis are hybrid in a way that their template protrusions are enclosed with a UV-blocking metal layer, by this means selective UV curing of the resist material is attainable. Hence, the ...
Dissertation
Load Reducing Control for Wind Turbines: Load Estimation and Higher Level Controller Tuning based on Disturbance Spectra and Linear Models
(2017-05-08)
The aim of this work is to demonstrate an efficient and pragmatic approach for design and optimization of load reducing controllers for wind turbines. Load reducing control means that, besides the classical rotor speed control loop, load or oscillation signals are included as additional input signals into the controller. Thereby, a multiple-input-multiple-output (MIMO) control design problem arises.
While the basic concepts of load reducing control and the achievable load reductions are essentially known from ...
Masterarbeit
Marketingstrategien zu In-Wert-Setzung von Naturschutzmaßnahmen in der ökologischen Direktvermarktung
(2017-01-31)
Forschungsgegenstand der Arbeit sind Marketingstrategien zur In-Wert-Setzung von Naturschutzmaßnahmen in der ökologischen Direktvermarktung. Vor diesem Hintergrund wurden anhand konkreter Beispiele unterschiedliche Marketingstrategien und Kommunikationskonzepte verglichen und untersucht, wodurch die Aufpreisbereitschaft der KundInnen für den Zusatznutzen „Naturschutz“ beeinflusst wird. Die Untersuchungen fanden zwischen November 2015 und Januar 2016 auf drei Höfen, die Naturschutzmaßnahmen in die ökologische ...
Aufsatz
Documenting the social and environmental consequences of oil palm plantations in Nicaragua
(Department of Organic Food Quality and Food Culture at the University of Kassel, Germany and Federation of German Scientists (VDW), 2017-12-29)
Palm oil production is expanding in many tropical areas worldwide. The implications for the landscape, social relations and living conditions in the respective areas are the subject of a controversial debate. This article explores the social, economic, and environmental impacts caused by palm oil production in two places in south-eastern Nicaragua: Kukra Hill and Boca de Sábalos. From a theoretical perspective of political ecology, this article reflects on the opportunities and limitations of non-governmental ...