Development of a wind gust model to estimate gust speeds and their return periods

Spatially dense observations of gust speeds are necessary for various applications, but their availability is limited in space and time. This work presents an approach to help to overcome this problem. The main objective is the generation of synthetic wind gust velocities. With this aim, theoretical wind and gust distributions are estimated from 10 yr of hourly observations collected at 123 synoptic weather stations provided by the German Weather Service. As pre-processing, an exposure correction is applied on measurements of the mean wind velocity to reduce the influence of local urban and topographic effects. The wind gust model is built as a transfer function between distribution parameters of wind and gust velocities. The aim of this procedure is to estimate the parameters of gusts at stations where only wind speed data is available. These parameters can be used to generate synthetic gusts, which can improve the accuracy of return periods at test sites with a lack of observations. The second objective is to determine return periods much longer than the nominal length of the original time series by considering extreme value statistics. Estimates for both local maximum return periods and average return periods for single historical events are provided. The comparison of maximum and average return periods shows that even storms with short average return periods may lead to local wind gusts with return periods of several decades. Despite uncertainties caused by the short length of the observational records, the method leads to consistent results, enabling a wide range of possible applications

Entwicklung von korrosionsstabilen Schutzschichten für oxidische Faserverbundwerkstoffe

Ziel der Arbeit war die Herstellung und Entwicklung von Schutzschichten ($\textit {environmental barrier coatings}$, EBC) für einen Aluminiumoxid-basierten Faserverbundwerkstoff ($\textit {ceramic matrix composite}$, CMC). Verschiedene Oberflächenbehandlungen des CMCs wurden untersucht, um die Anbindung der Schicht an das Substrat zu verbessern. Dabei stellte sich die Strukturierung mittels Laserablation als besonders geeignet heraus. YSZ, Gd$_2$Zr$_2$O$_7$, Y$_2$O$_3$ und YAlO$_3$ wurden als potenzielle Schutzschichten ausgewählt und mittels Plasmaspritzen auf Proben des CMC mit und ohne vorherige Strukturierung aufgebracht. Die Schichtsysteme wurden anschließend mittels Haftzugversuchen und thermischem Zyklieren untersucht. Darüber hinaus wurden ausgewählte Proben einem neu entwickelten Korrosionstestverfahren unterzogen, bei dem mithilfe eines HVOF-Brenners innerhalb kurzer Zeit aussagekräftige Korrosionsraten gemessen werden können.The focus of this work is the development of EBCs for an alumina-based CMC, especially the improvement of the adhesion between coating and CMC. First, different methods of surface structuring of the CMC were investigated. The structuring by laser ablation turned out to be promising and was investigated in detail. Subsequently, YSZ, Gd$_2$Zr$_2$O$_7$, Y$_2$O$_3$ und YAlO$_3$ were selected as potential protective coatings due to their low recession rates and matching thermal expansion coefficients. Samples of the CMC, with and without laser structuring were coated by plasma spraying. The suitability of these coating systems were investigated by tensile adhesion tests and thermal cycling. Since the corrosion stability is also important for an EBC, selected samples were used to validate a new corrosion test procedure in which significant corrosion rates can be measured within a short time by using a HVOF torch

Manufacturing technology for the fabrication a discrete actuator using a magnetic shape memory alloy

To demonstrate the feasibility of combining thin-film fabrication techniques and discrete MSM stripes, a hybrid actuator system was designed. The actuator system consists of four thin-film stators and two discrete MSM stripes mounted in a row. For creating an actuator motion, one of the MSM stripes has to be excited by a pair of stators to cause variant switching. This results in an elongation plus a compression of the second stripe and vice versa. The technologies required for fabricating the thin-film stator are sputter deposition, PECVD, electroplating, etching, and photolithography. This paper describes the fabrication process for a thin-film stator used for the MSM micro actuator system. It also presents technology study results indicating the feasibility of the planned fabrication sequence

The Role of Zinc Layer During Wetting of Aluminium on Zinc-coated Steel in Laser Brazing and Welding

AbstractThe zinc layer of zinc-coated steel is known to be a crucial factor for the spreading of liquid aluminium on the coated surface. For industrial brazing and welding processes these zinc-coatings enable a fluxless joining between aluminium and steel in many cases. Yet, the reason for the beneficial effect of the zinc to the wetting process is not completely understood. Fundamental investigations on the wetting behaviour of single aluminium droplets on different zinc-coated steel surfaces have revealed a distinct difference between coated surfaces at room temperature and at elevated temperature regarding the influence of different coating thicknesses.In this paper the case of continuous laser brazing and welding processes of aluminium and commercial galvanized zinc-coated steel sheets are presented. It is shown that in the case of bead-on-plate laser beam brazing, the coating thickness has a measureable effect on the resulting wetting angle and length but does not have a significant impact in case of overlap laser beam welding. This might be linked to different heat transfer conditions. The results also strongly indicate that proper initialbreakup of oxide layers is still required to accomplish good wetting on zinc-coated surfaces

An 18-year climatology of derechos in Germany

Derechos are high-impact convective wind events that can cause fatalities and widespread losses. In this study, 40 derechos affecting Germany between 1997 and 2014 are analyzed to estimate the derecho risk. Similar to the United States, Germany is affected by two derecho types. The first, called warm-season-type derechos, form in strong southwesterly 500 hPa flow downstream of western European troughs and account for 22 of the 40 derechos. They have a peak occurrence in June and July. Warm-season-type derechos frequently start in the afternoon and move either eastward along the Alpine forelands or northeastward across southern central Germany. Associated proximity soundings indicate strong 0–6 and 0–3 km vertical wind shear and a median of mixed-layer convective available potential energy (mixed-layer CAPE) around 500 J kg−1. The second derecho type, the cold-season-type derecho, forms in strong northwesterly 500 hPa flow, frequently in association with mid-tropospheric potential vorticity (PV) intrusions, and accounts for 18 of the 40 derechos. They are associated with a secondary peak from December to February. Cold-season-type derechos start over or close to the North Sea and primarily affect northern and central Germany; their start time is not strongly related to the peak of diurnal heating. Proximity soundings indicate high-shear–low-CAPE environments. A total of 15 warm-season-type and 9 cold-season-type derechos had wind gusts reaching 33 m s−1 in at least three locations. Although warm-season derechos are more frequent, the path length of cold-season-type derechos is on average 1.4 times longer. Thus, these two types of German derechos are likely to have similar impacts