Leibniz University Hannover Research Articles

Studying Geodesy and Geoinformatics - a German university perspective

Abstract. This paper describes the discipline of geodesy and geoinformatics with a focus on university studies and career opportunities. It is shown how the disciplines contributes to the solutions of major societal challenges such as sustainable development, mobility and global change. Then, the structure of German universities is shortly explained, followed by an overview of where in the country the discipline can be studied. Finally, the example of Leibniz University Hannover, one of the prime centres of excellence in the field, is described in more detail. We thus hope to encourage young talents to consider this exciting field of studies for their future career.

Höhere Achsdynamik durch Mehrkoordinatenantriebe in Werkzeugmaschinen

Abstract The Institute of Production Engineering and Machine Tools (IFW) at Leibniz University Hannover has developed a novel linear-rotary direct drive for machine tools. This combines linear and rotary movements in a single drive. As a result, the overall dynamics of machine axes can be improved and the installation space reduced.

Experimental Investigation of a Free-Form Honed Cylinder Liner for Heavy-Duty Engines

For future internal combustion engines, driven by regenerative fuels, efficiency is more important than ever. One approach to reduce the losses inside the piston cylinder unit (PCU) is to improve the alignment of the liner and the piston. Therefore, a cylinder liner with a free form was developed at the Institute of Technical Combustion (ITV) of the Leibniz University Hannover which compensates radial and linear deformations along the stroke. The layout is based on a FEM simulation. The liner was manufactured by the Institute of Production Engineering and Machine Tools (IFW) of Leibniz University of Hannover with a novel turn-milling process. The liner was investigated on the heavy-duty Floating-Liner engine of ITV with a displacement of 1991 ccm and a bore diameter of 130 mm. The experimental results show improvement in the friction losses over the whole engine map in the range of 9% and up to 17.3% compared to a serial liner. Sealing efficiency could be improved up to 28.8%, depending on the operational point. Overall, the investigation aims for lower fuel consumption which would in result fewer emissions.

STABILITY OF BURIED SCOUR PROTECTION IN SHALLOW COASTAL WATERS

Physical model tests were conducted to verify the stability of a conceptual scour protection design placed around monopiles in varying water depth and under combined wave and current load. The tests were performed in a 3D wave-current basin at Ludwig-Franzius-Institute, Leibniz University Hannover in a model scale of 1:35. As a unique boundary condition compared to previous studies, the scour protection around the monopile was placed below a mobile sand bed. Before waves and current could act on the scour protection, the overlaying sand layer had to be eroded. The scouring process of this sand layer was evaluated, and the stability of the armour layer of the underlying scour protection quantitatively assessed. The characteristics of the scour development and the damage of the armour layer were described as a function of several wave-current load combinations and different morphodynamic stages of the beach.

NUMERICAL ANALYSIS OF EARLY AGE MOVEMENT IN GROUTED CONNECTIONS

AbstractGrouted connections (GC) of offshore structures are susceptible to wave loading and current directly after grouting. Early age movement (EAM) during the curing process can affect the material properties and the interface quality between the grout and the steel structure. Such effects were observed in laboratory tests performed at the Leibniz University Hannover. Generally, EAM can be characterised by high shear rates and identified as a damage mechanism. In this contribution, the authors present a numerical approach that transfers the observed damage mechanism into an ultimate strength analyses of the GCs. For this purpose, EAM simulations and laboratory tests with fluid grout material are combined to determine areas of high shear rates, which mainly affect the grout properties. With an element‐based adjustment approach, the material law of each element is adjusted according to the level of shear rate. The adjusted finite element model is analysed on its maximum axial load resistance.

Numerical and laboratory investigations of thermally induced fractures in rock salt

In order to widen the basis for the dimensioning of storage caverns in rock salt, some special attention must be paid to the temperature developments within the cavity during its operation. Passed numerical investigations have shown that a temperature change in the storage medium, such as natural gas or hydrogen, can cause stress differences of several MPa in the rock mass surrounding the cavern. If, as a result, the difference between horizontal and vertical stresses is increased, the rock being additionally loaded by a fluid pressure can fracture orthogonally to the lowest principal stress. The knowledge of temperature conditions and heat flows in the cavern surrounding salt rock are therefore of great importance. A wide variety of factors must be considered when investigating such mechanisms. The novel test facility at the Institute of Geotechnical Engineering of the Leibniz University Hannover is used to simulate the situation that occurs at the caverns edge when storage medium is withdrawn. This means, the cooling and the gas pressure inside the cavity are reconstructed on hollow cylindrical test samples made of rock salt from various locations to draw conclusions about the propagation of fractures and thus the safety of salt caverns under gas-loading. Before inducing any mechanical loads by means of a triaxial cell, a comprehensive investigation of the temperature field, resulting from the locally limited, artificially induced cooling of a sample is carried out with the new testing facility. While local temperatures in the sample drop by about 20 Kelvin, it is free to contract inwards, causing thermal stresses to decrease significantly. Therefore, the facility needs to be optimized and the testing scheme is changed. Instead of reducing stresses thermally, one of the mechanical loadings is diminished while the gas pressure inside the borehole is kept constant. As a result, the material visibly fractures while mechanical loadings are at a fully-compressive state.

Fiber Reinforced Concrete with Natural Plant Fibers—Investigations on the Application of Bamboo Fibers in Ultra-High Performance Concrete

Natural plant fibers represent a sustainable alternative to conventional fiber reinforcement materials in cementitious materials due to their suitable mechanical properties, cost-effective availability and principle carbon neutrality. Due to its high tensile strength and stiffness as well as its worldwide distribution along with rapid growth, bamboo offers itself in particular as a plant fiber source. In experimental studies on concrete beams reinforced with plant fibers, a positive influence of the fibers on the flexural behavior was observed. However, the load-bearing effect of the fibers was limited by the poor bond, which can be attributed, among other things, to the swelling behavior of the fibers. In addition, the plant fibers degrade in the alkaline environment of many cementitious building materials. In order to improve the bond and to limit the alkalinity and to increase the durability, the use of ultra-high performance concrete (UHPC) offers itself. Since no tests have been carried out, investigations on the flexural behavior of UHPC with bamboo fibers were carried out at the Institute of Concrete Construction of Leibniz University Hannover. The test results show a significantly improved load-bearing behavior of the fibers and the enormous potential of the combination of UHPC and bamboo fibers.

Public opinion in the site selection process: survey methodologies

Abstract. The aim of the site selection process (StandAV) is to find the repository site for high-level radioactive waste with the best possible safety. Transparency and public involvement contribute to ensuring that the outcome of the site selection process is “supported by a broad consensus in society and can thus also be tolerated by those affected” (StandAG § 5 para. 1). In order for the public to be suitably informed and involved, BASE (Federal Office for the Safety of Nuclear Waste Management), in particular, as the body responsible for public participation, must base its work on the knowledge, attitudes and needs of the German population. In order to gain the respective knowledge, the company aproxima Gesellschaft für Markt- und Sozialforschung Weimar mbH was commissioned to conduct a repetitive representative survey (research project EWident). The study briefly deals with nuclear energy in general, but mainly focuses on questions related to the topic of final disposal (knowledge of the topic and especially the StandAV, interest in information and involvement, relevance of particular questions in the process, etc.). The first survey took place in summer 2020. A special focus was placed on the younger generation (14- to 29-year-olds). The results can be viewed on the BASE website (Götte and Ludewig, 2020). They show that the level of knowledge about and interest in StandAV are still rather feeble; at the same time, it is clear that questions of safety are a central driver for accepting a possible site in one's own region. A second survey is planned for autumn 2021. A comparably broadly drawn survey has not been carried out in recent years. However, the fact that the topic of final disposal is becoming increasingly important is shown by thematically related surveys that ask about attitudes towards nuclear energy or examine particular aspects of StandAV in more detail. Within the framework of the TRANSENS research association (Transdisciplinary Research on the Management of High-Level Radioactive Waste in Germany), for example, trust and confidence in the context of the site selection process were surveyed. The 60 min workshop is aimed at interested parties and researchers who would like to contribute their own experiences with related surveys. In the workshop we provide three incentives. aproxima introduces the methods and key results of EWident's first survey. BASE gives an insight into the practical use of these results. Roman Seidl of Leibniz University Hannover reports the results of the TRANSENS survey. Subsequently, we would like to discuss the following questions, among others, with the participants in the workshop. What is remarkable about the results? Do they differ from other StandAV surveys and experiences, and if so, how? How can this be explained? What further research needs can be identified? With what approaches could they be met?

CNN-based multi-class multi-label classification of sound scenes in the context of wind turbine sound emission measurements

Within the scope of the interdisciplinary project WEA-Akzeptanz, measurements of the sound emission of wind turbines were carried out at the Leibniz University Hannover. Due to the environment there are interfering components (e. g. traffic, birdsong, wind, rain, ...) in the recorded signals. Depending on the subsequent signal processing and analysis, it may be necessary to identify sections with the raw sound of a wind turbine, recordings with the purest possible background noise or even a specific combination of interfering noises. Due to the amount of data, a manual classification of the audio signals is usually not feasible and an automated classification becomes necessary. In this paper, we extend our previously proposed multi-class single-label classification model to a multi-class multi-label model, which reflects the real-world acoustic conditions around wind turbines more accurately and allows for finer-grained evaluations. We first provide a short overview of the data acquisition and the dataset. We then briefly summarize our previous approach, extend it to a multi-class multi-label formulation, and analyze the trained convolutional neural network regarding different metrics. All in all, the model delivers very reliable classification results with an overall example-based F1-score of about 80 % for a multi-label classification of 12 classes.

Earth’s Time-Variable Gravity from GRACE Follow-On K-Band Range-Rates and Pseudo-Observed Orbits

During its science phase from 2002–2017, the low-low satellite-to-satellite tracking mission Gravity Field Recovery And Climate Experiment (GRACE) provided an insight into Earth’s time-variable gravity (TVG). The unprecedented quality of gravity field solutions from GRACE sensor data improved the understanding of mass changes in Earth’s system considerably. Monthly gravity field solutions as the main products of the GRACE mission, published by several analysis centers (ACs) from Europe, USA and China, became indispensable products for quantifying terrestrial water storage, ice sheet mass balance and sea level change. The successor mission GRACE Follow-On (GRACE-FO) was launched in May 2018 and proceeds observing Earth’s TVG. The Institute of Geodesy (IfE) at Leibniz University Hannover (LUH) is one of the most recent ACs. The purpose of this article is to give a detailed insight into the gravity field recovery processing strategy applied at LUH; to compare the obtained gravity field results to the gravity field solutions of other established ACs; and to compare the GRACE-FO performance to that of the preceding GRACE mission in terms of post-fit residuals. We use the in-house-developed MATLAB-based GRACE-SIGMA software to compute unconstrained solutions based on the generalized orbit determination of 3 h arcs. K-band range-rates (KBRR) and kinematic orbits are used as (pseudo)-observations. A comparison of the obtained solutions to the results of the GRACE-FO Science Data System (SDS) and Combination Service for Time-variable Gravity Fields (COST-G) ACs, reveals a competitive quality of our solutions. While the spectral and spatial noise levels slightly differ, the signal content of the solutions is similar among all ACs. The carried out comparison of GRACE and GRACE-FO KBRR post-fit residuals highlights an improvement of the GRACE-FO K-band ranging system performance. The overall amplitude of GRACE-FO post-fit residuals is about three times smaller, compared to GRACE. GRACE-FO post-fit residuals show less systematics, compared to GRACE. Nevertheless, the power spectral density of GRACE-FO and GRACE post-fit residuals is dominated by similar spikes located at multiples of the orbital and daily frequencies. To our knowledge, the detailed origin of these spikes and their influence on the gravity field recovery quality were not addressed in any study so far and therefore deserve further attention in the future. Presented results are based on 29 monthly gravity field solutions from June 2018 until December 2020. The regularly updated LUH-GRACE-FO-2020 time series of monthly gravity field solutions can be found on the website of the International Centre for Global Earth Models (ICGEM) and in LUH’s research data repository. These operationally published products complement the time series of the already established ACs and allow for a continuous and independent assessment of mass changes in Earth’s system.

Cover Picture: Steel Construction 2/2021

AbstractApproach to the alpha ventus offshore wind farm. The support structures are exposed to extreme fatigue loads. © Leibniz University Hannover, Institute for Steel Construction (s. article p. 74 ff.).

Wave RUN-UP Measurements under very oblique wave incidence

Under the scope of the HYDRALAB+ transnational access project, the so-called RODBreak experiment was conducted in the multidirectional wave basin at the Marienwerden facilities of the Leibniz University Hannover (LUH). A stretch of a rubble-mound breakwater was built in the wave basin with a very gentle slope. Its armour layer was made of Antifer cubes, at the roundhead and adjoining trunk, and of rock, at the rest of the trunk. A set of tests was carried out to extend the range of wave steepness values analysed in wave run-up, overtopping and armour layer stability studies, focusing on oblique extreme wave conditions, with incident wave angles from 40º to 90º. The present study focuses on the analysis of measured wave run-up values obtained in the tests and on their on their variability as well as the influence of the wave obliquity and directional spreading. Keywords: rubble-mound breakwaters; run-up; oblique waves; physical modelling; RODbreak.

Multi-frequency multi-GNSS receiver antenna calibration at IfE: Concept - calibration results - validation

Multi-frequency and multi-constellation GNSS have the potential to boost the overall performance of GNSS-based positioning, navigation and timing. This has an impact on the realisation of global reference frames, geophysical monitoring applications as well as enabling new applications. To this end, all error sources should be adequately corrected for. However, currently multi-frequency multi-GNSS receiver antenna calibration values are still missing. In this paper, the newly developed multi-frequency multi-GNSS calibration process at Institut für Erdmessung (IfE), Leibniz University Hannover, is presented. The basic concept and the assumptions for the antenna calibration are described. Resulting phase centre corrections (PCC) for GPS and Galileo for typical antennas are presented. We show that the repeatability of the estimated patterns are almost better than 2 mm in terms of maximum deviation and that the used tracking strategies by the receivers have marginal impact on the patterns, at maximum 1.2 mm for the studied receiver-antenna combinations. Furthermore, applying phase centre corrections for multi-frequency multi-GNSS carrier phase observations reduces significantly (up to 37%) the antenna related biases as validated on short baselines. Moreover, a validation in the coordinate domain shows that with IfE PCC a short baseline can be computed with high accuracy: the topocentric coordinate differences to the known baseline are in most cases smaller than 1 mm for the horizontal components and smaller than 2.2 mm in vertical.

Gravity field modelling for the Hannover 10 m atom interferometer

Absolute gravimeters are used in geodesy, geophysics and physics for a wide spectrum of applications. Stable gravimetric measurements over timescales from several days to decades are required to provide relevant insight into geophysical processes. Users of absolute gravimeters participate in comparisons with a metrological reference in order to monitor the temporal stability of the instruments and determine the bias to that reference. However, since no measurement standard of higher-order accuracy currently exists, users of absolute gravimeters participate in key comparisons led by the International Committee for Weights and Measures. These comparisons provide the reference values of highest accuracy compared to the calibration against a single gravimeter operated at a metrological institute. The construction of stationary, large-scale atom interferometers paves the way for a new measurement standard in absolute gravimetry used as a reference with a potential stability up to 1,hbox {nm}{/}{hbox {s}^{2}} at 1 s integration time. At the Leibniz University Hannover, we are currently building such a very long baseline atom interferometer with a 10-m-long interaction zone. The knowledge of local gravity and its gradient along and around the baseline is required to establish the instrument’s uncertainty budget and enable transfers of gravimetric measurements to nearby devices for comparison and calibration purposes. We therefore established a control network for relative gravimeters and repeatedly measured its connections during the construction of the atom interferometer. We additionally developed a 3D model of the host building to investigate the self-attraction effect and studied the impact of mass changes due to groundwater hydrology on the gravity field around the reference instrument. The gravitational effect from the building 3D model is in excellent agreement with the latest gravimetric measurement campaign which opens the possibility to transfer gravity values with an uncertainty below the {10},hbox {nm}{/}{hbox {s}^{2}} level.

Compressive fatigue behaviour of high-strength concrete in a dry and wet environment

Fatigue verification becomes decisive for the design of slender structures of high-strength concretes. But there is a lack of knowledge regarding the fatigue behaviour, especially for wet environment. Only limited investigations are documented in literature. In addition, these investigations differ considerably regarding storage conditions and, therefore, are hard to compare. The fatigue behaviour of a high-strength concrete has been investigated at the Institute for Building Materials Science, Leibniz University Hannover, over the last years regarding different influences. In this paper, results of two research projects are comparatively analysed, focusing on the effect of surrounding water on the numbers of cycles to failure of a high-strength concrete. Thereby, additional influences of loading frequency and size effect are considered. The results clearly demonstrate that the fatigue resistance expressed by the numbers of cycles to failure is reduced by wet environment, meaning storing and testing submerged under water. In addition, indications were found that the moisture content of the high-strength concrete also affects the fatigue behaviour.

Scour around a monopile induced by directionally spread irregular waves in combination with oblique currents

The progressive expansion of offshore wind energy towards greater water depths demands for an optimization of foundation structure designs to a wider range of load conditions. In offshore waters, wind driven wave irregularity and directionality become important aspects of realistic sea states. To further improve the scour prediction in marine environment a novel experimental study was conducted to investigate the influence of directionally spread (3D) irregular waves on the scouring process. The tests were carried out in the 3D wave basin of the Ludwig-Franzius-Institute, Leibniz University Hannover, Germany. A monopile structure was simulated by a transparent pile made of acrylic glass. The study was set up to progress understanding and explore dissimilarities of scour development and patterns induced by directionally spread (3D) and unidirectional (2D) waves. The model tests were complemented by tests with superimposed oblique currents. Despite identical total wave energy in terms of m0 between the directionally spread and unidirectional wave spectra, minor but distinct differences in scour depth and rate could be observed, which inevitably can only be attributed to the presence and role of the wave spread. For wave-only conditions final scour depths S/D induced by directionally spread waves were on average 33% smaller than those for unidirectional waves. Furthermore, final scour depths decreased with increasing wave spreading and displayed a growing dependency on KC numbers with increasing wave spreading. In combined wave and current conditions of up to Ucw < 0.62 scour depths were found to be slightly larger and scour progression over time faster for directionally spread than for unidirectional waves. Differences regarding the scouring rates and depths have been found to decline with flow conditions further approaching current dominated regime. A prediction approach to estimate the maximum scour depth induced by either directionally spread or unidirectional random waves is proposed. Reasonable results and insights of the present study help advancing the understanding of scour development under more realistic, i.e. natural sea-state, conditions for a more reliable design of marine and offshore infrastructure.

Effects of Discharge Products on Zinc-Air Flow Cells

As one of the proposed post lithium-ion technologies, zinc-air batteries have received revived interest in the recent years, which is driven by the increased demand of high-performance, efficient, safe and environment-friendly energy storage solutions. These energy storage systems need to address variable power, capacity and profitability requests. Zinc-air flow batteries with higher specific energy density than lithium-ion and low-cost, highly available, eco-friendly active materials are suitable to fulfill these requirements. The energy density of most zinc-air flow batteries is limited by the low solubility of ZnO in the electrolyte. The use of a flow battery type with zinc-particles suspended in alkaline solution (zinc-slurry) in addition with high performance oxygen-reduction electrodes enables the development of high-power zinc-air batteries. This setup also enables the independent scaling of capacity and power.The cooperative project, ZnMobil, funded by the Federal Ministry of Economic Affairs and Energy in Germany, combines the experience of industrial and academic partners (Covestro Deutschland AG; Grillo-Werke AG; VARTA Microbattery GmbH; Accurec Recycling GmbH; Technical University Freiberg, Gottfried Wilhelm Leibniz University Hannover; University of Duisburg-Essen; the fuel cell research center ZBT GmbH) to develop a high-performance and low-cost zinc-air flow battery.The battery system consists of a 100 cm² copper plate as current collector for the zinc-suspension electrode and an oxygen reduction electrode with gas-diffusion layer (supplied by Covestro Deutschland AG). The zinc-slurry contains zinc particles (supplied by Grillo-Werke AG) suspended in alkaline solution (30 wt-% KOH) and stabilized with polyacrylic acid.Here we present a systematic study regarding the effects of the discharge products on the cell performance. Depending on the state of discharge and operation parameters the discharge product ZnO causes several important effects. The solid ZnO limits the discharge capacity in several ways. At high current densities, passivation is the main topic. If passivation is avoided, e.g. with the help of additives, the solid ZnO causes a viscosity increase until the cell is blocked. For this study we evaluated parameters like cell voltage, cell resistance, slurry viscosity, ZnO concentration in the electrolyte and other parameters in correspondence to the state of discharge. The understanding of these effects is essential to build a hydraulically rechargeable battery with high zinc usage and therefore high energy densities.Depending on parameters like current density and additive selection, it is possible to reach more than 70 % DOD, which corresponds to 287 mAh/gslurry or 574 mAh/gzinc. A demonstrator-cell was operated for more than 140 h. During this time it was successfully hydraulically recharged several times.

Deep learning for geometric and semantic tasks in photogrammetry and remote sensing

ABSTRACTDuring the last few years, artificial intelligence based on deep learning, and particularly based on convolutional neural networks, has acted as a game changer in just about all tasks related to photogrammetry and remote sensing. Results have shown partly significant improvements in many projects all across the photogrammetric processing chain from image orientation to surface reconstruction, scene classification as well as change detection, object extraction and object tracking and recognition in image sequences. This paper summarizes the foundations of deep learning for photogrammetry and remote sensing before illustrating, by way of example, different projects being carried out at the Institute of Photogrammetry and GeoInformation, Leibniz University Hannover, in this exciting and fast moving field of research and development.

Onlinelearning and Onlineteaching an der Rechtswissenschaftlichen Fakultät der Universität Wien unter COVID-19-Bedingungen von März bis November 2020 – ein persönlicher Erfahrungsbericht

In March 2020, the entire teaching had to be converted to “distance-learning” with a notice of a few hours. This was technically, organizationally and juridically challenging. This article provides a personal account on “lessons learned”, which I feel called to, as I have been engaging with its didactic, technical and juridical questions for a reasonable time, based on the professional background of my department (innovation and digitalization in law), my jurisprudential specialization in technology law and intellectual property rights, my experiences in strategic management of IT-processes at institutions of higher education (among them as CIO at the Leibniz University Hannover), and based on my decade-long engagement with juridical questions on online learning. These prior experiences, among others, have lead to me supporting the faculty’s endeavours in a consultative role.

Simulation and analysis of load shifting and energy saving potential of CO2-based demand-controlled ventilation in a sports training center

This paper aims to evaluate and characterize the impact of optimizing the operation of the HVAC system through maintaining dynamic CO2-based Demand-Controlled ventilation (DCV) on the electricity load profile and energy consumption of the sports training center of Leibniz University Hannover. The actual ventilation control scheme, in which the operation of the HVAC system is operated with a two-stage volume flow controller based on indoor CO2 concentration is improved through two steps to avoid overventilation and reduce power consumption. For this purpose, a detailed multi-zone model of the sports center and energy supply system has been developed in TRNSYS. In the first step, a multi-stage control scenario is implemented considering the occupancy schedules and indoor CO2 concentration measurement data. In the second step, based on an indoor CO2 concentration model, a predictive control scenario is developed and applied. Aiming at characterizing the influence of these operation scenarios on the power consumption of the building, the annual electricity load profiles of the simulation cases will be analyzed and compared with the actual load profile of the building based on the technical planning documents and data provided by building management system (BMS). Simulation results show that utilizing predictive CO2-based DCV leads to a reduction of the peak load electricity by almost 2 kW and the base load by 5 kW as well as decreasing the annual energy consumption by 40 %.