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A series of student-led research activities were performed using a cargo bicycle equipped with air chemistry instrumentation to study the dynamics of aerosol particles in urban air before, during, and after the COVID-19 lockdown periods. The studies examined a high-traffic route and a low-traffic route around the city center of Münster, Germany. A complex picture emerged for how the particle number concentrations (PN) and particle mass concentrations (PM, specifically PM10) were affected by the day of the week, the route selected, the meteorological conditions, and the traffic intensity. Traffic had the most impact on PN through the multitude of exhaust plumes from motorized vehicles. The impact of traffic on PM10 was rather low, which is also mirrored in the general pattern of the low response of PM10 to the pandemic lockdown in Germany. Instead, PM10 responded more to the day of the week. Presumably, PM10 responds either to a specific traffic component, such as commercial low-duty vehicles, or to other business, such as construction activity. Meteorological conditions exert their impact mostly through the relative humidity, which affects particle growth. As such, future research could examine PN and PM10 responses over all seasons of the year. In any case, this student-led study in which high-resolution data were acquired deepened our understanding and broadened our view on particle dynamics in urban air. Well-trained university graduates will contribute to meeting further challenges in studying and combatting air pollution.

The dynamic inflow effect describes the unsteady aerodynamic response to fast changes in rotor loading, due to the inertia of the wake. For pitch actuation and fast rotor speed changes this effect leads to load overshoots. The effect is suspected to be also relevant for gust situations, however this was never shown. The objective of the paper is to proove the dynamic inflow effect due to gusts and compare dynamic inflow engineering models to corresponding measurements. A 1.8 m diameter model turbine is used in the large wind tunnel of ForWind – University of Oldenburg with an active grid to impress rotor uniform gusts on the flow. The campaign features load and velocity measurements of the axial flow in the rotor plane. The unsteady dynamic inflow effect is investigated by comparing two experimental cases. Firstly, a dynamic measurement during a gust situation is performed. Secondly, quasi-steady loads and axial velocities are interpolated from a steady characterisation experiment according to the gust wind speed. By comparing both cases, the influence attributed to the dynamic inflow effect is isolated. Further comparisons to a typical Blade Element Momentum code and a higher fidelity Free VortexWake Model are performed. Based on analytical considerations an improvemed formulation of the Øye dynamic inflow model is proposed. The experiment shows a dynamic inflow effect due to gusts in the loads and axial velocity measurements. It leads to a reduction in load and axial velocity amplitudes and consequently also lower fatigue loading. The higher fidelity model shows a similar impact of the dynamic inflow effect. In contrast, the commonly used Øye engineering model in the BEM code predicts an increase in load amplitude and thus higher fatigue loads. The improved Øye engineering model however catches the observed dynamic inflow effect due to gusts in accordance to the experiment and FVWM simulations. An amplification of induced velocities, seen in the experiment and FVWM simulation, causes the reduced load amplitudes. Therefore, classic dynamic inflow models, which filter the induced velocity, cannot predict the effect. The proposed improvement to additionally consider the wake velocity for the filter of the dynamic inflow engineering model, proves to be a straight forward but also effective modification. In conclusion, these new experimental findings on dynamic inflow due to gusts and improvements to the Øye model enable improvements in wind turbine design by catching the related lower fatigue loads.

With the development of lidar technology for measuring wind disturbances ahead of a wind turbine, various feedforward control techniques have been proposed to utilize preview disturbance information for rotor speed regulation. Among these is model predictive control (MPC), which generates an optimal control trajectory subject to system constraints, but very few physical tests have been conducted using MPC for wind turbines presumably due to the challenges involved in solving an optimization problem online. In this paper, we test an optimal feedback/feedforward control algorithm that maintains the clear disturbance inclusion of MPC but results in a linear control law that can be implemented easily. On the other hand, the proposed controller is not able to handle physical system constraints explicitly—full MPC is the subject of future work. A scaled wind turbine model in a wind tunnel is used for testing.

The dynamic inflow effect denotes the unsteady aerodynamic response to fast changes in rotor loading due to a gradual adaption of the wake. This does lead to load overshoots. The objective of the paper was to increase the understanding of that effect based on pitch step experiments on a 1.8 m diameter model wind turbine, which we performed in the large open jet wind tunnel of ForWind – University of Oldenburg. We measured the flow in the rotor plane with a 2D Laser Doppler Anemometer and were able to extract the dynamic wake induction factor transients in axial and tangential direction. Further, integral load measurements with strain gauges and hot wire measurements in the near and close far wake were performed. Our results show a clear gradual decay of the axial induction factors after a pitch step, giving the first direct experimental evidence of dynamic inflow due to pitch steps. We fitted two engineering models to the induction factor transients to further investigate the relevant time constants of the dynamic inflow process.We discussed the radial dependency of the axial induction time constants as well as the dependency on the pitch direction. We confirmed that the nature of the dynamic inflow decay is better described by two rather than only one time constant. The dynamic changes in wake radius were connected to the radial dependency of the axial induction transients. In conclusion, the comparative discussion of inductions, wake deployment and loads facilitated the improved physical understanding of the dynamic inflow process for wind turbines. Furthermore, these measurements provide a new detailed validation case for dynamic inflow models and other types of simulations.

Abstract Active grids are commonly used in wind tunnels to generate turbulence with different characteristic features. In contrast to the common objective to generate turbulence with a very high Reynolds number, this work focuses on a method of blockage induced flow design for the generation of special flow structures. Particularly, we aim to investigate the underlying constraints of this excitation method. For this purpose, the scale dependency of the excitation is studied by clearly defined structures such as periodic sinusoidal velocity variations, velocity steps, and single gusts. It is shown that the generation process is limited by the reduced frequency of the active grid motion. For low values of reduced frequencies the imprinted flow structures remain undamped, whereas for higher reduced frequencies they are damped. This insight leads to the constraint that the active grid motion needs to be modified to compensate for the underlying dynamic damping effects. Thus, the inserted energy has to be increased for the corresponding reduced frequencies. This finding can be transferred to the generation of turbulent flows, for which an exemplary adaption is shown . Graphic abstract

We introduce an aerodynamic rotor concept for a 15 MW offshore wind turbine which is tailored for an increased power feed-in at low wind speeds. The main objective of the conceptual design is to limit the stationary loads (blade flapwise root bending moment (RBM) and thrust) to the maximum value of the IEA 15 MW offshore reference turbine, while greatly increasing the swept rotor aera. The outer part of the blade (e.g. outer 30% of the rotor) is designed for a higher design tip speed ratio (TSR) and a lower axial induction than the inner part. By operating at the high TSR in light winds, the slender outer part fully contributes to the increased power capture. In stronger winds the TSR is reduced and the torque generation is shifted to the inner section of the rotor. Moreover, the blade is designed in a way that makes the limitation of the flapwise RBM through peak shaving aerodynamically more efficient. With static blade element momentum simulations the characteristics of the rotor are investigated and the economic revenue of the turbine is estimated, considering a wind speed dependent feed-in price. Our results show that the revenue can be increased by 30% compared to the reference turbine with an increase of rotor diameter by 36%.

The standardized description of collections is an important means for prioritizing collection digitization on a supra-institutional level. Different organizational and systematic structures prevent easy comparison of collections sizes and foci, in order to make informed decisions on setting priorities and efficiently distributing tasks. In autumn 2018, the consortium of German Natural Sciences Collections (DCOLL), consisting of seven natural history collections holding institutions integrated as a National Research Infrastructure*1 performed a top-level description of the consortium's collections based on a subset of the criteria defined by the One World Collection Working Group (OWC). OWC is based on an initiative of the directors of the world's largest Natural Science Collections and aims at making collections and ressource allocations comparable. Categories relating to the staff structure of institutions were omitted, as those were considered sensitive and of little use for the purpose. The survey focused on collection size and geographical distribution only. Since some partners already had previously assembled the necessary data and since the OWC criteria are based on a reasonably high level, allowing the integration of heterogenous collections, the OWC dashboard presented an opportunity to achieve quick standardized results. However, as one purpose of the survey was to support decisions on the consortium's digitization strategy, arguably not the objective of OWC, a field to describe the digitization rate had to be added. Another shortcoming of the OWC dashboard for this purpose was the difficulty in subsuming some important sub-collections into the given criteria, namely digital collections (like animal sound archives) and non-biological or non-geological collections (e.g. historical objects and archives). On the positive side, the survey proved to be very helpful to indicate the consortium's collection focus in comparison with other institutions on an international level. This can provide valuable information for establishing an integrated collection development and digitization strategy on a supra-institutional level. It can be shown for example that 41% of the objects with origin from Europe in Natural Science Collections are held by DCOLL. While it may be possible to derive meaningful strategic goals from this information, it is a big challenge to implement practical objectives based on the same criteria. From a bottom-up perspective the OWC dashboard aggregates data, which were collected in a non-standardized form within institutions across organizational structures. Increasing the granularity from this level will result in an unreasonable effort. This presentation discusses the process of collecting information based on the OWC criteria and will present the collection structure of DCOLL.

With the ongoing increase in the size of wind turbines, experimental investigations have become more complicated and expensive. Therefore, computational models have proven to be a viable solution for design purposes. This article aims to validate CFD simulations of an experimental model wind turbine (MoWiTO 1.8) using Delayed Detached Eddy Simulation (DDES) and Improved DDES (IDDES) turbulence modelling approaches. For the purpose of validation, integral quantities (such as power, thrust, torque and blade-root bending moment in the flapwise direction) measured in the wind tunnel are compared with numerical results obtained with OpenFOAM. In general, the computational results show a very good agreement with the measurements for most of the monitored quantities. In particular, the blade-root bending moment presents the largest difference, taking into account that the simulation assumes the turbine blades are rigid. Nevertheless, the simulation does achieve in recreating the turbulent behavior as can be evidenced by the Power Spectral Density graphs, and the wake’s velocity measurements. In general, the IDDES turbulent model achieves a better agreement to the experimental results, while maintaining a very similar computational time as the DDES model.

In this paper a detailed comparison of the experimental and numerical results of a scaled wind turbine model in a wind tunnel subjected to fast pitching steps leading to the so-called dynamic inflow effect is presented. We compare results of an Actuator Line LES tool, a vortex code and four engineering models, to the experiment. We perform one and two time constant model analysis of axial wake induction and investigate the overshooting of integral loads. Our results show, that the effect is captured better by the two time constant models than by the one time constant models. Also the experiment and mid-fidelity simulations are best described by a two time constant fit. We identify the best dynamic inflow model to be the 0ye model. Different possibilities for the improvement of dynamic inflow models are discussed.

This paper deals with high-frequency (HF) thermoacoustic instabilities in swirl-stabilized gas turbine combustors. Driving mechanisms associated with periodic flame displacement and flame shape deformations are theoretically discussed, and corresponding flame transfer functions (FTF) are derived from first principles. These linear feedback models are then evaluated by means of a lab-scale swirl-stabilized combustor in combination with part one of this joint publication. For this purpose, the models are used to thermoacoustically characterize a complete set of operation points of this combustor facility. Specifically, growth rates of the first transversal modes are computed, and compared against experimentally obtained pressure amplitudes as an indicator for thermoacoustic stability. The characterization is based on a hybrid analysis approach relying on a frequency domain formulation of acoustic conservation equations, in which nonuniform temperature fields and distributed thermoacoustic source terms/flame transfer functions can be straightforwardly considered. The relative contribution of flame displacement and deformation driving mechanisms–i.e., their significance with respect to the total driving–is identified. Furthermore, promoting/inhibiting conditions for the occurrence of high frequency, transversal acoustic instabilities within swirl-stabilized gas turbine combustors are revealed.

Dynamic inflow is a phenomenon that leads to a load overshooting, for example by fast pitch angle changes. Established engineering models for this effect are known to still have deficits. Advanced control mechanisms for turbines like collective pitch control in order to alleviate tower loads or individual pitch control increase the need for accurate dynamic inflow models. We performed fast pitching step experiments with our MoWiTO 1.8 (Model Wind Turbine Oldenburg 1.8 m) in the WindLab wind tunnel of the University of Oldenburg and investigated the load overshooting of blade loads, torque and thrust as well as velocity transients in the near wake. Dynamic inflow phenomena were clearly visible and their non-dimensional time constants matched existing full scale and lab size experiments. The transient process of the different measured loads and flow in the near wake is disussed in the paper to gain a deeper understanding of the underlying physical phenomena.