Michael Hölling

The turbulent nature of the atmospheric boundary layer and its impact on the wind energy conversion process

Wind turbines operate in the atmospheric boundary layer, where they are exposed to turbulent atmospheric flows. As the response time of wind turbines is typically in the range of seconds, they are affected by the small-scale intermittent properties of turbulent wind. Consequently, basic features that are known for small-scale homogeneous isotropic turbulence, in particular the well-known intermittency problem, have an important impact on the wind energy conversion process. We report on basic research results concerning the small-scale intermittent properties of atmospheric flows and their impact on the wind energy conversion process. The analysis of wind data shows strong intermittent statistics of wind fluctuations. To achieve numerical modeling, a data-driven superposition model is proposed. For the experimental reproduction and adjustment of intermittent flows, the so-called active grid setup is presented. Its ability to generate reproducible properties of atmospheric flows on the smaller scales of lab...

Multi-scale generation of turbulence with fractal grids and an active grid

Turbulence plays an important role in our everyday life, yet it is still not well understood. Wind tunnel experiments can help to develop generalized descriptions of turbulent flows. However, creating turbulent flows with suitable characteristics for various experiments is still challenging. In this work, fractal and active grids were used to generate multi-scale turbulent flows. Using hot-wire measurements we investigated the influence of different boundary conditions, bar sizes and solidity for fractal grids. We found that the evolution of the flow generated by a fractal grid does not depend on the considered boundary conditions. An alternative to these rigid structured grids is an active grid, which allows for a dynamical generation of flow fields with comparable properties. Experiments were conducted with an active grid in which the distribution of the local solidity was actively changed. A transition between classical and fractal grid type decaying turbulence depending on the active grid excitation protocol was observed. We conclude that the distribution of the local solidity of these grids has a strong influence on the evolution of the generated turbulent flow.

Sphere anemometer - a faster alternative solution to cup anemometry

We present an anemometer technique characterized by an instrument in a sealed enclosure without moving parts. Measurements taken with our improved sphere anemometer in comparison to cup anemometer and hot-wire anemometer data subjected to wind gusts are discussed. The hot-wire anemometer serves as a reference with high temporal and spacial resolution. A manually driven gust generator produced gusts at low frequencies of about 1Hz. All measurements were carried out in the wind tunnel at the University of Oldenburg.

Comparative study on the wake deflection behind yawed wind turbine models

In this wind tunnel campaign, detailed wake measurements behind two different model wind turbines in yawed conditions were performed. The wake deflections were quantified by estimating the rotor-averaged available power within the wake. By using two different model wind turbines, the influence of the rotor design and turbine geometry on the wake deflection caused by a yaw misalignment of 30° could be judged. It was found that the wake deflections three rotor diameters downstream were equal while at six rotor diameters downstream insignificant differences were observed. The results compare well with previous experimental and numerical studies.

Experimental airfoil characterization under tailored turbulent conditions

Studies of the impact of turbulent inflow conditions on the airfoil characteristics were performed within the EU FP7 project AVATAR. The aim of this study is to provide data for the validation of simulations and the improvement of engineering tools. Chord-wise pressure distributions and highly-resolved force data of the wind turbine dedicated DU 00-W-212 profile were measured in the wind tunnel in two tailored turbulent inflow conditions generated with an active grid. A sinusoidal and an intermittent pattern with customized inflow angle fluctuations were generated providing two significantly different distributions of reduced frequencies. The obtained pressure distributions and polars from the unsteady patterns are compared to the laminar baseline case.

The level crossing and inverse statistic analysis of German stock market index (DAX) and daily oil price time series

The level crossing and inverse statistics analysis of DAX and oil price time series are given. We determine the average frequency of positive-slope crossings, να+, where Tα=1/να+ is the average waiting time for observing the level α again. We estimate the probability P(K,α), which provides us the probability of observing K times of the level α with positive slope, in time scale Tα. For analyzed time series, we found that maximum K is about ≈6. We show that by using the level crossing analysis one can estimate how the DAX and oil time series will develop. We carry out the same analysis for the increments of DAX and oil price log-returns (which is known as inverse statistics), and provide the distribution of waiting times to observe some level for the increments.

The Relevance of Turbulence for Wind Energy Related Research

Wind turbines are operating in the turbulent atmospheric boundary layer where they are exposed to wind speed changes in time and space on different scales [3]. This paper deals with a statistical description of the turbulent wind field on the one hand and with the interaction of turbulent wind fields with rotor blade elements with respect to dynamical changes of the lift coefficient c l on the other hand. In the first part we present a method to estimate the statistics of wind speed fluctuations u ′ = u – 〈u 〉 and more importantly of extreme events on different time scales based on 10-minute averaged values for the measured turbulence intensity of wind fields. In the second part we present measurement data that illustrates the effects of atmospheric like turbulence on a FX 79-W-151A airfoil under static angles of attacks.

Progress in Turbulence and Wind Energy IV

According to the Kolmogorov theory, the most important feature of high Reynolds number turbulent flows is the energy transfer from large to small scales. This energy cascade is believed to universally occur in a certain interval of scales, known as inertial range. This phenomenology has been shown to occur in a wide range of flows but not in wall-turbulence where a reverse cascade in the near-wall region is observed [1]. In order to analyse this new scenario, in the present work a study of a generalized Komogorov equation is performed. The results reveal an energy fluxes loop in the space of scales where the reverse cascade plays a central role. At the base of this phenomena it is found the anisotropic energy injection due to the action of the turbulent structures involved in the near-wall cycle. The data used for the analysis are obtained with a pseudo-spectral code in a channel at Reτ = 550. The computational domain is 8πh× 2h× 4πh with a resolution in the homogeneous directions of Δx = 13.5 and Δz = 6.7. 1 The Energy Transfer in Wall-Turbulence The most important contribution to the description of the energy transfer mechanisms in turbulence is the Kolmogorov theory. Under the assumption of a statistical isotropic condition, this theory is an exact quantitative result obtained by the balance of the second order structure function, 〈δu〉, where δui = ui(xs+rs)−ui(xs) is the fluctuating velocity increment and 〈·〉 denotes ensamble average. Although this is a well known result it is useful to go back A. Cimarelli · E. De Angelis DIEM, Università di Bologna, Viale Risorgimento, 40136 Bologna, Italy e-mail: andrea.cimarelli2@unibo.it,e.deangelis@unibo.it C.M. Casciola DIMA, Università di Roma ”La Sapienza” Via Eudossiana 18, 00184 Roma, Italy e-mail: carlomassimo.casciola@uniroma1.it M. Oberlack et al. (Eds.): Progress in Turbulence and Wind Energy IV, SPPHY 141, pp. 3–6. springerlink.com c

Validating subspace predictive repetitive control under turbulent wind conditions with wind tunnel experiment

To reduce the cost of wind energy, it is essential to reduce loads on turbine blades to increase lifetime and decrease maintenance cost. To achieve this, Individual Pitch Control (IPC) received an increasing amount of attention in recent years. In this paper, a data-driven IPC algorithm called Subspace Predictive Repetitive Control (SPRC) is used to alleviate periodic loads on a scaled 2-bladed wind turbine in turbulent wind conditions. These wind conditions are created in an open-jet wind tunnel with an active grid, enabling unique reproducible high turbulent wind conditions. Significant load reductions are achieved even under these high turbulent conditions.

Generation of user defined turbulent inflow conditions by an active grid for validation experiments

A wind tunnel experiment is presented which combines the use of controlled turbulent inflow conditions and a two-bladed model wind turbine utilizing a new control strategy called subspace predictive repetitive control (SPRC). The validation of the performance of SPRC was made under turbulent inflow conditions generated by an active grid. The 3m × 3m active grid is used in this experiment using a unique method to generate reproducible atmospheric- like turbulent wind fields to act on a medium sized model wind turbine. This contribution is focussing on the detailed description of the experiment and its components and the analysis of the turbulent inflow by means of one and two point statistics. Exemplarily the impact of the new control strategy to the generated turbulent test cases are discussed.