Christoph Strangfeld

Control of Thick Airfoil, Deep Dynamic Stall Using Steady Blowing

The utility of constant blowing as an aerodynamic load control concept for wind turbine blades was explored experimentally. A NACA 0018 airfoil model equipped with control slots near the leading edge and at mid-chord was investigated initially under quasi-static conditions at Reynolds numbers ranging from 1.25·105 to 3.75·105. Blowing from the leading-edge slot showed a significant potential for load control applications. Leading-edge stall was either promoted or inhibited depending on the momentum coefficient, and a corresponding reduction or increase in lift on the order of Δcl≈0.5 was obtained. Control from the mid-chord slot counteracted trailing-edge stall but was ineffective at preventing leading-edge separation. The impact of blowing from the leading-edge slot on dynamic stall was explored by means of unsteady surface pressure measurements and simultaneous particle image velocimetry above the suction surface. At a sufficiently high momentum coefficient, the formation and shedding of the dynamic sta...

Modeling Lift Hysteresis with a Modified Goman-Khrabrov Model on Pitching Airfoils

Wind tunnel experiments were conducted on two symmetric airfoils with different thickness ratios as a test of the ability of Goman-Khrabrov-type models to predict the lift coefficient history during pitching maneuvers. The primary difference between the two airfoils was that the thin airfoil had no hysteresis in its lift curve during quasi-steady maneuvers, but static hysteresis was observed with the thick airfoil over a range of 16 < α < 22. Both airfoils exhibited dynamic hysteresis in the lift coefficient when the wing was pitching. The existence of static hysteresis had a strong effect on the lift response during periodic pitching, and must be accounted for in the model. A modified version of the Goman-Khrabrov model was introduced that captured the static hysteresis behavior and the large-scale features of the dynamic hysteresis for the thick airfoil. Large-scale variations in the lift trajectory at different pitching frequencies are reproduced by the modified model, even when the thick airfoil is in a deep stall condition. Some deviations between model and experiment are observed at the highest angles of attack, which are attributed to the dynamic stall vortex and trailing edge vortex shedding.

Thick Airfoil Deep Dynamic Stall

Recent interest in vertical axis wind turbines (VAWTs) has surged due to their appearance in the built environment and their potential for deep water offshore applications. Their well-known advantages include insensitivity to the wind direction, proximity of the generator to the ground and relatively low noise levels. Furthermore, blade profile uniformity along the span can significantly reduce manufacturing costs, particularly for large scale utility machines. A drawback of VAWTs is the tendency of their blades to stall dynamically when they are pitched beyond their static stall angle. Dynamic stall is characterized by a strong vortex, the dynamic stall vortex (DSV), which forms near the leading-edge. When the DSV is convected downstream, a rapid drop in lift and severe pitching moment fluctuations result [1]. On VAWTs operating at low tip-speed ratios, dynamic stall occurs periodically throughout the rotation of the blades [2]. This situation is unique in that the blades experience pitch oscillations about zero angle of attack and the flow separates alternately on both sides. This results in a number of undesired effects: On the one hand, the full separation on the suction surface produces a sharp drop in cl and thus rotor torque. On the other hand, the unsteady aerodynamic loads cause fatigue damage to the generator and drive train [3].

Transmission characteristics of RFID sensor systems embedded in concrete

Completely embedded sensor systems for long-term operation offer innovative possibilities for structural health monitoring of concrete structures. Measuring of relevant parameters, e.g., temperature, humidity, or indication of corrosion can be performed with low energy sensors. This allows to implement passive RFID sensor systems without cable connection and battery, which are power supplied exclusively by the electromagnetic field from the external reader device. To evaluate characteristics and conditions of this concept, a systematical investigation of the transmission characteristics with variation of relevant parameters, as communication frequency, installation depth, type of concrete, moisture content, etc. is currently carried out in an interdisciplinary research project at BAM. First results are presented in this paper.

Airfoil subjected to high-amplitude free-stream oscillations: theory and experiments

A combined theoretical and experimental investigation is carried out with the objective of validating the two-dimensional airfoil theories of Isaacs and Greenberg subjected to high-amplitude harmonically varying free-stream velocity. To date, these theories have eluded full validation due to the significant experimental challenges associated with reliably producing high-amplitude unsteady flow. The theoretical approaches assume a flat plate in potential flow that is subjected to an oscillating free-stream. Unsteady forces is calculated using the approach described by Isaacs and the generalisation of van der Wall. In order to further understand the unsteady lift mechanism, the theory is extended here for the purposes of calculating the unsteady bound vorticity sheet strength along the airfoil chord. A closed-form solution is obtained which is valid for arbitrarily reduced frequencies and amplitudes. To validate the theories, experiments are conducted on a NACA 0018 airfoil in a wind tunnel that is designed specifically for the purpose of generating high-amplitude velocity oscillations. Unsteady pressure transducers are located at nominally identical chordwise locations to facilitate an estimation of the bound vorticity strength and loads. All experimental data are phase-averaged over O(10) cycles. The phase variations of lift and moment coefficients are reasonably well predicted at a 2◦ angle-of-attack. At 8◦ however, the experiments under-predicted the theoretical load fluctuations. The theoretical and measured unsteady bound vorticity sheet show that the process of lift overshoot starts at the trailing edge and propagates upstream.

Modeling Lift Hysteresis on Pitching Airfoils with a Modified Goman–Khrabrov Model

Wind-tunnel experiments were conducted on two symmetric airfoils with different thickness ratios as a test of the ability of Goman–Khrabrov-type models to predict the lift coefficient history during pitching maneuvers. The primary difference between the two airfoils was that the thin airfoil had no hysteresis in its lift curve during quasi-steady maneuvers, but static hysteresis was observed with the thick airfoil over a range of 16<α<22  deg. Both airfoils exhibited dynamic hysteresis in the lift coefficient when the wing was pitching. The existence of static hysteresis had a strong effect on the lift response during periodic pitching and must be accounted for in the model. A modified version of the Goman–Khrabrov model was introduced that captured the static hysteresis behavior and the large-scale features of the dynamic hysteresis for the thick airfoil, even when the thick airfoil was in a deep-stall condition. Some deviations between model and experiment were observed at the highest angles of attack, ...

Identification of Coherent Structures in a Turbulent Generic Swirl Burner using Large Eddy Simulations

mesh sizes and subgrid-scale models and the results were compared to experimental data. It was found that the overall inuence of the computational grid and the subgrid-scale model was neglectable and the simulations were in line with the experiments. A dominant frequency was found in the turbulent kinetic energy spectrum representing a coherent structure. Moreover, by applying the proper orthogonal decomposition (POD) this structure could be identied as a convective helical instability. This helical instability and can be represented by a pair of modes, and is assumed to be triggered by a precessing vortex core (PVC).

The Influence of Wind Tunnel Grid Turbulence on Aerodynamic Coefficients of Trains

In the design process of trains wind tunnel tests are indispensable in order to assess the cross-wind sensitivity. The aerodynamic forces and moments acting on the leading cars or trailer cars can be measured and aerodynamic coefficients determined. Usually the tests are carried out at low turbulence conditions (“smooth flow”). During recent wind tunnel tests on new high speed train models in the Hermann-Fottinger Institute Berlin undesired Reynolds number effects were observed. This paper describes an approach to reduce these effects by changing the turbulence conditions of the flow. Wind tunnel measurement on a high speed train model in scale 1:25 are carried out with different turbulence conditions: increased free stream turbulence and a tripped boundary layer on the model. The free stream turbulence level is varied from 0.5 to 8 % by using grids with different geometries up stream of the test section. The experiments showed that the Reynolds number dependency on the saftey relevant rolling moment can be well reduced by a simple rectangular grid installed in the contraction of the wind tunnel.

Data of embedded humidity sensors, sample weights, and measured pore volume distribution for eight screed types

Four cement-based and four calcium-sulphate-based screed types are investigated. The samples have a diameter of 300 mm and a height of 35 or 70 mm. Up to ten humidity sensors are embedded directly during the concreting of the screed samples. Thus, the humidity over the sample height is monitored during hardening, hydration, evaporation, and oven drying. Furthermore, the screed samples are weighed during every measurement to determine the total mass and the corresponding moisture loss. To define the pore system precisely, mercury intrusion porosimetry as well as gas adsorption is performed. According to the data, the entire pore volume distribution is known. The measured pore diameters range from 0.8 nm to 100 µm and the total porosity of the examined screeds ranges between 11% and 22%. Based on these measurement data, moisture transport, pore saturation as well as sorption isotherms and their hysteresis may be calculated quantitatively as described in “Monitoring of the absolute water content in porous materials based on embedded humidity sensors” (Strangfeld and Kruschwitz, 1921).

Parametric Investigations of the Leading Edge Vortex on a Delta Wing

A 70 -sweep delta wing was investigated experimentally by means of stereoscopic particle image velocimetry (StereoPIV). The velocity eld was measured at three angles of attack (25 , 30 , 35 ) and at three moderate chord Reynolds numbers (0.78, 1.2 and 1.55 10). The ow eld on delta wings, including vortex breakdown, is complex and highly threedimensional. Many qualitative observations exist (mostly ow visualisations), but only few measurements quantify the ow eld. This study will describe the evolution of crucial vortex parameters along the vortex axis, focusing on the vortex radius and the axial circulation. This study aims at generating a deeper insight into the ow structures of leading edge vorticies. The vortex was investigated upstream and downstream of vortex breakdown. The vortex centre was determined on the basis of ow visualisations as well as on StereoPIV data. Strong deviations between the two measurement techniques arise due to a mismatch between the positions of the maximum vorticity and the minimum azimuthal velocity. Thus, the leading edge vortex does not correspond to a regular Hamel-Oseen vortex and other approaches were used to detect the vortex. In the pre-vortex breakdown region, the radius remains almost constant and Reynolds number independent. In this area, a high concentration of vorticity is observed. Downstream of vortex breakdown, the vortex radius increases drastically and the axial velocity decreases towards zero. The circulation of the vortex tube was quanti ed along the vortex axis and a decrease of the axial circulation of the vortex tube is observed slightly upstream of the vortex breakdown position. Downstream of the vortex breakdown, the highly concentrated vorticity in the vortex tube spreads out whereas the circulation of the ow eld on the wing surface reaches an almost constant level. Such a behaviour is expected with a transformation from axial vorticity to azimuthal vorticity due to the breakdown. In the pre-VB region, a strong linear connection between the axial velocity in the vortex centre and the circulation of the vortex tube is observed.