Sorg, Michael
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Item type:Publication, Combined X-ray diffraction and photothermal radiometry methods for in situ analysis of nitriding treatment(EDP Sciences, 2018-09-18); ; ; ; Monitoring the nitriding treatment by analyzing directly the components’ surface state during the nitriding treatment is particularly interesting, since it allows a process monitoring and control based on the actual nitriding result. In the present study, two measuring methods are developed and combined with the aim of a direct surface state analysis during the nitriding treatment: the in situ X-ray diffraction (XRD) method and the photothermal radiometry. In order to validate the combined application of both methods during a nitriding treatment under controlled atmosphere, an experimental setup including a miniature nitriding furnace was developed. Two alloyed steels AISI 4140 and AISI H13 are treated with varying process atmosphere and nitriding potential leading to varying phase composition in the surface layer. As a result, the photothermal radiometry is shown to be sensitive with respect to the changing surface properties due to the growing compound layers and when porous layers are generated. It has a high potential to serve as surface sensor in industrial processes.journal articleBand:115Heft:478 50 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Die Biometrie des Auges als Ursache für systematische Messabweichungen bei der akustischen TonometrieZur Glaukomtherapie ist eine Überwachung des Augeninnendrucks erforderlich, wofür ein akustisches Selbsttonometer entwickelt worden ist. Die Laborversuche an Schweineaugen und die Patientenmessungen in einer klinischen Versuchsreihe belegen jedoch signifikante Querempfindlichkeiten von den biometrischen Parametern des Auges auf den gemessenen Augeninnendruck. Um die Individualität der Augen in der Auswertung der Messdaten zu berücksichtigen, werden Finite-Elemente-Simulationen des Auges für unterschiedliche geometrische Ausprägungen durchgeführt. Anhand der Simulationsergebnisse wird der Einfluss der Augengeometrie auf die Messunsicherheit des zu messenden Augeninnendrucks quantifiziert. Dadurch lässt sich bei Kenntnis der individuellen Augengeometrie des Patienten die systematische Messabweichung des Augeninnendrucks signifikant reduzieren und für das akustische Selbsttonometer eine Messunsicherheit in der Größenordnung aktueller klinischer Tonometer erzielen.journal articleBand:86Heft:481 51 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, IR thermographic flow visualization for the quantification of boundary layer flow disturbances due to the leading edge condition(Elsevier Science, 2019-08); ; ; ; The aerodynamic performance of wind turbine rotor blades is influenced by the leading edge condition. Contamination and erosion cause increased surface roughness, unevenness or defects, which affect the boundary layer flow and, thus, reduce lift and increase drag. Current approaches used to determine the disturbed boundary layer flow are based on invasive flow probes with limited spatial resolution; therefore, a non-invasive, camera-based measurement of the boundary layer flow disturbances on wind turbines in operation is proposed using thermographic flow visualization. The actual and the undisturbed laminar-turbulent transition positions are determined in the thermographic images and a subsequent assignment to the rotor blade geometry obtains chord-based information. The normalized difference of both transition positions can be used as a metric to describe the extent of the disturbed boundary layer flow. The approach is demonstrated on a multi-MW horizontal axis wind turbine with a laminar flow reduction of up to 90.4 %. Furthermore, the measurement results allow the estimation of the annual energy production loss due to the leading edge condition, which enhances the industrial standard of simply comparing clean and tripped aerodynamic polars. For the investigated wind turbine, the annual energy production loss amounts to 4.7 % at 6 m/s average wind speed.journal articleBand:13870 93 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Thermographic flow visualization by means of non-negative matrix factorization(Elsevier Science, 2020-04); ; ; ; In order to investigate the areas of different flow regimes in the boundary layer of an airfoil, thermography is a powerful flow visualization tool. However, the distinguishability between boundary layer flow regimes such as laminar or turbulent is limited due to systematic and random inhomogeneity in the measured temperature field, hindering a clear separation of the flow regimes. In order to increase the distinguishability of different flow regimes, a time series of thermographic images is evaluated by means of a non-negative matrix factorization. As a result, the non-negative matrix factorization creates images that contain the dominant features of the measured images, while reducing systematic temperature gradients within the flow regimes by up to a factor of five. This way an increase of the distinguishability between every pair of consecutive flow regimes can be achieved on the surface of a non-heated cylinder in cross-flow condition. As a further application example of the non-negative matrix factorization, the distinguishability between the flow and the laminar-turbulent transition zone on a heated helicopter airfoil is also increased by a factor of five. Hence, non-negative matrix factorization is capable of enhancing thermographic flow visualization for increasing the distinguishability of different flow phenomena.journal articleBand:8273 100 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Measurement uncertainty of IR thermographic flow visualization measurements for transition detection on wind turbines in operation(Elsevier Science, 2018-10); ; ; The application of thermographic flow visualization on wind turbines in operation differs from the well-established application in wind tunnel experiments. The necessary temperature difference between the rotor blade and the flow only relies on the absorbed solar radiation, which often leads to a low signal-to-noise ratio. Furthermore, a large distance of up to 400 m exists between the rotor blade and the thermographic camera for ground based measurements, mainly due to the height of the wind turbine. This results in a poor spatial resolution as well as a small numerical aperture, which means a small detectable radiant power further degrades the signal-to-noise ratio. In order to determine the limits of measurability for the localization of the laminar-turbulent transition, the fundamental effects on the measurement uncertainty are investigated. For this purpose, the measurement uncertainty budgets for three signal processing algorithms are derived and validated with wind tunnel experiments and field measurements on a 1.5 MW wind turbine under sunny and cloudy weather conditions. As a result, the achievable standard uncertainty of the laminar-turbulent transition position for a temperature difference of 2 K amounts to 0.16 pixels in this case, which corresponds to 0.17% chord length. In addition, the measurement uncertainty is currently not limited by the measurement system (detector noise and fixed pattern noise of the camera), but by flow induced temperature fluctuations and fluctuations of the laminar-turbulent transition position itself. Hence, the field measurements on wind turbines in operation are still limited by the flow characteristics, i.e., the measurement object itself. Finally, one of the presented signal processing methods allows a robust, automated flow characterization for wind turbines in operation with subpixel accuracy in all of the presented measurements.journal articleBand:9783 133 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, IR thermographic visualization of flow separation in applications with low thermal contrast(Elsevier Science, 2018-01); ; ; A measurement method for IR thermographic visualization of separated flow on rotor blades for wind turbines is demonstrated. Flow separation has a negative influence on the performance of airfoils, e.g., at wind turbine rotors. Thermographic flow visualization is a non-invasive measurement technique to identify different flow regimes, but the visualization of separated flow without explicit additional heating of the measured object has not been possible to date. For this reason, a measurement approach with an enhanced sensitivity is presented, which evaluates temporal temperature fluctuations from a thermographic images series by means of the standard deviation as well as the analysis of selected Fourier coefficients. The approach is validated by wind tunnel experiments with a non-heated circular cylinder as well as a 2D 6 digit NACA-airfoil. The flow and measurement conditions are chosen to be similar to wind turbines in operation. As a result, the flow regimes including the flow separation are resolved and are in agreement with reference measurements, while the sensitivity of standard thermographic flow visualization was too low. In addition, the Fourier analyses method results in an improvement of the contrast to noise ratio between turbulent and separated flow by 11.6 % compared to the evaluation of the temperature standard deviation. Further improvements are expected in future when taking the complete spatiotemporal temperature fluctuations into account.journal articleBand:88121 136 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Optical measurement of the corneal oscillation for the determination of the intraocular pressureMotivation: Glaucoma is currently the most common irreversible cause of blindness worldwide. A significant risk factor is an individually increased intraocular pressure (IOP). A precise measurement method is needed to determine the IOP in order to support the diagnosis of the disease and to monitor the outcome of the IOP reduction as a medical intervention. A handheld device is under development with which the patient can perform self-measurements outside the clinical environment. Method: For the measurement principle of the self-tonometer the eye is acoustically excited to oscillate, which is analyzed and attributed to the present IOP. In order to detect the corneal oscillation, an optical sensor is required which meets the demands of a compact, battery driven self-tonometer. A combination of an infrared diode and a phototransistor provides a high-resolution measurement of the corneal oscillation in the range of 10 μm–150 μm, which is compared to a reference sensor in the context of this study. By means of an angular arrangement of the emitter and the detector, the degree of reflected radiation of the cornea can be increased, allowing a measurement with a high signal-to-noise ratio. Results: By adjusting the angle of incidence between the detector and the emitter, the signal-to-noise ratio was improved by 40 dB which now allows reasonable measurements of the corneal oscillation. For low amplitudes (10 μm) the signal-to-noise ratio is 10% higher than that of the commercial reference sensor. On the basis of amplitude variations at different IOP levels, the estimated standard uncertainty amounts to <0.5 mm Hg in the physiological pressure range with the proposed measuring approach. Conclusion: With a compact and cost-effective approach, that suits the requirements for a handheld self-tonometer, the corneal oscillation can be detected with high temporal resolution. The cross-sensitivity of the sensor concept concerning a distance variation can be reduced by adding a distance sensor. Existing systematic influences of corneal biomechanics will be integrated in the sensor concept as a consecutive step.journal articleBand:64Heft:42651 2633 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Geographical comparison between wind power, solar power and demand for the German regions and data filling concepts(Elsevier Science, 2018-10); ; ; ; The rising penetration of renewable energies became an important issue in the German electricity sector within the past years. In order to plan the required infrastructure for the energy distribution, a detailed knowledge about the complete geographical and temporal power generation compared to the demand is crucial. However, the available data for the renewable power generation in Germany is insufficient due to the complexity of the energy system. For this reason, a comparison between the renewable power generation and the electricity demand is presented for 95 German zip code regions based on real input data with a sample time of 15 min from renewable energy generators. For enhancing the incomplete data, different model-based data filling methods using the data of neighboured regions or additional meteorological data are introduced and compared. As a result, a number of modelling methods, based either on a heuristic model, a wind speed model or a combination of both, has been investigated, leading to similar correlation coefficients of above 80%. Finally, the obtained data set is applied for an analysis with a high spatiotemporal resolution. For three use cases the resulting optimal flow of the inter-regional power transfers is calculated.journal articleBand:12683 101 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Failure probability prediction based on condition monitoring data of wind energy systems for spare parts supply(Elsevier Science, 2013-04-28); ; ; ; The feasibility of maintenance processes relies on the availability of spare parts. Spare part inventory planning is capital intensive. It is based on demand forecasting, which possesses a high potential in reducing inventories. Even if condition monitoring systems are installed in technical systems, condition monitoring information is barely used to predict the failure probability of units. Therefore, an enhanced forecast model, which integrates SCADA information, has been developed. This leads to more accurate spare part demand forecasts. The approach presented in the paper is based on data mining, the proportional hazards model (PHM) and a binomial distribution. It has been validated with maintenance data of wind energy systems.journal articleBand:62Heft:184 107 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Adaptive finite element eye model for the compensation of biometric influences on acoustic tonometryBackground and objective: Glaucoma is currently a major cause for irreversible blindness worldwide. A risk factor and the only therapeutic control parameter is the intraocular pressure (IOP). The IOP is determined with tonometers, whose measurements are inevitably influenced by the geometry of the eye. Even though the corneal mechanics have been investigated to improve accuracy of Goldmann and air pulse tonometry, influences of geometric properties of the eye on an acoustic self-tonometer approach are still unresolved. Methods: In order to understand and compensate for measurement deviations resulting from the geometric uniqueness of eyes, a finite element eye model is designed that considers all relevant eye components and is adjustable to all physiological shapes of the human eye. Results: The general IOP-dependent behavior of the eye model is validated by laboratory measurements on porcine eyes. The difference between simulation and measurement is below 8 µm for IOP levels from 5 to 40 mmHg. The adaptive eye model is then used to quantify systematic uncertainty contributions of a variation of eye length and central corneal thickness based on input statistics of a clinical trial series. The adaptive eye model provides the required relation between biometric eye parameters and the corneal deflection amplitude, which here is the measured quantity to trace back to the IOP. Implementing the relations provided by the eye model in a Gaussian uncertainty propagation calculation now allows the quantification of the uncertainty contributions of the biometric parameters on the overall measurement uncertainty of the acoustic self-tonometer. As a result, a systematic uncertainty contribution resulting from deviations in eye length dominate stochastic deviations of the sensor equipment by a factor of 3.5. Conclusion: As perspective, the proposed adaptive eye model provides the basis to compensate for systematic deviations of (but not only) the acoustic self-tonometer.journal articleBand:20091 137