Balaresque, Nicholas
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Item-typ:Veröffentlichung, 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.Wissenschaftlicher ArtikelBand:88121 137 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Temperature and pressure effects on the response behavior of anemometersCup anemometers are within the most used wind speed sensors for the wind energy industry. Wind tunnel calibrations under controlled conditions are required, but during operation uncontrolled environmental conditions occur. This is accounted for in the IEC 61400-50-1:2022 international wind measurements standard, which specifies sensor classification based on their response to external conditions, due to the influence parameters turbulence, air temperature, density, and upflow angle. Temperature and density effects are not covered appropriately in the IEC 61400-50-1:2022, since it assumes that air temperature only influences the bearing friction of a cup anemometer. No guidance is provided on evaluating variations in density, which depends on temperature and pressure. To investigate this, two cup anemometers are measured in Deutsche WindGuard’s Climatic Wind Tunnel, where density is changed by varying pressure and temperature independently. The results show that the sensor’s response to temperature can have other effects than an increase in ball bearing friction. Using pressure or temperature to modify density can even cause opposing results. Hence, varying temperatures and pressures independently is crucial to characterize a sensor’s response. The results correspond to cup anemometers, but the methodology is applicable on all sensors.Wissenschaftlicher ArtikelBand:90Heft:9138 122 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, 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.Wissenschaftlicher ArtikelBand:9783 134 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, 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.Wissenschaftlicher ArtikelBand:13870 93 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, 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.Wissenschaftlicher ArtikelBand:8273 100