# Thermographic Stall Detection by Model-Inspired Evaluation of the Dynamic Temperature Behaviour
###### Abstract  
Model-inspired signal processing approaches with an enhanced detectability of flow separation on thermographic images are presented. Flow separation causes performance loss, structural loads and increasing acoustic emissions on wind turbine rotor blades. However, due to the low thermal contrast between turbulent and separated flow regions, the non-invasive thermographic visualisation of flow separation is currently only possible for wind tunnel measurements, which are characterised by a high thermal contrast and a small measuring distance. The state-of-the-art signal processing approaches evaluate the surface temperature fluctuation of thermographic image series. However, understanding of the signal measurement chain with a distinct consideration of the influences on the dynamic surface temperature is incomplete. Therefore, designing model-inspired signal processing approaches which provide a high interpretability and a maximum contrast is an open task. The proposed signal processing approaches evaluate the surface response selectively, by using the amplitude information of the surface temperature response to an oscillating input signal or gradient-based for a transient input signal. The approaches are applied to wind tunnel measurements on a rotor blade profile at a near thermodynamic steady state and a transient thermodynamic behaviour at Reynolds numbers that are representative for operational wind turbines. The gradient-based evaluation shows an improved contrast for the detection of flow separation, but is only applicable to profiles with transient thermodynamic behaviour. The amplitude evaluation provides a high degree of interpretability of the processed images based on flow-dependent features and enables for an unambiguous identification of flow separation by a global amplitude minimum close to the separation point. Additionally, an increased spatial resolution for surface modifications is shown, while the contrast between flow regions is significantly decreased. Hence, the proposed approaches allow for an improved identifiability of flow separation with regard to future applications on wind turbines in operation. * [leftmargin=*] *
imaging measurement method with in-process capability is required for the detection and localisation of flow separation on wind turbine rotor blades. Long-established methods to detect and localise flow separation are tufts [5], stall flags [6], or oil solutions [7]. These methods are suitable for in-process measurements and are technologically easy to apply. However, the installation and precise positioning are time-consuming and costly. The techniques are also invasive, influence the boundary layer flow, and suffer from a poor spatial resolution along the rotor blade [8]. Thus, the available methods do not meet the requirements of a non-invasive, contactless, and imaging measurement system. A measurement method with the potential to meet all requirements for measuring flow separation on wind turbines, even in process, is infrared (IR) thermographic flow visualisation [9]. The measuring principle of thermographic flow visualisation is based on the relation between the convective heat flux density and the boundary layer flow region. Under the condition of the presence of convective heat flow, i.e., a temperature difference between the incoming flow and the blade, different convective heat flux densities cause different surface temperatures on the rotor blade. The surface temperature differences are non-invasively measurable by detecting the IR radiation with a respective camera [10]. As a result, the visualisation of different boundary layer flow regions on the rotor blade is possible with a high image resolution. In wind tunnel experiments, the measurement objects are usually actively, externally or internally, heated or cooled, to increase the necessary heat flux between the fluid and the rotor blade and, thus, to maximise the contrast of the thermographic images [8; 11].