# Thermographic Stall Detection by Model-Inspired Evaluation of the Dynamic Temperature Behaviour
## 5 Conclusions and Outlook  
Two signal processing approaches were proposed to achieve an enhanced detectability of flow separation using thermographic image series: a harmonic analysis of the small signal behaviour and a gradient-based evaluation of the large signal behaviour. By rigorously deriving both signal-processing approaches from an analysis of the signal measurement chain and evaluating the surface temperature response according to the dynamic behaviour of the incoming flow temperature, a higher degree of interpretability and distinguishability between the different flow regions is obtained.  
The amplitude information of the proposed harmonic signal processing approach around an operating point is shown to contain only the surface temperature response to the oscillating parts of the incoming flow temperature. Subsequently, the amplitude information provides a direct correlation with the flow-dependent mean value of the heat transfer coefficient. As a result, the harmonic signal-processing approach around an operating point enables an unambiguous identification of flow regions on the basis of flow-dependent features. In particular, the identifiability of flow separation is increased by a global minimum in the amplitude curve over the chord position near the separation point. In addition, the harmonic signal processing approach around an operating point leads to an increased spatial resolution for surface modifications. However, the contrast between the flow regions is significantly decreased compared to the classical signal processing approach, i.e., when evaluating the mean value of the surface temperature. Therefore, the harmonic signal processing approach around an operating point can be considered as a reference method for the identification of flow regions and is especially useful for measurement objects at near thermodynamic steady state, i.e., in applications with a low thermal contrast between the different flow regions.  
The second proposed signal processing approach, which evaluates the surface temperature response to a transient input signal (from the incoming flow temperature) on the basis of the temporal temperature gradient, provides an improved contrast for the thermographic detection of flow separation. However, the applicability of the approach is limited to measurement objects with a transient thermodynamic behaviour. As a result, the gradient approach provides an improved distinguishability and location of flow transitions on the basis of the CNR in case of a transient input signal. Instead, the harmonic approach enables an unambiguous distinction between attached and separated flow regions. Therefore, both proposed signal processing approaches are a supplement to the established evaluation of temporal mean temperatures.  
In the present work, the suitability of the novel signal processing approaches for the detection of flow separation in wind tunnel experiments was proven. Further research will focus on the method transfer to free-field measurements. The next aim is to detect flow separation on rotor blades of wind turbines in operation.  
Conceptualization, F.O., M.S. and A.F.; methodology, F.O. and A.F.; software, F.O.; validation, F.O., D.G. and A.F.; investigation, F.O., D.G., J.S., N.B. and A.F.; resources, F.O., J.S., N.B. and A.F.; writing--original draft preparation, F.O., M.S. and A.F.; writing--review and editing, F.O., J.S., N.B., M.S. and A.F.; visualization, F.O. and A.F.; supervision, M.S. and A.F.; project administration, F.O., M.S. and A.F.; funding acquisition, M.S. and A.F. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Deutsche Forschungsgemeinschaft (DFG project number FI 1989/5-1).  
Not applicable.  
The authors gratefully acknowledge the loan of the rotor blade model by the TU Delft.