## 1 Introduction
### State of the Art  
Conventional flow visualization methods used on wind turbines like pressure measurements [14], oil flow [15], or tuft [16] visualization cannot be used for an evaluation of the boundary layer flow because the pressure taps, oil film, or tufts act as artificial surface disturbances itself. A contactless measurement technique without the need of a surface preparation enabling a noninvasive flow measurement is thermographic flow visualization [17; 18; 19]. This measurement technique is already in use for a variety of applications in wind tunnel experiments for detecting the laminar-turbulent transition or flow separation [20; 21; 22; 23]. More recent studies begin to use thermography for analyzing surface defects on models of aerodynamic profiles [24; 25]. A transfer of the thermographic flow visualization from wind tunnel experiments to field measurement applications on wind turbines in operation was recently introduced by [PERSON] et al. [26; 27]. However, the state-of-the-art image processing method for evaluating the thermographic images is optimized for locating the flow transition occurring naturally without any premature transitions triggered by surface disturbances. For retrieving the flow transition position on the rotor blade surface, the image processing method conducts temperature profile evaluations of one-pixel-wide lines in the thermographic image parallel to the flow direction and, thus, almost perpendicular to the leading edge [9], see for instance the vertical lines in Figure 1 for  
Figure 2: Sketch of (**a**) the temperature profile and (**b**) the temperature gradient of the three evaluation lines A, B, and C in Figure 1. The star-symbol marks the location of the actual flow transition. The flat temperature rise in the profile B with a premature flow transition results in a lower gradient compared to the profile A and therefore has a lower sensitivity with respect to the flow transition position. For the low-contrast turbulence wedge example C, the location of the flow transition does not correspond with the global but a local gradient maximum, which is lower than for a strong turbulence wedge B.  
the cases A, B, and C. The position of the flow transition in each line is then determined by locating the maximum temperature gradient, as the transition between the laminar and turbulent flow regime creates a significant temperature step in flow direction. This image processing method was designed to evaluate the maximal temperature gradient for a flow transition perpendicular to the flow direction and evaluation line, because it minimizes the measurement uncertainty [28] and is subsequently referred to as the state-of-the-art method. A local surface disturbance, however, triggers a premature flow transition and creates a so-called turbulence wedge, see Figure 1. The flow transition along the flanks of the turbulence wedges is no longer perpendicular to the evaluation lines. In combination with an elongated transition process [7] due to the forced transition, the temperature step between the flow regimes is flattened in the evaluated temperature profile. To illustrate this finding, the temperature profile and temperature gradient in \(y\)-direction are compared in Figure 2a,b, respectively. Both are shown for an example of no premature flow transition (A) and an example with premature flow transition (B) with a star symbol showing the actual flow transition position for both cases. While the temperature gradient forms a spike at the position of the temperature gradient maxima in case A, the maximum gradient for case B is surrounded by a constant gradient level. Therefore, a localization of the gradient maxima in case B has a lower sensibility compared to the maxima in case A. As a result, the uncertainty of the flow transition localization in sections with a surface disturbance, hence a premature flow transition (case B), is increased.