## 3 Experimental Setup
### Measurement Objects  
The validation of the introduced image processing method is conducted on a cylinder object in cross-flow with a diameter of \(160\,\mathrm{mm}\) that is mounted in the middle of the closed test section of the wind tunnel, see Figure 2a. The used material for the cylinder is polyoxymethylene and has thermal properties suitable for the thermographic flow visualization. The selected freestream flow velocity is \(v_{\infty}=50\,\mathrm{m}\,\mathrm{s}^{-1}\), resulting in a Reynolds number of \(5.1\times 10^{5}\). For this flow condition, the boundary layer flow over the cylinder consists of laminar, turbulent and separated flow regimes as well as a laminar separation bubble in the region of laminar-turbulent flow transition, see Figure 3.  
In order to study a measurement object equivalent to the perspective application on wind turbine rotor blades, a second experiment is conducted with a DU96W180 airfoil, see Figure 2b, with a chord length of \(600\,\mathrm{mm}\) and the same material properties as a real rotor blade airfoil. The flow velocities in two test cases are chosen to yield Reynolds numbers that are typical for the flow situation on wind turbines in operation, \(\mathrm{Re}=2\times 10^{6}\) for test case 1 and \(\mathrm{Re}=3\times 10^{6}\) for test case 2. Note further that all measurements are conducted with no explicit heating that could enhance the thermal contrast between the flow regimes. Therefore, the thermographic images are similar to  
Figure 3: Oil paint measurement on a \(160\,\mathrm{mm}\) diameter cylinder in cross-flow at a Reynolds number of \(\mathrm{Re}=5.1\times 10^{5}\). The beginning and end of the different boundary layer flow regimes is visible by the accumulation of oil.  
Figure 2: Both measurement objects used in this work. (**a**) DU96W180 airfoil object with a chord length of \(600\,\mathrm{mm}\). (**b**) Cylinder object with a diameter of \(160\,\mathrm{mm}\) in the wind tunnel test section.  
on wind turbines in operation where the thermal conditions of surface and fluid cannot be manipulated easily. The existing thermal contrast between the fluid and surface in the wind tunnel experiments solely depends on the heating of the fluid by the wind tunnel fans and wall friction. The increase in the fluid temperature during test case 1 and test case 2 are \(\mathrm{d}T_{1}/\mathrm{d}t=1.9\times 10^{-5}\,\mathrm{K}\,\mathrm{s}^{-1}\) and \(\mathrm{d}T_{2}/\mathrm{d}t=3.5\times 10^{-4}\,\mathrm{K}\,\mathrm{s}^{-1}\), respectively. The angle of attack of the airfoil model in both test cases is \(\alpha=6^{\circ}\).  
The two different heating rates result in different thermal conditions in the test cases 1 and 2. The mean surface temperature of the airfoil measurement object during test case 1 is in an almost steady state and increases during the image acquisition by only \(\mathrm{d}T_{1}=0.02\,\mathrm{K}\). During test case 2 a transient state with a constant heating up increases the mean surface temperature by \(\mathrm{d}T_{2}=0.35\,\mathrm{\SIUnitSymbolCelsius}\). For the thermographic raw images this means that the thermal contrast between the laminar and turbulent flow regime in test case 1 is very low compared to the contrast in test case 2.  
Additionally, an external heating source is used to create a disturbing reflection near the leading edge of the airfoil in test case 2. The reflection in combination with a flow-depending non-constant heat flux generates a non-homogenous temperature field within the laminar flow regime. This way the PCA can be examined for its ability to reduce systematic gradients in order to increase the distinguishability, when the thermal contrast in the thermographic image is already high.