# Thermographic Stall Detection by Model-Inspired Evaluation of the Dynamic Temperature Behaviour
## 3 Experimental Setup  
### Wind Tunnel Setup and Experimental Procedures  
In order to investigate the potential of the presented signal-processing approaches, measurements at Deutsche WindGuard's aeroacoustically optimized wind tunnel (DWAA) in Bremerhaven, Germany are conducted. The Gottinger-type wind tunnel operates with a closed test section with turbulence intensities of less than 0.3% at wind speeds of up to 100 m s\({}^{-1}\).The test section has a dimension of 1.25 m \(\times\) 2.7 m \(\times\) 5 m. The aerodynamic models, with a maximal chord length of up to 0.9 m, are vertically mounted between two turntables at the ceiling and the floor of the test section, which allows for adjusting different angles of attack. The thermographic camera is positioned on the side of the wind tunnel test section, facing the measurement object through a window made of calcium fluoride. Figure 3 illustrates the experimental setup. Note that the friction in the wind tunnel due to operation is the only external heat source and leads to an increasing fluid temperature during the measurements. In addition to the large-scale fluid temperature gradient, there are small-scale fluid temperature oscillations that are probably a result of variations in the fan power. Due to the inertia of the power control, the oscillations are mainly in a low-frequency range from 0 to 0.2 Hz. For the presented measurements, a wind turbine airfoil of type DU96W180 is selected. The wind tunnel model has a chord length of 0.6 m, height of 1.25 m and is made out of carbon reinforced plastic with a polyurethane topcoat which is typical for rotor blades of wind turbines [8]. Figure 4 shows the mounted airfoil profile inside the wind tunnel measuring section. Note that the profile does not have a homogeneous surface, since a mylar band covers a flap-gap at the 0.82 normalised chord position. Piezo pressure sensors are attached to the upper and lower side of the airfoil as a reference measuring system for the detection and localisation of laminar-turbulent transition and flow separation. The 92 pressure sensors along the chord length in flow direction are connected to pressure tabs in the surface of the airfoil and provide a sampling rate of 10 Hz. The tabs are located not behind each other but with a spanwise offset, in order to avoid impacts on the boundary layer flow and subsequently the downstream pressure measurements. In order to characterise the proposed signal processing approaches, the experiments are designed for two free-field similar measurement conditions. Firstly, measurements with a low thermal contrast on a measurement object in a near thermodynamic steady state are conducted. Here, the focus is on the harmonic evaluation of the surface temperature response to small-scale oscillations of the input signal (_thermodynamic response around an operating point_). Secondly, the flow velocity is increased in order to increase the heating rate of the wind tunnel. Consequently, the fluid temperature is considered as a transient input signal and the gradient-based signal processing approach is characterised (_transient thermodynamic response_). Note that the harmonic signal processing approach around an operating point is not applicable to the heat-based signal processing approach. Figure 4: Section side of the mounted DU96W180 profile in the test section with marked field of view (FOV) of the thermographic camera. Figure 3: Schematic overview of the measurement setup in the closed test section of the Deutsche WindGuard’s aeroacoustic wind tunnel in Bremerhaven, Germany, adapted from [20]. operating point evaluates the surface temperature oscillations which occur independent of the large-scale behaviour of the surface temperature. Therefore, the harmonic approach is applicable for both measurement conditions, while the gradient-based approach is limited to transient input signals. The adjusted test parameters are the incoming flow velocity, the compensation time before starting the experiments and the angle of attack of the measurement object.