# Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil +
## 2 Experimental Setup  
The measurements of the far field noise as result of the turbulence-leading edge interaction of the aerofoil were conducted in the aeroacoustics wind tunnel at Brunel University London, UK [13]. The dimension of the anechoic chamber is 4 m \(\times\) 5 m \(\times\) 3.4 m. The open jet wind tunnel is driven by a centrifugal fan outside the chamber and the airflow inside the tunnel is acoustically treated by a 10 m long silencer. The cross section area of the nozzle exit is 10 cm (height) \(\times\) 30 cm (width). A maximum free jet velocity of approximately 80 ms\({}^{-1}\) can be reached. The range of the Reynolds numbers in the present aeroacoustics study is 2 \(\times\) 10\({}^{5}\)\(\leq\) Re \(\leq\) 5 \(\times\) 10\({}^{5}\) based on the chord length of the aerofoil, \(C\) (= 0.15 m), which is equivalent to free stream velocity \(U\) between 20 and 50 ms\({}^{-1}\), respectively. The aerofoil system was mounted horizontally between two parallel side plates. As shown in Figure 1a, a single pre-polarized condenser microphone (LarsonDavis 377B02) was located at 1.0 m from the leading edge of the aerofoil at the mid span and at a polar angle of 90 degree. The noise data from the microphone was sampled at frequency of 44 kHz for 10 s by a 16-bit Analog-Digital (A/D) card from the National Instrument. The far field acoustic data was windowed, Fast Fourier Transformed (1024 point) and averaged to compute the Power Spectral Density (PSD) with a 1 Hz bandwidth and frequency resolution of about 43 Hz. To determine the level of the freestream turbulence intensity (_Tu_) at the location coincides with the leading edge of the aerofoil (but without the presence of the aerofoil), a single hot wire was used to measure the streamwise velocity fluctuations \(u^{\prime}\). The diameter and length of the hot wire are 5 \(\upmu\)m and 1.25 mm, respectively. The hot wire was operated at an overheat ratio of 1.6. Placing the hot wire probe at 30 mm downstream of the nozzle exit, which corresponds to the leading edge of aerofoil when fixed to the side plates, the background _Tu_ produced by this wind tunnel (i.e., without placing turbulence grid inside the nozzle, and without the aerofoil) is between 0.1 and 0.2%. In order to generate elevated level of _Tu_, a bi-planar turbulence grid was placed inside the nozzle, as shown in Figure 2a. Using the same hot wire probe, and without the presence of the aerofoil, the _Tu_ has been identified for this study as the average value encompassing the two-dimensional _y_-_z_ plane, which is calculated as _Tu_\(\approx\)4.5%. The measured PSD of the fluctuating velocity, which is presented in Figure 2b, can be seen as nearly isotropic as demonstrated by the relatively good match with the von-Karman universal spectrum  
Figure 1: (**a**) Typical set up for the aerofoil noise experiments at the Brunel aeroacoustics wind tunnel for the turbulence–leading edge interaction noise; (**b**) load cell system used for the lift/drag measurements in a closed-section aerodynamic wind tunnel. for the longitudinal velocity fluctuation. This theoretical spectrum is represented by the following equation:  
\[\Phi_{u^{\prime}}=\frac{\overline{u^{\prime 2}}\Lambda_{uu}}{\pi u^{\prime}}\frac{1}{1 +k_{x}^{2}\Lambda_{uu}^{2}}\cdot G_{\gamma}G=exp\left[-\frac{9}{4}\cdot\left( \frac{k_{x}}{k_{\eta}}\right)^{2}\right],\text{ where }k_{\eta}\approx 4.8\times 10^{4} \text{m}^{-1}. \tag{2}\]  
where \(u^{\prime 2}\) is the mean-square of the velocity fluctuation, \(\Lambda_{uu}\) is the integral length scale, \(k_{x}\) = \(\omega/U\) is the streamwise wavenumber, and \(K_{\eta}\) is a parameter that controls the slope of the high-frequency roll-off. Aerodynamic forces (lift and drag) were measured in a separate wind tunnel. The wind tunnel is an open circuit, suction type with a closed-test section of 0.5 m (height) \(\times\) 0.5 m (width).