# Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil +
## 2 Experimental Setup
The air enters the wind tunnel through a nozzle with an area ratio of 1:3 after passing through several flow-conditioning fine meshes and a honeycomb to generate a uniform flow with \(T\)\(u\)\(\approx\)0.3%. Note that no turbulence grid is used for the aerodynamic measurements. The aerofoil model was placed horizontally across almost the entire width of the test section (2.5 mm gap was left at each side of the wind tunnel side window). This wind tunnel is not acoustically treated and is not suitable for the noise measurement. Instead, it is only used for measurements of the lift and drag produced by the aerofoil. A three-component strain gauge force balance was used to measure the aerodynamic forces produced by the aerofoil. As shown in the earlier Figure 1b, the aerodynamic forces will transmit to the three strain gauges via some pre-tensioned cables. The angle of attack of the aerofoil is rotated through a disc between \(-8^{\circ}\) and \(+30^{\circ}\). Wind tunnel blockage correction is applied in the data analysis. All tests are undertaken at angles of attack \(-15^{\circ}\leq\alpha\leq 15^{\circ}\) degrees. The freestream velocity \(U\) is set at 24 ms\({}^{-1}\). The rotating dial mechanism of the mounting plate is used to adjust the angle over this range with an estimated uncertainty of \(\pm 0.2^{\circ}\). Three data sets were taken for each test case so that the lift and drag coefficients presented in this paper will represent the averaged values. The NACA65 (12)-10 aerofoil model has a chord and span of \(C\) = 0.15 m and \(S\) = 0.495 m, respectively. The aerofoil model consists a main body with a length of 0.1 m and an interchangeable leading edge of 0.05 m. The geometrical modification only affects the leading edge region, and the remaining part of the aerofoil at rear is unaffected. The design of the serrations can be described by a sinusoidal curve along the leading edge with the characteristic shape of semi-cyclic serration tips. The sinusoidal curve is  
Figure 2: (**a**) Placement of the turbulence bi-planar grid inside the nozzle of the aeroacoustics wind tunnel; (**b**) comparison of the normalized turbulent energy spectra between the experimental results and the von-Kármán model for the longitudinal velocity fluctuations. The streamwise location of the measurement corresponds to the leading edge of the aerofoil (30 mm from the nozzle exit), but without the presence of the aerofoil. described by the serration amplitude \(A\) and the wavelength \(\lambda\). A design principle, which is to keep the same wetted area between the serrated leading edge and the baseline straight leading edge, is adopted in this study. This means that the serration peak will extend upstream of the baseline leading edge by \(A/2\), while the remaining \(A/2\) is cut into the aerofoil main body. This is reflected in Figure 3. In the current study, combinations of two serration amplitudes (\(A\) = 15 and 30 mm) and two serration wavelengths (\(\lambda\) = 5 and 10 mm) were investigated. Summary of the geometrical details are provided in Table 1.  
As shown in Figure 4a, mass airflow injection is delivered through the multiple orifices along the leading edge. Each of these orifices has the same diameter \(D\) = 2 mm. To study the effect of the spanwise distance between the orifices \(\lambda^{\prime}\), two straight leading edges with \(\lambda^{\prime}\) = 5 mm and 10 mm were manufactured. External air supply is connected to the aerofoil and the flow rate can be adjusted through a digital valve. The regulator and air supply are located outside of the anechoic chamber in order to isolate the valve noise. The bulk air supply into the aerofoil body is quantified by \(Q\) (liter/min), which is monitored by a digital flow rate meter. Through a system of air channels inside the aerofoil body, the air flow rate from each orifice (\(Q\) ) is approximately equal to \(Q/\varphi\), where \(\varphi\) is the total number of orifices along the leading edge.