# Sound Radiation of Aerodynamically Excited Flat Plates into Cavities
## 4 Experimental Results
### Flow Characterization
#### 4.1.1 Flow Parameters  
In general, the wind pressure merely acts towards the plate surface and thus, the pressure coefficient achieves positive values. The results show that first there is a sharp decline till the pressure coefficient is reaching a certain level. This suggests that the flow is not fully turbulent developed yet, but the transition area between laminar and turbulent flow is still present. A rough calculation of the critical Reynolds number confirms this assumption. In conclusion, there is no boundary layer with zero pressure gradient because the pressure coefficient is not close enough to zero. However, the values are only slightly increasing above 0.01 so that the deviation is relatively small. The relevant range between the front edge and the trailing edge of the flexible plate shows the least fluctuating pressure coefficient with values between 0.005 and 0.01. Consequently, the boundary layer along a flat plate is characterized by the fact that the velocity profiles measured at different distances to the leading edge of the rigid plate are similar.  
The spatial distribution and the accuracy of the flow velocity is very important in the aeroacoustic wind tunnel because the validity of the measurements is strongly dependent on whether a uniform velocity field with nearly constant velocity in the core region is formed in the test section. The resulting  
Figure 5: Measurement setup with Laser Scanning Vibrometers.  
velocity profiles are depicted in Figure 6 by means of contour plots linked to a simplified CAD model of the nozzle and the rigid plate. It is shown that the core area of the flow with a nearly constant velocity is shrinking, whereas the mixing zone in the transition area to the resting medium is broadening. The flow-excited flexible plate, which is exactly located between the two velocity profiles at \(x=350\) and \(520\,\mathrm{mm}\), is clearly lying in the core flow.  
The turbulence intensity for isotropic turbulence with equal average velocity fluctuations in all three dimensions \({u^{\prime}_{x}}^{2}={u^{\prime}_{y}}^{2}={u^{\prime}_{z}}^{2}\)[9; 15] is  
\[T_{u}=\frac{\sqrt{{\frac{{u^{\prime}_{x}}^{2}}{u_{\infty}}}}}{{u_{\infty}}} \times 100\quad\mathrm{in}\,\%, \tag{16}\]  
The distribution of the increasing turbulence intensity from \(0\,\%\) towards the edges is similiar to the decreasing velocity from \(45\,\mathrm{m}\,\mathrm{s}^{-1}\) (Figure 6). Moreover, the turbulence intensity is identically depicted as the velocity profiles (Figure 7) at the same position. The turbulence intensity values at the edges increase with distance to the nozzle. Accordingly, a contour with relatively small width and increased turbulence intensity, which is recreating the form of the nozzle outlet, is growing in width towards the middle of the profile. The first contour plot shows a high turbulence intensity at the edges. Considering the contour plots at the position \(x=350\,\mathrm{mm}\) and \(520\,\mathrm{mm}\), the turbulence intensity at the edges decreases with increasing distance to the nozzle. As already mentioned in connection with the flow velocity, a good comparability and transferability of the measurement results can be provided because the area of the flexible plate structure is exposed to low turbulence.  
Figure 6: Velocity profile at \(45\,\mathrm{m}\,\mathrm{s}^{-1}\) with distances \(x=100,350\) and \(520\,\mathrm{mm}\) from the nozzle outlet.