905461.fig.009
Figure 9: Power spectral density ( 1  /Hz) of the turbulent velocity fluctuations, measured at a nominal flow speed  m/s approximately at  m, 0.007 m upstream of the leading edge of the non-porous reference airfoil at mid-span (turbulence grids: (blue —) PPS 12/2, (red —) PPS 14/4).