Comparison between the normalised PSD of wall pressure fluctuations measured for sample 1 and sample 3 of (20–0.5–0–0) orifice at transducer positions transducer positions and 3 with orifice Reynolds number, Re_o = 22,000 as a function of Strouhal number . As shown in Figure 8, sample 1 and 3 have drastically different steady flow drag coefficients but the PSD of the wall pressure fluctuations are quite similar.