higher in the second case probably because the wall-shear stress has been determined via measurements of the mass-flow rate and pressure drop, while @ricco-wu-2004 have measured directly the mean streamwise velocity in the viscous sublayer. It is thus possible that the drag reduction variations have not been detected because of the high experimental uncertainty. This conjecture is further supported by the fact that our numerical uncertainty is lower than the ones by @ricco-wu-2004 and @choi-graham-1998 on account of the difficulty of such experiments and the high accuracy of our computations (refer to section 2.4 at page 256 in QR). We also note that the majority of the experiments by @ricco-wu-2004 have been conducted at relatively low $T^+$ ($T^+ \approx 50$, $D_m^+ \approx 240$, $W_m^+ \approx 15$ - see Fig. 13 in their paper), where the change of the drag reduction with the Reynolds number is weak, as shown in Fig. \[fig:figure\_8\].

Our results might also explain why numerical works have revealed higher drag reduction values than the experimental ones (see Fig. 14 at page 51 in Ricco and Wu, 2004), the latter generally conducted at higher Reynolds numbers. The effect of Reynolds number however warrants further investigation as we have 