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The widest spanwise scales in turbulent channel flows are studied through theuse of three periodic channel-flow simulations at friction Reynolds number$\mathrm{Re}_{\tau}=550$. The length and height of the channels are the same inall cases ($L_x/h=8\pi$ and $L_y/h=2$ respectively), while the width isprogressively doubled: $L_z/h = \{4\pi, 8\pi, 16\pi\}$. The effects ofincreasing the domain can not be determined with statistical significance inour simulations, since the difference in the statistics between the simulationsis of the same order as the errors of convergence. A channel flow similar tothe smaller one ($\textit{J. Fluid Mech.}$, vol. 500, 2004, pp. 135--144),which was averaged over a very long time, was used as a reference. Theone-dimensional spanwise spectrum of the streamwise velocity is computed withthe aim of assessing the domain-size effect on the widest scales. Our resultsindicate that $90\%$ of the total streamwise energetic fluctuations isrecovered without a significant influence of the size of the domain. Theremaining $10\%$ of the energy reflects that the widest scales in the outerlayer are the ones most significantly affected by the spanwise length of thedomain. The power-spectral density for $k_z = 0$ remains constant even if thesize of the domain in the spanwise direction is increased up to 4 times thestandard spanwise length, indicating that wide, spanwise coherent structuresare not an artifact of domain truncation.

Widest scales in turbulent channels