The lifetimes of velocity structures in turbulent channels*

As a continuation of previous work on the dynamics of the very large velocity structures in wall-bounded flows, we compute correlation lifetimes in two channels at Re_τ=950 and 5200, in a simulation box 8πh×3πh. The longest lifetimes collapse reasonably well across Reynolds numbers in terms of eddy turnovers h/u_τ, although h/U_b cannot be excluded. The longest living structures are those at the longest streamwise wavelength compatible with the computational domain and λ_z=O(2h), centered at y≈0.5h. The three velocity components live similar amounts at the same scales, even if the energy contents of those scales can be very different. Shorter wavelengths recover the known relation for the momentum-carrying structure u_τT∼λ_z, which also applies to the three velocity components.

It is found that lifetimes u_τT/h>2 are sensitive to the length on the smoothing temporal window used to compute the correlations. Their precise estimation requires simulation times of the order of hundreds eddy turnovers.