J. Fluid Mech. (2006), vol. 566, pp. 357–376. c  2006 Cambridge University Press doi:10.1017/S0022112006001534 Printed in the United Kingdom 357 Eﬀect of wall-boundary disturbances on turbulent channel ﬂows By OSCAR FLORES 1 AND JAVIER JIM ´ ENEZ 1,2 1 School of Aeronautics, Universidad Polit´ ecnica de Madrid, 28040 Madrid, Spain 2 Center for Turbulence Research, Stanford University, Stanford, CA 94305, USA (Received 4 January 2006 and in revised form 11 April 2006) The interaction between the wall and the core region of turbulent channels is studied using direct numerical simulations at friction Reynolds number Re τ ≈ 630. In these simulations the near-wall energy cycle is eﬀectively removed, replacing the smooth-walled boundary conditions by prescribed velocity disturbances with non-zero Reynolds stress at the walls. The proﬁles of the ﬁrst- and second-order moments of the velocity are similar to those over rough surfaces, and the eﬀect of the boundary condition on the mean velocity proﬁle is described using the equivalent sand roughness. Other eﬀects of the disturbances on the ﬂow are essentially limited to a layer near the wall whose height is proportional to a length scale deﬁned in terms of the additional Reynolds stress. The spectra in this roughness sublayer are dominated by the wavenumber of the velocity disturbances and by its harmonics. The wall forcing extracts energy from the ﬂow, while the normal equilibrium between turbulent energy production and dissipation is restored in the overlap region. It is shown that the structure and the dynamics of the turbulence outside the roughness sublayer remain virtually unchanged, regardless of the nature of the wall. The detached eddies of the core region only depend on the mean shear, which is not modiﬁed beyond the roughness sublayer by the wall disturbances. On the other hand, the large scales that are correlated across the whole channel scale with U LOG = u τ κ -1 log(Re τ ), both in smooth- and in rough-walled ﬂows. This velocity scale can be interpreted as a measure of the velocity diﬀerence across the log layer, and it is used to modify the scaling proposed and validated by del ´ Alamo et al. (J. Fluid Mech., vol. 500, 2004, p. 135) for smooth-walled ﬂows. 1. Introduction Wall-bounded turbulent ﬂows have been thoroughly studied in the past decade, with special emphasis on ﬂows over smooth walls. In the last few years increasing attention has been paid to the study of rough walls, which are commonly encountered not only in some industrial applications but also in the vast majority of geophysical ﬂows. There are also theoretical aspects of rough-walled ﬂows which might be useful for the understanding of the physics of the wall region, in particular its interaction with the outer ﬂow. Direct numerical simulations (DNS) of non-physical ﬂow conﬁgurations have been very useful in the study of inner-outer interactions, such as in the autonomous channel of Jim´ enez, del ´ Alamo & Flores (2004). From this point of view, the study of rough-walled ﬂows can be understood as the study of a core region without the structures of the smooth wall.