On the structure of turbulent and dispersive shear and associated Turbulent Kinetic Energy (TKE) flux across the roughness sublayer of a gravel-bed open-channel flow

摘要

In the present study, the structure of shear and associated vertical TKE flux in a gravel bed channel is compared to those obtained in standard k-type or d-type rough wall flows. This is done by comparing profiles across the roughness sublayer of Double-averaged (DA) turbulent and dispersive stresses, correlation coefficients and DA TKE fluxes (Reynolds and dispersive terms), to rough-wall flow results reported recently in the literature (Pokrajac et al. 2008, Djenidi et al. 2008). Furthermore, level-crossing conditional statistics of the turbulent shear and associated TKE flux (Hurther et al. 2007) is used to examine the bed zone signatures of ejection and sweep type events. It is shown that bed zones located downstream local bed protuberances with a k-type behaviour, exhibit very intense sweep activity. This process is found to be at the origin of the negative value (i.e. downward orientation) of the DA TKE flux in the lower interfacial layer of the roughness sublayer.