Dispersion experiments are compared for two transparent model fractures with identical complementary rough walls but with a relative shear displacement $\vec{ \delta}$ parallel ($\vec{ \delta}\parallel \vec{ U}$) or perpendicular ($\vec{ \delta} \perp \vec{ U}$) to the flow velocity $\vec{ U}$. The structure of the mixing front is characterized by mapping the local normalized local transit time $\bar t(x,y)$ and dispersivity $\alpha(x,y)$. For $\vec{ \delta} \perp \vec{ U}$, displacement fronts display large fingers: their geometry and the distribution of $\bar t(x,y)U/x$ are well reproduced by assuming parallel channels of hydraulic conductance deduced from the aperture field. For $\vec{ \delta} \parallel \vec{ U}$, the front is flatter and $\alpha(x,y)$ displays a narrow distribution and a Taylor-like variation with $Pe$.
Categories: physics.geo-ph physics.flu-dyn