Anisotropic Decay of Turbulence in Plane Couette-Poiseuille Flow

We report the results of an experimental investigation into the decay of turbulence in Couette-Poiseuille flow using so-called 'quench' experiments where the flow laminarises after a sudden reduction in Reynolds number. We measured the velocity field in the $xz$ plane, where $x$ is the streamwise and $z$ the spanwise directions respectively. We show that the decay of turbulence is anisotropic: the spanwise velocity $u_z$, corresponding to streamwise vortices (or rolls), decays faster than the streamwise velocity $u_x$, corresponding to elongated regions of higher or lower velocity named streaks. We define turbulent fractions $F_x$ and $F_z$ from the streamwise $x$ and spanwise $z$ velocity components, respectively, and examine their decay as a function of the Reynolds number. The decay of $F_z$ is linear and always faster than the one of $F_x$, while the decay of the spanwise energy $E_z$ fits an exponential. We characterized the decay rate $A_z$ of $E_z$ and the decay slope $a_z$ of $F_z$ as a function of $Re$. We found that the obtained values are independent of the noise levels. Both the decay rates $A_z$ and the decay slopes $a_z$ scale in the form $\propto(Re_*-Re)$, with $Re_*$ close to $Re=670$ above which turbulence becomes self-sustained.