Intermittency and spontaneous interface formation in stratified open channel flow

We investigate and quantify the effects of spatio-temporal intermittency resulting from stable stratification in stratified open channel flow through direct numerical simulations. We find that due to the vertical inhomogeneity of the momentum and buoyancy flux profiles in our flow configuration, intermittency spontaneously manifests as a deformed horizontal interface between the upper quiescent flow where Re_B ≤ Ο(1) and the lower bulk turbulent flow where Re_B ≥ Ο(10) , where Re_B is the buoyancy Reynolds number. We hence adapt the local unstable density criterion of Portwood et al 2016 to robustly identify the turbulent and quiescent regions within instantaneous realisations of the flow. We quantify local intermittency through a depth varying `turbulent' volume fraction which we find is well predicted by a local Monin-Obhukov length normalized in viscous wall-units Λ<sup>+</sup>, such that intermittency is observed within the range of 2.5 ≤ Λ<sup>+</sup> ≤ 260. We find that the region of intermittency is defined by vigorous and efficient mixing through Kelvin-Helmholtz type overturning instabilities that form due to the critical conditions at the turbulent/quiescent interface and accordingly find that the thickness of the interfacial layer scales with the Ellison length L<sub>E</sub>. This region of critical intermittency is described by a constant gradient Richardson number of Ri<sub>g</sub> ≈ 0.2, and where the local stream-wise velocity and density gradients develop such that the `turbulent' and `quiescent' flow are defined by Ri_g marginally lower and higher than the critical value respectively. Similarly we find that irrespective of the external parameter set, the turbulent Froude number approaches a clear lower limit at the turbulent/quiescent interface of Fr ≈ 0.3. Hence, we investigate and quantify the effects of intermittency within this region on a Fr based parametrization of the flux coefficient Γ.