Intermittency and critical mixing in internally heated stratified channel flow

Through direct numerical simulations we investigate the effects of spatiotemporal intermittency as a result stable stratification in surface heated stratified open channel flow. By adapting density inversion criterion method Portwood et al. [ J. Fluid Mech. , vol. 807, 2016, R2] for our flow, demonstrate that flow may be robustly separated into regions active turbulence which $Re_B \gtrsim {O}(10)$ and surrounding quiescent fluid, where $Re_B$ is buoyancy Reynolds number. The spontaneously manifests deformed horizontal interface between upper lower turbulent characterised by vigorous mixing from ‘overturning’ shear instabilities. resulting vertical profile accurately predicted local Monin–Obukhov length normalised viscous wall units $\varLambda ^+$ such displays within parameter range $2.5 \lesssim \varLambda ^+ 260$ . considering conditional averages ‘turbulent’ ‘quiescent’ separately, find this region to described constant critical gradient Richardson Froude numbers $Ri_{g,c} \approx 0.2$ $Fr_c 0.3$ We continues display $\varGamma \sim Fr^{-1}$ relationship when $Fr < Fr_c$ whereas shows no correlation $\varGamma$ $Fr$ flux coefficient. Hence, directly emergence an asymptotic ‘saturated’ regime limit low ‘global’ occurs due increasing contributions measurements