Direct Numerical Simulation of Global Intermittent Turbulence in Stably Stratified Flows

The existence of global intermittent turbulence (a.k.a. turbulent bursts) is well known in the atmospheric boundary layer (ABL) literature. This intriguing phenomenon is usually characterized as alternately quiescent and bursty portions of an observed time series, representing laminar and turbulent states of the ABL flow, respectively. In recent years, various mesoscale atmospheric phenomena have been identified as possible triggers for turbulent bursts (e.g., low-level jets, density currents, and gravity waves). Even though these physical processes are extremely relevant for atmospheric intermittent turbulence, in this study we would like to address a more fundamental question: is intermittent turbulence an inherent property of stably stratified boundary layer flows? In other words, we would like to find out if this phenomenon can be simulated in the absence of any external forcings (e.g., topography, inertial oscillation, and mesoscale atmospheric phenomena). To this end, we perform direct numerical simulation of stably stratified channel flows with various Reynolds numbers and Richardson numbers. Clear signatures of turbulent bursts are observed in the simulation results for a range of stabilities. To further probe its characteristics, spatio-temporal variations of global intermittent turbulence are shown at various vertical locations and their correlation is discussed. Instantaneous gradient Richardson number, a local stability parameter, is utilized to predict the transition between laminar and turbulent flows. Lastly, the extended self-similarity analyses are performed to evaluate the impact of global intermittency on small-scale intermittency for various stratification levels.