Quantifying and predicting near-wall global intermittency in stably stratified channel flow

In this study we investigate stably stratified channel flow (SCF) between two solid walls, focusing on its transition from fully developed turbulence to the onset of global intermittency, characterized by the coexistence of laminar and turbulent patches in the flow. With direct numerical simulations, we examine this transition across various friction Reynolds (180 < Re-tau < 960) and shear Richardson (Ri(tau)) numbers, parameters that are observed to impact intermittency dynamics. To quantify intermittency, we measure the volume fraction of turbulent patches using enstrophy as a criterion and examine the variation of the turbulent fraction along the wall-normal direction. Our findings reveal that intermittency in SCF can originate independently from either near-wall or midchannel regions, depending on the values of Re(tau )and Ri(tau). With increased stratification, intermittency originating from both regions may merge across the channel's depth. Particular attention is paid to near-wall intermittency (NWI) and identifying its occurrence boundary within the Re-tau-Ri(tau) parameter space. We assess various dimensionless parameters for their ability to predict NWI, discovering that intermittency consistently occurs when the Nusselt number falls below a critical value of approximately 3.0. To establish the intermittency boundary following this Nusselt number criterion, a Reynolds-averaged Navier-Stokes model is formulated based on a first-order closure scheme. This model proves effective in predicting the occurrence of NWI in SCF in terms of Rey and Riy. Furthermore, we verify the Nu scaling recently proposed by Zonta et al. [J. Fluid Mech. 945, A3 (2022)], which leads to an intermittency boundary in the form of Re(tau)(2)Ri(tau)(-1 )= const.