Wall turbulence response to surface cooling and formation of strongly stable stratified boundary layers

This paper investigates the processes by which stable boundary layers are formed through strong surface cooling imposed on neutrally stratified wall-bounded turbulence using high-resolution direct numerical simulation at a moderate Reynolds number. The adjustment of the flow to the imposed strong surface cooling is investigated. We further focus on a strongly stable case where turbulence partially collapses. We show that, due to a significant reduction in turbulence production, turbulence becomes patchy, with a band of turbulence coexisting with quiet regions. The nature of the quiet regions, which are often characterized as laminar, is investigated and shown to be consistent with viscously coupled stratified turbulence. The one-dimensional longitudinal streamwise velocity spectrum exhibits kx−5 and kx−3 behavior in the buffer and logarithmic layers, respectively, adjacent to an active region of three-dimensional turbulence with a kx−5/3 spectrum. Scenarios for turbulence recovery from such a patchy state are also discussed. We show that the presence of outer layer turbulence above z+ ≈ 300 is a key requirement for recovery. For higher values of stratification, it is shown that inner layer turbulence is damped entirely and outer layer turbulence is damped subsequently.This paper investigates the processes by which stable boundary layers are formed through strong surface cooling imposed on neutrally stratified wall-bounded turbulence using high-resolution direct numerical simulation at a moderate Reynolds number. The adjustment of the flow to the imposed strong surface cooling is investigated. We further focus on a strongly stable case where turbulence partially collapses. We show that, due to a significant reduction in turbulence production, turbulence becomes patchy, with a band of turbulence coexisting with quiet regions. The nature of the quiet regions, which are often characterized as laminar, is investigated and shown to be consistent with viscously coupled stratified turbulence. The one-dimensional longitudinal streamwise velocity spectrum exhibits kx−5 and kx−3 behavior in the buffer and logarithmic layers, respectively, adjacent to an active region of three-dimensional turbulence with a kx−5/3 spectrum. Scenarios for turbulence recovery from such a patchy state...