Route to turbulence in Strongly Stratified Slope Flows

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Turbulence, or high-speed chaotic fluid motion, is the norm rather than the exception in nature. Describing and predicting fluid turbulence is complex and requires computer simulations. Many advances have been achieved in computer modeling of turbulence under well-mixed atmospheric flow conditions. However, a serious concern in weather forecasting has been inadequate modeling of turbulence within a stratified atmosphere and in regions of complex terrain. In a stratified atmosphere, buoyancy force (due to density variations) acts in the same direction as gravity, hindering the formation of well-mixed flow conditions. Stratified conditions are commonly observed during nighttime in mountainous terrain and over large ice sheets and glaciers. Accurate prediction of winds (air flow) over Antarctica and Greenland is an important aspect of understanding the impact of climate change, because the strength and structure of winds are directly connected to the ablation of ice sheet and the associated sea level rise. Moreover, prediction of nocturnal winds in mountainous terrain has important implications for air quality, agriculture, and defense operations. This project is expected to lead to improved prediction of winds under stratified conditions in computer models of weather. Outreach activities are planned for existing precollege preparatory programs. The project will result in open-source educational materials related to stratified fluid turbulence.The chief technical objective is to investigate newly discovered fluid instabilities over sloping terrain and unravel their role in progression toward an intermittent and patchy turbulent state. Inclination of the surface alters the turbulence dynamics due to obliqueness of stratification relative to flow shear, leading to further departure from a classical boundary layer profile. The project will adopt contemporary techniques from hydrodynamic stability theory and direct numerical simulations to arrive at an atlas of fluid instabilities as a function of the surface inclination and the newly introduced dimensionless stratification perturbation parameter. The investigation is expected to reveal new flow features emerging at sufficiently high stratification perturbation numbers and improve the comprehension of flow intermittency in the form of bursting phenomena and patchy turbulence. The project will culminate with a quantitative criterion to characterize intermediate flow regimes that range from laminar to fully turbulent conditions, thus filling the current knowledge gap pertinent to partially turbulent stratified slope flows. The outcomes of this project are expected to lead to future improvements of subgrid-scale parameterizations of stably stratified conditions in numerical weather prediction models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.