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Turbulence characteristics of non-neutral/ non-stationary flows in the atmospheric surface layer

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TURW04 - Wall-bounded turbulence: beyond current boundaries

Under the influence of environmental and meteorological conditions, non-neutral (in the sense of thermal equilibrium) and non-stationary (in the sense of statistics) flows are ubiquitous in the atmospheric surface layer (ASL). This talk will introduce measurements on clean and sand-storm wind flows in the ASL based on a unique observation facility, i.e., the Qingtu Lake Observation Array (QLOA) specifically designed to probe the turbulence characteristics of wind flows in ASL . Preliminary results on the turbulence coherent structures of non-neutral clean-air flows, and the evolution of the turbulence features during the whole process of sand-storm events characterized by non-stationary flows will be present. Firstly, the similarity between large-scale flow structures and temperature field structures in the ASL with various scales under different stratification stability conditions were analyzed, and the results show that large-scale flow structures have scale similarity at different scales with unstable stratification conditions, and the scale ratio decreases with the thermal buoyancy. Further, with unstable stratification conditions the inclination angles of large-scale flow structures are scale-dependent, i.e., the inclination angle of large-scale flow structure decreases with the scale. For the temperature field structures, their streamwise scales display a linear increase with height but a decrease with the thermal buoyancy. Then, this talk will take a sand-storm as an example to demonstrate the variations of the turbulent kinetic energy during the growth, stabilization and decay stages. Analysis show that the large-scale coherent structures originally exist in the growth stage of sandstorms with an energy fraction of 75% rather than gradually forming. In addition to carrying a substantial portion of energy, the large-scale structures are active structures with strong nonlinear energy transfer. Structures gain energy from the nonlinear interaction in the growth stage, but as time evolves, the nonlinear interaction in the stabilization and decay stages gradually weakens but maintains a balanced budget of energy. The energy fraction reduced to 40% in the decay stage. These results will provide an insight for understanding the turbulent characteristics of wind flows and sandstorm process in the ASL under the influence of heat.

This talk is part of the Isaac Newton Institute Seminar Series series.

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