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Scale interactions between the lower atmosphere and the urban canopy

Abstract : The urban boundary layer consists of complex coherent structures, such as large-scale low momentum regions and intermittent turbulent sweeps and ejections, which are responsible for the transport of heat, momentum and pollution. Although these structures have been well identified, their quantitative relationship with the flow inside the canopy is still unknown. Wind tunnel modelling of flow over simplified rough terrain consisting of either three-dimensional or twodimensional roughness elements are able to reproduce these structures and are used to investigate the nonlinear relationship between large-scale momentum regions and small-scales induced by the presence of the roughness. As the temporally resolved small-scale signalis not available Linear Stochastic Estimation is used to decompose the flow into large and small-scales and confirm that the large-scale structures within the overlying boundary layer influence the small-scales close to the roughness through a non-linear mechanism similar to amplitude modulation. Changing terrain configuration from 3D to 2D roughness results in a modification of the non-linear relationship closer to the shear layer that develops near the top of the obstacles. The full turbulent kinetic energy (TKE) budget including dissipation is calculated using an LE-PIV model and demonstrates that small-scale structures within the shear layer are important to the production, transport and dissipation of energy. Finally, triple decomposition of theTKE budget confirms that the non-linear relationship that exists between large-scale momentum regions and small-scales close to the roughness is related to energy transfer between these structures.
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Submitted on : Wednesday, October 28, 2020 - 11:43:47 AM
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Karin Blackman. Scale interactions between the lower atmosphere and the urban canopy. Fluids mechanics [physics.class-ph]. École centrale de Nantes, 2017. English. ⟨NNT : 2017ECDN0028⟩. ⟨tel-02981790⟩

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