Velocity and Surface Shear Stress Distributions Behind a Rough-to-Smooth Surface Transition: A Simple New Model |
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Authors: | Leonardo P Chamorro Fernando Porté-Agel |
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Institution: | (1) Saint Anthony Falls Laboratory, Department of Civil Engineering, University of Minnesota, 2, Third Avenue SE, Minneapolis, MN 55414, USA;(2) National Center for Earth-Surface Dynamics, Minneapolis, MN, USA |
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Abstract: | A simple new model is proposed to predict the distribution of wind velocity and surface shear stress downwind of a rough-to-smooth
surface transition. The wind velocity is estimated as a weighted average between two limiting logarithmic profiles: the first
log law, which is recovered above the internal boundary-layer height, corresponds to the upwind velocity profile; the second
log law is adjusted to the downwind aerodynamic roughness and local surface shear stress, and it is recovered near the surface,
in the equilibrium sublayer. The proposed non-linear form of the weighting factor is equal to ln(z/z
01)/ln(δ
i
/z
01), where z, δ
i
and z
01 are the elevation of the prediction location, the internal boundary-layer height at that downwind distance, and the upwind
surface roughness, respectively. Unlike other simple analytical models, the new model does not rely on the assumption of a
constant or linear distribution for the turbulent shear stress within the internal boundary layer. The performance of the
new model is tested with wind-tunnel measurements and also with the field data of Bradley. Compared with other existing analytical
models, the proposed model shows improved predictions of both surface shear stress and velocity distributions at different
positions downwind of the transition. |
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Keywords: | Atmospheric boundary layer Large-eddy simulation Roughness transition Surface shear stress model |
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