NDLI: On Displacement Height, from Classical to Practical Formulation: Stress, Turbulent Transport and Vorticity Considerations

Content Provider	Springer Nature Link
Author	Sogachev, Andrey Kelly, Mark
Copyright Year	2015
Abstract	Displacement height (d) is an important parameter in the simple modelling of wind speed and vertical fluxes above vegetative canopies, such as forests. Here we show that, aside from implicit definition through a (displaced) logarithmic profile, accepted formulations for d do not consistently predict flow properties above a forest. Turbulent transport can affect the displacement height, and is an integral part of what is called the roughness sublayer. We develop a more general approach for estimation of d, through production of turbulent kinetic energy and turbulent transport, and show how previous stress-based formulations for displacement height can be seen as simplified cases of a more general definition including turbulent transport. Further, we also give a simplified and practical form for d that is in agreement with the general approach, exploiting the concept of vortex thickness scale from mixing-layer theory. We assess the new and previous displacement height formulations by using flow statistics derived from the atmospheric boundary-layer Reynolds-averaged Navier–Stokes model SCADIS as well as from wind-tunnel observations, for different vegetation types and flow regimes in neutral conditions. The new formulations tend to produce smaller d than stress-based forms, falling closer to the classic logarithmically-defined displacement height. The new, more generally defined, displacement height appears to be more compatible with profiles of components of the turbulent kinetic energy budget, accounting for the combined effects of turbulent transport and shear production. The Coriolis force also plays a role, introducing wind-speed dependence into the behaviour of the roughness sublayer; this affects the turbulent transport, shear production, stress, and wind speed, as well as the displacement height, depending on the character of the forest. We further show how our practical (‘mixing-layer’) form for d matches the new turbulence-based relation, as well as correspondence to previous (stress-based) formulations.
Starting Page	361
Ending Page	381
Page Count	21
File Format	PDF
ISSN	00068314
Journal	Boundary-Layer Meteorology
Volume Number	158
Issue Number	3
e-ISSN	15731472
Language	English
Publisher	Springer Netherlands
Publisher Date	2015-10-22
Publisher Place	Dordrecht
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Canopy flow Displacement height Mixing-layer theory Turbulent kinetic energy Turbulent transport Atmospheric Sciences Meteorology Atmospheric Protection/Air Quality Control/Air Pollution
Content Type	Text
Resource Type	Article
Subject	Atmospheric Science

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