Top-down and bottom-up eddy motion in wall bounded turbulence

Abstract

Many people argue that turbulence structure in a turbulent boundary layer is dominated by motion created by viscous instabilities in the near-wall region (see [4] and references quoted there). This analysis is based on observations made for a smooth wall up to heigth of order 50 y⁺, and generalised to rough walls. This is what is called the bottom-up theory of turbulent boundary layers.

On the other hand, other people think that the turbulence characteristics in the boundary layer are dominated by the way eddies coming from a higher layer are distorted when impinging onto the ground, in a top-down manner. It is mainly supported by people coming from the meteorological community and practitionners, who are influenced by the way wind patterns evolves on the sea surface or on agricultural fields (so called ”cat-paws”), see [5], and by work on very high Reynolds number rough boundary layers, with typical viscous height v/u* ≈ 10-4 m, roughness height from 0.1 to 10 m, and measurement heigth from 10 to 100 m.