Vertical Coherence of Turbulence in the Atmospheric Surface Layer: Connecting the Hypotheses of Townsend and Davenport

TL;DR

This paper connects classical hypotheses of turbulence in the atmospheric boundary layer with recent analyses, examining the effects of stratification on turbulence coherence and scaling, with implications for wind engineering applications.

Contribution

It demonstrates the consistency of Davenport's empirical formulation with Townsend's attached-eddy hypothesis and explores stratification effects on turbulence coherence in the atmospheric surface layer.

Findings

Davenport's formulation aligns with recent turbulence analysis.

Stable conditions show no self-similar scaling, challenging existing models.

Unstable conditions exhibit self-similar scaling and sensitivity to stability parameters.

Abstract

Statistical descriptions of coherent flow motions in the atmospheric boundary layer have many applications in the wind engineering community. For instance, the dynamical characteristics of large-scale motions in wall-turbulence play an important role in predicting the dynamical loads on buildings, or the fluctuations in the power distribution across wind farms. Davenport (Quarterly Journal of the Royal Meteorological Society, 1961, Vol. 372, 194-211) performed a seminal study on the subject and proposed a hypothesis that is still widely used to date. Here, we demonstrate how the empirical formulation of Davenport is consistent with the analysis of Baars et al. (Journal of Fluid Mechanics, 2017, Vol. 823, R2) in the spirit of Townsend's attached-eddy hypothesis in wall turbulence. We further study stratification effects based on two-point measurements of atmospheric boundary-layer flow over the Utah salt flats. No self-similar scaling is observed in stable conditions, putting the application of Davenport's framework, as well as the attached eddy hypothesis, in question for this case. Data obtained under unstable conditions exhibit clear self-similar scaling and our analysis reveals a strong sensitivity of the statistical aspect ratio of coherent structures (defined as the ratio of streamwise and wall-normal extent) to the magnitude of the stability parameter.