Mean Flow and Turbulence in Unsteady Canopy Layers

TL;DR

This study investigates how unsteady pressure gradients influence mean flow and turbulence over urban surfaces using large-eddy simulations, revealing parameter effects on roughness length, displacement height, and turbulence structures.

Contribution

It introduces new phenomenological models for predicting roughness length and Stokes layer thickness under unsteady flow conditions in the atmospheric boundary layer.

Findings

Roughness length increases with oscillation amplitude and frequency.

Turbulence within the Stokes layer is out of equilibrium with the mean flow.

Models successfully capture flow unsteadiness effects on surface parameters.

Abstract

Non-stationarity is the rule in the atmospheric boundary layer (ABL). Under such conditions, the flow may experience departures from equilibrium with the underlying surface stress, misalignment of shear stresses and strain rates, and three-dimensionality in turbulence statistics. Existing ABL flow theories are primarily established for statistically stationary flow conditions and cannot predict such behaviors. Motivated by this knowledge gap, this study analyzes the impact of time-varying pressure gradients on mean flow and turbulence over urban-like surfaces. A series of large-eddy simulations of pulsatile flow over cuboid arrays is performed, programmatically varying the oscillation amplitude $\alpha$ and forcing frequency $\omega$. The analysis focuses on both longtime-averaged and phase-dependent flow dynamics. Inspection of longtime-averaged velocity profiles reveals that the aerodynamic roughness length $z_0$ increases with $\alpha$ and $\omega$, whereas the displacement height $d$ appears to be insensitive to these parameters. In terms of phase-averaged flow statistics, it is found that $\alpha$ primarily controls the oscillation amplitude of the streamwise velocity and Reynolds stresses, but has a negligible impact on their wall-normal structure. On the other hand, $\omega$ determines the size of the region affected by the unsteady forcing, which identifies the so-called Stokes layer thickness $\delta_s$. Within the Stokes layer, phase-averaged resolved Reynolds stress profiles feature substantial variations during the pulsatile cycle, and the turbulence is out of equilibrium with the mean flow. Two phenomenological models have been proposed that capture the influence of flow unsteadiness on $z_0$ and $\delta_s$, respectively.