Turbulent characteristics within roughness sublayer over spanwise homogeneous transversal bars

TL;DR

This study investigates turbulent flow characteristics within the roughness sublayer over spanwise homogeneous transversal bars, revealing how element spacing influences turbulence structure, mixing, and flow transition from canopy to channel flow.

Contribution

It introduces a simplified spanwise homogeneous roughness model to analyze flow transitions and turbulence structures at different element spacings, advancing understanding of roughness effects on turbulence.

Findings

Turbulent flow transitions from canopy to channel flow as element spacing decreases.

Turbulent mixing is enhanced and coherency decreases with smaller element spacing.

The attached-eddy hypothesis is valid for certain small spacing cases.

Abstract

Turbulent characteristics within shear layer have been studied, recently, within vegetative canopy, buildings, dunes. Kevin-Helmholtz instability triggered hairpin vortex shedding has been widely concluded as the "signature" of mixing layer analogy. However, convoluted roughness types complicate the observing practice of turbulent evolving progress. To simplify that, spanwise homogeneous roughness has been adopted in this work to capture the streamwise flow alternations under different streamwise roughness element distances. Streamwise successive object distance $\lambda_x$ is used to show the roughness element distance. The transitional trends from turbulent channel flow to canopy flow have been observed from Case 1 to Case 4. However, when $\lambda_x/h<1.0$, as successive distance decreases, the evidences indicate turbulent flow is transferring from canopy flow into channel flow from Case 4 to Case 6. Instantaneous results verifies the enhancement of turbulent mixing and decreasing turbulent coherency. The attached-eddy hypothesis (AEH) becomes valid in $\lambda_x/h<1.0$ Cases. Meanwhile, Reynolds-averaged flow results show the roughness shear "curve" effectiveness will decrease when $\lambda_x$ keeps decreasing after $\lambda_x/h<1.0$. The probability density function (PDF) of three fluctuating variables on a sampling point over element display the "increasing-decreasing" trend of decreasing spacing effect on upper region, wherein the maximal effect emerges in Case 2 and 3. And the swirling structural visualizations also demonstrate the progress of turbulent structure changing in roughness sublayer at different streamwise successive object distances. Finally, the instantaneous results show wall-normal momentum flux transports and turbulence swirling structures are correlated to the element spacing.