Direct numerical simulation of high aspect ratio spanwise-aligned bars

Abstract

We conduct minimal-channel direct numerical simulations of turbulent flow over two-dimensional rectangular bars aligned in the spanwise direction. This roughness has been often described as $d$-type, as the roughness function $\Delta U^{+}$ is thought to depend only on the outer-layer length scale (pipe diameter, channel half height or boundary layer thickness). This is in contrast to conventional engineering rough surfaces, named $k$-type, for which $\Delta U^{+}$ depends on the roughness height, $k$. The minimal-span rough-wall channel is used to circumvent the high cost of simulating high Reynolds number flows, enabling a range of bars with varying aspect ratios to be investigated. The present results show that increasing the trough-to-crest height ($k$) of the roughness while keeping the width between roughness bars, $\mathcal{W}$, fixed in viscous units, results in non-$k$-type behaviour although this does not necessarily indicate $d$-type behaviour. Instead, for deep surfaces with $k/\mathcal{W}\gtrsim 3$, the roughness function appears to depend only on $\mathcal{W}$ in viscous units. In these situations, the flow no longer has any information about how deep the roughness is and instead can only `see' the width of the fluid gap between the bars.

arXiv commentAccepted in the Journal of Fluid Mechanics

References in corpus (3)

• A fast direct numerical simulation method for characterising hydraulic roughness
• Turbulent flow over transitionally rough surfaces with varying roughness densities
• The minimal-span channel for rough-wall turbulent flows

Cited by in corpus (10)

• DNS-based characterization of pseudo-random roughness in minimal channels
• Turbulent flows over dense filament canopies
• Modulation of near-wall turbulence in the transitionally rough regime
• Turbulence and added drag over acoustic liners
• Turbulent drag reduction by spanwise wall forcing. Part 1: Large-eddy simulations
• On the Flow Past an Array of Two-Dimensional Street Canyons Between Slender Buildings
• Turbulent flows over porous lattices: alteration of near-wall turbulence and pore-flow amplitude modulation
• Examination of outer-layer similarity in wall turbulence over obstructed surfaces
• The wind-shade roughness model for turbulent wall-bounded flows
• Spanwise wall forcing can reduce turbulent heat transfer more than drag