Flows across high aspect ratio street canyons: Reynolds number independence revisited

The Reynolds number for flow in a street canyon, Re = U ref H/ν (where U ref is a reference velocity, H the street canyon height, and ν the kinematic viscosity), cannot be matched between reduced-scale experiments and full-scale field measurements. This mismatch is often circumvented by satisfying the Re independence criterion, which states that above a critical Re (Re c ), the flow field remains invariant with Re. Re c = 11,000 is often adopted in reduced-scale experiments. In deep street canyons with height-to-width aspect ratio ≥ 1.5, reduced-scale experiments have shown two recirculation vortices induced by the mean flows, but full-scale field measurements have observed only one vortex. We investigated this discrepancy by conducting water channel experiments with Re between 10 4 and 10 5 at three aspect ratios. The canyons with aspect ratio 1.0 have Re c = 11,000, the canyons with aspect ratio 1.5 have Re c between 31,000 and 58,000, while the canyons with aspect ratio 2.0 have Re c between 57,000 and 87,000. Therefore, the widely adopted Re c = 11,000 is not applicable for canyons with aspect ratio greater than 1.5. Our results also confirm that there is only one vortex in deep canyons at high Re. This single-vortex flow regime could change our fundamental understanding of deep canyons, which are often assumed to exhibit multiple-vortex flow regimes. Applications such as numerical model validation based on the multiple-vortex regime should be revisited. Our experimental data with Re up to 10 5 could be used to validate numerical models at high Re.