Comprehensive Evaluation of Fast-Response, Reynolds-Averaged Navier–Stokes, and Large-Eddy Simulation Methods Against High-Spatial-Resolution Wind-Tunnel Data in Step-Down Street Canyons

• Published in: Boundary-layer meteorology 2017-08, Vol.164 (2), p.217-247
• Main Authors: Hayati, Arash Nemati, Stoll, Rob, Kim, J. J., Harman, Todd, Nelson, Matthew A., Brown, Michael J., Pardyjak, Eric R.
• Format: Article
• Language: English
• Subjects: Aspect ratio; Atmospheric Protection/Air Quality Control/Air Pollution; Atmospheric Sciences; Computational fluid dynamics; Computer applications; Computer based modeling; Earth and Environmental Science; Earth Sciences; Experimental data; Flow topology; Flow velocity; Fluid dynamics; Fluid flow; Fluid mechanics; Fluid-structure interaction; GEOSCIENCES; High rise buildings; Hydrodynamics; Inflow; Large eddy simulation; Mathematical models; Meteorology; Methods; Modelling; Navier-Stokes equations; Oceanic eddies; Research Article; Resolution; Reynolds averaged Navier-Stokes method; Reynolds-averaged Navier–Stokes; Saddle points; Secondary flow; Separation; Simulation; Spatial discrimination; Stokes law (fluid mechanics); Street canyons; Street-canyon flow; Tall buildings; Topology; Tunnels; Turbulence (Fluid dynamics); Updraft; Velocity; Vertical velocities; Width; Wind tunnels
• ISSN: 0006-8314; 1573-1472
• DOI: 10.1007/s10546-017-0245-2

Three computational fluid dynamics (CFD) methods with different levels of flow-physics modelling are comprehensively evaluated against high-spatial-resolution wind-tunnel velocity data from step-down street canyons (i.e., a short building downwind of a tall building). The first method is a semi-empirical fast-response approach using the Quick Urban Industrial Complex (QUIC-URB) model. The second method solves the Reynolds-averaged Navier–Stokes (RANS) equations, and the third one utilizes a fully-coupled fluid-structure interaction large-eddy simulation (LES) model with a grid-turbulence inflow generator. Unlike typical point-by-point evaluation comparisons, here the entire two-dimensional wind-tunnel dataset is used to evaluate the dynamics of dominant flow topological features in the street canyon. Each CFD method is scrutinized for several geometric configurations by varying the downwind-to-upwind building-height ratio ( H d / H u ) and street canyon-width to building-width aspect ratio ( S  /  W ) for inflow winds perpendicular to the upwind building front face. Disparities between the numerical results and experimental data are quantified in terms of their ability to capture flow topological features for different geometric configurations. Overall, all three methods qualitatively predict the primary flow topological features, including a saddle point and a primary vortex. However, the secondary flow topological features, namely an in-canyon separation point and secondary vortices, are only well represented by the LES method despite its failure for taller downwind building cases. Misrepresentation of flow-regime transitions, exaggeration of the coherence of recirculation zones and wake fields, and overestimation of downwards vertical velocity into the canyon are the main defects in QUIC-URB, RANS and LES results, respectively. All three methods underestimate the updrafts and, surprisingly, QUIC-URB outperforms RANS for the streamwise velocity component, while RANS is superior to QUIC-URB for the vertical velocity component in the street canyon.