Large-Eddy Simulation of Coherent Flow Structures within a Cubical Canopy

Abstract

Instantaneous flow structures "within" a cubical canopy are investigated via large-eddy simulation. The main topics of interest are, (1) large-scale coherent flow structures within a cubical canopy, (2) how the structures are coupled with the turbulent organized structures (TOS) above them, and (3) the classification and quantification of representative instantaneous flow patterns within a street canyon in relation to the coherent structures. We use a large numerical domain (2,560 m × 2,560 m × 1,710 m) with a fine spatial resolution (2.5 m), thereby simulating a complete daytime atmospheric boundary layer (ABL), as well as explicitly resolving a regular array of cubes (40 m in height) at the surface. A typical urban ABL is numerically modelled. In this situation, the constant heat supply from roof and floor surfaces sustains a convective mixed layer as a whole, but strong wind shear near the canopy top maintains the surface layer nearly neutral. The results reveal large coherent structures in both the velocity and temperature fields "within" the canopy layer. These structures are much larger than the cubes, and their shapes and locations are shown to be closely related to the TOS above them. We classify the instantaneous flow patterns in a cavity, specifically focusing on two characteristic flow patterns: flushing and cavity-eddy events. Flushing indicates a strong upward motion, while a cavity eddy is characterized by a dominant vortical motion within a single cavity. Flushing is clearly correlated with the TOS above, occurring frequently beneath low-momentum streaks. The instantaneous momentum and heat transport within and above a cavity due to flushing and cavity-eddy events are also quantified.

The Parallelized Large-Eddy Simulation Model (PALM) version 4.0 for atmospheric and oceanic flows: model formulation, recent developments, and future perspectives

“…The research fields ranged from development of better urban parametrization schemes to the investigation of the ventilation at pedestrian level in densely built-up cities. The flow around street canyons and idealized buildings has been the subject of several studies (e.g., Inagaki et al, 2011;Abd Razak et al, 2013;Park et al, 2012;Park and Baik, 2013;Yaghoobian et al, 2014). With increasing computational resources, it also has become possible to use PALM to simulate entire city quarters Kanda et al, 2013).…”

Interaction between turbulent flow and sea breeze front over urban‐like coast in large‐eddy simulation

“…Small eddies with strong instantaneous flow form in the urban canopy layer, owing to the discrete nature of buildings [ Eliasson et al ., ]. Given favorable conditions, such eddies may grow coherently to form turbulent organized structures of large size in the CBL [ Inagaki et al ., ]. This organized structures manifest as low‐speed streaks coupled with updrafts that are aligned with a streamwise orientation [ Kanda , ; Park and Baik , ].…”

“…This organized structures manifest as low‐speed streaks coupled with updrafts that are aligned with a streamwise orientation [ Kanda , ; Park and Baik , ]. Such turbulent flow is related to the characteristic length of roughness elements [ Finnigan et al ., ] and reaches a size several times larger than an individual building [ Castillo et al ., ; Inagaki et al ., ]. A common type is horizontal convective rolls that can be observed over urban and/or coastal areas [ Miao and Chen , ; Iwai et al ., ; Chen et al ., ].…”

Wind-Direction Effects on Urban-Type Flows

“…We remark finally that one might ask, for example, how the 'portal vortices' identified by Hagishima et al (2009) might be affected by wind direction, or how the strong link between the outer flow organized longitudinal turbulent structures and the near-ground canopy flow identified by Inagaki et al (2012) might be affected by wind flow directions that do not yield along-wind streets in the surface morphology. Whilst such questions are not without fundamental interest, from a practical perspective the crucial global issues concern, rather, how wind direction affects the variability in total (mean) surface drag, consequent changes to inertial layer parameters, and overall canopy flows.…”