Abstract
In this work, we adopt the integration of the L-system fractal tree generation, 3D printed wind tunnel modeling, and computational fluid dynamics (CFD) simulation approach to model the wind effect on a single tree. We compare the agreement between CFD simulations and wind tunnel measurements of rigid branched structures resembling trees. First, fractal tree mesh models based on species growth and branching patterns are developed to represent tree species for wind–tree modeling. Subsequently, a scaled-down fractal tree is generated with 3D-printing and subjected to tunnel testing with load cell and particle image velocimetry measurement data under the wind speed of 10 m/s and 15 m/s. Finally, CFD based on Reynolds-Average Navier–Stokes (RANS) simulation with a full closure model and Large Eddy Simulation (LES) using appropriate momentum sink and turbulence source terms for the volumetric tree is carried out. We use both the volume-average porous media and the volume-splitting discretized zones (split number 10 × 10 × 10) to reproduce the momentum sink effect in the numerical simulation. Three tree species, namely, Peltophorum pterocarpum (yellow flame), Khaya senegalensis (African mahogany), and Hopea odorata (ironwood), are tested, and a reasonable agreement of drag force prediction and velocity profiles is obtained when comparing the CFD simulation results with wind tunnel data. The RANS modeled drag force results exhibit 20% of over-prediction, while the normalized velocity profiles display a good match of velocity decay at the tree leeward sides. On the other hand, LES produces much better results with only 3% discrepancy with the experimental results. A comparison of experimental results among the tree species is also carried out. Due to the actual random wind direction, tree slenderness representation, and structural flexibility issues, the current methodology still has the limitation for validation with urban on-site measurement. Nonetheless, this integrated approach is the first step in establishing modeling tool applicability to examine the effect of the forest structure and composition on wind loads.
Highlights
Urban vegetation plays an important role in improving the urban landscape
A possibly first of its kind approach using the integration of L-system fractal tree generation, 3D printed wind tunnel modeling, and the computational fluid dynamics (CFD) simulation methodology to model the wind effect on a single tree and predict the tree drag force has been proposed and analyzed
This integrated approach uses species modeling based on tree species growth and branching patterns to generate 3D fractal tree models, which were meshed and downscale printed (70:1) for wind tunnel particle image velocimetry (PIV) and drag force measurement, and employs both Reynolds-Average Navier–Stokes (RANS) simulation and Large Eddy Simulation (LES) for CFD flow modeling and drag calculation for further validation
Summary
Urban vegetation plays an important role in improving the urban landscape. Besides enhancing the aesthetics of the city, urban greenery promotes psychological wellbeing among urban dwellers, mitigates the urban heat effect, and improves air quality. trees must withstand forces imparted by the moving wind. Recent developments in computational fluid dynamics (CFD), experimental validation, and field studies have sought to understand the complex and dynamic wind–tree interaction, in order to estimate the aerodynamic force that the tree can endure in given locations and assess the tree management scenario and the risk of tree failure. It is essential to include the irregular tree branch and leaf geometry for wind load prediction on urban greenery, the computational resource and cost for meshing and CFD simulation make it impossible to model every single geometrical detail. 3. 3D CFD wind–tree modeling at the wind tunnel and urban landscape to develop, simulate, validate, and implement the CFD large scale wind–tree study using the tree fractal approach, with momentum sink terms being modeled as functions of the tree anisotropy, Leaf Area Density (LAD) properties, and reflecting the realistic directional effects from. The CFD analysis of flow over a single fractal tree with a high fidelity Large Eddy Simulation (LES) grid-filtering turbulence model will be investigated.
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