Computational Fluid Dynamics Analysis of an Innovative Multi-Purpose Green Roof ^†

Abstract

In this study, to improve the application and performance of conventional green roof systems, a novel multi-purpose green roof system was simulated numerically using computational fluid dynamics (CFD). The innovative multi-purpose green roof contains a soil layer and water filter, meaning the water retention time not only depends on the soil media but also depends on the filter’s pore size, improving the impact on runoff quality and quantity. In this regard, after mesh sensitivity analysis, the developed model was validated using experimental data, and the results show the accuracy of CFD in the simulation of porous media and filters. Comparisons between experimental and numerical results demonstrate the impact of proper porosity values in the simulation of a porous environment and reveal the source of errors in the numerical prediction of capillary flow in soil media, which can be minimized by adaptive consideration of the parameters, such as wall adhesion and appropriate wettability.

1. Introduction

The advantages of nature-based solutions (NBSs) in urban environments are plenty, and green roofs are among the best choices; these have several impacts, mainly including runoff quantity and quality [1], thermal impacts [2], life quality, and heat island impact [3]. One suitable method for evaluating the performance of green roofs and optimizing their design strategies is by using numerical simulations [4]. Beyond common hydrological models such as HYDRUS and SWMM [5], many studies emphasize different numerical solutions, like the Lattice Boltzmann method, for predicting water infiltration in porous media such as green roofs [6]. Some studies used the CFD approach to evaluate rainwater infiltration [7]. Many researchers have also used the coupled CFD-DEM method to predict water flow in porous media on a pore scale, as in one the study [8]. However, using the CFD porous media approach in the literature for modeling water infiltration is rare. This study aims to evaluate the performance of an innovative green roof with a string wound filter and the applicability of the CFD porous model approach, comparing the results from 2D simulations with experimental results.

2. Methodology

For the evaluation of the innovative green roof’s performance, the test bed, which is shown in Figure 1, was evaluated using CFD simulations. The test bed contains a 1 μm string wound filter in the center.

Model Setup

The 3D geometry of the test bed is shown in Figure 2. As the geometry of the test bed was symmetrical, a 2D model of the test bed was created, cut into half, and used, as demonstrated in Figure 3.

In the next step, the mesh was created, as shown in Figure 4, and mesh sensitivity analysis was conducted to achieve accurate results.

For the simulation, the transient VOF (Volume of Fluid) multiphase model and k-Ꜫ method were selected, and the boundary conditions are presented in

Table 1. Boundary conditions.

Name	Boundary Condition
Inlet	Mass-flow inlet
Outlet	Pressure outlet
Wall	No-slip
Soil	Porous zone 1
Filter	Porous zone 2

.

The porosity of the soil (Vulcaflor) was 50%, and hydraulic resistance in unsaturated conditions was more than 0.6 mm/min [9]. For the CFD simulations, two porous zones for soil and filter were considered with different permeabilities (10⁻⁹ [m²] for the soil and 10⁻¹⁰ [m²] for the filter) and viscous resistances (10⁹ [m⁻²] for the soil and 10¹⁰ [m⁻²] for the filter) in the 2D geometry of the test bed. The boundary condition for the inlet was set based on the rainfall intensity, and the simulation results were compared with the experimental results.

3. Results

The simulation and experimental hydrographs for rainfall with an intensity of 0.68 mm/min are shown in Figure 5, and the volume fraction of water in a 2D model for two-time steps is presented in Figure 6.

4. Conclusions

The simulation results agreed well with the experimental data from the rain event simulations. However, the simplifications considered for the CFD simulation, such as the soil medium being completely homogenous instead of employing the complicated geometry of the soil and also being completely unsaturated, might have caused the deviation in the simulation and in the experimental results, such as initial outflow time and the peak outflow value. Moreover, comparisons between experimental and numerical results demonstrated the impact of proper porosity values in the simulation of a porous environment and revealed the source of errors in the numerical prediction of capillary flow in soil media, which can be minimized by adaptive consideration of the parameters such as wall adhesion and appropriate wettability.