La pregunta por la religión: origen y elementos

For a proper understanding of the simulation processes, there were various verification processes undertaken in this study to confirm the assumptions made in the literature as regards the use of CFD for indoor simulations of livestock buildings. The verification processes were considered necessary in order to critically assess the applicability of the assumptions and the accuracy of the CFD predictions in this study. The framework of the verifications in this study include; (1) the inlet configurations, (2) airflow through the inlets on right and left sidewalls, (3) addition of indoor obstacles such as drinker and feeder lines in the CFD simulation and (4) finally the positioning of measurement planes. Previous studies have indicated that multiple sidewall inlets can be represented by a long sidewall inlet (Bjerg et al., 2002; Blanes-Vidal et al., 2008), airflows through the sidewall openings are identical (Harral and Boon, 1997) and indoor obstacles such as drinker and feeder lines can be neglected during the CFD simulation based on their negligible effect on the airflow (Blanes-Vidal et al., 2008). These assumptions were first verified to determine appropriate broiler building geometry and building set up for further CFD simulations.

4.2.2.1.1. Development of broiler building geometries

Figure 4.1 shows the geometries of broiler buildings with different inlet configurations and indoor obstacles. The geometries were developed with SolidWorks 2016. The building is 21.75 m long, 18.30 m wide, 2.36 m eave height and the inlets are 1.60 m above the floor. The detailed description of the experimental building can be found in chapter 3. Figure 4.1a shows a broiler building with two long sidewall inlets with the dimension 20.36 m by 0.21 m placed at 1.60 m above the floor. Figure 4.1b shows a typical broiler building with twenty-four 0.52 m by 0.21 m sidewall inlets. Figure 4.1c shows a broiler building with feeder and drinker lines raised to the height of 0.3 m above the floor with their locations from the sidewall as they

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are in the experimental broiler building. These geometries were used in the CFD simulations to verify the assumptions earlier discussed and for simulating the indoor air velocities of the empty broiler building.

Figure 4.1: Geometries of broiler building with (a) long bottom-hinged sidewall inlets (b) multiple sidewall inlets and (c) drinker and feeder lines. All dimensions are in metres.

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4.2.2.1.2. Mesh generation

The CAD geometries of the broiler building (shown in Figure 4.1) were imported into the Star CCM+ 12 (Siemens, 2017), a CFD software package, for simulations. The software was adopted in this study based on it wide usage in the agricultural industry by previous

researchers (Ji et al., 2013; Norton et al., 2013, 2009). The internal parts, which represents the computational domain, of the imported CAD geometry were extracted for surface and volume discretisation. The discretised surfaces and volume were used by the solver (turbulence models) to obtain the physical solutions of the airflow in the broiler occupied zones within the sidewall inlet and roof exhaust ventilated broiler building.

For each of the building geometries, three unstructured coarse mesh densities were generated to discretise the computational domain (see Table 4.1). The mesh density refinement was done based on the number of cells within the prism layer. The successive growth rate of prism layer was maintained at a stretch ratio of one and a half from the wall surface. For the inlet mesh size, 25 % of the relative target size was used as the inlet relative minimum mesh size. As shown in Table 4.1, Mesh 3 has the highest mesh densities in all the building geometries test than Meshes 1 and 2. Therefore, in order to ensure that CFD

predictions were precise and accurate for all the turbulence models, Mesh 3 has been used in all the CFD simulations.

Table 4.1: Mesh densities used for model verification

Case Multiple inlets Long inlets Internal obstacles

Mesh 1 656,983 605,335 656,724

Mesh 2 791,754 738,144 853,522

Mesh 3 925,448 870,795 1,050,996

Figure 4.2 shows the meshes of the broiler building with different inlet configurations and internal obstructions. During volume meshing, unstructured polyhedral grids were used to improve and optimise the overall quality of the cell surfaces and the volume mesh model. Polyhedral grids were adopted based on it high accuracy, lesser computational time, fewer cells and the allowances it gives for conformal mesh interface between separate regions (Siemens, 2017). A prism layer mesh was used to generate prismatic cells near wall surfaces to improve the accuracy of the flow solution closer to all wall surfaces.

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Figure 4.2: Polyhedral meshes of the computational model used for the verification of airflow in the broiler building with; (a) long bottom-hinged inlet (b) multiple bottom-hinged inlets and (c) internal obstacles.

4.2.2.1.3. Turbulence models, measurement planes and convergence criteria

To verify the conditions under which indoor conditions of broiler building were simulated, three, widely adopted turbulence models were selected (Bjerg et al., 2002; Li et al., 2016a; Norton et al., 2009; Rong et al., 2016). Their selection for the evaluation of airflow in the broiler building was necessary in order to determine the most appropriate turbulence model for the indoor environment of a sidewall inlet and roof exhaust ventilated broiler building. These models include standard (Std) 𝑘 − 𝜀, realisable (Re) 𝑘 − 𝜀, and shear stress transport (SST) 𝑘 − 𝜔 turbulence models. The model that predicted the closest air velocity magnitude to the mean air velocities obtained in the field experiments has been used for further CFD simulation.

The fundamental principles (continuity and momentum and energy) of Naiver-Stoke equations governing the fluid flow in a sidewall inlet and roof exhaust ventilated broiler building were solved using the turbulence models. The indoor environment of the broiler building was simulated based on inlet configurations and internal obstacles as reported in the literature. Four measurement planes were created in the simulated broiler building to verify the assumptions reported in the literature. Figure 4.3 represents the measurement planes

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created to determine airflow variations at different locations from the sidewalls. The locations of the measurement planes, irrespective of the building geometry, are similar. Planes A, B, C and D are 4.50 m, 7.30 m, 10.88 m and 17.20 m from the front door respectively. Plane C represents the plane of symmetry of the building. The planes were created to represent the locations where inlets are not aligned with the outlets and places closer to the door walls. These were very important as they would enable the ventilation engineers to strictly consider the effects that inlets and outlets locations could have on the air distribution within the broiler buildings.

Figure 4.3: Measurement planes indicating measurement locations within the sidewall inlet and roof exhaust ventilated broiler building.

For the purpose of solution monitoring and convergence criteria, a global residual of 0.001 (0.1 %), for all fundamental equations (conservation of mass, momentum and energy) was defined. The computations were not terminated until the residuals were lesser than 0.001 and the air velocity magnitudes in the broiler occupied zones were also stabilised. The air velocity magnitudes in the broiler occupied zones, where broiler chickens experience heat stress during hot weather periods, were only considered in this study.

4.2.2.1.4. Boundary conditions

The boundary conditions specified in this study are shown in Table 4.2. These include the air velocity at the inlet, pressure at the outlet and building wall surfaces. Air turbulence intensity of 0.10 was imposed at the inlet. Similar inlet turbulence intensity in the broiler building has been reported in the literature (Blanes-Vidal et al., 2008; Li et al., 2016b). The same

turbulence intensity was obtained during the field experimentation at the inlet of the sidewall inlet and roof exhaust ventilated broiler building.

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Table 4.2: Boundary conditions specifications

Building surfaces Boundary conditions

Inlet Velocity inlet

Air velocity of 4.91 m s-1 _{at the inlet}

Inlet turbulence intensity of 0.10

Outlet Pressure outlet

Pressure (0 Pa) Building walls, floor and roof No-slip and smooth wall

4.2.2.2. Part 2: CFD modelling and validation of airflow in an experimental broiler