CFD NUMERICAL SIMULATION OF TEMPERATURE AND AIRFLOW DISTRIBUTION 
IN PIGSTY BASED ON GRID INDEPENDENCE VERIFICATION

Grid independence verification was implemented to improve the reliability of CFD numerical simulation in pigsty. The effects of four different grid numbers on airflow and temperature simulation of 0.4m, 1.0m and 1.6m heights were compared. The results showed that the third method of mesh generation and the grid numbers about 2.09 million were more suitable for this pigsty model. The average relative error of airflow velocity and temperature between the simulated and the measured alues were 7.1% and 3.8% respectively, the average NMSE were 0.0012 and 0.0066 respectively. Therefore, grid independence verification is of great significance for CFD numerical simulation. 摘要 为了提高猪舍 CFD 数值模拟的可靠性,对猪舍网格独立性进行了验证。 比较了四种不同网格数对 0.4m、 1.0m 和 1.6m 高度的气流和温度场模拟的影响。 结果表明,第三种网格生成的方法和大约 209 万的网格数量更 适合该猪舍模型。 气流速度、温度的模拟值和实测值的平均相对误差分别为 7.1%和 3.8%,均方误差分别为 0.0012 和 0.0066。 可见,网格独立性验证对提高 CFD 数值模拟的可靠性具有重要的意义。 INTRODUCTION With the rapid development of large-scale and intensive pig husbandry, higher requirements have been put forward for the environment in pigsty. Temperature and airflow velocity are two of the most important environmental factors in pig farming which are directly related to the economic benefits and pigs’ welfare. So, it is necessary to evaluate the environmental indexes efficiently and provide a comfortable environment for the pigs. There are three main methods to study the airflow and temperature distributions in livestock buildings: field test, laboratory test (or wind tunnel test) and Computational Fluid Dynamics (CFD) numerical simulation (Hong et al., 2013; Nielsen., 2015; Ntinas., 2017). Although the field test should be closer to the real conditions in the pigsty, it is easily affected by the external environment and the accuracy of the sensors. As for the laboratory test (wind tunnel test), under stable conditions, it can work better while requiring a lot of time and cost to repeat the different structural configurations and different weather conditions. CFD offers a feasible way to overcome the shortcomings of these two methods mentioned above; it has been widely used to study airflow and temperature distribution within livestock buildings (Nielsen et al., 2015; Rojano et al., 2016; Sejun et al., 2014; Sapounas et al., 2013;). However, due to the heavy workload of mesh generation, the accuracy and efficiency of the simulation are all greatly affected by the gird numbers and the mesh quality (Yao et al., 2016; Li et al., 2018). The impact of three mesh types (hexahedral, tetrahedral, and hybrid) and five grid numbers on the accuracy and computing costs of air distribution simulations has been conducted in a firstclass aircraft cabin; the results showed that the hexahedral meshes can get the best result but also the highest computing costs (Duan et al., 2015). Yu et al. investigated the computational accuracies and convergence rates of triangular and quadrilateral meshes, they concluded that the number of triangular meshes needs to be 4/3 times that of quadrilateral to obtain similar accuracy (Yu et al., 2012). 1 Min Jin, Ph.D. Stud. Eng.; Chunguang Wang*, Prof. Ph.D. Eng.; Pengpeng Wang , Ms. Stud. Eng. Vol. 61, No. 2 / 2020 INMATEH – 242 The effect of airflow velocity on heat exchange between cows’ standing and tilting was also studied by CFD numerical simulation; the results indicated that CFD can be used as a useful way to get ideal results based on optimal parameters and reasonable mesh model (Wang et al., 2018). Therefore, prior to conduct the numerical simulation of the pigsty, grid independence verification must be done to minimize the workload of the mesh generation while not affecting the reliability and the accuracy of the results. In this study, field test and CFD numerical simulation are combined to verify the importance of grid independence test for improving the reliability of CFD numerical simulation in pigsty. MATERIALS AND METHODS ⚫ Data source The experiment was carried out on a pig farm in Hohhot, Inner Mongolia, China (40°40 '26 "N, 111°21' 46"E). The dimensions of the pigsty are 9.2m (length) × 9.0m (width) × 3.6m (height), with negative pressure ventilation. There are 14 adjustable air inlets (265mm×655mm) at the top of the pigsty, many conveying pipes in the upper space, 2 automatic feeding equipment on the left side, 4 cylindrical finned heating pipes, 2 variable speed fans on the side walls and a 2m deep manure pit beneath the floor. Meanwhile, each pen is surrounded by solid wall and partially slatted concrete floor. The internal structure of the experimental pigsty is demonstrated in Fig.1. The field test was conducted on January 9, 2018. The environmental temperature was measured and recorded by temperature sensors (HSTL-102WS, China) every 2s. In addition, an intelligent hot wire anemometer (9565-P, TSI, USA) was used to measure the air velocity every 0.2 s and averaged per second for a measurement period of 60s at each measuring point. As shown in Fig.2, the sensors were located in the centre of each pig pen as well as middle of the aisle at 3 different heights: 0.4m (height of lying pig), 1m (height of standing pig) and 1.6m (height of breeder). The 14 adjustable air inlets were set as the velocity inlets and the four outlets set as pressure outlets in GAMBIT. The heating was regarded as a constant heat source in the pigsty and its averaged surface temperature was measured by the infrared thermometer (MT4, Raytek, American) every 5min. Pigs’ model was regarded as non-slip wall with constant temperature. In addition, the density of the pigs was 1100 kg.m, the specific heat was 3500 J.(kg.K) and the thermal conductivity as 0.464 W (m.K). Fig. 1 Internal structure of the pigsty Fig. 2 Sensor layout scheme in the pigsty ⚫ Numerical simulation ➢ Selection of governing equation No matter complex or simple the fluid flow is governed by the law of conservation. The basic conservation law mainly includes the mass conservation law, the momentum conservation law, energy conservation law, collectively known as the governing equation. In terms of the control equation of CFD, the continuity equation, the momentum equation and the energy equation are mainly applied (Osorio Saraz et al., 2013): Vol. 61, No. 2 / 2020 INMATEH – 243 (1) Continuity equation: