## 4 Results and Discussion
### Uniformity of Air Velocity Distribution  
The air velocity above the crop in the plant factory is one of the key factors affecting the growth rate of the crop. Relevant research has shown that air movement could enhance the exchange of substances between air and plants and enhance photosynthesis when the horizontal velocity of the air above the plant is between 0.3 m\(\cdot\)s\({}^{-1}\) and 0.5 m\(\cdot\)s\({}^{-1}\)[51]. In plant factory farming systems, the obstruction of indoor airflow by the cultivation trays and the crop itself leads to a decrease in air velocity and increasing non-uniformity of air velocity distribution above the crop planting areas. Hence, it is crucial to investigate the impact of the air change rate (\(ACH\)) of an indoor ventilation system and the planting density in crop areas on indoor airflow patterns. The objective uniformity parameter (\(OU\)) is utilized to evaluate the uniformity of canopy air velocity distribution in indoor plant cultivation areas. As \(OU\) indicates the deviation of the air velocity above the crop from the suitable velocity for each cultivation tray, a higher \(OU\) value signifies a more uniform air velocity distribution above the crop. Figure 5: Comparison of CFD results with the experimental data from literature [54]: **(a)** horizontal velocity component at y/L = 0.5; **(b)** vertical velocity component at x/L = 0.5; **(c)** temperature at y/L = 0.5; **(d)** temperature at x/L = 0.5. Figure 6 presents a comprehensive overview of the variations in the overall \(OU\) with \(ACH\) above the crop across all five different cultivation densities. It can be observed that, in general, \(OU\) is increasing with increasing \(ACH\) values in the plant factory and the uniformity of the air velocity distribution over the crop varies between 48% and 76%. (Figure 6). The velocity distribution for the case of \(P_{d}\) = 0.5 in the indoor crop area is chosen as the basis for analyzing the variation in indoor air velocity distribution with increasing \(ACH\). This particular case, at \(P_{d}\) = 0.5 in the crop area, effectively represents the overall trend of \(OU\) values with increasing \(ACH\) for each planting density (Figure 6). As depicted in Figure 7, it is evident that the overall indoor air velocity experiences a significant increase with higher \(ACH\) values in the room. Furthermore, the indoor mixed flow is enhanced as the \(ACH\) increases. Additionally, the deviation of the air velocity from the optimal air velocity (\(U_{0}\) = 0.4 m\(\cdot\)s\({}^{-1}\)) above each cultivation tray decreases. This ultimately contributes to an increase in \(OU\) in the room (Figure 6). As the \(ACH\) increased, the \(OU\) of indoor did not consistently increase. Specifically, in the case of indoor crop area with \(P_{d}\) = 0.1, the \(OU\) achieved a maximum value of 72.6% at \(ACH\) = 80 h\({}^{-1}\) and then decreased as the \(ACH\) further increased. This is due to the fact that, as the \(ACH\) increases, the average velocity of the airflow over the crop area of the lowest indoor cultivation tray is much greater than \(U_{0}\) = 0.4 m\(\cdot\)s\({}^{-1}\), which leads to a lower overall indoor \(OU\) value. It can be observed that smaller \(P_{d}\) values result in larger \(OU\) values in the indoor under the same \(ACH\) conditions. To further analyze this phenomenon, let us discuss the case of \(ACH\) = 80 h\({}^{-1}\). Figure 8 depicts the two-dimensional velocity distributions of the five \(P_{d}\) values of the crop in the x-z plane above the cultivation tray. It is evident that, in the cases where \(P_{d}\) > 0.5 in the crop area, the higher planting density leads to increased resistance to airflow, resulting in lower air velocities above each cultivated tray. Consequently, the overall increase in \(OU\) above the crop area is less pronounced when \(ACH\) is increased in these cases (Figure 6).