*3.2. Analysis of Simulation Results*

3.2.1. Analysis of Variance and Range

The range analysis of the simulation experiment results [21] is shown in Table 3. The selection of the optimal parameter combination is based on the following information: the larger the average airflow velocity and the minimum airflow velocity of the seed-sucking hole, the lower the airway resistance and pressure loss and airflow velocity standard deviation of the seed-sucking hole; the smaller the difference in seed sucking performance, the better the sucking stability. According to this information, it was concluded that the optimal parameter combination of the airway structure parameters for the three experiment indicators is A2B2C3, which is the fifth level of the orthogonal simulation experiment. The simulation results show that the average airflow velocity of the seed-sucking hole is 102.59 m · <sup>s</sup><sup>−</sup>1, the minimum airflow velocity of the seed-sucking hole is 101.58 <sup>m</sup> · <sup>s</sup><sup>−</sup>1, and the airflow velocity standard deviation of the seed-sucking hole is 0.54 <sup>m</sup> · <sup>s</sup><sup>−</sup>1, which are the best values in every group. Therefore, the optimal airway structural parameters can be


34 mm.

Further variance analysis was conducted on the experimental data to determine the significance of each factor. The variance analysis is shown in Table 4.

determined as follows: the diameter of the horizontal air pipe must be 15 mm, the angle of the air pipe must be 105◦, and the diameter of the negative pressure aperture must be


**Table 4.** Variance analysis.

Note: In the table, the critical value of significance judgment is F0.001 (2, 2) = 999, F0.01 (2, 2) = 99, and F0.05 (2, 2) = 19. \*\* indicates that the influence is very significant. \* indicates that the influence is significant. The data in the table were rounded up, but in order to ensure accuracy, the calculation was based on the original value.

From the analysis of variance results, it can be observed that the angle of the air pipe and the diameter of the negative pressure aperture have a significant influence on the average airflow velocity index of the seed-sucking hole. The diameter of the horizontal air pipe does not influence the average airflow velocity of the seed-sucking hole. For the minimum airflow velocity of the seed-sucking hole, the angle of the air pipe and the diameter of the negative pressure aperture have less significant influence, while the diameter of the horizontal air pipe has no influence on the experiment results, and the degree of influence on the average airflow velocity of the seed-sucking hole is ranked as follows: angle of air pipe > diameter of negative pressure aperture > diameter of horizontal air pipe. From the results of the variance analysis for the airflow velocity standard deviation index of the seed-sucking hole, it can be observed that three factors do not influence this index. By analyzing the simulation experiment data, it was found that there is little difference in airflow velocity between the 12 seed-sucking holes, resulting in a small standard deviation value, indicating that the distribution of airflow in the air chamber is very uniform, which is beneficial for the collection of seeds.

3.2.2. Analysis of Airflow Velocity Difference of Various Seed-Sucking Holes

Taking the optimal parameter combination as the research object, the airflow velocity at the center of the 12 seed-sucking holes with negative pressure was calculated, as shown in Figure 8. It can be found from the figure that the position of the seed-sucking hole has no significant influence on the airflow velocity of the seed-sucking hole. The deviation in the airflow velocity value of the seed-sucking hole compared with the average value is less than 1%, which shows that the air pressure stability of the seeding-wheel-type seed precision placing device is higher.

**Figure 8.** The line chart of the airflow velocity of seed-sucking holes with the optimal parameter combination.

#### 3.2.3. Airflow Trajectory Analysis

Taking the airflow inlet (seed-sucking hole) as the starting point, it is possible to study the airflow trajectory of air that enters the negative pressure fluid domain and flows out the airflow outlet (negative pressure port). A streamlined diagram of the fluid domain is shown in Figure 9.

**Figure 9.** Streamline diagram of fluid domain.

It can be observed from the streamlined diagram that after the airflow flows in from the seed-sucking hole, the flow velocity gradually decreases after diffusion occurs twice. When entering the vertical air pipe, most of the airflow flows into the air chamber after a 90◦ turn, but a vortex composed of a small amount of airflow is formed near the wall, and the streamline is chaotic, resulting in a large local pressure loss. After entering the arc-shaped air chamber, the airflow flows to the negative pressure aperture. In this trajectory, a clear light blue area to the right of the negative pressure aperture can be observed, indicating

that the airflow in this area accelerates. It is speculated that the reason for this phenomenon is that there were more seed-sucking holes contained on the right side, resulting in a large airflow, and a longer flow distance than on the left side. When all the airflow converges at the negative pressure aperture, the airflow swirls upward, generating a cyclone, and also causes pressure loss, but a larger space means fewer collisions; therefore, the optimal level was recorded at the upper part of the negative pressure aperture.

#### *3.3. Study on Negative Pressure Characteristics of Locally Connected Seed-Sucking Holes*

During operation, a seed-sucking hole that is locally connected can be observed, but its airflow velocity does not significantly differ from other seed-sucking holes. Therefore, it can be determined that when more than half of the air-passing aperture of the longitudinal air pipe is connected to the air chamber, the seed-sucking hole's performance will not be affected. However, this phenomenon will lead to a certain deviation in the seeding position compared with the design value, making it impossible to cast the seed precisely at the designated position. To compensate for this deviation, it is necessary to further determine when the air-passing aperture of the longitudinal air pipe and the air chamber are in any of the connection states, as the airflow velocity of the seed-sucking hole will drop significantly, thus triggering seeding.

A dynamic simulation was carried out that gradually reducing the connected part of the air-passing aperture, and the time when the negative pressure of the seed-sucking hole dropped significantly and sharply was determined as the trigger time of seed casting. According to the actual working situation, we simulated the rotation of the seeding device, set the fluid domain of the upper air chamber to remain static, and the lower fluid domain to rotate, and studied the variation law of the negative pressure of the seed-sucking hole in the lower sub-fluid domain. Separate fluid domain extraction was performed for each sub-airway, obtaining 14 fluid domain units, as shown in Figure 10a.

**Figure 10.** Diagram of simulation process and results: (**a**) fluid domain flow diagram; (**b**) interface setting; (**c**) dot–line graph of experiment results.

To observe the spatial position changes in rotating objects in real time, the sliding mesh method [38–41] was adopted. The surface of the air-passing aperture of the lower sub-airway and the lower surface of the fluid domain of the upper air chamber were set as the interfaces, and data exchange could be performed, as shown in Figure 10b. The rotational speed of the lower sub-airway was set to 2.31 rad · <sup>s</sup><sup>−</sup>1, and the time step was 1.5 × <sup>10</sup>−<sup>4</sup> s. The simulation time was 0.141 s, which could cover the complete process of lower sub-airway separation. We plotted the experimental data as a dot–line graph, as shown in Figure 10c. We recorded the significant negative pressure changes in the seed-sucking holes for groups 41–48, and calculated the corresponding opening angles of

the air-passing aperture according to the structural parameters. The results are shown in Table 5.


**Table 5.** Sliding mesh simulation results for groups 41–48.

It can be observed from Figure 10c and Table 5 that the air pressure of the seed-sucking hole remains stable for most of the time, fluctuating only slightly. However, there is a certain decline after 0.12 s, but the range is not large, and after 0.129 s (the corresponding opening angle of the air-passing aperture is 1.326◦), it falls sharply, and the negative pressure of the seed-sucking hole is only −5.13 Pa until 0.138 s. The negative pressure distribution of groups 41–48 is shown in Figure 11.

**Figure 11.** Negative pressure distribution chart of groups 41–48: (**a**) group 41; (**b**) group 42; (**c**) group 43; (**d**) group 44; (**e**) group 45; (**f**) group 46; (**g**) group 47; (**h**) group 48.

On the whole, when only a small part of the air-passing aperture is connected to the upper air chamber, the seed-sucking hole still retains the ability to maintain seed adsorption. The trigger of seed casting may be associated with the period when the negative pressure of the seed-sucking hole drops sharply, but it is difficult to judge its exact time. However, the period in which the negative pressure of the seed-sucking hole sharpy drops is very short and only lasts about 0.01 s, so the setting error is small.
