# Numerical Simulation and Optimization of the Airflow Field of a Forage Drum Mower
## 1 Introduction  
A mower conditioner is an essential device for forage harvesting, whose work performance has a direct impact on the outcome of forage harvesting and the quality of grass products. Currently, mowers are divided into drum mowers and reciprocating mowers, depending on the type of cutters used. As for the former, it relies on the rotary motion of the drum cutter to cut the stalk, which leads to a greater cutting capacity, lower stubble, and less vibration during service [1]. Therefore, it has been widely applied to forage harvesting. The primary factors that affect the cutting performance of a drum cutter include its structure and operating parameters. In some studies, it has been shown that harvest loss is related to blade shape [2; 3; 4; 5], cutter surface [6; 7], and cutter speed [8; 9; 10; 11]. The key factors affecting the shape and size of the cutting area are cutter size, blade arrangement, and cutting speed ratio [12; 13; 14], all of which should be thoroughly analyzed and taken into account for design. It has been demonstrated that the combination of twisted blades and tilting discs is effective in reducing harvest loss for leguminous forages such as alfalfa [15; 16; 17; 18]. The twisted blades could lift the plant at the moment of cutting to reduce recutting, and the tilting discs are conducive to reducing stubble height and the smooth feeding of the plant to the subsequent condition roller. In addition to the structure and operating parameters of the drum cutter, the airflow field around the cutter has a considerable impact on its cutting performance as well. Atpresent, lawnmowers are the focus of research on the airflow field of cutters. Through Computational Fluid Dynamics (CFD) and other techniques, scholars have investigated the distribution of airflow [19; 20; 21] and air pressure [22; 23] inside lawnmowers to figure out the effects of blade speed [24] and blade geometric parameters [25] on the airflow. Furthermore, the structural parameters of the cutter are optimized to determine the optimal parameters of the airflow field [26; 27]. The lawnmower products even used airflow auxiliary systems such as airflow field management devices [28]. In addition, studies have shown that the internal airflow field has a substantial impact on the vibration and noisiness of lawnmowers, with the improvement of the airflow field demonstrated as one of the most effective solutions to reducing the noise made by lawnmowers [29; 30; 31; 32]. In spite of this, there are still few studies conducted on the airflow field of forage drum mowers. It has been revealed that the aerodynamic action generated by the drum cutter is comparable to the weight of alfalfa, which means the airflow can affect the movement of alfalfa after cutting [16], thus affecting performance-related indicators such as recutting rate and loss rate. According to previous studies, when the cutter rotates, the airflow intersects and collides in front of the cutter, creating a lifting airflow and a feeding airflow above and behind the intersection point; these two airflows serve to reduce recutting and assist in feeding, respectively [17]. However, there is still a lack of clarity on the relationship between the cutter structure and operating parameters in the above-mentioned airflow field. Also, its effect on the airflow field is ignored by the existing design and optimization process of the cutter structure. In summary, a double-drum cutter of 9 GYZ-1.2 forage mower conditioner, equipped with twisted blades, tilting disc, and guide plates, is taken as the research object in this study to conduct numerical simulation and analysis of the airflow field around the cutter through CFD technology. Furthermore, the impact of the structure and operation parameters of the cutter on the airflow field is studied to optimize the design of the cutter structure and operating parameters, with consideration given to the airflow field parameters.