# Experimental and Numerical Study of Vacuum Resin Infusion for Thin-Walled Composite Parts
## 1 Introduction
During infusion of large-sized thin-walled aircraft structures, the resin can transform from a liquid to a gel state, without having time to fill the entire preform due to the long path [2; 10; 23; 33]. This fact makes it necessary to thoroughly analyze the thermal-kinetic and rheological state of the resin during the infusion process. A change in the compressing state along the porous preform and the resin rheology during the process can cause difficulty in ensuring the uniformity of the product's thickness and the homogenous distribution of the reinforcing and resin fractions [8; 34; 35]. The solution of the listed problems using the trial and error method requires considerable time, financial expenses due to the high cost of the components, raising the importance of the development of process modeling tools that allow taking into account with maximum accessible precision the following process features:  
1. Geometry of preform and mold;
2. Their mechanical and thermal properties, which determine the effect of pressures and temperatures on the spread of liquid resin in a medium with varying porosity and permeability;The evolution of the thermal-kinetic and rheological properties of the resin;
4. A process scheme including numbers, locations and shapes of the liquid resin gates and vacuum events. Such a modeling system would make it possible to reconstruct the nature of the motion of the resin front and its state throughout the entire technological cycle, to select the optimal process parameters to ensure the production of defect-free products. Historically, the first such system was the LIMS software [3; 9; 12; 36], the theoretical foundations of which were presented in the 1990s of the 20 th century and presented in [37]. Lately, a kind of similar systems have been developed and used by some scientists: PAM-RTM [9; 13; 15; 38]; RTM-WORKS [2; 24; 26]; 3 DINFIL [39]; RTMSim [40]; Moldex3D RTM [41] and the ANSYS Fluent module [23; 25; 32]. All of these systems are based on the use of the so-called finite element/control volume (FE/CV) method, which uses special means of monitoring the front movement of the resin to eliminate the need to work with the moving boundaries and the deformed FE mesh. Some features of the FE/CV method are presented here to justify the proposed approach. In the fundamental work [36], the modeling approach that was used to describe the resin flow process is based on the FE/CV method. The modeled part's geometry was assumed as shell-like, the thickness of which is much smaller than other dimensions. This approach was implemented in almost all later works [10; 25; 27; 38], including the most recent ones [5; 16; 20]. The FE/CV method proposed in [36] and used in all subsequent works does not require the rebuilding of the mesh as the resin front moves in areas with complex geometries. A key feature of the FE/CV method is a rough approximation of the shape of the modeled region, combined with careful consideration of mass conservation. Three basic steps are needed to simulate an FE/CV flow [37]: (1) use the FE solution to obtain the pressure distribution in the area filled with resin; (2) calculate the resin flow rate; and (3) trace the front of the resin flow. The commonly used approach to construct control volumes introduces an error when using skewed elements, which is almost always observed in the case of complex part geometry. For such cells, the mass conservation condition may be violated. In some works, a modification of the method is proposed, according to which the finite elements and control volumes coincide. Tracking the interface between air and resin is the main goal of our study. In [10; 13; 25; 27; 37; 38], which use the FE/CV approach, the tracking was implemented by solving the continuity equation for the volume fraction of resin \(V_{r}\).