## 3 Fundamentals of CFD in the Context of Mechanical Processes
### Boundary Conditions, Mesh Generation, Solver Types  
Accurate CFD simulations of mechanical processes in food engineering critically rely on precise definitions of boundary conditions, mesh topology, and numerical solver settings, all of which dictate numerical stability, convergence, and physical fidelity [24; 25].  
Boundary conditions are crucial for replicating operational environments, with typical applications including velocity inlets and pressure outlets for continuous flows, no-slip conditions at solid surfaces for viscous effects, and defined wall temperatures or heat transfer coefficients for thermal considerations [26]. Symmetry and periodic conditions can reduce computational load for repetitive geometries, while moving wall or dynamic mesh techniques are essential for systems with mechanical motion. Mesh generation, a pivotal aspect, involves choosing between structured meshes for simple domains and unstructured meshes for complex geometries like extrusion screws or baffled chambers, with fine resolution near walls being vital for capturing boundary layer phenomena in non-Newtonian materials. Dynamic meshing strategies such as sliding or overset meshes are employed for moving components, and mesh independence studies are routinely conducted to ensure solution accuracy [27]. The selection of numerical solvers is equally vital; pressure-based solvers are commonly used for incompressible food systems, while density-based solvers are reserved for compressible flows. Solver strategies can be segregated for weakly coupled systems or coupled for strong interdependencies, with transient solvers necessary for unsteady dynamics.  
The accuracy and reliability of CFD simulations critically depend on mesh quality, which influences convergence behavior, computational efficiency, and the fidelity of predicted physical fields (e.g., velocity, temperature, and shear stress). Poor mesh resolution, especially in regions with high gradients or complex geometries, can lead to numerical diffusion, under-resolved boundary layers, or even divergence [28; 29]. Different types of meshes are employed based on the geometry and nature of the simulation. Structured meshes offer high accuracy and efficiency for simple, orthogonal domains due to their regular topology. However, they are less suited for complex geometries typical in food processing equipment. Unstructured meshes, composed of tetrahedral, hexahedral, or polyhedral elements, provide greater flexibility for irregular domains but may require additional refinement and careful mesh quality control. Hybrid meshes, which combine structured and unstructured regions, are increasingly used to balance accuracy and computational cost [30]. For mechanical food processes involving moving parts--such as rotating impellers or screw elements--dynamic meshing techniques (e.g., sliding mesh or overset mesh) are essential to capture time-dependent interactions. Mesh refinement is particularly important near walls and interfaces, where boundary layers, shear gradients, or phase interfaces are present. Mesh independence studies are recommended to confirm that simulation results do not significantly change with finer mesh resolution [31].  
Finally, spatial and temporal discretization schemes, along with numerical parameters like under-relaxation factors and convergence criteria, must be carefully tuned to maintain solver robustness, and rigorous experimental validation is paramount for establishing predictive reliability and model credibility [25].