# A Systematic Review of Computational Fluid Dynamics Models in the Stomach and Small Intestine
## 3 Results
### Stomach  
A total of 15 articles were identified for the stomach and these are summarized in Table 1 with their relevant characteristics. The study by [PERSON] et al. [34], which developed the first simplified 2D CFD model of the stomach incorporating contractions based on in vivo MRI data, was excluded from the analysis due to the utilization of a 2D geometry. However, the study is a significant contribution to gastric flow modeling since the motility patterns from this study have been used in \(10/15\) of the subsequently developed CFD models of the stomach. CFD models of gastric motility assess different aspects of gastric motility, such as the different contraction properties [35], fluid properties [36; 37], and their effects on the flow patterns and mixing. The effect of gastric motility and other factors such as posture [38] on emptying [31; 39; 40], mixing and disintegration of food [41; 42; 43], and drugs [32; 44] in the human stomach were also studied. Well-established numerical techniques such as the Finite Volume Method (FVM) were utilized in the CFD models of the stomach, with its use in both single-phase [36; 45] and multi-phase [41; 42] simulations. Particle-based CFD methods such as the Lattice-Boltzmann method (LBM) have been used [35; 38; 39], as well as the Smoothed Particle Hydrodynamics (SPH) method [40]. Due to their mesh-free formulation, SPH and LBM are well-suited for models with moving and deforming boundaries. On the other hand, models using the FVM are limited in simulating large amplitude contractions [42]. More recently, [PERSON] and [PERSON] [44] and [PERSON] et al. [46] have used the Immersed Boundary Method to resolve fluid-structure interactions between a pill and stomach wall, respectively. Several different anatomically realistic geometries were used to represent the stomach in these CFD models i.e., 7/15 studies generated an idealized geometry that was specific to that study, such as the one utilized by [36] (Figure 2a), some studies used a segmented geometry from the Visible Human Project (VHP) [47] (Figure 2b), and a geometry from the Virtual Population Library [48] (Figure 2c). Most studies simulated peristaltic contractions by prescribing the motion of points on the boundary, and this motion is typically prescribed mathematically with respect to the centerline. The parameters defining this motion i.e., speed (2.5 mm/s), period (20 s), and amplitude (90% radial contraction) have been primarily obtained from one MRI study of a healthy subject from [PERSON] et al. [34]. [PERSON] et al. [35] conducted high-resolution mapping of gastric slow waves within 10 subjects, and these recordings were averaged and used to derive peristaltic contractions. [PERSON] et al. [46] incorporated circular and longitudinal muscle layers to the stomach wall, and via the activation of muscle fibers embedded in these layers, gastric peristalsis was generated. Fluid flow within the stomach due to these muscle contractions was computed using fluid-structure interactions. All simulation results showed two main flow patterns induced by gastric motility, as outlined in Figure 3. These two patterns were the'retropulsive jet' located in the distal antrum and recirculating regions between pairs of antral contractions. Retropulsion is when chyme is squirted back into the stomach due to the combined effect of peristaltic contractions reaching the pylorus and the pylorus itself also being closed [49]. Thus, a'retropulsive jet' refers to the resulting flow pattern due to the retropulsion motion of digesta. These fluid motions are responsible for the mixing and emulsifying of food with gastric juices and cause grinding and rubbing between food particles and/or the stomach wall [49].