# A Systematic Review of Computational Fluid Dynamics Models in the Stomach and Small Intestine
## 3 Results
### Small Intestine  
Table 1 summarizes the 15 articles identified for the CFD models of the small intestine. Unlike the stomach, which has relatively slow and infrequent contractions (around  
Figure 4: Enzymatic hydrolysis, via pepsin (red), of a protein-rich meal (blue) predicted by [PERSON] et al. [31]. Reproduced with permission from [PERSON] et al., Physics of Fluids; published by AIP Publishing, 2022.  
three contractions per minute or cpm), the small intestine exhibits more frequent contractions, ranging from 10-12 cpm [53; 54]. The small intestine has a more convoluted geometry; although topologically it is still a tube, its surface area is significantly larger than the stomach. Mesh-based methods such as the FVM [55; 56; 57] and Finite Element Method (FEM) [58; 59; 60; 61] been used to model intestinal flow. Particle-based methods such as the LBM have also been utilized in [62; 63]. SPH has also been utilized to model intestinal flow with [PERSON] et al. [64], coupling it with the discrete element method to model the solid phase, and [PERSON] et al. [33] coupling SPH with a lattice spring model for the intestinal wall. A small segment (3.6-25 cm in length) of the small intestine is typically modeled except for [PERSON] et al. [60] who modeled a 120 cm jejunum and 180 cm ileum. These CFD models were based on a variety of animal species, including rats [62], rabbits [58; 59], chickens [61], guinea pigs [55], pigs [56], and humans [63; 64; 33; 65]. Most models used an axisymmetric 2D geometry to represent the intestinal segment, with others also using a 2D rectangular channel [55; 62], and a 3D cylinder [63; 64; 33]. The validity of using an axisymmetric condition about the center of the intestine lumen should be reconsidered since this assumes that both sides of the intestinal wall are undergoing the same contraction. On the contrary, it has been experimentally observed that the contraction amplitude differs between the side of the intestinal wall which attaches to the mesentery vs. the opposite side [66]. All studies used a simplified geometry, while [PERSON] et al. [65] used an idealized 3D geometry with aspects of realistic anatomy, which consisted of a 'C' shaped cylinder to represent the duodenum. [PERSON] et al. [67] was the first study to incorporate an anatomically realistic geometry of the duodenum derived from the VHP (Figure 5). Three distinct motility patterns are observed within the small intestine that facilitate food digestion and absorption. Peristalsis is responsible for the propulsion of digesta along the intestine, and these contractions were studied by 10/15 studies. The majority of these studies only simulate a single peristaltic contraction moving in an antegrade direction, while [PERSON] et al. [63] modeled the effect of multiple contractions and [67; 61] also investigated the fluid mechanics of colliding peristaltic waves. Segmentation contractions involve localized contractions and relaxations with no overall propulsion along the intestine, and their role in mixing of digesta with surrounding fluids (enzymes and chemicals secreted into the intestinal lumen) has been studied by [56; 57; 59; 62]. Pendular contractions consist of a 'to-and-fro' motion of the intestinal wall as identified experimentally by [PERSON] [68], and the fluid mechanical consequences of these contraction patterns have been analyzed in a few studies [58; 59; 62]. [PERSON] et al. [59] considered the effect of combining pendular and segmentation motions. These motility patterns are prescribed into the CFD model as boundary deformations, which range from sinusoidal waves [63; 64; 65], Gaussian distribution function [60], fluid-structure interactions [33], and more realistic deformation patterns obtained from experimental recordings of ex-vivo intestinal samples [56; 58; 61; 62]. A model of slow wave propagation was utilized by [PERSON] et al. [67] to obtain anatomically realistic boundary deformations to be applied in the model of duodenum motility (Figure 5a).