# A Systematic Review of Computational Fluid Dynamics Models in the Stomach and Small Intestine
## 3 Results
### Stomach
Several studies have varied the viscosity of digesta, which is typically represented as a Newtonian fluid with viscosities ranging from \(\mathcal{O}(10^{-3})\) to \(\mathcal{O}(10^{0})\) Pa s. An increase in the viscosity resulted in a reduction in the spatial extent of the flow structures [36; 40], and a reduction in the amount of gastric emptying [39]. Negligible differences in the flow patterns were observed for a non-Newtonian fluid (shear-thinning fluid) representing tomato juice compared to a highly viscous Newtonian fluid (representing honey) [45]. Gastric mixing has been studied through several approaches, such as the evolution of particle tracers [38], evaluation of the stretching of material elements via the computation of the deformation gradient tensor [45], time integration of the rate of strain tensor [35; 37],and relative vertical location changes of SPH particles [40]. Factors such as posture [38], the viscosity of digesta [40,45], velocity and acceleration of antral contractions [35] and frequency of peristaltic contractions [37] were all found to have an impact on the gastric mixing evaluated within these studies. The content found in the stomach is a multiphase fluid consisting of gastric juices, food, and other ingested materials. The gastric juices are a complex mixture of enzymes, acids, and other components that contribute to the overall heterogeneous and dynamic nature of the gastric fluid [49]. Few studies have considered the mixing and interplay between the gastric fluid phases. [PERSON] and [PERSON] [42] developed a CFD model of the stomach with the inclusion of gastric acid (H\({}^{+}\) ion) secretion at the stomach wall, and this was the first study to model the folds in the mucosal surface of the stomach wall (called rugae or gastric folds) as a porous medium. Simulations conducted for three liquid foods (water, orange juice and whole milk) resulted in varying pH distributions due to density variations i.e., the H\({}^{+}\) ions were transported from the top of the stomach to the bottom when the food is heavier than gastric juice. [PERSON] et al. [41] also used the previously discussed model to quantify the mixing between five different food boluses and water. This study treated the individual boluses as fluids (with varying densities, diffusion coefficients, and viscosities) and solved an individual scalar transport equation per food bolus. This study assumed that large food particles were deposited in the distal stomach, and this was modeled as a porous matrix. This food matrix was found to have a larger flow resistance to food boluses than to water; therefore, rapid  
Figure 3: Fluid flow patterns predicted by the CFD model developed by [PERSON] and [PERSON] [36], using (**a**) low-viscosity, and (**b**) high-viscosity Newtonian fluid. Reproduced with permission from [PERSON] and [PERSON], Journal of Food Science; published by [PERSON], 2010. Figure 2: Comparison of three main stomach geometries used for CFD simulations: (**a**) Idealized 3D stomach geometry constructed by [PERSON] and [PERSON] [36], (**b**) gastroduodenal geometry segmented from the Visible Human Project by [PERSON] et al. [39] and (**c**) stomach geometry segmented from the Virtual Population Library by [PERSON] and [PERSON] [44]; (**a**) has been reproduced with permission from [PERSON] and [PERSON], Journal of Food Science; published by [PERSON], 2010; (**b**,**c**) were published under a Creative Commons Attribution 4.0 license [39,44]. emptying of water was observed. This study also identified an important phenomenon related to gastric emptying known as the'magenstrasse' [50], which is the fast pathway of emptying for liquids (such as water) along the lesser curvature of the stomach. In a subsequent study, [PERSON] and [PERSON] [43] further improved their previous model to evaluate the disintegration of small meat particles (treated as a fluid) due to the following factors; hydrogen ions (secreted from the gastric wall), enzymes and temperature effects on enzyme activity.