# A Systematic Review of Computational Fluid Dynamics Models in the Stomach and Small Intestine
###### Abstract  
The use of in silico models to improve our understanding of the fluid dynamics within the gastrointestinal tract has increased over the last few decades. Computational fluid dynamics (CFD) is an in silico technique that can be used to characterize and model the fluid mechanics driving the digestion of food and absorption of nutrients. This systematic review outlines the current methodologies used to develop CFD models of the stomach and small intestine, and summarizes the flow and mixing patterns predicted from these models. A literature search was conducted on Scopus, and 15 stomach CFD studies and 15 small intestine CFD studies were included in this review after the literature selection and exclusion process. Two primary flow patterns; retropulsive flow and recirculation regions, were identified within the stomach CFD models. The flow patterns within the small intestine were depended on the type of motility pattern present. The shortcomings of the current models are discussed, and considerations for future gastric and intestinal flow modeling are provided. computational fluid dynamics; gastric digestion; small intestine; human stomach 2023202331  
common approach for treating many diseases, and understanding how different foods and nutrients can impact the bioavailability of these drugs is an important area of study. GI diseases are a huge burden on the healthcare system, with annual healthcare expenditures for gastric problems comparable to that of cardiac diseases [5; 6]. Approximately a quarter of the general population suffer from indigestion [7], and this can be linked to GI motility disorders [8]. GI motility disorders are often chronic or recurring in nature and can dramatically affect quality of life. Chronic intestinal pseudo-obstruction is an example of a chronic digestive disorder that is characterized by signs of intestinal obstruction without evidence of an actual mechanical obstruction in the intestinal wall [9]. Irritable bowel syndrome (IBS) is a common disorder of the small and large intestines that is characterized by abdominal pain and altered bowel habits [10]. The pathophysiology of IBS is not entirely understood and there are no effective treatments or diagnostic tests for this condition [10]. In vivo studies of intestinal motility are conducted using approaches such as advanced mapping techniques [11], magnetic resonance imaging (MRI) [12], scintigraphy [13] and endoscopy [14]. There are limited in vivo studies that measure the flow dynamics within the intestines. These range from luminal flow rate measurements [15; 16], manometry pressure measurements [17] and flow patterns via combined videofluoroscopy, manometry, and multiple intraluminal impedance [18]. While advanced mapping techniques and real-time imaging tools have enabled in vivo analysis of different GI functions, the capability of these techniques to analyze and quantify the fluid dynamic behavior of the GI contents is still limited. There is also limited ability to control or manipulate experimental conditions within in vivo studies. In vitro digestion models are typically categorized into static and dynamic models. Static models have been traditionally used to study the biochemical aspect of digestion, but these systems typically are rigid, cannot model absorption and they do not accurately mimic the physiological conditions of gastric digestion [19]. In contrast, dynamic models have been developed to incorporate more realistic conditions, such as the anatomy and peristalsis of the stomach [20; 21] and these systems can track the structural and physicochemical changes of foods during digestion. Numerous review papers have examined various in vitro models and emphasized their ethical advantages and reproducibility [2; 22; 23]. However, simulating realistic gastric emptying and mixing on the laboratory benchtop remains a significant challenge due to current limitations in experimental techniques [24].