Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session L04: Physiological Fluid Mechanics I: General |
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Chair: William Schultz, University of Michigan Room: Ballroom D |
Monday, November 25, 2024 8:00AM - 8:13AM |
L04.00001: Modeling transport in physiologically realistic tumor microenvironment Mohammad Mehedi Hasan Akash, Mohammad Yeasin, Pei Ran, Anna-Blessing Merife, Anupam Pandey, Pranav Soman, Saikat Basu We present a first principles model for perfusion in tumor extracellular matrix (ECM), with realistic morphology. Our study conducts high-fidelity Eulerian multiphase simulations of blood flow in both idealized and imaging-based tumor vessels and numerically tracks the subsequent intratumoral plasma transport. The tumor vessel simulations enforce pulsatile flow, treating red and white blood cells and plasma as distinct phases under viscous-laminar transient conditions. We also incorporate glycocalyx patches along luminal vessel surfaces and the resulting electrohydrodynamic effects. Using plasma flux data from intra-vessel simulations at endothelial gaps, we next model plasma penetration into the ECM, bearing physiologically feasible interstitial pressure and fiber packing. The derived perfusion trends are compared with our analytical convection-diffusion model. The results indicate an inverse correlation between plasma percolation rates and intratumoral diffusion distances, validated by experiments in a millimeter-scale 3D-printed ECM topology and microfluidic chambers with ECM-mimicking collagen gel. In the latter, our model predicts mechanical response trends in the ECM domain during pulsed droplet motion through adjoining vessels. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L04.00002: Investigation of microphone placement on assessment of restricted pulsatile flow Felix Goldmann, Clayton Byers This study seeks to assess the severity of a restriction through acoustic measurements, inspired by aortic stenosis. Combining the acoustic power spectrum, the bispectrum, and associated bicoherence exposes different characteristics in the obtained signals. This study investigates the influence of measurement location together with the severity of restrictions to create an understanding of these correlating factors. Varying blockages are created with 3D printed semi-triangular valve models, inspired by the tricuspid shape of a heart valve, spanning from completely open to 82 percent restricted. Initial findings show that the distribution of energy over frequencies below 300 Hz are indicative of the restriction present, regardless of the microphone placement or the Reynolds number of the flow. Previous investigations have found the amount of restriction to be inversely proportional to the bicoherence of the acoustic signal. Current analysis shows that the level of coherence between frequencies decreases with increasing restriction percentage, consistent with prior studies. Using a combination of these three measures, a predictive model for non-invasively assessing the severity of the restriction is sought. |
Monday, November 25, 2024 8:26AM - 8:39AM |
L04.00003: Translation of a viscoelastic cell surrounded by a Newtonian fluid Ellen Jolley, James M Oliver, Sarah L Waters Information on the effect of fluid stresses on cells is needed for progress in regenerative stem cell therapies, wherein stem cells are injected into the bloodstream with the aim of reaching a target damaged organ and subsequently repairing the organ. This is hoped to avoid organ transplants. The fluid stresses experienced by the cell on its journey are known to directly impact its biomechanical response at the target site and hence the success of the therapy. However, this is not currently well-understood. In this presentation, we introduce a new mathematical model to capture the response of a biological cell to the stresses exerted on it by a Newtonian fluid. The cell is modelled as a sac of viscoelastic fluid, obeying the Upper Convected Maxwell constitutive law. The surrounding fluid is a viscous Newtonian fluid, obeying the Stokes equations. Exploiting the larger viscosity of the cell compared with the surrounding fluid enables the cell constitutive law to be linearized. New leading-order solutions for the intracellular flow and stress distributions over time for given initial conditions can then be obtained by primarily analytical means. |
Monday, November 25, 2024 8:39AM - 8:52AM |
L04.00004: How subcutaneous fluid injection stretches and shapes the skin Taeki Kim, Paul E Barbone, Edward Tang, James C Bird We evaluate three mathematical models of skin surface deformation and subsurface fluid flow due to subcutaneous injection of fluid. We focus on a regime where the fluid volume injected is large enough to create a visible bleb on the skin surface whose lateral dimensions are significantly larger than the depth of the injection. The goal of the modeling is to improve our understanding of the mechanisms of bleb formation that might contribute to patient discomfort. The models considered are: (1) a poroelastic model, (2) pressurized void in an elastic half space, and (3) a blister model based on a membrane / plate supported by an inelastic foundation and overlying a pool of fluid. The bleb height, area, and area stretch ratios predicted by the mathematical models are compared to the authors' recent measurements on high volume injections into ex vivo porcine tissue over various viscosities and flow rates. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L04.00005: Towards a Digital Twin of the Stomach with application to Digestion and Gastric Reflux Sharun Kuhar, Jung-Hee Seo, Rajat Mittal The stomach is responsible for physically and chemically processing the ingested meal before controlled emptying of the contents into the duodenum through the pyloric sphincter. If the orifice is unable to close due to dysfunction or surgery, contents could flow back from the duodenum into the stomach. The reverse flow could alter the low pH environment of the stomach, erode the mucosal lining, and the regurgitated contents, in some cases, may even reach the esophagus. In this work, an imaging data-based stomach model “StomachSim” is used to study the mechanism of duodenogastric reflux. The effect of variations in food properties and pre-existing motility disorders on the reflux are investigated. The primary driver of reflux was the relaxation of the antrum after a stomach contraction terminated near the pylorus. The region of the stomach walls exposed to the regurgitated contents changed significantly based on the density of stomach contents with respect to the duodenal contents. Concomitant stomach motility disorders led to weaker relaxation of the walls which in turn also affected the amounts of reflux. When the stomach contents were of higher viscosity, the proximal stomach applied increased pressure on the contents to achieve the same emptying rate which reduced the amount of duodenogastric reflux. The work illustrates the utility of in-silico models in identifying and investigating the effect of dietary and motility changes in gastrointestinal disorders. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L04.00006: Modeling flow, transport, and microbial dynamics in realistic intestinal geometries Bibhas Kumar, Saikat Mukherjee The intestinal surface comprises a complex architecture of villi (finger-like projections) and crypts (cavities between the villi). In this work, we aim to understand the role of the intestinal surface in nutrient absorption and microbial dynamics in the presence of an unsteady flow field. To model the flow, we solve the Navier-Stokes equation with an axisymmetric formulation in a cylindrical domain consisting of radially protruding villi. We prescribe a Womersley solution for the inlet condition of the flow to model the effect of peristalsis due to gut motility. Nutrient and bacterial dynamics are solved by coupled nonlinear reaction-advection-diffusion equations with Monod kinetics. The research provides fundamental insights into the fluid dynamics and transport in the gut in healthy and diseased conditions and has implications for developing therapeutic strategies against gastrointestinal disorders. Our numerical approach is flexible and can be adapted to model a range of intestinal flow conditions and reaction kinetics for nutrients and the gut microbiome. |
Monday, November 25, 2024 9:18AM - 9:31AM |
L04.00007: Food Digestion in the Stomach after Bariatric Surgery: Insights from Multiphase Flow Modeling Weixuan Li, Sharun Kuhar, Jung-Hee Seo, Rajat Mittal The geometry and contraction behavior of the stomach profoundly affect the digestion of liquid food with gastric solvent. Sleeve gastrectomy, a weight-loss surgery, reduces stomach size by 75-80%, aiding patients in consuming less food and losing weight. This study aims to investigate the impact of changes in stomach shape and contraction behavior on digestion within the stomach after a specific type of bariatric surgery – the laparoscopic sleeve gastrectomy. Based on the original model, modifications were made to reflect the post-surgical stomach. The first modification involved resecting a portion of the stomach and suturing the remaining part to form a new stomach geometry. The second modification altered the stomach's contraction pattern from bilateral to unilateral contraction. This change was made because, post-surgery, the resected side of the stomach no longer contracts. Thus, the new stomach model features a non-contracting resected side, while the remaining side retains its contraction capability. Multiphase flow simulations were conducted on this modified stomach model to study how the altered geometry and contraction behavior affect the digestion of gastric contents. These results were compared with those from the original, unmodified stomach model. The simulation results suggest that the reduced stomach volume leads to faster transport of contents from the proximal stomach to the antrum. The decreased peristaltic behavior also results in a slower mixing of gastric contents. This study demonstrates the potential of in-silico models in planning and understanding the effects of surgery. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L04.00008: Mechano-pathogenesis of esophageal hypertrophy and remodeling Xinyi Liu, Leroy Jia, Sourav Halder, Peter J Kahrilas, John E Pandolfino, Neelesh A Patankar Esophageal hypertrophy is a chronic condition resulting from abnormal esophageal contractions during swallowing. In this study, we develop and utilize a finite growth tissue remodeling model that incorporates fiber contraction to simulate the hypertrophy of the esophagus caused by abnormal swallowing. Specifically, we investigate the differential impacts of stretch-driven and stress-driven growth mechanisms, which are respectively triggered by volume overload and pressure overload conditions. Our analysis reveals distinct growth trajectories that lead to various equilibrium states, each depicting different pathological outcomes. To validate our model, we examined the radial and circumferential components of growth, employing ultrasound clinical data from patients diagnosed with esophageal hypertrophies associated with Achalasia, Distal Esophageal Spasm (DES), and hypercontractility disorders. Clinical observations demonstrate that hypercontractility can induce a range of abnormal esophageal deformations. Extending our model, we can describe additional morphogenetic alterations in the esophagus, such as those observed in Jackhammer esophagus. This condition is characterized by hypercontractile activity leading to a winding staircase of the esophageal lumen, resembling a corkscrew. |
Monday, November 25, 2024 9:44AM - 9:57AM |
L04.00009: Toward a Digital Twin for Liver Resection Pre-Operative Planning Himanshi Saini, Jeffrey Tithof, Joseph Sushil Rao, Timothy Pruett The liver maintains homeostasis by filtering blood, producing bile, and metabolizing substances. It receives dual blood supply from the hepatic artery and portal vein, distributing blood through sinusoids (liver microvasculature) before draining into the hepatic vein. Cells lining the sinusoids detect haemodynamic changes, initiating liver regeneration after surgical resections. However, liver regeneration ability is limited; excessive tissue removal can lead to complications like small-for-size syndrome (SFSS). SFSS is associated with elevated portal pressure, and we hypothesize that sinusoidal wall shear stress (WSS) above some critical threshold results in cell death which impairs regeneration, leading to liver failure. We use a lumped parameter hydraulic network model to quantify the extent of elevated sinusoidal WSS under different surgical scenarios. Our versatile and computationally efficient approach facilitates future model extensions that will incorporate patient-specific anatomy to improve surgical outcomes for liver resection. By performing computational pre-operative planning, we will help clinicians ensure that WSS levels promote regeneration without causing excess damage that may lead to liver failure. |
Monday, November 25, 2024 9:57AM - 10:10AM |
L04.00010: Practically perfect peristaltic pump performance William W Schultz, Kourosh Kalayeh, Haotian Xie, J. Brian Fowlkes, Bryan S Sack The ureter transports urine from the kidney to the bladder through peristalsis. The musculature of its wall is often considered as multiple layers of helically-oriented smooth muscle: an inner layer oriented primarily longitudinally and other layers oriented more circumferentially. While observation of the ureter during peristalsis shows both lateral and longitudinal movement, the first model to include longitudinal motion was developed by Kalayeh et al. (2023), expanding on Shapiro's (1967) earlier 2D sinusoidal wall motion using lubrication theory. Shapiro's model had impracticalities, such as achieving near 100% efficiency only for near zero pressure rise and having reflux at high efficiency. Kalayeh's model improved performance modestly. We now show that a slight modification of the form of longitudinal motion leads to almost 100% efficiency without reflux over the entire pressure range. We discuss further model refinements and propose future in vivo experimental verification. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L04.00011: Peristaltic pumping in vessels of changing length Aaron C Winn, Paheli Desai-Chowdhry, Purba Chatterjee, Stanislaw Zukowski, Laureline Julien, Annemiek Cornelissen, Eleni Katifori The radial and longitudinal contractions of biological vessels can drive instantaneous fluid flows in an organism. When these contractions propagate as peristaltic waves, net volume can be transported throughout the system, as is the case, for example, with peristaltic pumping in the human esophagus. However, traditional models of peristaltic pumping only account for changes in radius. For some networks, such as the gastrovascular system of the jellyfish, length changes of vessels may be just as important in determining flow properties. We will outline a method of incorporating longitudinal peristalsis that relies only on the Lagrangian coordinates of the deforming material. This description of contractions can be coupled to mechanical properties of the deforming solid boundary such as the Poisson ratio. We explore the results of this model in some simple single-vessel examples. We then demonstrate a novel way of studying flow networks of length-changing vessels using time-dependent node positions. |
Monday, November 25, 2024 10:23AM - 10:36AM |
L04.00012: Inverse Method for Ureteral Peristalsis Determination from Pressure Measurements Haotian Xie, William W Schultz, Kourosh Kalayeh, J. Brian Fowlkes, Bryan S Sack We present a method to determine ureteral peristalsis wave shapes, incorporating the longitudinal wall motion of Kalayeh et al. (2023). With synthetic pressure data, our inverse algorithm accurately identifies wave shapes, longitudinal motion, and potential reflux. In vivo pressure measurements to compare to the model would require a ureteral catheter with two pressure ports separated by an appropriate distance to find the wave speed and the pressure offset over one wavelength. |
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