Knowledge of the spatial and temporal dynamics of the gut microbiome is essential to understanding the state of human health, as over a hundred diseases have been correlated with changes in microbial populations. Unfortunately, due to the complexity of the microbiome and the limitations of in vivo and in vitro experiments, studying spatial and temporal dynamics of gut bacteria in a biological setting is extremely challenging. Thus, in silico experiments present an excellent alternative for studying such systems. In consideration of these issues, we have developed a user-friendly agent-based model, GutLogo, that captures the spatial and temporal development of four representative bacterial genera populations in the ileum. We demonstrate the utility of this model by simulating population responses to perturbations in flow rate, nutrition, and probiotics. While our model predicts distinct changes in population levels due to these perturbations, most of the simulations suggest that the gut populations will return to their original steady states once the disturbance is removed. We hope that, in the future, the GutLogo model is utilized and customized by interested parties, as GutLogo can serve as a basic modeling framework for simulating a variety of physiological scenarios and can be extended to capture additional complexities of interest.
Although the behaviour of fluid-filled vesicles in steady flows has been extensively studied, far less is understood regarding the shape dynamics of vesicles in time-dependent oscillatory flows. Here, we investigate the nonlinear dynamics of vesicles in large amplitude oscillatory extensional (LAOE) flows using both experiments and boundary integral (BI) simulations. Our results characterize the transient membrane deformations, dynamical regimes and stress response of vesicles in LAOE in terms of reduced volume (vesicle asphericity), capillary number ( ${Ca}$ , dimensionless flow strength) and Deborah number ( ${De}$ , dimensionless flow frequency). Results from single vesicle experiments are found to be in good agreement with BI simulations across a wide range of parameters. Our results reveal three distinct dynamical regimes based on vesicle deformation: pulsating, reorienting and symmetrical regimes. We construct phase diagrams characterizing the transition of vesicle shapes between pulsating, reorienting and symmetrical regimes within the two-dimensional Pipkin space defined by ${De}$ and ${Ca}$ . Contrary to observations on clean Newtonian droplets, vesicles do not reach a maximum length twice per strain rate cycle in the reorienting and pulsating regimes. The distinct dynamics observed in each regime result from a competition between the flow frequency, flow time scale and membrane deformation time scale. By calculating the particle stresslet, we quantify the nonlinear relationship between average vesicle stress and strain rate. Additionally, we present results on tubular vesicles that undergo shape transformation over several strain cycles. Broadly, our work provides new information regarding the transient dynamics of vesicles in time-dependent flows that directly informs bulk suspension rheology.
The combustion processes present in modern propulsion systems exhibit very complex turbulent chemically reacting unsteady multiphase flow with fuel spray evaporation and heat and mass transfer. The understanding and design of such systems represents a challenging research area. Ultra-short compact, high performance combustion systems are desirable for advanced propulsion systems. AFRL has proposed placing an Ultra-Compact Combustor (UCC) between a high pressure turbine stage and low pressure turbine stage, to create an innovative Inter-Turbine Burner (ITB) concept. This analysis focuses on ITB combustor technologies that can enable the development of compact, high-performance combustion systems. Compact combustors weigh less and require less volume for space-limited turbine engine aero applications. For turbulent conditions flame speed is directly proportional to the square root of G and high G flames exhibit increased flame speeds, which can aid in the design of shorter combustion systems. This paper presents the Ultra-Compact Combustor (UCC), a novel design based on a high G flame driven by high swirl in a circumferential cavity (CC) utilized to enhance mixing rates via high G-loading on the order of 3000 G’s within the CC. Flow field predictions utilizing FLUENT are presented for the UCC-ITB for a variety of operating conditions: (1) the addition of curved Radial Vanes (CRV) in the combustor flow path, (2) a comparison performance of the combustor at various fuel/air ratios. The effect of various equivalence ratios on the resulting G load distribution, predicted total pressure loss, the entrainment and the calculated exit temperature profile are discussed. The analysis and comparison with available rig test data supplements the understanding of the design space required for future engine designs that may use these novel, compact, high-G combustion systems.
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