Physics of Fluids最新文献

The glottal flow is often simplified as one-dimensional (1D) in phonation models to reduce computational cost. Although previous studies showed that a 1D flow model can predict voice production by a three-dimensional (3D) flow combined with a simplified two-mass vocal fold model, its validity in voice production involving more realistic 3D vibrations remains unclear. The goal of this study is to investigate the accuracy of the 1D flow model in predicting vocal fold vibration and voice production in a vocal fold model exhibiting a more realistic 3D vibration pattern, by comparing its prediction to that from a mechanical experiment and a 3D Navier-Stokes compressible flow model. The results showed that the 1D flow model predicted overall vibratory pattern similar to that observed in experiment and simulations based on the 3D flow model. However, the 1D flow model predicted slightly larger displacements and greater glottal flow fluctuations than the 3D flow model. The 3D flow model revealed strong variations in surface pressure along the anterior-posterior direction, particularly during the closing phase, which was not captured by the 1D flow model. Despite these differences, the 1D flow model adequately reproduced major aerodynamic and vibratory features under typical normal phonatory conditions, supporting its use in phonation models for efficient voice simulations.

The contact pressure experienced by the vocal folds during phonation is considered a major factor contributing to vocal fold injuries and lesions. Understanding the spatiotemporal distribution of vocal fold contact pressure across the medial surface and its dependence on laryngeal geometrical and mechanical properties (such as glottal gap, vocal fold vertical thickness, vocal fold length, and vocal fold stiffness) is essential to identifying strategies that minimize contact pressure and reduce the risk of vocal injury. This study aims to characterize the spatiotemporal distribution of intraglottal pressure across the medial surface in excised larynges. The intraglottal pressure was measured using a modified probe microphone at different locations within the vertical plane containing the glottal centerline (mid-sagittal plane), following a grid-like pattern. The resulting pressure distribution maps indicate small variations of intraglottal pressure in the anterior-posterior dimension, but large, complex variations in the vertical dimension. A criterion, derived from applying Bernoulli's equation to vocal fold vibration with prescribed vocal fold contact, was developed to identify the contact pressure peak as a rapid increase in the intraglottal pressure preceded by a negative pressure in the intraglottal pressure waveform. This criterion also allows estimation of the vertical span of vocal fold contact (by extension the vocal fold vertical thickness) and the mucosal wave speed. The preliminary results from this study indicate that the vocal fold vertical thickness has a large impact on the peak contact pressure value, which corroborates findings from previous computational studies.

Transport during flow of generalized Newtonian fluids (GNFs) appears often in systems that can be treated in a simplified form as either cylindrical tubes or slit openings between parallel plates. Based on the pioneering work of Taylor, analytical solutions for transport in these simplified systems were derived generally. This includes analytical solutions for advection dominated transport, as well as a computation of the enhanced molecular diffusion coefficient in low Peclet number systems. These generally derived solutions were developed without assuming any specific fluid rheology and can predict transport when only a steady velocity field is known. The newly derived general solutions for species transport were applied to Cross and Carreau model fluids using a semi-analytical solution for velocity of these fluids. The semi-analytical solutions derived herein were compared to microscale simulations and showed agreement with the numerical error of those simulations. Because of the general nature of the transport solutions derived herein, these solutions can be applied to other non-Newtonian fluids, such as viscoelastic or viscoplastic fluids, as a straightforward extension of this work.

The non-Newtonian properties of blood flow have been widely debated in hemodynamic research, particularly for congenital heart defects. Many studies comparing Newtonian and non-Newtonian models have overlooked dimensional group consistency, resulting in comparisons influenced by inconsistent Reynolds numbers rather than viscosity effects. In this study, we address this issue by applying a generalized Reynolds number formulation to ensure consistent dimensionless group comparisons. We compare flow structures and hemodynamic metrics in 20 pediatric Fontan circulations using the non-Newtonian Casson model against both conventional and generalized Reynolds number-corrected Newtonian models. Our results show that the conventional Newtonian model significantly overestimates flow rotation and underestimates stagnation regions, potentially misrepresenting thrombosis risk. The generalized Reynolds number method, however, predicts flow structures, wall shear stress, and energy-based metrics more in line with the non-Newtonian model. Percentage of power loss estimates from the generalized method (17.7 [10.1, 22.7]; $p<0.05$ ) align more closely with the non-Newtonian model (12.9 [7.0, 17.1]) than with the conventional approach (8.5 [4.3, 10.2]; $p<0.001$ ), offering a more clinically relevant prediction. Additionally, indexed viscous dissipation from the generalized method (2.14 [1.17, 3.69] n.d.) is statistically indistinguishable (p=0.97) from the non-Newtonian model (2.42 [1.07, 3.60] n.d.; $p<0.05$ ). Our analysis highlights that while the generalized Reynolds number method cannot fully replicate local shear-thinning effects, it substantially improves upon the conventional Newtonian approach by correcting for viscosity mismatch. We emphasize the importance of dimensionless group consistency before drawing conclusions in hemodynamic studies and advocate for broader adoption of non-Newtonian models to obtain critical clinical insights.

As the diffusion of fluids is hindered by semipermeable membranes, the long-time behavior of the diffusion coefficient is influenced by the arrangement of the membranes. We develop methods that predict this long-time instantaneous diffusivity from bulk diffusivity, the membranes' locations, and their permeabilities. We studied this problem theoretically and expressed the instantaneous diffusivity analytically as an infinite sum. An independent numerical scheme was employed. Several types of disorder in the membranes' positions were considered including a new disorder family that generalizes hyperuniform and short-range disorders. Our theoretical and numerical findings are in excellent agreement. Our methods provide an alternative means for studying time-dependent diffusion processes.

The glomerulus is a critical filtration unit in the kidney, yet its complex three-dimensional architecture has long hindered a comprehensive understanding of its function and regulation. Here, we present an integrated framework that combines in vivo imaging based three-dimensional modeling, computational fluid dynamics simulations, and in vitro reconstruction to elucidate the structural and hemodynamic complexity of the glomerulus. Our analyses reveal that the inherent asymmetry between afferent and efferent arterioles is critical for establishing a precise pressure-flow relationship and regulating hemodynamics. We further successfully fabricated a perfusable, anatomically accurate mouse glomerulus within a microphysiological system, demonstrating proof-of-concept for perfusion analysis and vascularization. These findings establish a transformative platform for studying glomerular diseases and pave the way for therapeutic interventions.

Molecular-scale simulations of pressure-driven transport through polyamide nanogaps (5-100 Å) were performed to investigate fundamental transport mechanisms. Results show that transport in nanogaps $\le$  10 Å is always subdiffusive, but superdiffusive transport was observed in nanogaps $\ge$  20 Å. Near typical operating pressures for applications ( $\Delta p$  = 100 atm), only the 100 Å nanogap exhibited superdiffusive behavior. Since openings in common membrane materials are typically <20 Å, results indicate that subdiffusive to diffusive transport dominates for typical applications, such as reverse osmosis.

For univentricular heart patients, the Fontan circulation presents a unique pathophysiology due to chronic non-pulsatile low-shear-rate pulmonary blood flow, where non-Newtonian effects are likely substantial. This study evaluates the influence of non-Newtonian behavior of blood on fluid dynamics and energetic efficiency in pediatric patient-specific models of the Fontan circulation. We used immersed boundary-lattice Boltzmann method simulations to compare Newtonian and non-Newtonian viscosity models. The study included models from twenty patients exhibiting a low cardiac output state (cardiac index of 2 L/min/m²). We quantified metrics of energy loss (indexed power loss and viscous dissipation), non-Newtonian importance factors, and hepatic flow distribution. We observed significant differences in flow structure between Newtonian and non-Newtonian models. Specifically, the non-Newtonian simulations demonstrated significantly higher local and average viscosity, corresponding to a higher non-Newtonian importance factor and larger energy loss. Hepatic flow distribution was also significantly different in a subset of patients. These findings suggest that non-Newtonian behavior contributes to flow structure and energetic inefficiency in the low cardiac output state of the Fontan circulation.