118 research outputs found
A fully resolved active musculo-mechanical model for esophageal transport
Esophageal transport is a physiological process that mechanically transports an ingested food bolus from the pharynx to the stomach via the esophagus, a multilayered muscular tube. This process involves interactions between the bolus, the esophagus, and the neurally coordinated activation of the esophageal muscles. In this work, we use an immersed boundary (IB) approach to simulate peristaltic transport in the esophagus. The bolus is treated as a viscous fluid that is actively transported by the muscular esophagus, and the esophagus is modeled as an actively contracting, fiber-reinforced tube. Before considering the full model of the esophagus, however, we first consider a standard benchmark problem of flow past a cylinder. Next a simplified version of our model is verified by comparison to an analytic solution to the tube dilation problem. Finally, three different complex models of the multi-layered esophagus, which differ in their activation patterns and the layouts of the mucosal layers, are extensively tested. To our knowledge, these simulations are the first of their kind to incorporate the bolus, the multi-layered esophagus tube, and muscle activation into an integrated model. Consistent with experimental observations, our simulations capture the pressure peak generated by the muscle activation pulse that travels along the bolus tail. These fully resolved simulations provide new insights into roles of the mucosal layers during bolus transport. In addition, the information on pressure and the kinematics of the esophageal wall resulting from the coordination of muscle activation is provided, which may help relate clinical data from manometry and ultrasound images to the underlying esophageal motor function
A moving control volume approach to computing hydrodynamic forces and torques on immersed bodies
We present a moving control volume (CV) approach to computing hydrodynamic
forces and torques on complex geometries. The method requires surface and
volumetric integrals over a simple and regular Cartesian box that moves with an
arbitrary velocity to enclose the body at all times. The moving box is aligned
with Cartesian grid faces, which makes the integral evaluation straightforward
in an immersed boundary (IB) framework. Discontinuous and noisy derivatives of
velocity and pressure at the fluid-structure interface are avoided and
far-field (smooth) velocity and pressure information is used. We re-visit the
approach to compute hydrodynamic forces and torques through force/torque
balance equation in a Lagrangian frame that some of us took in a prior work
(Bhalla et al., J Comp Phys, 2013). We prove the equivalence of the two
approaches for IB methods, thanks to the use of Peskin's delta functions. Both
approaches are able to suppress spurious force oscillations and are in
excellent agreement, as expected theoretically. Test cases ranging from Stokes
to high Reynolds number regimes are considered. We discuss regridding issues
for the moving CV method in an adaptive mesh refinement (AMR) context. The
proposed moving CV method is not limited to a specific IB method and can also
be used, for example, with embedded boundary methods
A unified constraint formulation of immersed body techniques for coupled fluid-solid motion: continuous equations and numerical algorithms
Numerical simulation of moving immersed solid bodies in fluids is now
practiced routinely following pioneering work of Peskin and co-workers on
immersed boundary method (IBM), Glowinski and co-workers on fictitious domain
method (FDM), and others on related methods. A variety of variants of IBM and
FDM approaches have been published, most of which rely on using a background
mesh for the fluid equations and tracking the solid body using Lagrangian
points. The key idea that is common to these methods is to assume that the
entire fluid-solid domain is a fluid and then to constrain the fluid within the
solid domain to move in accordance with the solid governing equations. The
immersed solid body can be rigid or deforming. Thus, in all these methods the
fluid domain is extended into the solid domain. In this review, we provide a
mathemarical perspective of various immersed methods by recasting the governing
equations in an extended domain form for the fluid. The solid equations are
used to impose appropriate constraints on the fluid that is extended into the
solid domain. This leads to extended domain constrained fluid-solid governing
equations that provide a unified framework for various immersed body
techniques. The unified constrained governing equations in the strong form are
independent of the temporal or spatial discretization schemes. We show that
particular choices of time stepping and spatial discretization lead to
different techniques reported in literature ranging from freely moving rigid to
elastic self-propelling bodies. These techniques have wide ranging applications
including aquatic locomotion, underwater vehicles, car aerodynamics, and organ
physiology (e.g. cardiac flow, esophageal transport, respiratory flows), wave
energy convertors, among others. We conclude with comments on outstanding
challenges and future directions
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