71 research outputs found
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
Hydrodynamics of Suspensions of Passive and Active Rigid Particles: A Rigid Multiblob Approach
We develop a rigid multiblob method for numerically solving the mobility
problem for suspensions of passive and active rigid particles of complex shape
in Stokes flow in unconfined, partially confined, and fully confined
geometries. As in a number of existing methods, we discretize rigid bodies
using a collection of minimally-resolved spherical blobs constrained to move as
a rigid body, to arrive at a potentially large linear system of equations for
the unknown Lagrange multipliers and rigid-body motions. Here we develop a
block-diagonal preconditioner for this linear system and show that a standard
Krylov solver converges in a modest number of iterations that is essentially
independent of the number of particles. For unbounded suspensions and
suspensions sedimented against a single no-slip boundary, we rely on existing
analytical expressions for the Rotne-Prager tensor combined with a fast
multipole method or a direct summation on a Graphical Processing Unit to obtain
an simple yet efficient and scalable implementation. For fully confined
domains, such as periodic suspensions or suspensions confined in slit and
square channels, we extend a recently-developed rigid-body immersed boundary
method to suspensions of freely-moving passive or active rigid particles at
zero Reynolds number. We demonstrate that the iterative solver for the coupled
fluid and rigid body equations converges in a bounded number of iterations
regardless of the system size. We optimize a number of parameters in the
iterative solvers and apply our method to a variety of benchmark problems to
carefully assess the accuracy of the rigid multiblob approach as a function of
the resolution. We also model the dynamics of colloidal particles studied in
recent experiments, such as passive boomerangs in a slit channel, as well as a
pair of non-Brownian active nanorods sedimented against a wall.Comment: Under revision in CAMCOS, Nov 201
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
multi-layered 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, which is modeled as an
actively contracting, fiber-reinforced tube. A simplified version of our model
is verified by comparison to an analytic solution to the tube dilation problem.
Three different complex models of the multi-layered esophagus, which differ in
their activation patterns and the layouts of the mucosal layers, are then
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 due to the coordination of muscle activation is provided, which may help
relate clinical data from manometry and ultrasound images to the underlying
esophageal motor function
An Immersed Interface Method for Discrete Surfaces
Fluid-structure systems occur in a range of scientific and engineering
applications. The immersed boundary(IB) method is a widely recognized and
effective modeling paradigm for simulating fluid-structure interaction(FSI) in
such systems, but a difficulty of the IB formulation is that the pressure and
viscous stress are generally discontinuous at the interface. The conventional
IB method regularizes these discontinuities, which typically yields low-order
accuracy at these interfaces. The immersed interface method(IIM) is an IB-like
approach to FSI that sharply imposes stress jump conditions, enabling
higher-order accuracy, but prior applications of the IIM have been largely
restricted to methods that rely on smooth representations of the interface
geometry. This paper introduces an IIM that uses only a C0 representation of
the interface,such as those provided by standard nodal Lagrangian FE methods.
Verification examples for models with prescribed motion demonstrate that the
method sharply resolves stress discontinuities along the IB while avoiding the
need for analytic information of the interface geometry. We demonstrate that
only the lowest-order jump conditions for the pressure and velocity gradient
are required to realize global 2nd-order accuracy. Specifically,we show
2nd-order global convergence rate along with nearly 2nd-order local convergence
in the Eulerian velocity, and between 1st-and 2nd-order global convergence
rates along with 1st-order local convergence for the Eulerian pressure. We also
show 2nd-order local convergence in the interfacial displacement and velocity
along with 1st-order local convergence in the fluid traction. As a
demonstration of the method's ability to tackle complex geometries,this
approach is also used to simulate flow in an anatomical model of the inferior
vena cava.Comment: - Added a non-axisymmetric example (flow within eccentric rotating
cylinder in Sec. 4.3) - Added a more in-depth analysis and comparison with a
body-fitted approach for the application in Sec. 4.
How wavelength affects the hydrodynamic performance of two accelerating mirror-symmetric slender swimmers
Fish schools are capable of simultaneous linear acceleration. To reveal the
underlying hydrodynamic mechanism, we numerically investigate how Reynolds
number , Strouhal number and wavelength
affect the mean net thrust and net propulsive efficiency
of two side-by-side hydrofoils undulating in anti-phase. In total,
cases are simulated using immersed boundary method. The thrust increases
significantly with wavelength and Strouhal number, yet only slightly with the
Reynolds number. We apply a symbolic regression algorithm to formulate this
relationship. Furthermore, we find that mirror-symmetric schooling can achieve
a \textit{net} thrust more than ten times that of a single swimmer, especially
at low Reynolds numbers. The highest efficiency is obtained at and
, where is consistent with that observed in the
linear-accelerating natural swimmers, \eg Crevalle jack. Six distinct flow
structures are identified. The highest thrust corresponds to an asymmetric flow
pattern, whereas the highest efficiency occurs when the flow is symmetric with
converging vortex streets.Comment: This paper has been accepted by Physics of Fluids. This is the
accepted versio
Measuring performance on the Healthcare Access and Quality Index for 195 countries and territories and selected subnational locations: A systematic analysis from the Global Burden of Disease Study 2016
Background: A key component of achieving universal health coverage is ensuring that all populations have access to quality health care. Examining where gains have occurred or progress has faltered across and within countries is crucial to guiding decisions and strategies for future improvement. We used the Global Burden of Diseases, Injuries, and Risk Factors Study 2016 (GBD 2016) to assess personal health-care access and quality with the Healthcare Access and Quality (HAQ) Index for 195 countries and territories, as well as subnational locations in seven countries, from 1990 to 2016. Methods Drawing from established methods and updated estimates from GBD 2016, we used 32 causes from which death should not occur in the presence of effective care to approximate personal health-care access and quality by location and over time. To better isolate potential effects of personal health-care access and quality from underlying risk factor patterns, we risk-standardised cause-specific deaths due to non-cancers by location-year, replacing the local joint exposure of environmental and behavioural risks with the global level of exposure. Supported by the expansion of cancer registry data in GBD 2016, we used mortality-to-incidence ratios for cancers instead of risk-standardised death rates to provide a stronger signal of the effects of personal health care and access on cancer survival. We transformed each cause to a scale of 0-100, with 0 as the first percentile (worst) observed between 1990 and 2016, and 100 as the 99th percentile (best); we set these thresholds at the country level, and then applied them to subnational locations. We applied a principal components analysis to construct the HAQ Index using all scaled cause values, providing an overall score of 0-100 of personal health-care access and quality by location over time. We then compared HAQ Index levels and trends by quintiles on the Socio-demographic Index (SDI), a summary measure of overall development. As derived from the broader GBD study and other data sources, we examined relationships between national HAQ Index scores and potential correlates of performance, such as total health spending per capita. Findings In 2016, HAQ Index performance spanned from a high of 97\ub71 (95% UI 95\ub78-98\ub71) in Iceland, followed by 96\ub76 (94\ub79-97\ub79) in Norway and 96\ub71 (94\ub75-97\ub73) in the Netherlands, to values as low as 18\ub76 (13\ub71-24\ub74) in the Central African Republic, 19\ub70 (14\ub73-23\ub77) in Somalia, and 23\ub74 (20\ub72-26\ub78) in Guinea-Bissau. The pace of progress achieved between 1990 and 2016 varied, with markedly faster improvements occurring between 2000 and 2016 for many countries in sub-Saharan Africa and southeast Asia, whereas several countries in Latin America and elsewhere saw progress stagnate after experiencing considerable advances in the HAQ Index between 1990 and 2000. Striking subnational disparities emerged in personal health-care access and quality, with China and India having particularly large gaps between locations with the highest and lowest scores in 2016. In China, performance ranged from 91\ub75 (89\ub71-93\ub76) in Beijing to 48\ub70 (43\ub74-53\ub72) in Tibet (a 43\ub75-point difference), while India saw a 30\ub78-point disparity, from 64\ub78 (59\ub76-68\ub78) in Goa to 34\ub70 (30\ub73-38\ub71) in Assam. Japan recorded the smallest range in subnational HAQ performance in 2016 (a 4\ub78-point difference), whereas differences between subnational locations with the highest and lowest HAQ Index values were more than two times as high for the USA and three times as high for England. State-level gaps in the HAQ Index in Mexico somewhat narrowed from 1990 to 2016 (from a 20\ub79-point to 17\ub70-point difference), whereas in Brazil, disparities slightly increased across states during this time (a 17\ub72-point to 20\ub74-point difference). Performance on the HAQ Index showed strong linkages to overall development, with high and high-middle SDI countries generally having higher scores and faster gains for non-communicable diseases. Nonetheless, countries across the development spectrum saw substantial gains in some key health service areas from 2000 to 2016, most notably vaccine-preventable diseases. Overall, national performance on the HAQ Index was positively associated with higher levels of total health spending per capita, as well as health systems inputs, but these relationships were quite heterogeneous, particularly among low-to-middle SDI countries. Interpretation GBD 2016 provides a more detailed understanding of past success and current challenges in improving personal health-care access and quality worldwide. Despite substantial gains since 2000, many low-SDI and middle- SDI countries face considerable challenges unless heightened policy action and investments focus on advancing access to and quality of health care across key health services, especially non-communicable diseases. Stagnating or minimal improvements experienced by several low-middle to high-middle SDI countries could reflect the complexities of re-orienting both primary and secondary health-care services beyond the more limited foci of the Millennium Development Goals. Alongside initiatives to strengthen public health programmes, the pursuit of universal health coverage hinges upon improving both access and quality worldwide, and thus requires adopting a more comprehensive view-and subsequent provision-of quality health care for all populations
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