1,337 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
Simulating water-entry/exit problems using Eulerian-Lagrangian and fully-Eulerian fictitious domain methods within the open-source IBAMR library
In this paper we employ two implementations of the fictitious domain (FD)
method to simulate water-entry and water-exit problems and demonstrate their
ability to simulate practical marine engineering problems. In FD methods, the
fluid momentum equation is extended within the solid domain using an additional
body force that constrains the structure velocity to be that of a rigid body.
Using this formulation, a single set of equations is solved over the entire
computational domain. The constraint force is calculated in two distinct ways:
one using an Eulerian-Lagrangian framework of the immersed boundary (IB) method
and another using a fully-Eulerian approach of the Brinkman penalization (BP)
method. Both FSI strategies use the same multiphase flow algorithm that solves
the discrete incompressible Navier-Stokes system in conservative form. A
consistent transport scheme is employed to advect mass and momentum in the
domain, which ensures numerical stability of high density ratio multiphase
flows involved in practical marine engineering applications. Example cases of a
free falling wedge (straight and inclined) and cylinder are simulated, and the
numerical results are compared against benchmark cases in literature.Comment: The current paper builds on arXiv:1901.07892 and re-explains some
parts of it for the reader's convenienc
Tunneling magnetoresistance in (La,Pr,Ca)MnO3 nanobridges
The manganite (La,Pr,Ca)MnO3 is well known for its micrometer scale phase
separation into coexisting ferromagnetic metallic and antiferromagnetic
insulating (AFI) regions. Fabricating bridges with widths smaller than the
phase separation length scale has allowed us to probe the magnetic properties
of individual phase separated regions. We observe tunneling magnetoresistance
across naturally occurring AFI tunnel barriers separating adjacent
ferromagnetic regions spanning the width of the bridges. Further, near the
Curie temperature, a magnetic field induced metal-to-insulator transition among
a discrete number of regions within the narrow bridges gives rise to abrupt and
colossal low-field magnetoresistance steps at well defined switching fields.Comment: 13 pages, 3 figures, submitted to Applied Physics Letter
A low Mach enthalpy method to model non-isothermal gas-liquid-solid flows with melting and solidification
Modeling phase change problems numerically is vital for understanding many
natural (e.g., ice formation, steam generation) and engineering processes
(e.g., casting, welding, additive manufacturing). Almost all phase change
materials (PCMs) exhibit density/volume changes during melting, solidification,
boiling, or condensation, causing additional fluid flow during this transition.
Most numerical works consider only two phase flows (either solid-liquid or
liquid-gas) for modeling phase change phenomena and some also neglect
volume/density change of PCMs in the models. This paper presents a novel low
Mach enthalpy method for simulating solidification and melting problems with
variable thermophysical properties, including density. Additionally, this
formulation allows coupling a solid-liquid PCM with a gas phase in order to
simulate the free surface dynamics of PCMs undergoing melting and
solidification. We revisit the two-phase Stefan problem involving a density
jump between two material phases. We propose a possible means to include the
kinetic energy jump in the Stefan condition while still allowing for an
analytical solution. The new low Mach enthalpy method is validated against
analytical solutions for a PCM undergoing a large density change during its
phase transition. Additionally, a few simple sanity checks are proposed to
benchmark computational fluid dynamics (CFD) algorithms that aim to capture the
volume change effects of PCMs
Cerebral phaeohyphomycosis: An important differential of tubercular brain abscess
Cerebral phaeohyphomycosis (CP), caused by dematiaceous fungi, is a serious form of central nervous system fungal infection. It is a rare disease with male predominance, no specific symptoms or signs and is associated with grim prognosis irrespective of the immune status of the patient. The disease is difficult to diagnose antemortem, and many cases are accidentally diagnosed during surgery or autopsy. The early diagnosis and appropriate treatment remain a challenge. The authors report a misinterpreted case of CP in a 53-year-old man without immunodeficiency who showed a favorable outcome after surgical excision and antifungal therapy. Therefore, CP should be an important differential in cases of brain abscess
- …