4,744 research outputs found
Bifurcations in valveless pumping techniques from a coupled fluid-structure-electrophysiology model in heart development
We explore an embryonic heart model that couples electrophysiology and
muscle-force generation to flow induced using a fluid-structure
interaction framework based on the immersed boundary method. The propagation of
action potentials are coupled to muscular contraction and hence the overall
pumping dynamics. In comparison to previous models, the electro-dynamical model
does not use prescribed motion to initiate the pumping motion, but rather the
pumping dynamics are fully coupled to an underlying electrophysiology model,
governed by the FitzHugh-Nagumo equations. Perturbing the diffusion parameter
in the FitzHugh-Nagumo model leads to a bifurcation in dynamics of action
potential propagation. This bifurcation is able to capture a spectrum of
different pumping regimes, with dynamic suction pumping and peristaltic-like
pumping at the extremes. We find that more bulk flow is produced within the
realm of peristaltic-like pumping.Comment: 11 pages, 13 figures. arXiv admin note: text overlap with
arXiv:1610.0342
Reynolds number limits for jet propulsion: A numerical study of simplified jellyfish
The Scallop Theorem states that reciprocal methods of locomotion, such as jet
propulsion or paddling, will not work in Stokes flow (Reynolds number = 0). In
nature the effective limit of jet propulsion is still in the range where
inertial forces are significant. It appears that almost all animals that use
jet propulsion swim at Reynolds numbers (Re) of about 5 or more. Juvenile squid
and octopods hatch from the egg already swimming in this inertial regime. The
limitations of jet propulsion at intermediate Re is explored here using the
immersed boundary method to solve the two-dimensional Navier Stokes equations
coupled to the motion of a simplified jellyfish. The contraction and expansion
kinematics are prescribed, but the forward and backward swimming motions of the
idealized jellyfish are emergent properties determined by the resulting fluid
dynamics. Simulations are performed for both an oblate bell shape using a
paddling mode of swimming and a prolate bell shape using jet propulsion.
Average forward velocities and work put into the system are calculated for
Reynolds numbers between 1 and 320. The results show that forward velocities
rapidly decay with decreasing Re for all bell shapes when Re < 10. Similarly,
the work required to generate the pulsing motion increases significantly for Re
< 10. When compared actual organisms, the swimming velocities and vortex
separation patterns for the model prolate agree with those observed in Nemopsis
bachei. The forward swimming velocities of the model oblate jellyfish after two
pulse cycles are comparable to those reported for Aurelia aurita, but
discrepancies are observed in the vortex dynamics between when the 2D model
oblate jellyfish and the organism
Pulsing corals: A story of scale and mixing
Effective methods of fluid transport vary across scale. A commonly used
dimensionless number for quantifying the effective scale of fluid transport is
the Reynolds number, Re, which gives the ratio of inertial to viscous forces.
What may work well for one Re regime may not produce significant flows for
another. These differences in scale have implications for many organisms,
ranging from the mechanics of how organisms move through their fluid
environment to how hearts pump at various stages in development. Some
organisms, such as soft pulsing corals, actively contract their tentacles to
generate mixing currents that enhance photosynthesis. Their unique morphology
and intermediate scale where both viscous and inertial forces are significant
make them a unique model organism for understanding fluid mixing. In this
paper, 3D fluid-structure interaction simulations of a pulsing soft coral are
used to quantify fluid transport and fluid mixing across a wide range of Re.
The results show that net transport is negligible for , and continuous
upward flow is produced for .Comment: 8 pages, 8 figure
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ALL-AMERICAN VACATIONLAND: AFRICAN AMERICAN, PUERTO RICAN, AND ITALIAN RESORTS IN THE CATSKILL MOUNTAINS, 1920-1980
In the twentieth century, New York State’s Catskill Mountain resort area was an “All-American” vacationland. Each summer, many different racial and ethnic minorities sought a brief respite from their lives and labor in New York City at boarding houses, resorts, and bungalows scattered throughout the mountains. Collectively, these groups contributed to the development of a highly segregated resort area that reflected, on an exaggerated scale, the racial, ethnic, and class divisions within New York City and the nation as a whole in the twentieth century.
This dissertation examines the Catskills resort landscape through a comparative analysis of African American, Puerto Rican, and Italian summer resorts from 1920 to 1980. It draws on oral history interviews, newspaper accounts, and archival sources to trace the history of these resorts from their origins as modest boarding houses in the 1920s and 1930s, to their immense growth in popularity after World War II, and their decline in the final decades of the twentieth century. All three groups created resorts where they sought to foster and sustain a sense of collective pride and identity in insulated recreational environments, free from the racism and nativism of dominant white society. Summer resorts catered to and were shaped by each group’s distinct social, cultural, and political needs; these needs evolved according to changes in vacationers’ lives in urban and suburban areas around New York City.
Considered alongside one another, these histories demonstrate that summer resorts were not solely a stepping-stone for ethnic minorities on their way to assimilation and acceptance in American society. In the decades following World War II, Italians successfully reconfigured the meanings of their ethnic identity to gain acceptance as white Americans. By contrast, racial minorities found that racism continued to hamper their efforts at upward mobility, well after legal barriers to their success were dismantled. Summer resorts built upon and helped naturalize patterns of segregation and inequality that structured vacationers’ everyday lives in the New York metropolitan area. In this sense, too, the resort landscape was “All-American”—a striking reflection of the country’s deeply entrenched racial hierarchy
Flow Structure and Transport Characteristics of Feeding and Exchange Currents Generated by Upside-Down Cassiopea Jellyfish
Quantifying the flows generated by the pulsations of jellyfish bells is crucial for understanding the mechanics and efficiency of their swimming and feeding. Recent experimental and theoretical work has focused on the dynamics of vortices in the wakes of swimming jellyfish with relatively simple oral arms and tentacles. The significance of bell pulsations for generating feeding currents through elaborate oral arms and the consequences for particle capture are not as well understood. To isolate the generation of feeding currents from swimming, the pulsing kinematics and fluid flow around the benthic jellyfish Cassiopea spp. were investigated using a combination of videography, digital particle image velocimetry and direct numerical simulation. During the rapid contraction phase of the bell, fluid is pulled into a starting vortex ring that translates through the oral arms with peak velocities that can be of the order of 10 cm s–1. Strong shear flows are also generated across the top of the oral arms throughout the entire pulse cycle. A coherent train of vortex rings is not observed, unlike in the case of swimming oblate medusae such as Aurelia aurita. The phase-averaged flow generated by bell pulsations is similar to a vertical jet, with induced flow velocities averaged over the cycle of the order of 1–10 mm s–1. This introduces a strong near-horizontal entrainment of the fluid along the substrate and towards the oral arms. Continual flow along the substrate towards the jellyfish is reproduced by numerical simulations that model the oral arms as a porous Brinkman layer of finite thickness. This two-dimensional numerical model does not, however, capture the far-field flow above the medusa, suggesting that either the three-dimensionality or the complex structure of the oral arms helps to direct flow towards the central axis and up and away from the animal
Three-dimensional low Reynolds number flows near biological filtering and protective layers
Mesoscale filtering and protective layers are replete throughout the natural
world. Within the body, arrays of extracellular proteins, microvilli, and cilia
can act as both protective layers and mechanosensors. For example, blood flow
profiles through the endothelial surface layer determine the amount of shear
stress felt by the endothelial cells and may alter the rates at which molecules
enter and exit the cells. Characterizing the flow profiles through such layers
is therefore critical towards understanding the function of such arrays in cell
signaling and molecular filtering. External filtering layers are also important
to many animals and plants. Trichomes (the hairs or fine outgrowths on plants)
can drastically alter both the average wind speed and profile near the leaf's
surface, affecting the rates of nutrient and heat exchange. In this paper,
dynamically scaled physical models are used to study the flow profiles outside
of arrays of cylinders that represent such filtering and protective layers. In
addition, numerical simulations using the Immersed Boundary Method are used to
resolve the 3D flows within the layers. The experimental and computational
results are compared to analytical results obtained by modeling the layer as a
homogeneous porous medium with free flow above the layer. The experimental
results show that the bulk flow is well described by simple analytical models.
The numerical results show that the spatially averaged flow within the layer is
well described by the Brinkman model. The numerical results also demonstrate
that the flow can be highly 3D with fluid moving into and out of the layer.
These effects are not described by the Brinkman model and may be significant
for biologically relevant volume fractions. The results of this paper can be
used to understand how variations in density and height of such structures can
alter shear stresses and bulk flows.Comment: 28 pages, 10 figure
The Role of the Pericardium in the Valveless, Tubular Heart of the Tunicate, \u3cem\u3eCiona savignyi\u3c/em\u3e
Tunicates, small invertebrates within the phylum Chordata, possess a robust tubular heart which pumps blood through their open circulatory systems without the use of valves. This heart consists of two major components: the tubular myocardium, a flexible layer of myocardial cells that actively contracts to drive fluid down the length of the tube; and the pericardium, a stiff, outer layer of cells that surrounds the myocardium and creates a fluid-filled space between the myocardium and the pericardium. We investigated the role of the pericardium through in vivo manipulations on tunicate hearts and computational simulations of the myocardium and pericardium using the immersed boundary method. Experimental manipulations reveal that damage to the pericardium results in aneurysm-like bulging of the myocardium and major reductions in the net blood flow and percentage closure of the heart\u27s lumen during contraction. In addition, varying the pericardium-to-myocardium (PM) diameter ratio by increasing damage severity was positively correlated with peak dye flow in the heart. Computational simulations mirror the results of varying the PM ratio experimentally. Reducing the stiffness of the myocardium in the simulations reduced mean blood flow only for simulations without a pericardium. These results indicate that the pericardium has the ability to functionally increase the stiffness of the myocardium and limit myocardial aneurysms. The pericardium\u27s function is likely to enhance flow through the highly resistive circulatory system by acting as a support structure in the absence of connective tissue within the myocardium
Large Amplitude, Short Wave Peristalsis and Its Implications for Transport
Valveless, tubular pumps are widespread in the animal kingdom, but the mechanism by which these pumps generate fluid flow is often in dispute. Where the pumping mechanism of many organs was once described as peristalsis, other mechanisms, such as dynamic suction pumping, have been suggested as possible alternative mechanisms. Peristalsis is often evaluated using criteria established in a technical definition for mechanical pumps, but this definition is based on a small-amplitude, long-wave approximation which biological pumps often violate. In this study, we use a direct numerical simulation of large-amplitude, short-wave peristalsis to investigate the relationships between fluid flow, compression frequency, compression wave speed, and tube occlusion. We also explore how the flows produced differ from the criteria outlined in the technical definition of peristalsis. We find that many of the technical criteria are violated by our model: Fluid flow speeds produced by peristalsis are greater than the speeds of the compression wave; fluid flow is pulsatile; and flow speed have a nonlinear relationship with compression frequency when compression wave speed is held constant. We suggest that the technical definition is inappropriate for evaluating peristalsis as a pumping mechanism for biological pumps because they too frequently violate the assumptions inherent in these criteria. Instead, we recommend that a simpler, more inclusive definition be used for assessing peristalsis as a pumping mechanism based on the presence of non-stationary compression sites that propagate unidirectionally along a tube without the need for a structurally fixed flow direction
IB2d : a Python and MATLAB implementation of the immersed boundary method
The development of fluid-structure interaction (FSI) software involves trade-offs between ease of use, generality, performance, and cost. Typically there are large learning curves when using low-level software to model the interaction of an elastic structure immersed in a uniform density fluid. Many existing codes are not publicly available, and the commercial software that exists usually requires expensive licenses and may not be as robust or allow the necessary flexibility that in house codes can provide. We present an open source immersed boundary software package, IB2d, with full implementations in both MATLAB and Python, that is capable of running a vast range of biomechanics models and is accessible to scientists who have experience in high-level programming environments. IB2d contains multiple options for constructing material properties of the fiber structure, as well as the advection-diffusion of a chemical gradient, muscle mechanics models, and artificial forcing to drive boundaries with a preferred motion
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