Investigation of periodic characteristics of perturbed flow over a slender body.

The asymmetric flow over a slender body was particularly sensitive to the nose at a high angle of attack (AoA). Two patterns of separation occurred on the noses of the pointed-nosed slender body and blunt-nosed slender body as open- and close-type separation, respectively. The effects of the bluntness were investigated at high AoA (α = 50°) to clarify the evolution of the separated pattern from open-to close-type separation by the nose and by the periodic characteristics of perturbed flow. Wind tunnel experimental tests were conducted to investigate the periodic characteristics of asymmetric flow at a Reynolds number ReD = 1.54 × 105, based on incoming free-stream velocity (U∞) and the diameter (D) of the model. A particle was attached to the tip of the nose to induce the perturbed flow and attain a definite and predictable asymmetric flow in experimental tests. The pressure scanning and surface oil-flow visualization techniques were used to capture the pressure distributions and flow separations. The major findings were that axial flow increases with the increase of bluntness, resulting in open-type separation turning into close-type separation, and the perturbation moved from downstream to upstream of starting points of the separation line. The critical bluntness of separation pattern switching from open-type to close-type located between 1.5 and 3. Thus, the management of perturbation on asymmetric flow pattern switched from directly participating in separation to influencing separation through micro-flow. Therefore, the locations of perturbation and starting points of the separation line were closely related to asymmetric flow management by perturbation, then affecting the periodic characteristics of perturbed flow.

Engineering Themes in Plant Forms and Functions.

Living structures constantly interact with the biotic and abiotic environment by sensing and responding via specialized functional parts. In other words, biological bodies embody highly functional machines and actuators. What are the signatures of engineering mechanisms in biology? In this review, we connect the dots in the literature to seek engineering principles in plant structures. We identify three thematic motifs-bilayer actuator, slender-bodied functional surface, and self-similarity-and provide an overview of their structure-function relationships. Unlike human-engineered machines and actuators, biological counterparts may appear suboptimal in design, loosely complying with physical theories or engineering principles. We postulate what factors may influence the evolution of functional morphology and anatomy to dissect and comprehend better the why behind the biological forms.

Primary cilium: a paradigm for integrating mathematical modeling with experiments and numerical simulations in mechanobiology.

Primary cilia are non-motile, solitary (one per cell) microtubule-based organelles that emerge from the mother centriole after cells have exited the mitotic cycle. Identified as a mechanosensing organelle that responds to both mechanical and chemical stimuli, the primary cilium provides a fertile ground for integrative investigations of mathematical modeling, numerical simulations, and experiments. Recent experimental findings revealed considerable complexity to the underlying mechanosensory mechanisms that transmit extracellular stimuli to intracellular signaling many of which include primary cilia. In this invited review, we provide a brief survey of experimental findings on primary cilia and how these results lead to various mathematical models of the mechanics of the primary cilium bent under an external forcing such as a fluid flow or a trap. Mathematical modeling of the primary cilium as a fluid-structure interaction problem highlights the importance of basal anchorage and the anisotropic moduli of the microtubules. As theoretical modeling and numerical simulations progress, along with improved state-of-the-art experiments on primary cilia, we hope that details of ciliary regulated mechano-chemical signaling dynamics in cellular physiology will be understood in the near future.

Hydrodynamics of a twisting slender swimmer.

Sea snakes propel themselves by lateral deformation waves moving backwards along their bodies faster than they swim. In contrast to typical anguilliform swimmers, however, their swimming is characterized by exaggerated torsional waves that lead the lateral ones. The effect of torsional waves on hydrodynamic forces generated by an anguilliform swimmer is the subject matter of this study. The forces, and the power needed to sustain them, are found analytically using the framework of the slender (elongated) body theory. It is shown that combinations of torsional waves and angle of attack can generate both thrust and lift, whereas combinations of torsional and lateral waves can generate lift of the same magnitude as thrust. Generation of lift comes at a price of increasing tail amplitude, but otherwise carries practically no energetic penalty.

Slender-body theory for plasmonic resonance.

We develop a slender-body theory for plasmonic resonance of slender metallic nanoparticles, focusing on a general class of axisymmetric geometries with locally paraboloidal tips. We adopt a modal approach where one first solves the plasmonic eigenvalue problem, a geometric spectral problem which governs the surface-plasmon modes of the particle; then, the latter modes are used, in conjunction with spectral-decomposition, to analyse localized-surface-plasmon resonance in the quasi-static limit. We show that the permittivity eigenvalues of the axisymmetric modes are strongly singular in the slenderness parameter, implying widely tunable, high-quality-factor, resonances in the near-infrared regime. For that family of modes, we use matched asymptotics to derive an effective eigenvalue problem, a singular non-local Sturm-Liouville problem, where the lumped one-dimensional eigenfunctions represent axial voltage profiles (or charge line densities). We solve the effective eigenvalue problem in closed form for a prolate spheroid and numerically, by expanding the eigenfunctions in Legendre polynomials, for arbitrarily shaped particles. We apply the theory to plane-wave illumination in order to elucidate the excitation of multiple resonances in the case of non-spheroidal particles.

Chirality-specific lift forces of helix under shear flows: Helix perpendicular to shear plane.

Chiral objects in shear flow experience a chirality-specific lift force. Shear flows past helices in a low Reynolds number regime were studied using slender-body theory. The chirality-specific lift forces in the vorticity direction experienced by helices are dominated by a set of helix geometry parameters: helix radius, pitch length, number of turns, and helix phase angle. Its analytical formula is given. The chirality-specific forces are the physical reasons for the chiral separation of helices in shear flow. Our results are well supported by the latest experimental observations.

Hydraulic resistance of a plant root to water-uptake: A slender-body theory.

A slender-body theory for calculating the hydraulic resistance of a single plant root is developed. The work provides an in-depth discussion on the procedure and the assumptions involved in calculating a root׳s internal hydraulic resistance as well as the physical and the mathematical aspects of the external three-dimensional flow around the tip of a root in a saturated soil and how this flow pattern enhances uptake and reduces hydraulic resistance. Analytical solutions for the flux density distribution on the stele-cortex interface, local water-uptake profile inside the stele core, the overall water-uptake at the base of the stele, and the total hydraulic resistance of a root are obtained in the slender-body limit. It is shown that a key parameter controlling a root's hydraulic resistance is the dimensionless axial conductivity in the stele, which depends on the permeabilities of the stele and the cortex as well as the root's radial and axial dimensions. Three-dimensional tip effect reduces a root's hydraulic resistance by as much as 36% when compared to the radial flow theory of Landsberg and Fowkes. In addition, the total hydraulic resistance cannot be generally decomposed into the direct sum of a radial resistance and an axial resistance.

Magnetic Propulsion of Microswimmers with DNA-Based Flagellar Bundles.

We show that DNA-based self-assembly can serve as a general and flexible tool to construct artificial flagella of several micrometers in length and only tens of nanometers in diameter. By attaching the DNA flagella to biocompatible magnetic microparticles, we provide a proof of concept demonstration of hybrid structures that, when rotated in an external magnetic field, propel by means of a flagellar bundle, similar to self-propelling peritrichous bacteria. Our theoretical analysis predicts that flagellar bundles that possess a length-dependent bending stiffness should exhibit a superior swimming speed compared to swimmers with a single appendage. The DNA self-assembly method permits the realization of these improved flagellar bundles in good agreement with our quantitative model. DNA flagella with well-controlled shape could fundamentally increase the functionality of fully biocompatible nanorobots and extend the scope and complexity of active materials.

A quantitative approach to study endothelial cilia bending stiffness during blood flow mechanodetection in vivo.

Primary cilia are necessary for shear stress sensing in different developing organs such as the kidneys and blood vessels. In endothelial cells (ECs), primary cilia bend in response to blood flow forces and are necessary for flow sensing as well as for the control of angiogenesis. The different parameters guiding cilia bending reflect the forces generated at the surface of the ECs and the mechanical properties of the endothelial cilia. Here, we present an approach allowing the calculation of the bending rigidity of endothelial cilia based on live imaging. The method relies on segmentation and mathematical modeling to extract the critical parameters needed for the calculation.

Modification of mean wake flow behind a very slender axisymmetric body of revolution by imposed nonlinear unstable helical modes.

For a sufficiently slender axially symmetric body placed in a uniform stream, only convectively unstable modes are found in previous experiments. This work imposes theoretically and computationally a pair of most unstable helical modes, symmetrically and asymmetrically. The Reynolds stress modification of the developing laminar mean wake flow is modified into an elliptic-like cross section for symmetrical forcing; the consequences of unequal upstream amplitudes are also explored. Energy-transfer mechanisms between the mean flow and the relevant dominant modes and between the modes through 'triad interactions' are studied. The results from dynamical considerations provide the physical understanding of the generation of a standing wave mode at twice the azimuthal wavenumber; it is necessary that the wave envelopes of participating modes, including that of the mean flow, overlap in their spatial development, which is a necessary supplement to kinematical conditions for such interactions to take place effectively. Standing wave motions, which are otherwise only found naturally in wakes behind blunt-trailing-edge axisymmetric bodies, can be rendered present through appropriate forcing and nonlinear interactions behind very slender axisymmetric bodies.

Passive elastic mechanism to mimic fish-muscle action in anguilliform swimming.

Swimmers in nature use body undulations to generate propulsive and manoeuvring forces. The anguilliform kinematics is driven by muscular actions all along the body, involving a complex temporal and spatial coordination of all the local actuations. Such swimming kinematics can be reproduced artificially, in a simpler way, by using the elasticity of the body passively. Here, we present experiments on self-propelled elastic swimmers at a free surface in the inertial regime. By addressing the fluid-structure interaction problem of anguilliform swimming, we show that our artificial swimmers are well described by coupling a beam theory with the potential flow model of Lighthill. In particular, we show that the propagative nature of the elastic wave producing the propulsive force is strongly dependent on the dissipation of energy along the body of the swimmer.

The Effect of Body Build and BMI on Aerobic Test Performance in School Children (10-15 Years).

Body Mass Index (BMI) has often questionably been used to define body build. In the present study body build was defined more specifically using fat free mass index (FFMI = fat free mass normalised to the stature) and fat mass index (FMI = fat mass normalised to stature). The body build of an individual is 'solid' in individuals with a high FFMI for their FMI and is 'slender' in individuals with a low FFMI relative to their FMI. The aim of the present study was to investigate the association between aerobic test performance and body build defined as solid, average or slender in 10 to 15 year old children. Five-hundred-and-two children (53% boys) aged 10 to 15 years of age were included in the study. Aerobic test performance was estimated with an incremental cycle ergometer protocol and a shuttle run test. BMI and percentage fat (by skin folds) were determined to calculate FMI and FFMI. After adjustment for differences in age, gender and body mass the solid group achieved a significantly higher maximal power output (W) and power output relative to body mass (W/kg) during the cycle test (p < 0.05) and a higher shuttle-run score (p < 0.05) compared to the slender group. The power output relative to FFM (W/kg FFM) was comparable (p > 0.05) between different body build groups. This study showed that body build is an important determinant of the aerobic test performance. In contrast, there were no differences in aerobic test performance per kilogramme FFM over the body build groups. This suggests that the body build may be determined by genetic predisposition. Key PointsChildren with a solid body build perform better in aerobic exercise tests than slender children.The power output relative to fat free mass was comparable in the solid, slender and average group.Besides body composition, body build should be considered related to other performance measurements.