Helical Geometry Research Articles

Transverse Magnetic Tweezers Allowing Coincident Epi-Fluorescence Microscopy on Horizontally Extended DNA.

Longitudinal magnetic tweezers (L-MT) have seen wide-scale adoption as the tool of choice for stretching and twisting a single DNA molecule. They are also used to probe topological changes in DNA as a result of protein binding and enzymatic activity. However, in the longitudinal configuration, the DNA molecule is extended perpendicular to the imaging plane. As a result, it is only possible to infer biological activity from the motion of the tethered paramagnetic microsphere. Described here is a "transverse" magnetic tweezers (T-MT) geometry featuring simultaneous control of DNA extension and spatially coincident video-rate epi-fluorescence imaging. Unlike in L-MT, DNA tethers in T-MT are extended parallel to the imaging plane between two micron-sized spheres, and importantly protein targets on the DNA can be localized using fluorescent nanoparticles. The T-MT can manipulate a long DNA construct at molecular extensions approaching the contour length defined by B-DNA helical geometry, and the measured entropic elasticity agrees with the wormlike chain model (force <35 pN). By incorporating a torsionally constrained DNA tether, the T-MT would allow both the relative extension and twist of the tether to be manipulated, while viewing far-red emitting fluorophore-labeled targets. This T-MT design has the potential to enable the study of DNA binding and remodeling processes under conditions of constant force and defined torsional stress.

AC losses of Roebel and CORC® cables at higher AC magnetic fields and ramp rates

We have measured ReBCO coated conductor-based conductor on round core (CORC®) and Roebel cables at 77 K in a spinning magnet calorimeter, which subjected the tapes in the samples to a radial magnetic field of 566 mT (peak) at frequencies up to 120 Hz (272 T s−1, cyclic average) with an approximately sinusoidal waveform. The samples were oriented such that the field applied to the tapes within the cables was entirely radial, simplifying subsequent analysis. An expression for loss which included hysteretic, flux creep and eddy current losses was fit to both the CORC® and the Roebel cables. This expression allowed easy comparison of the relative influence of eddy currents and flux creep (or power-law behavior) effects. The loss of both the CORC ® and Roebel cables measured here were seen to be essentially the sum of the hysteretic loss, flux creep effects, and the normal metal eddy current losses of the individual tapes. The losses of these cables were measured at high B × dB/dt with no coupling current loss observed under the present preparation conditions. The influence of flux creep effects on loss were not negligible. The losses of the CORC® cable per meter of tape were seen to be reduced from the case of a flat tape because of the helical geometry of the tapes.

Whither Second-Sphere Coordination?

Whither Second-Sphere Coordination?

Stretchable Textile Yarn Based on UHF RFID Helical Tag

In the context of wearable technology, several techniques have been used for the fabrication of radio frequency identification (RFID) tags such as 3D printing, inkjet printing, and even embroidery. In contrast to these methods where the tag is attached to the object by using sewing or simple sticking, the E-Thread® technology is a novel assembling method allowing for the integration of the RFID tag into a textile yarn and thus makes it embeddable into the object at the fabrication stage. The current E-Thread® yarn uses a RFID tag in which the antenna is a straight half-wave dipole that makes the solution vulnerable to mechanical strains (i.e., elongation). In this paper, we propose an alternative to the current RFID yarn solution with the use of an antenna having a helical geometry that answers to the mechanical issues and keeps quite similar electrical and radiative properties with respect to the present solution. The RFID helical tag was designed and simulated taking into consideration the constraints of the manufacturing process. The helical RFID tag was then fabricated using the E-Thread® technology and experimental characterization showed that the obtained structure exhibited good performance with 10.6 m of read range in the ultra high frequency (UHF) RFID band and 10% of tolerance in terms of elongation.

Light‐Fueled Climbing of Monolithic Torsional Soft Robots via Molecular Engineering

Azobenzene‐functionalized liquid crystalline polymer networks (azo‐LCNs) are promising candidates for light‐fueled contactless manipulation of miniaturized soft robots through embedding photoactive molecular switches into alignment‐programmable LCNs. In particular, the 3D helical geometry of azo‐LCNs is reported to achieve rapid photomotility by introducing rolling resistance. However, the maximum height of the obstacle that soft robot can overcome is limited by the helix diameter and the stress–strain responsivity. Herein, the helical diameter per unit length and photogenerated stress through molecular engineering of photoactive molecular switches are maximized. The carbon number of aliphatic spacers in the photoactive molecular switches is varied from two to eight to systematically investigate the structure–property–performance relations by studying the molecular geometry, physical properties of polymers, and photomotility of polymers. Furthermore, a finite‐element analysis simulation is presented to understand the rolling locomotion of helical torsional soft robots. Through molecular engineering, the helix diameter per unit length of 0.2 mg soft robots is maximized, demonstrating high Young's modulus (≈2 GPa) and photogenerated stress (&gt;1 MPa), as well as large velocity per body length, compared with the previously reported soft robots. Finally, the molecularly engineered soft robots successfully climb stairs, which is a key task in robotic systems.

Experimental investigation of a rotor blade tip vortex pair

This paper presents the results of an experimental study of two closely spaced vortices generated by a rotating blade with a modified tip geometry. The experiments are carried out in two water channel facilities and involve a generic one-bladed rotor operating in a regime near hover. It is equipped with a parametric fin placed perpendicular to the pressure surface near the tip, which generates a co-rotating vortex pair having a helical geometry. Based on previous results obtained with a fixed wing, a series of small-scale experiments is first carried out, to validate the method of vortex pair generation also for a rotating blade, and to obtain a qualitative overview of its evolution going downstream. A more detailed quantitative study is then performed in a larger facility at three times the initial scale. By varying the fin parameters, it was possible to obtain a configuration in which the two vortices have almost the same circulation. In both experiments, the vortex pair is found to merge into a single helical wake vortex within one blade rotation. Particle image velocimetry measurements show that the resulting vortex has a significantly larger core radius than the single tip vortex from a blade without fin. This finding may have relevance in the context of blade–vortex interactions, where noise generation and fatigue from fluid–structure interactions depend strongly on the vortex core size.

Optimisation of helical geometry of inspection probe for steam generator tubes of the PFBR

The steam generator (SG) tubes of the prototype fast breeder reactor (PFBR) located in Kalpakkam, India, need to be periodically inspected using the remote field eddy current (RFEC) technique. During the pre-service inspection of the SG tubes, it was found that the RFEC probes experienced frequent mechanical breakages. To avoid these failures, changes in the existing structural design of the RFEC probe were required. A helical groove design was proposed to obtain a smooth transition in the variation of stress across the probe during the inspection. It was difficult to calculate the flexural stiffness of the proposed helical geometry probe due to the varying cross-section along its length. In this paper, the smearing approach adopted to calculate the stiffness of the RFEC probe and the sensitivity analysis carried out to determine the optimal design of the probe are discussed. A probe was fabricated based on the helical groove design and tested to qualify its suitability for the SG inspection. The RFEC probe with helical grooves was employed for the pre-service inspection of the SG tubes of the PFBR. More than 200 tubes have been inspected using the proposed design and no mechanical failure of the probe has been observed.

Modeling and estimation of cutting forces in ball helical milling process

Milling forces play an important role in the milling process and are generally calculated by the mechanistic or numerical methods; reliable model of cutting forces is very important for the simulation of milling process, which has big scientific significance to further improve machining quality. Ball helical milling technology is used to make holes based on the cutting principle of helical milling using ball end cutter, and due to the influence of spherical surface machining characteristic, the modeling of cutting force in ball helical milling is difficult. Therefore, the main purpose of this paper is to establish an analytical cutting force model in the ball helical milling process. Considering cutting characteristics in the axial feed, the kinematics of ball helical milling is first presented, then the chip thickness distribution in different directions along the cutting edges is predicted. Furthermore, based on the characteristics of helical milling technology and geometry shape of ball end cutter and the classical mechanical cutting force model, through the study on the ball-end milling mechanics, a new relatively accurate theoretical cutting force model is established. At the same time, cutting force coefficients are identified through instantaneous force method according to the Ti-alloy experimental research result. Finally, higher simulation precision of cutting force model in ball helical milling process is received.

A multi-director continuum beam finite element for efficient analysis of multi-layer strand cables

This study presents a multi-director continuum beam finite element developed for efficient analysis of multi-layer strand cables. The beam element is derived from the continuum mechanics based beam formulation incorporating multi-directors and cross-sectional elements to describe the helical geometries of wires that constitute the strand cables. The main advantage of the present continuum beam formulation is that complex geometries of strand cable and inter-wire motions can be addressed in the framework of the beam kinematics regardless of the numbers of layers and subordinate helical wires. This approach enables simple modeling together with efficient analysis, and allows both geometric and material nonlinearities to be considered. In addition to its excellent accuracy, the proposed beam element model requires a significantly reduced number of degrees of freedom compared to conventional three-dimensional solid finite element models for cable analysis. Through several numerical examples, the predictive ability of the proposed beam model is demonstrated.

Challenges in studying steric interactions in highly strained perhalogenated alkanes and silanes

AbstractThe decomposition of the molecular total energy in their hyperconjugative, steric and electrostatic effects can lead to interesting interpretations about the stereoelectronic effects that govern their geometry and properties. In this work, we have studied homologous series of perfluoroalkanes, perchloroalkanes, perfluorosilanes and perchlorosilanes, and all molecules have preference for helical geometries. According to Natural Bond Orbitals (NBO) calculations, the silanes have their helical geometry stabilized by hyperconjugative interactions, as well as the perfluoroalkanes. However, it was surprisingly difficult to disclose that steric interactions are ruling the helical geometry preference in perchloroalkanes by comparing the NBO analysis and the Quantum Theory of Atoms ins Molecules (QTAIM). Although perchloroalkanes have extremely intense steric interactions between Cl lone pairs, some of them were underestimated by the NBO analysis, which showed the opposite behavior compared with QTAIM that indicates steric effects as the leading forces to helical geometry preference.

Liner implosion experiments driven by a dynamic screw pinch

This paper expands upon recent experimental results [Campbell et al., Phys. Rev. Lett. 125, 035001 (2020)], where thin-foil liner implosions were driven by a dynamic screw pinch (DSP) and found to have magneto-Rayleigh–Taylor instability (MRTI) amplitudes up to three times smaller than in implosions driven by a standard z-pinch (SZP). The expanded discussion presented herein includes: (1) a detailed comparison of the MRTI growth measured in the experiment with that calculated from theory; (2) measurements of axial magnetic field injection into the liner interior prior to the implosion, as well as the subsequent compression of this field during the implosion; (3) an in-depth description of how the helical geometry of the DSP can result in earlier implosion and stagnation times relative to the SZP; and (4) particle-in-cell simulations showing different electron drift behavior in the anode–cathode gap of the DSP relative to the SZP, and how this difference may be related to the different current waveforms recorded during the experiments.

Effect of cable surface geometry and ice accretion shapes on the aerodynamic behaviour of inclined stay cables

A wind-tunnel study has been conducted to evaluate changes in the aerodynamic behaviour of stay cables of a bridge due to ice accretion from freezing rain and, due to different cable surface geometries. Numerical simulations were performed to predict the three-dimensional ice shapes resulting from freezing rain for a variation of environmental conditions: wind speed, wind direction, air temperature, velocity of precipitation and duration of precipitation. Stay-cable models were fabricated using laser selective sintering for three different surface geometries (double helical fillet, concentric rings, and double helical strakes) with ice formed under the same environmental conditions. Additional models with the helical fillet geometry were fabricated to evaluate the effect of the air temperature at which ice was formed, the effect of wind direction during the precipitation event and the effect of the duration of the precipitation. The aerodynamic drag and lift forces acting on the models were measured for different wind speeds and cable-wind angles. The overall study evaluated the mean aerodynamic force coefficients of ten models in smooth flow, and two of the models were also tested in turbulent flow. The critical Reynolds number regime for the models of stay cable with ice accretion shapes occurred at a lower Reynolds number (below 100,000) than for the un-iced cables and the drag coefficient was less sensitive to increases in Reynolds numbers than the equivalent un-iced cables. These effects were attributed to the combined role of the ice-shape in determining the separation point combined with the higher surface roughness of the ice. An important observation was that for the three surface geometries and the range of test conditions that were considered, the maximum drag coefficients for ice-accreted models ranged from approximately 1.1 to 1.3, and were up to 50% greater than those observed without ice. The model with helical strakes and ice accretion exhibited the highest drag coefficient and exhibited more variation in lift coefficient with changes in Reynolds number than the other two cable surface geometries. The aerodynamic forces of an ice-accreted model followed similar trends in smooth and turbulent flow, but the drag coefficients were greater in turbulent flow. The presence and shape of the ice appeared to have a more dominant effect on the separation behaviour of the models than the presence of turbulence (for an ice-accreted model). The aerodynamic coefficients did not change greatly between the cases where ice was accreted at −0.5°C and −1.5°C due to the similar ice shape and thickness near the separation locations. The ice shape formed at −5°C was thicker but more localized and resulted in drag and lift coefficients that had greater variation with changes in the cable-wind plane. The effect of ice duration from 10 h to 20 h resulted in a increased variation in mean lift coefficient behaviour due to the asymmetric shape that resulted from the ice accretion parameters.

Unique Transmembrane Domain Interactions Differentially Modulate Platelet αIIbβ3 and αvβ3 Function

Lateral transmembrane (TM) helix-helix associations play essential roles in membrane-protein folding, assembly, and signal transduction. Thus, dissociation of the TM helices of resting integrins is central to integrin activation. Atomic-level models have revealed that a small residue-X3-small residue motif G-x3-G mediates the interaction of αIIb TM helix with β3, whereas a reciprocating large residue-small residue motif V-x3-I-x3-G mediates the interaction of the β3 helix with αIIb. Platelets contain a second β3 integrin αvβ3. Like αIIβ3, αvβ3 on resting platelets is inactive until platelets are stimulated by agonists such as ADP and thrombin after which it is able to interact with immobilized ligands such as osteopontin (OPN) and vitronectin. It is noteworthy that besides a V-x3-I-x3-G motif, the β3 TM domain contains the small residue-X3-small residue motif, S-x3-A and a small residue-X3-small residue motif in the β7 TM domain mediates its association with α4. Accordingly, we asked whether the motifs mediating the interaction of the β3 TM domain with the TM domains of αIIb and αv are necessarily the same. To address this question, we scanned the TM helix of intact β3 with leucine and alanine replacements, stably co-expressed the β3 mutants with wild-type human αv in CHO cells, and used optical trap-based force spectroscopy to measure the constitutive interaction of αvβ3 with either immobilized OPN or the activation-dependent αvβ3-specific monoclonal antibody WOW-1 (a kind gift from Dr. Sanford Shattil). The latter method measures the probability of receptor-ligand binding and the forces required to rupture single receptor-ligand bonds. Initially, we measured the interaction of wild-type (WT) αvβ3 in its low-affinity state with OPN. The majority of measured rupture forces were in the range of 0-40 pN with a cumulative probability of detecting meaningful rupture forces >10 pN of only 1.1±0.7%. However, when αvβ3 was shifted to its high-affinity state by 1 mM Mn2+, the range of rupture forces increased up to 100 pN with a peak in the force spectrum at 45-50 pN and a cumulative probability of detecting rupture forces >10 pN of 3.1±1.3%. We then scanned the β3 TM domain with leucine and alanine residues and found a periodic increase in the cumulative probability of constitutive OPN binding that was comparable to Mn2+-stimulated WT binding and with probability peaks at S699L, A703L, and I707L. To confirm the latter result, we replaced OPN with WOW-1. Again, there was a periodic increase in the cumulative probability of constitutive WOW-1 binding with peaks at S669L, V700A, A703L, I704L, I707L, and G708L. Thus, in contrast to αIIbβ3, these data indicate that the β3 TM domain associates with the αv TM domain via its S-x3-A motif. To investigate the structure of the αvβ3 TM complex, we built a molecular model for the αvβ3 TM and cytoplasmic domains from an αIIbβ3 NMR model by threading the aligned αv sequence onto αIIb. The model was then subjected to cycles of minimization and rotamer optimization using the Rosetta modeling suite. The resulting structures deviated from the TM domain interface used by αIIbβ3, suggesting it is not favorable for αvβ3. Next, to investigate the preferred interface of the αvβ3 TM helices, we used the lowest energy model to build an all-atom water-lipid-protein system and then conducted unbiased molecular dynamics simulations. For the majority of these simulations, the helices were tightly associated and maintained a relatively uniform conformation. While the dimer complex was stable, the orientation of the αv and β3 TM helices rapidly equilibrated to the small residue-X3-small residue heterodimer interface. No additional forces were applied to this system to bias the formation of helical geometries. Strikingly, the αvβ3 interface observed throughout the simulation correlated well with the helical face suggested by the optical force spectroscopy experiments. In conclusion, we found that compared to αIIbβ3, there is a clear shift in the helical register and periodicity of the αvβ3 TM heterodimer such that both the αv and β3 helices interacted via small residue-X3-small residue motifs. Subsequent molecular modeling and molecular dynamics simulations were consistent with the results of the scanning mutagenesis experiments. Thus, these studies provide insight into how unique but homologous integrins regulate signal activation across diverse biological processes. DisclosuresNo relevant conflicts of interest to declare.

Large eddy simulation of fully-developed flow in a helical segmented-fin tube bundle

The Large Eddy Simulation approach was used to perform a numerical simulation of a flow around a circular tube bundle. The model is based on numerical tools, such as the immersed boundary technique and the extrapolation of pressure in the solid region of the tube, to represent a helical segmented-fin tube geometry. The simulation was conducted in an area away from the boundaries of the tube bundle, where the flow is fully-developed, and the use of periodic boundary conditions was possible. The mean velocity and turbulence values of the results and experimental data were compared and showed good general agreement. From the instantaneous results, it was possible to observe and study different turbulent structures presented in the flow. Two counter-rotating horseshoe vortices were identified in the space between the fins, which can improve heat transfer. Furthermore, a relevant transfer of momentum in the inter-segment space and contiguous zones was observed.

THE EFFECT OF THE CIRCULAR FEED ON THE SURFACE ROUGHNESS AND THE MACHINING TIME

The surface roughness is analysed in different feeds and turning procedures (rotational and conventional) in this paper. Cutting experiments were made on different cutting speeds and feed rates with 2 cutting tool with helical edge geometry and 1 traditional turning tool. The measured 2D surface roughness values were compared between the different cutting tools. The benefit of the circular feed application is showed by the decrease of roughness parameters and machining time.

External-field-induced dynamics of a charged particle on a closed helix.

We investigate the dynamics of a charged particle confined to move on a toroidal helix while being driven by an external time-dependent electric field. The underlying phase space is analyzed for linearly and circularly polarized fields. For small driving amplitudes and a linearly polarized field, we find a split up of the chaotic part of the phase space, which prevents the particle from inverting its direction of motion. This allows for a nonzero average velocity of chaotic trajectories without breaking the well-known symmetries commonly responsible for directed transport. Within our chosen normalized units, the resulting average transport velocity is constant and does not change significantly with the driving amplitude. A very similar effect is found in case of the circularly polarized field and low driving amplitudes. Furthermore, when driving with a circularly polarized field, we unravel a second mechanism of the split up of the chaotic phase space region for very large driving amplitudes. There exists a wide range of parameter values for which trajectories may travel between the two chaotic regions by crossing a permeable cantorus. The limitations of these phenomena, as well as their implication on manipulating directed transport in helical geometries are discussed.

Enhancing the short-beam strength of composite laminates using helical carbon nanotubes

The main disadvantage of traditional composite laminated structures is the lack of reinforcement in out-of-plane direction in between the laminae and the fiber filaments. This will result in poor interlaminar properties leading to delamination and disintegration of the laminates, when subjected to out-of-plane external loads. Most prior research has been focused on use of straight carbon nanotubes (CNTs) and other nanoparticles as additional reinforcement in resin systems. Our prior research has shown that the helical CNTs are more effective as a nanoscale reinforcement, as compared to other nanomaterials. The goal of this research was to improve the interfacial bonding effectiveness between the laminae using the unique characteristics and coil-like geometry of the helical CNTs. They can wrap around each other, entangle with the micro-fiber reinforcements, and get stuck inside the solidified resin. Because of their inertness, CNTs interaction with the resin is generally weak and the helical geometries of CNTs can provide unique mechanical interlocking mechanisms within the composite material systems. This characteristic can substantially improve their interfacial and out-of-plane properties and introduce additional multifunctionalities. In this research, helical CNTs with various weight percentages were used as additional reinforcements in composite laminates. ASTM standard D2344/2344M − 16 was used to investigate the short-beam strength of the helical CNTs reinforced laminates. Test specimens were fabricated, cut, and tested as per ASTM standard and then analyzed. The comparison of test results showed up to 11.3%, 11.9%, and 6.9% improvements for short-beam strength, tangent modulus, and strain at failure, respectively, for the nanocomposite samples with 0.05 wt% HCNTs loading. These results prove that the use of helical CNTs as additional reinforcement is a promising approach for the improvement of the interlaminar properties of laminated composites and structures.

Black hole magnetic fields and their imprint on circular polarization images

ABSTRACT The circular polarization of black hole accretion flows can encode properties of the underlying magnetic field structure. Using general relativistic magnetohydrodynamic (GRMHD) simulations, we study the imprint of magnetic field geometry on circular polarization images potentially observable by the Event Horizon Telescope (EHT). We decompose images into the different mechanisms that generate circular polarization in these models that are sensitive to both the line-of-sight direction and twist of the magnetic field. In these models, a stable sign of the circular polarization over time, as observed for several sources, can be attributed to a stability of these properties. We illustrate how different aspects of a generic helical magnetic field geometry become imprinted on a circular polarization image. We also identify novel effects of light bending that affect the circular polarization image on event horizon scales. One consequence is the sign flipping of successive photon rings in face-on systems, which if observable and uncorrupted by Faraday rotation, can directly encode the handedness of the approaching magnetic field.

Thermohydraulic performance and second law analysis of a tube embedded with multiple helical tape inserts

ABSTRACT The present investigation deals with the analysis of the heat exchanger tube by inserting the multiple helical tape geometry to determine the heat transfer, friction factor, thermal performance, and entropy generation experimentally. The helical tapes designed for current investigation are analyzed in terms of geometrical parameters namely number of tape (1,2,3), twist ratio (2,3,4), and width ratio (0.4). The experiments are conducted in the range of Reynolds number lying in turbulent region from 5000 to 29000. The experimentation affirms an enhancement of the thermal performance. The experimental data reveal that there is 3.95 times enhancement in the heat transfer for twist ratio of 2 and number of tapes of 3. The thermal performance factor is enhanced by 1.23 times over smooth tube for the twist ratio of 2 and number of tapes of 1. It is observed that the maximum heat transfer enhancement is found for low and high values of twist ratio and number of tapes, respectively. The rate of entropy generation is less for the tube equipped with helical tape inserts for all possible cases than smooth tube. The minimum entropy generation value of 0.27 is observed for twist ratio of 2 and number of tapes 3. The developed empirical correlations for Nusselt number, friction factor, and thermal performance factor are in good agreement with experimental results.

Preliminary design of a helical coil steam generator mock-up for the CIRCE facility for the development of DEMO LiPb heat exchanger

In the framework of the EU DEMO fusion reactor technology development, a new steam generator consisting in a helical tube bundle is currently under study for the lithium-lead loop of the Dual Coolant Lithium Lead (DCLL) and Water Cooled Lithium Lead (WCLL) breeding blankets. This solution turns out to be very interesting for high power plants, since the helical geometry is very compact and it assures a high thermal power exchanged, taking up the minimum amount of space.In this framework, the ENEA Brasimone Research Centre supports the development of this innovative component by means of experimental activities, involving CIRCE, a large scale pool-type facility using Lead-Bismuth Eutectic as primary fluid and pressurized water as secondary fluid. The main components of a new test section named THETIS are: a Fuel Pin Simulator, acting as primary heating source, a vertical mechanical pump for the primary fluid circulation inside the main pool and a new prototypical helical coil steam generator mock-up, designed to be relevant for the DCLL and WCLL lithium-lead loop, acting as primary heat sink. In such configuration, the facility will be involved in a set of tests aiming at demonstrating the technological feasibility and thermal-hydraulic performances of this prototypical steam generator, as well as the suitability of the component for the lithium-lead loop in DEMO. The experiments will also provide a database for system thermal-hydraulic codes validation.The aim of this paper is to present the main layout of the CIRCE facility, to describe the preliminary design of the test section and the main features of the helical coil steam generator mock-up. Furthermore, a preliminary test analysis carried out by the system thermal hydraulic code RELAP5/Mod3.3 is presented. A numerical 1-D model of the helical coil steam generator has been set-up in order to test the performance of the component from a thermal-hydraulic point of view. An additional 3D CFD analysis was performed for the flow field of the HCSG to assess the secondary flow behaviour in the component and the pressure losses.