Helical Angle Research Articles

Quantifying the ultrastructure of carotid arteries using high-resolution micro-diffusion tensor imaging—comparison of intact versus open cut tissue

Diffusion magnetic resonance imaging (dMRI) can provide insights into the microstructure of intact arterial tissue. The current study employed high magnetic field MRI to obtain ultra-high resolution dMRI at an isotropic voxel resolution of 117 µm3 in less than 2 h of scan time. A parameter selective single shell (128 directions) diffusion-encoding scheme based on Stejskel-Tanner sequence with echo-planar imaging (EPI) readout was used. EPI segmentation was used to reduce the echo time (TE) and to minimise the susceptibility-induced artefacts. The study utilised the dMRI analysis with diffusion tensor imaging (DTI) framework to investigate structural heterogeneity in intact arterial tissue and to quantify variations in tissue composition when the tissue is cut open and flattened. For intact arterial samples, the region of interest base comparison showed significant differences in fractional anisotropy and mean diffusivity across the media layer (p < 0.05). For open cut flat samples, DTI based directionally invariant indices did not show significant differences across the media layer. For intact samples, fibre tractography based indices such as calculated helical angle and fibre dispersion showed near circumferential alignment and a high degree of fibre dispersion, respectively. This study demonstrates the feasibility of fast dMRI acquisition with ultra-high spatial and angular resolution at 7 T. Using the optimised sequence parameters, this study shows that DTI based markers are sensitive to local structural changes in intact arterial tissue samples and these markers may have clinical relevance in the diagnosis of atherosclerosis and aneurysm.

Application of response surface method and multi-objective genetic algorithm to configuration optimization of Shell-and-tube heat exchanger with fold helical baffles

A kind of shell-and-tube heat exchangers with fold baffles was proposed to eliminate the triangular leakage zones between adjacent baffles. An effective algorithm combing second-order polynomial response surface method and multi-objective genetic algorithm was adopted to study the effect of fold baffle configuration parameters on the performance of flow and heat transfer. The helical angle, overlapped degree and shell-side inlet velocity were chosen as design parameters, and the Nusselt number and shell-side pressure drop were considered as objective functions. The results show that both the Nusselt number and shell-side pressure drop increase with the decrease of helical angle and shell-side inlet velocity, and increase with the increasing overlapped degree. A set of Pareto-optimal points were obtained, and the optimization results illustrate a good agreement with CFD simulation data with the relative deviation less than ±3%. And the empirical correlations of Nusselt number and friction coefficient were obtained based on response surface method, the helical angle and overlapped degree were fitted into empirical correlations as correction factors for the first time. It is found that the adjusted coefficient of determination of the Nusselt number and friction coefficient is 0.943 and 0.999, respectively, which illustrate the fitting is correct and reliable.

Relationship between streamwise and azimuthal length scales in a turbulent pipe flow

The statistical relationships among the turbulence structures of the streamwise velocity fluctuations along the streamwise and azimuthal directions in a turbulent pipe flow were examined using direct numerical simulation data at Reτ = 3008. Two-point correlations of the streamwise velocity fluctuations showed a linear relationship between the streamwise and azimuthal length scales (lx and lθ), where lθ/lx = 0.07 along the wall-normal distance, indicating the long coherent structures called very-large-scale motions (VLSMs). The one-dimensional pre-multiplied energy spectra of the streamwise velocity fluctuations showed that the streamwise and the azimuthal wavelengths (λx and λθ) grew linearly along the wall-normal distance, λx/y = 20 and λθ/y = 7, respectively. The ratio between the two linear relationships was determined to be λθ/λx = 0.35, indicative of large-scale motions (LSMs). The energetic modes obtained from a proper orthogonal decomposition (POD) analysis using the translational invariance method showed that the averaged helical angles of the wall mode (ix &amp;lt; iθ; β &amp;lt; 0.1 rad, where ix and iθ are the streamwise and azimuthal mode numbers and β is the helical angle) and lift mode (ix ≥ iθ; β ≥ 0.1 rad) were related to lθ/lx = 0.07 (VLSMs) and λθ/λx ≈ 0.35 (LSMs), respectively. The superposition of the energetic POD modes showed the superimposed X-shaped patterns. The helical angle of the wall mode in the near-wall region was similar to that in the outer region, implying the existence of the VLSMs in the entire wall-normal distance. The LSMs showed more inclined X-shaped patterns. The LSMs were concatenated with the azimuthal offsets to form meandering VLSMs. Most of the VLSMs and LSMs in the near-wall region inclined smaller and larger than 10° (0.17 rad), respectively. In the core region, VLSMs were distributed more helically along the azimuthal direction due to the space limitations of the pipe geometry.

Study on the criterion to determine the bottom deployment modes of a coilable mast

A practical design criterion that allows the coilable mast bottom to deploy in local coil mode was proposed. The criterion was defined with initial bottom helical angle and obtained by bottom deformation analyses. Discretizing the longerons into short rods, analyses were conducted based on the cylinder assumption and Kirchhoff's kinetic analogy theory. Then, iterative calculations aiming at the bottom four rods were carried out. A critical bottom helical angle was obtained while the angle changing rate equaled to zero. The critical value was defined as a criterion for judgement of bottom deployment mode. Subsequently, micro-gravity deployment tests were carried out and bottom deployment simulations based on finite element method were developed. Through comparisons of bottom helical angles in critical state, the proposed criterion was evaluated and modified, that is, an initial bottom helical angle less than critical value with a design margin of −13.7% could ensure the mast bottom deploying in local coil mode, and further determine a successful local coil deployment of entire coilable mast.

Coupling between myosin head conformation and the thick filament backbone structure.

The recent high-resolution structure of the thick filament from Lethocerus asynchronous flight muscle shows aspects of thick filament structure never before revealed that may shed some light on how striated muscles function. The phenomenon of stretch activation underlies the function of asynchronous flight muscle. It is most highly developed in flight muscle, but is also observed in other striated muscles such as cardiac muscle. Although stretch activation is likely to be complex, involving more than a single structural aspect of striated muscle, the thick filament itself, would be a prime site for regulatory function because it must bear all of the tension produced by both its associated myosin motors and any externally applied force. Here we show the first structural evidence that the arrangement of myosin heads within the interacting heads motif is coupled to the structure of the thick filament backbone. We find that a change in helical angle of 0.16° disorders the blocked head preferentially within the Lethocerus interacting heads motif. This observation suggests a mechanism for how tension affects the dynamics of the myosin heads leading to a detailed hypothesis for stretch activation and shortening deactivation, in which the blocked head preferentially binds the thin filament followed by the free head when force production occurs.

Study on a novel screw rotor with variable cross-section profiles for twin-screw vacuum pumps

In order to improve the performance of screw rotors, a novel screw rotor with variable cross-section profiles, in which the screw pitch and cross-section profile varying with the helical angle, is proposed for twin-screw vacuum pumps in this paper. The screw pitch of the proposed screw rotor decreases, on the contrary, its screw width increases continuously from the suction side to the discharge side. Equations of its cross-section profile cluster varying with two variables, the angular parameter and helical angle, are derived. The proposed screw rotor is compared with the existing variable pitch screw rotor. The study results show that the proposed screw rotor can realize completely correct meshing, and has better comprehensive performance in terms of the built-in volume ratio, gas sealing and mechanical property. Thus, the screw rotor designed and the data obtained can be applied to the design and optimization of twin-screw vacuum pumps.

On The Temporal and Spectral Characteristics of Micro-Milling Dynamics

Due to different chip formation mechanisms, increased tool-radius to feed-rate ratio, and higher spindle speeds, micro-milling is a highly nonlinear process which can produce multiple and broadband frequencies that negatively impact the process. Micro-milling is investigated through the development and analysis of a nonlinear micromilling dynamic model. A lumped mass-spring-damper system is assumed for modeling the dynamic properties of the tool. The force mechanism utilized is a slip-line field model that provides the advantage of being highly dynamic by accounting for the constantly changing effective rake angle and slip-line variables. Accurate prediction of the chip thickness is important in correctly predicting the dynamics of the system since the force mechanism and its variables are a function of the chip thickness. A novel approach for calculating the instantaneous chip thickness which accounts for the tool jumping out of the cut and elastic recovery of the workpiece is presented. The derivation for the effective rake angle is given and the helical angle is accounted for resulting in a three dimensional micro-milling model. The model generates the high frequency force components that are seen in experimental data available in literature. The effect that the helical angle and system stiffness has on the resulting cutting forces is also investigated. It is shown that dynamic instability has the greatest impact on tool performance and improving the dynamic response is a necessity for achieving high speed ultra-stable micro-machining.

The end of the unique myocardial band: Part I. Anatomical considerations.

The concept of the 'unique myocardial band', which proposes that the ventricular myocardial cone is arranged like skeletal muscle, provides an attractive framework for understanding haemodynamics. The original idea was developed by Francisco Torrent-Guasp. Using boiled hearts and blunt dissection, Torrent-Guasp created a single band of ventricular myocardium extending from the pulmonary trunk to the aortic root, with the band thus constructed encircling both ventricular cavities. Cooked hearts can, however, be dissected in many ways. In this review, we show that the band does not exist as an anatomical entity with defined borders. On the contrary, the ventricular cardiomyocytes are aggregated end to end and by their branching produce an intricate meshwork. Across the thickness of the left ventricular wall, the chains of cardiomyocytes exhibit a gradually changing helical angle, with a circumferential zone formed in the middle. There is no abrupt change in helical angle, as could be expected if the wall was constructed of opposing limbs of a single wrapped band, nor does the long axis of the cardiomyocytes consistently match with the long axis of the unique myocardial band. There are, furthermore, no connective tissue structures that could be considered to demarcate its purported boundaries. The unique myocardial band should be consistent with evolution, and although the ventricular wall of fish and reptiles has one or several distinct layers, a single band is not found. In 1965, Lev and Simpkins cautioned that the ventricular muscle mass of a cooked heart can be dissected almost at the whim of the anatomist. We suggest that the unique myocardial band should have ended there.

Numerical investigation of heat transfer and pressure drop of heat transfer oil in smooth and micro-finned tubes

The laminar, non-isothermal flow of the heat transfer oil in horizontal smooth and micro-finned tubes with different fin heights and helical angles are investigated numerically for the constant wall temperature boundary condition. Simulations have been performed for the Reynolds number ranging from 100 to 1000, the fin height varying from 0.2 to 0.5 mm, and the fin helix angle between 5 and 45°. The results are presented in the form of streamlines, temperature contours, relative Nusselt number, and friction factor. The numerical procedure is validated by comparing the simulation results for the flow and heat transfer of the oil through the micro-finned and smooth tubes with the corresponding experimental results from the literature. The comparisons demonstrate good agreements between the simulation and the experimental results for both the smooth and micro-finned tubes. The results indicate maximum heat transfer enhancement of 44 percent and friction factor increase of 69 percent for the flow of heat transfer oil through the micro-finned tubes in comparison with heat transfer and friction factor of the fluid flow through the corresponding smooth tube at the Reynolds number of 103.

Simulation of TE6,2-to-Gaussian mode launcher for a millimeter-wave gyrotron

The short-length TE6,2-to-Gaussian mode launcher for a millimeter-wave transverse output gyrotron was designed and simulated using two different software tools, namely Launcher Optimization Tool (LOT) and Surf3d. The geometrical parameters were calculated using geometric optics theory and the dimensions were optimized using the tools: LOT and Surf3d to achieve maximum possible (>95%) energy conversion and Gaussian fit. The launcher length to cut length ratio has been optimized as 4.15, taper angle 0.23° and the helical cut angle 70.6° for achieving energy conversion of 96 and 97%, and Gaussian fit of 98 and 97%, respectively, using LOT and Surf3d. Further, the radial variations in azimuth as well as in axial direction due to dimpled profiles obtained using the two tools were compared.

Helicity analysis of single, double, and triple helical iodine chains inside single-walled silicon carbide nanotubes

The helicity of encapsulated single, double, and triple helical iodine chains inside single-walled silicon carbide nanotubes is studied using the van der Waals interaction potential and density functional theory (DFT). Our results show that the optimal radius of the helical iodine chain increases linearly with tube radius, which produces a constant separation between the encapsulated chain and tube wall. The optimal helical angle cannot be determined with a single helical chain, due to the absence of the inter-chain interaction. For the double and triple helical chains, a small optimal helical angle can be induced by a large inter-chain interaction for the same tubes or a large tube for the same chain structure. We also find that the helicity is insensitive to the tube chirality. The DFT calculation further justifies our classical force field analysis and provides a more comprehensive understanding of this helical chains configuration.

A hybrid deterministic-probabilistic approach to model the mechanical response of helically arranged hierarchical strands

Very recently, a Weibull-based probabilistic strategy has been successfully applied to bundles of wires to determine their overall stress-strain behaviour, also capturing previously unpredicted nonlinear and post-elastic features of hierarchical strands. This approach is based on the so-called “Equal Load Sharing (ELS)” hypothesis by virtue of which, when a wire breaks, the load acting on the strand is homogeneously redistributed among the surviving wires. Despite the overall effectiveness of the method, some discrepancies between theoretical predictions and in silico Finite Element-based simulations or experimental findings might arise when more complex structures are analysed, e.g. helically arranged bundles. To overcome these limitations, an enhanced hybrid approach is proposed in which the probability of rupture is combined with a deterministic mechanical model of a strand constituted by helically-arranged and hierarchically-organized wires. The analytical model is validated comparing its predictions with both Finite Element simulations and experimental tests. The results show that generalized stress-strain responses - incorporating tension/torsion coupling - are naturally found and, once one or more elements break, the competition between geometry and mechanics of the strand microstructure, i.e. the different cross sections and helical angles of the wires in the different hierarchical levels of the strand, determines the no longer homogeneous stress redistribution among the surviving wires whose fate is hence governed by a “Hierarchical Load Sharing” criterion.

Configuration optimization of shell-and-tube heat exchangers with helical baffles using multi-objective genetic algorithm based on fluid-structure interaction

The configuration parameters of helical angle and overlapped degree of shell-and-tube heat exchangers with helical baffles have been discussed for the thermal-structural comprehensive performance. Based on fluid-structure interaction theory, a method on configuration optimization of shell-and-tube heat exchangers with helical baffles is introduced using second-order polynomial regression response surface combined with Multi-objective Genetic Algorithm. The results show that the heat transfer coefficient per unit pressure drop of shell-and-tube heat exchangers with helical baffles increases firstly and then decreases with the increase of helical angle, and decreases with the increase of overlapped degree under certain shell-inlet velocity. And the performance of flow and heat transfer is more sensitive to helical angle compared with overlapped degree. The maximum shear stress increases with helical angle, but it is almost unaffected by overlapped degree for mechanical properties of helical baffles. The objectives of optimization are the heat transfer coefficient per unit pressure drop maximizing and maximum shear stress minimizing with scope of allowable stress, and three optimal structures are obtained. The optimal results indicate that the heat transfer coefficient per unit pressure drop increases averagely by 14.1%, the maximum shear stress decreases averagely by 4.1%, which provides theoretical guidance for industrial design of shell-and-tube heat exchangers with helical baffles.

Boundary plasma response in RFX-mod to 3D magnetic field perturbations

The edge of the RFX-mod (R=2m, a=0.46m) Reversed Field Pinch device is characterized by weak magnetic chaos affecting ion and electron diffusion. Edge particle transport is strongly influenced by a toroidal and poloidal asymmetry caused by magnetic islands and an ambipolar radial electric field ensures local neutrality. At higher plasma current (Iϕ>1MA) a spontaneous resonant dominant mode m/n=1/7, slowly rotating, develops in the inner region. The edge electron pressure Pe and floating potential Vf show a shape of the Plasma Wall Interaction (PWI) with the same toroidal periodicity, which follows the edge local ideal magnetic displacement Δ1,7.Detailed measurements along the poloidal direction of Pe and Vf have been undertaken newly, their time behavior present respect to Δ1,7 a time lag, which depends on the poloidal angle θ. The mode analysis in terms of helical angle reveals a role of the 0/7 mode in determining a poloidal phase lag respect to Δ1,7 of Pe and Vf. Since the 0/7 is the largest toroidal sideband of the 1/7 mode, this work suggests a role of toroidal coupling in determining the plasma response to a MP.

Numerical simulation of R410A condensation in horizontal microfin tubes

ABSTRACTThe heat transfer characteristics of condensation for R410A inside horizontal microfin tubes with 0° and 18° helical angles were investigated numerically. The numerical data fit well with the experimental results and with the empirical correlations. The results indicate that local heat transfer coefficients increase with increasing mass flux, vapor quality, and helical angle. The heat transfer enhancement in the helical microfin tubes is more pronounced at higher mass flux and vapor quality. The centrifugal force induced by the microfin with a 18° helical angle tends to spread the liquid from the bottom to the top, leading to a nearly symmetrical liquid–vapor interface during condensation. Swirling flows in the liquid phase are observed in the tube with the 18° helical angle, but the liquid phase tends to flow to the bottom due to gravity in the tube with the 0° helical angle.

Partial phase transition and quantum effects in helimagnetic films under an applied magnetic field

We study the phase transition in a helimagnetic film with Heisenberg spins under an applied magnetic field in the c direction perpendicular to the film. The helical structure is due to the antiferromagnetic interaction between next-nearest neighbors in the c direction. Helimagnetic films in zero field are known to have a strong modification of the in-plane helical angle near the film surfaces. We show that spins react to a moderate applied magnetic field by creating a particular spin configuration along the c axis. With increasing temperature (T), using Monte Carlo simulations we show that the system undergoes a phase transition triggered by the destruction of the ordering of a number of layers. This partial phase transition is shown to be intimately related to the ground-state spin structure. We show why some layers undergo a phase transition while others do not. The Green's function method for non collinear magnets is also carried out to investigate effects of quantum fluctuations. Non-uniform zero-point spin contractions and a crossover of layer magnetizations at low T are shown and discussed.

Voltage-induced torsion of a fiber-reinforced tubular dielectric elastomer actuator

A fiber-reinforced torsional dielectric elastomer (DE) actuator was conceived by embedding one family of helical fibers into a DE tube in the present work. Due to the constraint of the fibers, the application of an electrical voltage will induce torsional deformation which is coupled with the longitudinal and hoop stretches of the actuator. By employing an energy method, the voltage-induced torsion and snap-through instability of the DE tubular actuator were modeled and analyzed. Based on the formulation, the effects of the fiber stiffness and helical angle as well as externally applied mechanical loads on the voltage-induced torsional deformation were studied in detail. It was found that when snap-through instability occurs, the voltage-induced twist angle jumps from a smaller value in the unbulged state to a larger value in the budged state, and the voltage-induced twist angle can be effectively tuned by varying the fiber stiffness and helical angle. Moreover, the voltage-induced torsion of the actuator in the budged state is almost not affected by externally applied axial force or torque. The revealed results are expected to provide guide for the rational design and utilization of fiber-reinforced torsional DE actuators.

Modeling and Optimization of Shell and Tube Heat Exchanger in Treatment Unit of South Pars Fifth Refinery by Fluent Software

In this thesis, the numerical solution of navier stockes equations and turbulent equations have also been investigated in fluent software, to investigate baffle type change from cut off to helical in shell and tube heat exchanger. RNG K-e turbulence model was used to perturbations modelling. The main objective of baffle changing in thesis is increasing propane temperature in pipes outlet. 4 heat exchanger type with different helical angles (35, 40, 45 and 50) compared with simple baffle type. Studies indicate that heat exchanger with helical angle of 50°, maximum outlet temperature of propane will result and have the maximum heat transfer rate in shell. Exchangers with helical angle of 40 degrees have the highest ratio h / Δp, which reflects the heat transfer rate to pressure drop ratio.

Uncoiling of helical boron nitride-graphene nanoribbons in a single-walled carbon nanotube.

Molecular dynamics simulation has been employed to study the encapsulation of boron nitride-graphene nanoribbons (BNCNRs) in a single-walled carbon nanotube (SWNT). The simulation results show that a helical BNCNR with large curvature can uncoil repeatedly and spontaneously in the SWNT, like the unwinding of the DNA in the nucleus. The uncoiling of the BNCNRs is accompanied by a system energy exchange between non-bonding energy and elastic potential energy due to the competition between the induction of graphene nanoribbon (GNR) segments and the resistance of boron nitride nanoribbon (BNNR) segments. The electronic transmission capacity of the BNCNR changes with the helical angle of the BNCNR, suggesting a changing electrical signal in the uncoiling and spiraling process. This study provides the opportunity to understand the encapsulation of the BNCNR in the SWNT in detail.

Changes in overall ventricular myocardial architecture in the setting of a porcine animal model of right ventricular dilation

BackgroundChronic pulmonary regurgitation often leads to myocardial dysfunction and heart failure. It is not fully known why secondary hypertrophy cannot fully protect against the increase in wall stress brought about by the increased end-diastolic volume in ventricular dilation. It has been assumed that mural architecture is not deranged in this situation, but we hypothesised that there might be a change in the pattern of orientation of the aggregations of cardiomyocytes, which would contribute to contractile impairment.MethodsWe created pulmonary valvular regurgitation by open chest, surgical suturing of its leaflets in seven piglets, performing sham operations in seven control animals. Using cardiovascular magnetic resonance imaging after 12 weeks of recovery, we demonstrated significantly increased right ventricular volumes in the test group. After sacrifice, diffusion tensor imaging of their hearts permitted measurement of the orientation of the cardiomyocytes.ResultsThe helical angles in the right ventricle approached a more circumferential orientation in the setting of right ventricular RV dilation (p = 0.007), with an increased proportion of surface-parallel cardiomyocytes. In contrast, this proportion decreased in the left ventricle. Also in the left ventricle a higher proportion of E3 angles with a value around zero was found, and conversely a lower proportion of angles was found with a numerical higher value. In the dilated right ventricle the proportion of E3 angles around −90° is increased, while the proportion around 90° is decreased.ConclusionContrary to traditional views, there is a change in the orientation of both the left ventricular and right ventricular cardiomyocytes subsequent to right ventricular dilation. This will change their direction of contraction and hinder the achievement of normalisation of cardiomyocytic strain, affecting overall contractility. We suggest that the aetiology of the cardiac failure induced by right vetricular dilation may be partly explained by morphological changes in the myocardium itself.