Variable Pitches Research Articles

Femtosecond Laser Nanomachining of High‐Aspect‐Ratio Channels in Bulk Fused Silica

AbstractThee‐dimensional nano‐structuring of bulk fused silica glass remains a challenging issue due to limitations of traditional fabrication methods. In the present work the fabrication of hollow mm‐length channels is demonstrated in bulk fused silica by femtosecond laser irradiation followed by chemical etching, achieving feature sizes of 300 nm. The tight focusing and threshold energy conditions allow to reveal unexplored fabrication regimes. The robustness and repeatability of the process are demonstrated through the fabrication of fused silica diffraction gratings, made of parallel hollow channels with variable pitches even in the sub‐wavelength regime.

Hybrid electromagnetic Halbach array for wireless power transfer (WPT) system using variable and uniform pitch coils

AbstractThe electromagnetic Halbach array (EHA) for wireless power transfer (WPT), designed to improve the transferred power and power transfer distance, has been proven to be effective and can be applied in various consumer applications. This paper presents a hybrid form of the EHA, aimed at improving the quality of power generated and transmitted. The expression for the determination of the magnetic field of a variable pitch flat coil for wireless power transfer and other uses has been derived. The average magnetic field, magnetic flux, and power transfer efficiency are investigated. The power transfer efficiency and the average magnetic field generated by four different design considerations were compared. The hybrid electromagnetic Halbach array wireless power transfer system (HEHA‐WPTS) has also been studied and results are compared with the conventional arrangements. Simulations and experiments were carried out to verify the analytical results obtained. According to the results obtained, the HEHA‐WPTS, with a base coil with variable pitches and uniform side coils, produces a more uniform average magnetic field and a better average power transfer efficiency over a longer power transfer distance.

Enhancing recuperators performance in supercritical CO2 Brayton cycle of solar power plants through ribbed heat transfer plates

Sixteen diverse combinations of ribbed heat transfer plates (HTPs), comprising continuous and discontinuous rib configurations, are investigated as potential replacements for traditional smooth HTPs in both the low-temperature recuperator (LTR) and high-temperature recuperator (HTR) of supercritical CO2 Brayton cycles. The findings indicate that the impacts of the introduced HTPs are more significant under the operating conditions of the LTR than those of the HTR. Generally, the incorporation of denser ribs at the upstream sections of both the hot and cold flow paths leads to a significant enhancement in j-factor values. On the hot side of the LTR, these increases exceed 60%, while on the cold side, they surpass 40%. It is also observed that variations in rib pitch exert a more substantial influence on thermal performance compared to alterations in rib length. When comparing non-uniform rib patterns, it is found that the use of ribs with variable lengths increases f-factor values, whereas the design of HTPs with variable rib pitches decreases these values. The design of HTPs with a non-uniform arrangement of rib pitch yields the best overall performance in the LTR. The performance index values for the hot-side and cold-side of this model reach 1.39 and 1.26, respectively.

A Design Methodology for Irregularly Shaped Windings in Inductive Wireless Power Transfer Systems

Inductive wireless power transfer systems often incorporate unconventional, irregularly shaped transmitter windings for the purposes of covering a designated area, fitting into special enclosures and enhancing the tolerance of misalignment. To design and optimise the winding structures, the inductive parameters must be extracted and linked to the design objectives. Conventionally, these parameters can be extracted using three-dimensional finite element analysis, which often requires subjective manual tweaks and prolonged trial and error procedures. The efficacy is therefore greatly dependent on the experience of the designer. In this paper, a case study for modelling and optimising the spatial coverage by scuplturing the winding shape is demonstrated via a Christmas tree model, utilising the parametric formation equations and line-integral based numerical solvers. A cone-shaped winding with variable interturn pitches was used as the transmitter and a receiver winding was designed to be fit into a bubble that can be hung on the tree. A two-stage optimisation method with simplified degree-of-freedom parameters and brutal force search was used to find the optimal design candidate. Heatmaps of receiver output voltages were generated in a time-efficient way, intuitively helping the designers to make adjustments for the winding structures. A practical prototype was built to verify the open loop voltage distribution on the receiving winding at various positions and another demonstration was made to show the continuum of power coverage around the Christmas tree.

Experimental and numerical studies of air flow and heat transfer due to insertion of novel delta-winglet tapes in a heated channel

The use of vortex-generators is one of the most effective ways to enhance the thermal performance of heat exchangers. The vortex-generators are commonly designed and made in two forms: wings and winglets. In this work, an investigation is carried out on a heated channel equipped with the novel delta-winglet tapes (DWTs). Eight non-uniform arrangements of the delta-winglets with variable pitches and heights along the axial direction are proposed and compared with the original model as the uniform case. All cases are studied experimentally, and also numerical simulations are used to analyze the corresponding mechanisms. The thermal and hydrodynamic characteristics are reported using the dimensionless parameters of Nusselt number, friction factor, and overall performance index for the Reynolds number range between 1643 and 8215. The results show that equipping the channel with the non-uniform DWTs affects the performance of the channel, and in some cases, it leads to better thermal performance compared to the uniform tape. The range of the variations is between 0.88 and 1.14 times for the Nusselt number and between 0.95 and 1.49 times for the friction factor. In general, placing the delta-winglets with longer height or shorter pitches at the upstream of DWTs improves the channel's thermal performance. The maximum Nusselt number ratio of 1.62 is recorded for the DWT in which the height of delta-winglets is from High to Low with the uniform arrangement of pitches. On the other hand, the DWT with the uniform heights of delta-winglets and Low to High arrangement of pitches possess the best overall performance among the studied cases. The performance index for this model is in the range of 1.44 to 1.07.

Design and stiffness analysis of a pitch-varying folded flexure hinge (PFFH)

Based on the theory and characteristics of the coil spring, we proposed a pitch-varying folded flexure hinge (PFFH), which is formed by a series of folded flexure units of variable pitches. With the increase of deformation, different pitch segments are contacted in order to achieve the purpose of variable stiffness. After giving an example design of the hinge structure, a theoretical calculation model of its equivalent spring constant is then derived accordingly. The hinge's stiffness characteristics are discussed and analyzed through both theoretical calculation and finite element analysis (FEA). The results showed that the stiffness of the hinge is varying, and the theoretical calculations results are well-matched with the results of FEA. In addition, the influence of different pitches on the stress and the trend of its stress changes are also analyzed.

A Critical Reanalysis of Uncontrollable Washboarding Phenomenon in Metal Band Sawing.

The article analyzes the cutting process of hard bars. Investigations conducted in industrial conditions demonstrated the presence of surface errors in the machined workpieces in the form of washboard patterns. The purpose of this study was to analyze the results of cutting on band sawing machines with different band saw blades. The cutting processes were conducted on three different horizontal band sawing machine types. Analyzed material was an alloy steel Ø40 mm rod with a hardened surface covered with a thin layer of chromium. The hardness of the outer layer was 547 HV with a core hardness of 180 HV. The surface topography measurements of the processed workpieces were carried out with the 3D Optical Profiler, which supplied information on the irregularities of the processed material texture. In each of the analyzed cases, a corrugated surface was obtained after sawing, which is the effect of the occurrence of the washboarding phenomenon, despite the fact that the teeth of each band saw had variable pitches. The washboarding phenomenon when cutting rods with hard surfaces is caused by the phenomenon of wave regeneration. Despite the use of variable pitch saw blades, the cutting process results in rippling of the sawn surface, which is caused by the high hardness of the outer layer of the workpiece, as well as by the type of tool with spring setting of teeth.

Anisotropic Synthesis of Armchair Graphene Nanoribbon Arrays from Sub-5 nm Seeds at Variable Pitches on Germanium.

At widths below 10 nm, armchair graphene nanoribbons become semiconductors. One promising route to synthesize nanoribbons is chemical vapor deposition (CVD) of hydrocarbons on Ge(001), and synthesis from seeds reduces nanoribbon polydispersity. In this contribution, we advance the seed-initiated synthesis of nanoribbons and explore the impact of seed size and nanoribbon spacing on growth kinetics. Periodic arrays of graphene seeds are lithographically patterned and etched to reduce their diameter. The viability of initiating synthesis from sub-5 nm seeds is demonstrated, and the pitch between nanoribbons is reduced from 500 to 50 nm to show that crowding effects do not perturb nanoribbon growth kinetics. The invariance of kinetics with pitch in combination with density functional theory (DFT) calculations indicate that (1) the growth species for synthesis has a diffusion length of ≪50 nm and/or (2) the kinetics are strongly attachment-limited. These results demonstrate that seed-initiated synthesis on Ge(001) is a promising route for creating dense arrays of armchair graphene nanoribbons for semiconductor electronics applications.

Application of variable teeth pitch face mill as chatter suppression method for non-rigid technological system

The article describes the results of experimental studies on the effects of variation type for variable teeth pitches on low-rigidity workpiece chatter suppression efficiency in a feed direction and in a direction of the normal to the machined surface. Mill operation performance was identified by comparing the amplitudes of dominant chatter harmonics using constant and variable teeth pitches. The following variable pitch formation variants were studied: alternative, linear rising, and linear rising falling. The angle difference of adjacent teeth pitches ranged from 0 to 10°, from 5 to 8° and from 5 to 10° with interval of 1°. The experiments showed that for all variants, machining dynamics performance resulted from the difference of adjacent pitches corresponding to a half the chatter wavelength along the cutting surface. The alternative nature of a variable teeth pitch is most efficient as it almost completely suppresses the chatters. Theoretical explanations of the results are presented

Analytical Modeling, Design and Performance Evaluation of Chatter-Free Milling Cutter With Alternating Pitch Variations

Machining efficiency could be increased and chatter vibrations could be suppressed by employing irregular pitch angles onto milling cutters. Applying variable pitch cutters could improve machining stability by disrupting the regenerative chatter mechanism and reducing cutting forces due to decreased nonlinear dynamic chip thicknesses. Previous studies on variable pitch cutters have been mainly investigating vibration suppression and dynamic modeling. There is a lack of efficient and practical variable pitch cutter design methods for practical industrial applications. An analytical method for designing milling cutters with alternating variable pitches has been proposed in this paper. In this method, a new variable, the generic sum of phase difference between the inner and outer waves, has been defined to model the machining dynamics and the stability of the variable pitch cutter. The phase difference value has then been investigated to obtain its generic optimal solution to the stability limit considering different cutting conditions. Based on this, the analytical variable pitch cutter design method is derived. The experimental results have manifested that, compared with uniform pitch cutter, the variable pitch cutter, designed using the proposed method, was able to achieve: 1) a critical stable axial depth of cut improvement of 126% under the desired spindle speed; 2) a cutting force reduction of 53%; 3) a decrease of 75% and 52% in Sq (the root mean square length of the scale limited surface) and Sz (the maximum height of the scale limited surface) of the machined surfaces, respectively. These significant improvements approved the superiority and robustness of the proposed design method for chatter suppression and may promote the application of variable pitch cutter technologies.

Stretchable Persistent Spin Helices in GaAs Quantum Wells

The Rashba and Dresselhaus spin-orbit (SO) interactions in 2D electron gases act as effective magnetic fields with momentum-dependent directions, which cause spin decay as the spins undergo arbitrary precessions about these randomly-oriented SO fields due to momentum scattering. Theoretically and experimentally, it has been established that by fine-tuning the Rashba $\alpha$ and Dresselhaus $\beta$ couplings to equal $\it{fixed}$ strengths $\alpha=\beta$, the total SO field becomes unidirectional thus rendering the electron spins immune to dephasing due to momentum scattering. A robust persistent spin helix (PSH) has already been experimentally realized at this singular point $\alpha=\beta$. Here we employ the suppression of weak antilocalization as a sensitive detector for matched SO fields together with a technique that allows for independent electrical control over the SO couplings via top gate voltage $V_T$ and back gate voltage $V_B$. We demonstrate for the first time the gate control of $\beta$ and the $\it{continuous\,locking}$ of the SO fields at $\alpha=\beta$, i.e., we are able to vary both $\alpha$ and $\beta$ controllably and continuously with $V_T$ and $V_B$, while keeping them locked at equal strengths. This makes possible a new concept: "stretchable PSHs", i.e., helical spin patterns with continuously variable pitches $P$ over a wide parameter range. The extracted spin-diffusion lengths and decay times as a function of $\alpha/\beta$ show a significant enhancement near $\alpha/\beta=1$. Since within the continuous-locking regime quantum transport is diffusive (2D) for charge while ballistic (1D) for spin and thus amenable to coherent spin control, stretchable PSHs could provide the platform for the much heralded long-distance communication $\sim 8 - 25$ $\mu$m between solid-state spin qubits.

Enhanced pool boiling heat transfer on mini- and micro- structured surfaces

The surfaces used for investigating nucleate pool boiling for four working fluids had mini- and micro-fins of variable configurations, cross-sections and pitches, restrained by perforated foil or mesh cloth with various pore/opening diameters. Unique enhanced structures on these surfaces formed a system of interconnected horizontal and vertical tunnels. Four structured surfaces were proposed, each being a system of subsurface tunnels connected to 10 and 5 mm fins or 1 and 0.5 mm mini-fins. Measurement results for boiling water, ethanol, Fluorinert FC-72 and R-123 from more than 60 samples constituted the database used to verify the proposed theoretical models. These models were based on the results from the visualization studies, including internal visualization allowing observation of bubble nucleation, growth and displacement inside the tunnels, and on the analysis of existing boiling models for mini- and micro-structures.

Parameter Optimization Method of Screw Axis with Variable Diameters and Different Pitches Based on PSO-BP NN

Parameter optimization of screw axis with variable diameters and different pitches is very important to improve the performance of material conveying equipment. But it is a trouble thing with classic theory modeling because of the complexity of material fluidity and screw structure. A PSO-trained BP algorithm is applied to establish the screw axis parameter optimization neural network (NN) model.Taking asphalt transfer vehicle as an example, PSO-BP NN is applied to set up the relationship between input parameters and aim parameter. Practical example shows that the PSO-BP NN has faster convergence and higher computational precision than the other three investigated algorithms, and it provides a powerful parameter optimization approach for screw axis with variable diameters and different pitches.

Numerical investigations of the mechanical properties of a braided non-vascular stent design using finite element method

This paper discusses various issues relating to the mechanical properties of a braided non-vascular stent made of a Ni–Ti alloy. The design of the stent is a major factor which determines its reliability after implantation into a stenosed non-vascular cavity. This paper presents the effect of the main structural parameters on the mechanical properties of braided stents. A parametric analysis of a commercial stent model is developed using the commercial finite element code ANSYS. As a consequence of the analytical results that the pitch of wire has a greater effect than other structural parameters, a new design of a variable pitch stent is presented to improve mechanical properties of these braided stents. The effect of structural parameters on mechanical properties is compared for both stent models: constant and variable pitches. When the pitches of the left and right quarters of the stent are 50% larger and 100% larger than that of the central portion, respectively, the radial stiffness in the central portion increases by 10% and 38.8%, while the radial stiffness at the end portions decreases by 128% and 164.7%, the axial elongation by 25.6% and 56.6% and the bending deflection by 3.96% and 10.15%. It has been demonstrated by finite element analysis that the variable pitch stent can better meet the clinical requirements.

Anterior screw fixation for an odontoid fracture using an Acutrak 4/5 screw: a case report

The direct anterior screw fixation of odontoid fractures by a single cancellous screw, especially for osteoporotic vertebrae, has a potential risk of leading to insufficient stability and implant failures. We experienced good results following surgery using a single Acutrak 4/5 screw to obtain sufficient stability for an odontoid fracture in a patient with osteopenia. The screw is a cannulated self-tapping headless screw and has a tapered profile and full threads with variable pitches, and it can yield sufficient compression force as the screw is inserted. The preoperative severe neck pain of the patient was diminished immediately after the surgery. The patient achieved bone union in a short time and had a good clinical result for at least 3 years. Some biomechanical studies showed that the compression force of Acutrak standard screws was stronger than that of both 4.0-mm cancellous screws inserted with the lag screw technique and Herbert screws, and other studies showed that the compression force of Acutrak 4/5 screws was equivalent to that of 4.5-mm cortical screws. However, there has been no clinical report of surgery using an Acutrak 4/5 screw for odontoid fractures. This is the first clinical report of fixation by an Acutrak 4/5 screw.

The System Design and Parameterized Modeling of Juice Extractor Screw

The ordinary screw designed in traditional method will increase the power consumption and decrease the juice yield in the process of squeeze. Presented the system design of juice extractor firstly, then used the UG software and dimension driving technology to construct the 3d model entity of the screw with variable pitches and diameters, followed by systems three-dimensional model. Finished the static analysis of the screw blade by ANSYS, completing a follow-up study of parameterized modeling.

Study on the Interaction Mechanism between Screw Conveyor and Asphalt Mixture

From the aspect of mechanics characteristics and the tumbling mechanism of the transportation process, the force, the acceleration, the friction and the speed characteristics of asphalt mixture flow were analyzed by using theoretical mechanics and hydromechanics. The influence of the working speed and diameter-to-pitch ratio of the screw conveyor and mixing effect of the material was researched; the relation between the working speed of screw conveyor and the material conveying performance was discussed under the condition of ensuring even flow rate of material. The segregation technical reasons of asphalt mixture and improvement measures were derived. In the experiment, the full-scale test was conducted by using the AC25 mixture. And through changing the using parameters compare the paving operations, the reasonable parameter will greatly improve the uniformity of paving, and the surface proportion of structural depth of mixture decrease at 2.13-1.60. The design method of the screw conveyor with variable diameters and variable pitches is proposed, and the parametric design model of the screw conveyor with variable diameters and variable pitches is established, and which provides a theoretical basis for the structure design of screw shaft with variable diameter and variable pitch.

Geometrical frustration of chiral ordering in cholesteric droplets

Frustration of chiral ordering is explored in cholesteric liquid crystal droplets with planar degenerate anchoring using numerical modeling. Droplets of variable pitches are studied, demonstrating the role of a gradually increasing cholesteric pitch and the corresponding equilibrium structures. All previously known structures are identified but with notable differences. The structures presented with director fields are complemented with a detailed description of the defect regions. The characteristic half-diameter +2 disclination from previous studies is found to be in fact a double-helix of two λ+1 disclination lines, whereas the full-diameter +1 disclination is composed of an alternating series of τ−1/2 and λ+1/2 disclination rings. Finally, two new meta-stable cholesteric structures -Lyre and Yeti- are found, which are characterised by complex compositions of cholesteric disclinations.

Experimental investigation of bolted cold formed steel frame apex connections under pure moment

An extensive experimental investigation on bolted moment connections between cold formed steel sections was carried out. The study was performed on portal frame rafter with double C cold formed sections. The investigation focused on the apex connection. A total of 10 rafter sections as part of portal frames were used in the study. The study aimed to investigate the effect of different factors on both the frame capacity and connection failure mode. The effects of variable bolt pitches, thickness of gusset plates, effect of bearing or hole deformation around the bolt were investigated. A parallel theoretical study was carried out using the same boundary conditions and the results were compared with the experimental ones. The compatibility between the results was good. The flexural failure of the connected sections was always the critical.

Concept of minimal heart rate for each pitch value to avoid interpolation artifact when using dual-source CT: a phantom study

Interpolation artifact is known to occur when the heart rate is decreased lower than the critical value for the specific pitch. The purpose of our study is to determine the minimum heart rate (minHR) for the specific pitch that provides images without interpolation artifact when using dual-source computed tomography (DSCT). We scanned the 'thin slice thickness block' of the CT performance phantom provided by the American Association of Physicists in Medicine using DSCT for variable pitches. Change in heart rate was simulated through ECG editing by changing R-R interval. Axial, sagittal, and coronal image sets were reconstructed and assessed for the presence and extent of interpolation artifact. MinHR at which no interpolation artifact was detected for each pitch value was determined. Length of interpolation artifact (LOA) on sagittal view was also measured when the heart rate was simulated at 10 bpm lower than the minHR on each pitch setting. MinHRs for each pitch value were 9-10 bpm from the estimated heart rate. However, minHR for the lowest pitch value 0.2, estimated heart value 40 bpm was 37 bpm. LOA was larger in the low heart rate condition. Measured values of minHR were correlated exactly with the calculated values. MinHRs that provide images without interpolation artifact for each pitch value when using DSCT were determined. The concept of minHR is important for obtaining high quality images of cardiac CT angiography when using DSCT.