Small Pitch Angles Research Articles

Adiabatic Heating of Electrons in the Magnetospheric Current Sheet

Electron dynamics and acceleration in an electromagnetic field configuration modeling the current sheet configuration of the Earth’s magnetotail region is investigated. A focus is made on the role of the dawn−dusk magnetic field component By in the convection electron heating by an electric field Ey. For numerical integration of a large number of test particle trajectories over long time intervals, the equations of motion written in the guiding center approximation are used. It is shown that the presence of a By ≠ 0 magnetic field significantly changes the electron heating and allows electrons with small pitch angles to gain energy much more efficiently than the equatorial electrons. As a result, the convection heating in the current sheet with By ≠ 0 leads to the formation of an accelerated anisotropic population of particles with energies higher than a few hundred electronvolts. The obtained results and spacecraft observations in the Earth’s magnetotail are compared, and possible limitations in the proposed model approaches are discussed.

Molecular dynamics study of wettability and pitch effects on maximum critical heat flux in evaporation and pool boiling heat transfer

ABSTRACTMolecular dynamics simulations were employed to investigate the effects of wettability (contact angle) and pitch on nanoscale evaporation and pool boiling heat transfer of a liquid argon thin film on a horizontal copper substrate topped with cubic nano-pillars. The liquid–solid potential was incrementally altered to vary the contact angle between hydrophilic (∼0°) and hydrophobic (∼127°), and the pitch (distance between nano-pillars) was varied between 21.7 and 106.6 Å to observe the resultant effect on boiling heat transfer enhancement. For each contact angle, the superheat was gradually increased to initiate nucleate boiling and eventually pass the critical heat flux (CHF) into the film boiling regime. The CHF increases with pitch, and tends to decrease substantially with increasing contact angle. A maximum overall heat flux of 1.59 × 108 W/m2 occurs at the largest pitch investigated (106.6 Å), and as the contact angle increases the superheat required to reach the CHF condition also increases. Finally, in certain cases of small pitch and large contact angle, the liquid film was seen to transition to a Cassie–Baxter state, which greatly hindered heat transfer.

On the synchrotron emission in kinetic simulations of runaway electrons in magnetic confinement fusion plasmas

Developing avoidance or mitigation strategies of runaway electrons (REs) in magnetic confinement fusion (MCF) plasmas is of crucial importance for the safe operation of ITER. In order to develop these strategies, an accurate diagnostic capability that allows good estimates of the RE distribution function in these plasmas is needed. Synchrotron radiation (SR) of RE in MCF, besides of being one of the main damping mechanisms for RE in the high energy relativistic regime, is routinely used in current MCF experiments to infer the parameters of RE energy and pitch angle distribution functions. In the present paper we address the long standing question about what are the relationships between different REs distribution functions and their corresponding synchrotron emission simultaneously including: full-orbit effects, information of the spectral and angular distribution of SR of each electron, and basic geometric optics of a camera. We study the spatial distribution of the SR on the poloidal plane, and the statistical properties of the expected value of the synchrotron spectra of REs. We observe a strong dependence of the synchrotron emission measured by the camera on the pitch angle distribution of runaways, namely we find that crescent shapes of the spatial distribution of the SR as measured by the camera relate to RE distributions with small pitch angles, while ellipse shapes relate to distributions of runaways with larger the pitch angles. A weak dependence of the synchrotron emission measured by the camera with the RE energy, value of the q-profile at the edge, and the chosen range of wavelengths is observed. Furthermore, we find that oversimplifying the angular dependence of the SR changes the shape of the synchrotron spectra, and overestimates its amplitude by approximately 20 times for avalanching runaways and by approximately 60 times for mono-energetic distributions of runaways.

Eight new astrometry results of 6.7 GHz CH3OH and 22 GHz H2O masers in the Perseus arm

AbstractWe report astrometric results for seven 6.7 GHz CH3OH and one 22 GHz H2O masers in the Perseus arm with VLBA and VERA observations. Among the eight sources, we succeeded in obtaining trigonometric parallaxes for all sources, except G098.03+1.44 at 6.7 GHz band. By combining our results with previous astrometry results (Choi et al. 2014), we determined an arm width of 0.41 kpc and a pitch angle of 8.2 ± 2.5 deg for the Perseus arm. By using a large sample of the Perseus arm (26 sources), we examined the three-dimensional, non-circular motions (defined as U, V and W) of sources in the Perseus arm as a function of the distance (D) perpendicular to the arm. Interestingly, we found a weighted mean of <U > = 12.7 ± 1.2 km s−1 for 14 sources with D < 0 kpc (i.e. sources on the interior side of the arm) and <U > = −0.3 ± 1.5 km s−1 for 12 sources with D > 0 kpc (i.e. sources exterior to the arm). These findings might be the first observational indication of the ”damping phase of a spiral arm” suggested by the non-steady spiral arm model of Baba et al. (2013). The small pitch angle of the Perseus arm (< 10 deg) also supports the damping phase, based on ”pitch angle vs. arm amplitude” relation shown in Grøsbol et al. (2004).

An investigation of misalignment effects on the performance of acetal gears

Abstract This paper concentrates on the effects of misalignment on meshing behaviour of acetal gears as hardly any misalignment investigations on polymer gears in the existing literature. The experimental results show that the wear of acetal gears is insensitive to radial and axial misalignments but sensitive to yaw and pitch misalignments which degrade the conjugate contact action. Yaw misalignment leads to ‘scoop’ wear marks near tooth pitch points. Pitch misalignment causes ‘superimposed palisade’ wear marks and micro cracks near tooth roots. Compared with metal gears, the effects of small pitch angle on acetal gears are insignificant which may be linked closely to polymer's low elastic modulus. Strikingly different wear striations and various debris morphologies are observed by using scanning electronic and optical microscope (SEM, OM) and misalignment effects can be noted.

Wind turbines ice distribution and load response under icing conditions

Wind turbines operating in cold climate are susceptible to icing events. Under icing conditions, in order to gain a better understanding of the ice distribution affected by different parameters, and the load response of different components, the NREL Phase VI was selected as the test case. Firstly, the ice distribution affected by different parameters with the multi-dispersed water droplets size was investigated. Results show that the ice mass and the ice thickness increase approximately linear from the blade root to the blade tip. Higher free stream wind speed, smaller pitch angle, higher liquid water content, larger water droplets median volumetric diameter and lower temperature have bigger effect on the blade icing. Secondly, the ice shapes on the blade tip region (r/R = 80%–100%) were simulated. Then the lift coefficients and the drag coefficients of the clean airfoils and the iced airfoils were calculated. Finally, the load response of the asymmetric icing (one blade is covered with ice, the other is free) and the symmetric icing (two blades are covered with ice) of the blade and the tower were analyzed. Results show that icing can decrease the rotor thrust force, the blade root edgewise moment and the blade root flatwise moment. For the low speed shaft, the asymmetric icing can induce additional imbalance shear force. For the tower base, the symmetric icing can decrease the fore after moment, whereas the asymmetric icing can increase the side to side moment. The asymmetric load can increase the blade and the tower fatigue damage up to 97.6% and 70.8%, respectively.

A space-saving steering method for underwater gliders in lake monitoring

An increasing number of underwater gliders have been applied to lake monitoring. Lakes have a limited vertical space. Therefore, good space-saving capacity is required for underwater gliders to enlarge the spacing between monitoring waypoints. This paper presents a space-saving steering method under a small pitch angle (SPA) for appearance-fixed underwater gliders. Steering under an SPA increases the steering angle in per unit vertical space. An amended hydrodynamic model for both small and large attack angles is presented to help analyze the steering process. Analysis is conducted to find the optimal parameters of net buoyancy and roll angle for steering under an SPA. A lake trial with a prototype tiny underwater glider (TUG) is conducted to inspect the applicability of the presented model. The trial results show that steering under an SPA saves vertical space, unlike that under a large pitch angle. Simulation results of steering are consistent with the trial results. In addition, multiple-waypoint trial shows that monitoring with steering under an SPA covers a larger horizontal displacement than that without steering.

Resolved magnetic structures in the disk-halo interface of NGC 628

Magnetic fields are essential to fully understand the interstellar medium (ISM) and its role in the disk-halo interface of galaxies is still poorly understood. Star formation is known to expel hot gas vertically into the halo and these outflows have important consequences for mean-field dynamo theory in that they can be efficient in removing magnetic helicity. We perform new observations of the nearby face-on spiral galaxy NGC 628 with the Karl G. Jansky Very Large Array (JVLA) at S-band and the Effelsberg 100-m telescope at frequencies of 2.6 GHz and 8.35 GHz. We obtain some of the most sensitive radio continuum images in both total and linearly polarised intensity of any external galaxy observed so far in addition to high-quality images of Faraday depth and polarisation angle from which we obtained evidence for drivers of magnetic turbulence in the disk-halo connection. Such drivers include a superbubble detected via a significant Faraday depth gradient coinciding with a HI hole. We observe an azimuthal periodic pattern in Faraday depth with a pattern wavelength of 3.7$\pm$ 0.1 kpc, indicating Parker instabilities. The lack of a significant anti-correlation between Faraday depth and magnetic pitch angle indicates that these loops are vertical in nature with little helical twisting, unlike in IC 342. We find that the magnetic pitch angle is systematically larger than the morphological pitch angle of the polarisation arms which gives evidence for the action of a large-scale dynamo where the regular magnetic field is not coupled to the gas flow and obtains a significant radial component. We additionally discover a lone region of ordered magnetic field to the north of the galaxy with a high degree of polarisation and a small pitch angle, a feature that has not been observed in any other galaxy so far and is possibly caused by an asymmetric HI hole.

Doubly fed induction generator (DFIG) wind turbine controlled by artificial organic networks

The main goal of this paper is to show the control capabilities of artificial organic networks when they are applied to variable speed wind generators. Since doubly fed induction generator (DFIG) is one of the most important variable wind generators, it requires to include advanced controllers which allow to improve its performance during operation. On the other hand, the artificial organic controllers (AOC) are intelligent controllers based on ensembles of fuzzy inference systems and artificial hydrocarbon networks. To understand AOC, this paper introduces the fundamentals of artificial hydrocarbon networks, describes the fuzzy-molecular inference ensemble, and discusses artificial organic controllers when they are deployed in variable speed wind generators. Additionally, DFIG wind turbine model is completely derived in order to test the AOC. A conventional proportional–integral–derivative (PID) controller is compared with the proposed PID-based AOC (PID-AOC) for wind generators under linear and nonlinear wind profiles. Five parameters were used for evaluation: pitch angle, stator power, rotor power, generator’s speed and power coefficient. Results showed the superior control performance in wind generators when artificial organic networks are implemented. Particularly, the PID-AOC response obtained higher values of rotor and stator powers, small pitch angle response meaning less energy consumption, high power coefficient values, and smooth starting phase minimizing risks of damage in the DFIG. The proposed PID-AOC can be applied in DFIG to minimize the undesired fluctuation on the electric grid, to reduce the mechanical stress in the blades preventing mechanical damages and to perform good sensitivity when noise in the wind is included.

Entropy-Based Registration of Point Clouds Using Terrestrial Laser Scanning and Smartphone GPS.

Automatic registration of terrestrial laser scanning point clouds is a crucial but unresolved topic that is of great interest in many domains. This study combines terrestrial laser scanner with a smartphone for the coarse registration of leveled point clouds with small roll and pitch angles and height differences, which is a novel sensor combination mode for terrestrial laser scanning. The approximate distance between two neighboring scan positions is firstly calculated with smartphone GPS coordinates. Then, 2D distribution entropy is used to measure the distribution coherence between the two scans and search for the optimal initial transformation parameters. To this end, we propose a method called Iterative Minimum Entropy (IME) to correct initial transformation parameters based on two criteria: the difference between the average and minimum entropy and the deviation from the minimum entropy to the expected entropy. Finally, the presented method is evaluated using two data sets that contain tens of millions of points from panoramic and non-panoramic, vegetation-dominated and building-dominated cases and can achieve high accuracy and efficiency.

Study of Hybrid Air Vehicle Stability Using Computational Fluid Dynamics

This paper uses computational fluid dynamics to predict aerodynamic damping of airships or hybrid air vehicles. This class of aircraft is characterized by large lifting bodies combining buoyancy and circulatory lift. Damping is investigated via forced oscillations of the vehicle in pitch and yaw. The employed method is verified using data for lighter-than-air vehicles. The use of fins and stabilizers is found to be beneficial. The rear part of the body is dominated by separated flow that contains more frequencies than the forcing frequency imposed on the body. The final design is seen to be dynamically stable across a range of conditions for small pitch angles.

STRONG EVIDENCE FOR THE DENSITY-WAVE THEORY OF SPIRAL STRUCTURE IN DISK GALAXIES

ABSTRACT The density-wave theory of galactic spiral-arm structure makes a striking prediction that the pitch angle of spiral arms should vary with the wavelength of the galaxy’s image. The reason is that stars are born in the density wave but move out of it as they age. They move ahead of the density wave inside the co-rotation radius, and fall behind outside of it, resulting in a tighter pitch angle at wavelengths that image stars (optical and near-infrared) than those that are associated with star formation (far-infrared and ultraviolet). In this study we combined large sample size with wide range of wavelengths, from the ultraviolet to the infrared to investigate this issue. For each galaxy we used an optical wavelength image (B-band: 445 nm) and images from the Spitzer Space Telescope at two infrared wavelengths (infrared: 3.6 and 8.0 μm) and we measured the pitch angle with the 2DFFT and Spirality codes. We find that the B-band and 3.6 μm images have smaller pitch angles than the infrared 8.0 μm image in all cases, in agreement with the prediction of density-wave theory. We also used images in the ultraviolet from Galaxy Evolution Explorer, whose pitch angles agreed with the measurements made at 8 μm.

Wing Kinematics, Aerodynamic Forces and Vortex-wake Structures in Fruit-flies in Forward Flight

Wing kinematics in forward-flying fruit-flies was measured using high-speed cameras and flows of the flapping wing were calculated numerically. The large lift and thrust coefficients produced by the wing were explained. The wing flaps along a forward-tilting stroke plane. In the starting portion of a half-stroke (an upstroke or downstroke), the wing pitches down to a small pitch angle; during the mid portion (the wing has built up its speed), it first fast pitches up to a large pitch angle and then maintains the pitch angle; in the ending portion, the wing pitches up further. A large aerodynamic force (normal to the wing surface) is produced during the mid portion of a half-stroke. The large force is produced by the fast-pitching-up rotation and delayed-stall mechanisms. As a result of the orientation of wing, the thrust that propels the insect is produced by the upstroke and the major part of the vertical force that supports the weight is produced by the downstroke. In producing the thrust the upstroke leaves a “vortex ring” that is almost vertical, and in producing the vertical force the downstroke leaves a “vortex ring” that is almost horizontal.

Stability of drift-cyclotron loss-cone waves in H-mode plasmas

The drift-cyclotron loss-cone mode was first studied in mirror machines. In such devices, particles with small pitch angles are not confined, creating a hole in the velocity distribution function that is a source of free energy and leads to micro-instabilities in the cyclotron-range of frequencies. In the edge region of tokamak devices operating under H-mode conditions, ion loss also occurs. In this case, gradient drift carries ions moving opposite to the plasma current preferentially into the divertor, creating a one-sided loss cone. A simple analysis shows that for the quiescent H-mode plasmas in DIII-D the critical gradient for instability is exceeded within 2 cm of the separatrix, and the maximum growth rate at the separatrix is 3 × 107 s−1.

Extreme asymmetry in the polarized disk of V1247 Orionis

Abstract We present the first near-infrared scattered-light detection of the transitional disk around V1247 Ori, which was obtained using high-resolution polarimetric differential imaging observations with Subaru/HiCIAO. Our imaging in the H band reveals the disk morphology at separations of ∼0${^{\prime\prime}_{.}}$14–0${^{\prime\prime}_{.}}$86 (54–330 au) from the central star. The polarized intensity image shows a remarkable arc-like structure toward the southeast of the star, whereas the fainter northwest region does not exhibit any notable features. The shape of the arm is consistent with an arc of 0${^{\prime\prime}_{.}}$28 ± 0${^{\prime\prime}_{.}}$09 in radius (108 au from the star), although the possibility of a spiral arm with a small pitch angle cannot be excluded. V1247 Ori features an exceptionally large azimuthal contrast in scattered, polarized light; the radial peak of the southeastern arc is about three times brighter than the northwestern disk measured at the same distance from the star. Combined with the previous indication of an inhomogeneous density distribution in the gap at ≲46 au, the notable asymmetry in the outer disk suggests the presence of unseen companions and/or planet-forming processes ongoing in the arc.

Equatorial electron loss by double resonance with oblique and parallel intense chorus waves

AbstractPuzzling satellite observations of butterfly pitch angle distributions and rapid dropouts of 30–150 keV electrons are widespread in the Earth's radiation belts. Several mechanisms have been proposed to explain these observations, such as enhanced outward radial diffusion combined with magnetopause shadowing or scattering by intense magnetosonic waves, but their effectiveness is mainly limited to storm times. Moreover, the scattering of 30–150 keV electrons via cyclotron resonance with intense parallel chorus waves should be limited to particles with equatorial pitch angle smaller than 70°–75°, leaving unaffected a large portion of the population. In this paper, we investigate the possible effects of oblique whistler mode waves, noting, in particular, that Landau resonance with very oblique waves can occur up to ∼89°. We demonstrate that such very oblique chorus waves with realistic amplitudes can very efficiently nonlinearly transport nearly equatorially mirroring electrons toward smaller pitch angles where nonlinear scattering (phase bunching) via cyclotron resonance with quasi‐parallel waves can take over and quickly send them to much lower pitch angles <40°. The proposed double resonance mechanism could therefore explain the formation of butterfly pitch angle distributions as well as contribute to some fast dropouts of 30–150 keV electrons occurring during moderate geomagnetic disturbances at L = 4–6. Since 30–150 keV electrons represent a seed population for a further acceleration to relativistic energies by intense parallel chorus waves during storms or substorms, the proposed mechanism may have important consequences on the dynamics of 100 keV to MeV electron fluxes in the radiation belts.

Formation of sub-ion scale filamentary force-free structures in the vicinity of reconnection region

In this paper we review the results of spacecraft observations of current sheets (CSs) of sub-ion spatial scales in the Earth’s magnetotail as well as experiments with these structures in laboratory devices. We demonstrate that such sub-ion CSs having a thickness less than the ion gyroradius are usually formed in the vicinity of the magnetic reconnection region and are supported by strong electron currents flowing along magnetic field lines. The magnetic field configuration of sub-ion CSs is close to the force-free configuration, with a strong shear magnetic field component in the CS central region. Spacecraft observations suggest that parallel electron currents are generated by electron beams (pronounced enhancement of the phase space density for electrons with small pitch angles and energies ∼1–3 keV). We discuss several models describing such force-free sub-ion CSs.

Resonant scattering of central plasma sheet protons by multiband EMIC waves and resultant proton loss timescales

AbstractThis is a companion study to Liang et al. (2014) which reported a “reversed” energy‐latitude dispersion pattern of ion precipitation in that the lower energy ion precipitation extends to lower latitudes than the higher‐energy ion precipitation. Electromagnetic ion cyclotron (EMIC) waves in the central plasma sheet (CPS) have been suggested to account for this reversed‐type ion precipitation. To further investigate the association, we perform a comprehensive study of pitch angle diffusion rates induced by EMIC wave and the resultant proton loss timescales at L = 8–12 around the midnight. Comparing the proton scattering rates in the Earth's dipole field and a more realistic quiet time geomagnetic field constructed from the Tsyganenko 2001 (T01) model, we find that use of a realistic, nondipolar magnetic field model not only decreases the minimum resonant energies of CPS protons but also considerably decreases the limit of strong diffusion and changes the proton pitch angle diffusion rates. Adoption of the T01 model increases EMIC wave diffusion rates at > ~ 60° equatorial pitch angles but decreases them at small equatorial pitch angles. Pitch angle scattering coefficients of 1–10 keV protons due to H+ band EMIC waves can exceed the strong diffusion rate for both geomagnetic field models. While He+ and O+ band EMIC waves can only scatter tens of keV protons efficiently to cause a fully filled loss cone at L > 10, in the T01 magnetic field they can also cause efficient scattering of ~ keV protons in the strong diffusion limit at L > 10. The resultant proton loss timescales by EMIC waves with a nominal amplitude of 0.2 nT vary from a few hours to several days, depending on the wave band and L shell. Overall, the results demonstrate that H+ band EMIC waves, once present, can act as a major contributor to the scattering loss of a few keV protons at lower L shells in the CPS, accounting for the reversed energy‐latitude dispersion pattern of proton precipitation at low energies (~ keV) on the nightside. The pitch angle coverage for H+ band EMIC wave resonant scattering strongly depends on proton energy, L shell, and field model. He+ and O+ band EMIC waves tend to cause efficient scattering loss of protons at higher energies, thereby importantly contributing to the isotropic distribution of higher energy (> ~ 10 keV) protons at higher L shells on the nightside where the geomagnetic field line is highly stretched. Our results also suggest that scattering by H+ band EMIC waves may significantly contribute to the formation of the reversed‐type CPS proton precipitation on the dawnside where both the wave activity and occurrence probability is statistically high.

Experimental investigation into the effects of blade pitch angle and axial distance on the performance of a counter-rotating tidal turbine

In order to investigate the influence of blade pitch angle and axial distance on the performance of the counter-rotating type horizontal-axis tidal turbine (HATT), a series of experiments were conducted in a wind tunnel under different installation conditions. The blades were designed based on the blade element momentum (BEM) theory, and the effects of configuration factors were actualized in two aspects: different combinations of the front and rear blade pitch angles and different axial distances of dual rotors. The test results indicate that the proper increase of blade pitch angles and axial distance can enhance the performance of a counter-rotating turbine by increasing the peak CP (power coefficient) value and widening high CP value area. However, more experiments are still needed in order to investigate the relationship between CP and axial distance under small blade pitch angles more precisely. Flow field investigation is also required to understand the wake structure and intensity around the blades with small pitch angles. The result in the wind tunnel is a reference for the experiment in the water tunnel.

Simplified 7 DOF Model of Car Vertical Vibrations for Small Pitch and Roll Angles

This paper concerns a simplified 7 DOF model for car vertical vibrations. The classical 7 DOF of the considered 3D vertical model are: the vertical displacement of the gravity center of the car body, the roll and pitch angles of the car body and the four vertical displacements of the wheels centers. Using the x-y-z sequence of rotations parameterization, the Euler’s rotation equations concerning the roll and pitch angle are easily obtained. Small differences are observed between our car body rotation equations concerning the two pitch and roll angles and the same equations provided by Demić et al. [6]. For small pitch and roll angles, no differences were observed with respect with [6]. Also, no differences were observed concerning the other 5 dynamics equations of the 7 DOF model, the ones for the vertical displacements.The simplified 7 DOF car vertical dynamics model, comprising two rotation equations (for pitch and roll angles) and five dynamics equations concerning vertical displacements/accelerations, were integrated/simulated in Matlab. For small pitch and roll angles (a rather flat and smooth straight road profile was considered in our case study), the results obtained using the in-house 7 DOF model Matlab simulator are in very good agreement with the results provided by CARSIM.