Instability of the printing jet during the three-dimensional-printing process

Euler’s instability criterion is widely used in engineering to design a column. However, this criterion is not suitable for judging the instability of a three-dimensional printing process because the axial motion of the printing jet has to be considered. A variational principle is established, and an equivalent Eulerian load is obtained. The theoretical results show that a higher printing velocity makes the moving jet much more stable, and an experiment is designed to verify our theoretical prediction.

Dropping in electrospinning process: A general strategy for fabrication of microspheres

The dropping mechanism in the electrospinning process is elucidated. A moving jet becomes thinner at the initial stage due to the acceleration caused by the electrostatic force. When the jet diameter reaches a threshold, beyond which the jet breaks into drops and daughter jets, dropping occurs. The drops will finally form microspheres. Effects of applied voltage, flow rate, polymer?s concentration and receptor?s distance on the dropping process are theoretically analyzed and experimentally verified. This paper gives a general strategy for fabrication of smooth fiber, microspheres, and their mixture.

Performance of Hydrodynamics Flow on Flip Buckets Spillway for Flood Control in Large Dam Reservoirs

Flip buckets are usually used in high head dams to dissipate the destructive energy of high speed jets. These structures are fixes at the end of the outlet conduits to direct the moving jet into the atmosphere. The process of energy dissipation also resumes, while the jet entering into its downstream plunge pool. Although studies of flow over flip buckets turn back to many years ago, but still there are uncertainties regarding the flow behaviour over these structures with various geometries and flow conditions. In this study, experimental measurements of static and dynamic pressures and their distribution over these structures are investigated. Measurements were made along two different simple flip buckets with various Froude numbers to determine the effects of the geometry and flow characteristics on pressure field. Maximum pressures are also presented and the results are compared with those of other investigations. The results of this study can be used to increase the safety of large dams that remain sustainable in the process of exploitation such as irrigation, human consumption, industrial use, aquaculture, and navigability. Doi: 10.28991/HEF-2020-01-01-05 Full Text: PDF

Asymmetric jet production in the active galactic nucleus of NGC 1052

Context. Few active galactic nuclei (AGN) reveal double-sided jet systems. However, these systems are crucial to understand basic physical properties of extragalactic jets. Aims. We address the questions whether jets in AGN are symmetric in nature, how well they are collimated on small scales, and how they evolve with time. Methods. We monitored the sub-parsec scale morphology of NGC 1052 with the Very Long Baseline Array at 43 GHz from 2005 to 2009. Results. A detailed study of 29 epochs show a remarkable asymmetry between both jets. A kinematic analysis of the outflows reveals higher apparent velocities for the eastern (approaching) jet as compared to the western (receding) jet, i.e., βej = 0.529 ± 0.038 and βwj = 0.343 ± 0.037, respectively. Contradictory to previous studies, we find higher flux densities for the western jet as compared to the eastern. The distribution of brightness temperature and jet width features well-collimated jets up to 1 mas distance to the dynamic center and a nearly conical outflow further outward. By combining flux density ratios and velocities of the jet flows, we were unable to find a combination of intrinsic velocities and inclination angles of the jets that is consistent for all four years of observation; this contradicts findings for symmetrically evolving jets. Spectral index maps between quasi-simultaneous 22 GHz and 43 GHz observations support the existence of an optically thick absorber covering the innermost ≃1.6 mas around the 43 GHz central feature and an optically thin jet emission with a spectral index of ≤−1. Conclusions. Our results fit into a picture in which we expect larger internal energy and/or magnetic flux in the western jet and higher kinetic energy in the eastern jet. Previous observations at lower frequencies have found slower velocities of the moving jet features as compared to this work. Considering the different velocities in different areas, we suggest a spine-sheath structure with a faster inner layer and slower outer layer.

Fabrication of Beltlike Fibers by Electrospinning

Electrospinning is always used to fabricate one-dimensional nanofibers. Cylindrical fibers are formed during the spinning process due to the minimal-surface principle. However, when the moving jet has high rigidity, which can counteract the surface tension for a minimal surface, beltlike fibers can be obtained. Using the Hall–Petch effect, the rigidity of the moving jet can be greatly enhanced by adding nanoparticles. Polyethylene glycol with a nanometric crystallite size of 4 nm and ZrO2 nanoparticles are used as additives in the experiment, a theoretical analysis is carried out, and the theoretical predictions are verified experimentally.

On the Role of Inertial Instability in Stratosphere–Troposphere Exchange near Midlatitude Cyclones*

In simulations of midlatitude cyclones with the University of Wisconsin Nonhydrostatic Modeling System (UWNMS), mesoscale regions with large negative absolute vorticity commonly occur in the upper troposphere and lower stratosphere (UTLS), overlying thin layers of air with stratospheric values of ozone and potential vorticity (PV). These locally enhanced stratosphere–troposphere exchange (STE) events are related to upstream convection by tracing negative equivalent potential vorticity (EPV) anomalies along back trajectories. Detailed agreement between the patterns of negative absolute vorticity, PV, and EPV—each indicators of inertial instability in the UTLS—is shown to occur in association with enhanced STE signatures. Results are presented for two midlatitude cyclones in the upper Midwest, where convection develops between the subpolar and subtropical jets. Mesoscale regions of negative EPV air originate upstream in the boundary layer. As they are transported through convection, EPV becomes increasingly negative toward the tropopause. In association with the arrival of each large negative EPV anomaly, a locally enhanced poleward surge of the subpolar jet occurs, characterized by high turbulent kinetic energy and low Richardson number. Isosurfaces of wind speed show that gravity waves emanating from inertially unstable regions connect with and modulate the subpolar and subtropical jets simultaneously. Inertially unstable convective outflow surges can facilitate STE locally by fostering poleward acceleration in the UTLS, with enhanced folding of tropospheric air over stratospheric air underneath the poleward-moving jet.

Solitary waves on a ferrofluid jet

The propagation of axisymmetric solitary waves on the surface of an otherwise cylindrical ferrofluid jet subjected to a magnetic field is investigated. An azimuthal magnetic field is generated by an electric current flowing along a stationary metal rod which is mounted along the axis of the moving jet. A numerical method is used to compute fully nonlinear travelling solitary waves, and the predictions of elevation waves and depression waves made by Rannacher and Engel (New J. Phys., vol. 8, 2006, pp. 108–123) using a weakly nonlinear theory are confirmed in the appropriate ranges of the magnetic Bond number. New nonlinear branches of solitary wave solutions are identified. As the Bond number is varied, the solitary wave profiles may approach a limiting configuration with a trapped toroidal-shaped bubble, or they may approach a static wave (i.e. one with zero phase speed). For a sufficiently large axial rod, the limiting profile may exhibit a cusp.