Resonant Instability of Kink Oscillations in Magnetic Flux Tubes with Siphon Flow

We study kink oscillations of a straight magnetic tube in the presence of siphon flows. The tube consists of a core and a transitional or boundary layer. The flow velocity is parallel to the tube axis, has constant magnitude, and confined in the tube core. The plasma density is constant in the tube core and it monotonically decreases in the transitional layer to its value in the surrounding plasma. We use the expression for the decrement/increment previously obtained by Ruderman and Petrukhin (Astron. Astrophys.631, A31, 2019) to study the damping and resonant instability of kink oscillations. We show that, depending on the magnitude of siphon-velocity, resonant absorption can cause either the damping of kink oscillations or their enhancement. There are two threshold velocities: When the flow velocity is below the first threshold velocity, kink oscillations damp. When the flow velocity is above the second threshold velocity, the kink oscillation amplitudes grow. Finally, when the flow velocity is between the two threshold velocities, the oscillation amplitudes do not change. We apply the theoretical result to kink oscillations of prominence threads. We show that, for particular values of thread parameters, resonant instability can excite these kink oscillations.

Mathematics Modelling and Performance Analysis of the Heat Transfer on Vacuum Tube Collector of Water Heater Application

The aim of this work is to study a mathematical modelling and simulation for predicting the thermal performance heat-pipe evacuated a solar collector for water heater, which was considered the temperature distribution along the tube axis and radius. This model used to help in studying the dynamic behaviour of the system design and the effect of influential parameter on the water heater process. The result showed that water heating is affected by the surface of collector, metrological conditions such as solar radiation and outdoor temperature, even the thermal performance of the collector.

Electron diffraction patterns from scroll nanotubes: interpretation peculiarities

This article describes the structure of scroll nanotubes and associated diffraction effects in the context of electron diffraction from a single nanotube. It is suggested that the effect of multiple equidistant splitting of diffuse reflections into cone series be used as a diffraction criterion for conical scroll structure identification. For cylindrical scroll structure determination, the effect of the azimuthal dependence of the intensity of basal diffraction spots is proposed as a characteristic sign. Good agreement between specific oscillations in both theoretical and experimental profiles of basal diffraction spots was achieved. It was also established that there are special values of chiral angles in cylindrical scroll nanotubes that lead to order enhancement in their structure along the tube axis, whereas even a small deviation from these angles results in degradation of diffraction conditions for some diffraction spots in the diffraction pattern.

On the Damage Criteria and their Critical Values for Flowforming of ELI Grade Ti64

Cold flowforming is a chipless forming process that deforms tubular parts by reducing theirouter diameter and thickness while increasing their length. It consists of a rotating mandrel and oneor more rollers that are translated along the tube axis, thus plastically deforming it. Flowforming ofTi-6Al-4V (also known as Ti64) is of great interest for improving the mechanical properties of thematerial, such as yield stress and fatigue strength. However this alloy is known to have poor ductilityat room temperature. Therefore, flowforming of Ti64 without failure or crack is a great challenge. Inthis present paper, the authors have attempted to predict the different failure modes occurring duringflowforming. An experimental machine has been built at the Center forMaterial Forming (CEMEF) inorder to monitor the force on the single roller, the torque on the mandrel and the actual rotation speedof the roller as well. Numerous flowforming tests have been performed using different processingparameters, such as working depth, roller feed and initial geometry, in order to investigate the criticalvalues which lead to the failure of the flowformed tube. In addition, numerical simulations of theprocess have been performed using the FORGE FEM solver. The results of the simulations have beenused to evaluate the relevance of usual failure criteria (Crockford-Latham, Rice-Tracey and Oyane).

Three Components- and Tomographic-PIV Measurements of a Cyclone Cooling Flow in a Swirl Tube

Swirl cooling is a very efficient method for turbine blade cooling. However, the flow in such a system is quite complicated. In order to gain understanding of the flow structure, the velocity field in a leading edge swirl cooling chamber with two tangential inlet ducts is experimentally studied via Particle Image Velocimetry (PIV). The examined swirl tube is 1 m long and has a diameter of 50 mm. It represents an upscaled generic model of a leading edge swirl chamber. The Reynolds number, defined by the bulk velocity and the swirl tube diameter, ranges from 10,000 to 40,000, and the swirl number is 5.3. Velocity fields are measured in the center plane of the tube axis with stereo- and tomographic-PIV using two and four CCD cameras respectively. Tomographic-PIV is a three-dimensional PIV technique relying on the illumination, recording, reconstruction and cross correlation of a tracer particle distribution in a measurement volume opposed to a plane in stereo-PIV. For statistical analysis 2,000 vector maps are calculated and evaluations show a sample size of 1,000 ensembles is sufficient. Our experiment showed, that the flow field is characterized by a vortex system around the tube axis. Near the tube wall we observed an axial flow towards the outlet with a circumferential velocity component in the same order of magnitude. In contrast the vortex core consists of an axial backflow (vortex breakdown). The gained understanding of the flow field allows to predict regions of enhanced heat transfer in swirl chambers.

Submicrocristalline Structure and Dynamic Recovery of Cold Flowformed ELI Grade Ti-6Al-4V

Flowforming is a means to produce seamless tubes by plastic deformation at room temperature. It consists in reducing the thickness of a tubular part mounted on a mandrel by deforming it using several rollers translating along the tube axis, while the tube is rotating along its axis. Thanks to the high compressive stresses, and to the incremental nature of the deformation process, flowforming can lead to a high thickness reduction and thus to high elongation of the deformed tubes. Ti-6Al-4V (Extra Low Interstitial grade) tubes have been deformed by cold flowforming, with a thickness reduction ratio higher than 60%, and their microstructures have been investigated using light optical microscopy (LOM), scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Based on EBSD data, a post-processing analysis has been performed in order to study the texture of the flowformed parts. Optical Microscopy showed that the material could be deformed without displaying flow instability such as adiabatic shear banding, despite the fact that it has been processes out of the stable processing maps (high strain rate and low temperature). It also evidenced a major deformation along the tube axis accompanied with a slight twist due to torsion stress. EBSD analysis indicated the occurrence of continuous dynamic recrystallization, which is rarely reported in the α-β domain of such alloys. The recovery/ recrystallization effects resulted in a submicrocrystalline equiaxed structure, which is consistent with that previously reported for Ti-6Al-4V subjected to severe plastic deformation (SPD). The texture of the hexagonal α-phase appeared to be similar to that obtained on extruded Ti-6Al-4V, with a basal component perpendicular to the tube axis.