DEVELOPMENT OF A MATHEMATICAL MODEL FOR RESEARCHING THE PROCESS OF REDUCTION OF TUNGSTEN HEXAFLUORIDE BY GASEOUS HYDROGEN

В работе рассмотрена математическая модель, описывающая движение течения стационарной, ламинарной, вязкой, несжимаемой смеси газа в трехмерном осесимметричном канале. Математическая модель, описывающая этот процесс, состоит из уравнений Навье – Стокса, уравнения неразрывности и массообмена, которые записаны в безразмерной форме с учетом осесимметричности в цилиндрической системе координат. Решение уравнений осуществляется в физических переменных «скорость – давление» на разнесенной разностной сетке. Показано влияние характерных параметров на распределение концентрации смеси газа гексафторида вольфрама и водорода в канале. Полученная математическая модель позволяет проводить численные исследования по выбору оптимальных условий осуществления процесса восстановления гексафторида вольфрама водородом. The paper considers a mathematical model describing the flow motion of a stationary, laminar, viscous, incompressible gas mixture in a three-dimensional axisymmetric channel. The mathematical model describing this process consists of the Navier-Stokes equations, the continuity and mass transfer equations, which are written in dimensionless form taking into account axisymmetry in a cylindrical coordinate system. The equations are solved in the physical variables "velocity - pressure" on a spaced difference grid. The influence of characteristic parameters on the concentration distribution of a mixture of tungsten hexafluoride gas and hydrogen in the channel is shown. The obtained mathematical model makes it possible to conduct numerical studies on the choice of optimal conditions for the process of reduction of tungsten hexafluoride with hydrogen.

Visualization and Sound Measurements of Vibration Plate in a Boiling Bubble Resonator

We developed a boiling bubble resonator (BBR) as a new heat transfer enhancement method aided by boiling bubbles. The BBR is a passive device that operates under its own bubble pressure and therefore does not require an electrical source. In the present study, high-speed visualization of the flow motion of the microbubbles spouted from a vibration plate and the plate motion in the BBR was carried out using high-speed LED lighting and high-speed cameras; the sounds in the boiling chamber were simultaneously captured using a hydrophone. The peak point in the spectrum of the motion of the vibration plate and the peak point in the spectrum of the boiling sound were found to be matched near a critical heat-flux state. Therefore, we found that it is important to match the BBR vibration frequency to the condensation cycle of the boiling bubble as its own design specification for the BBR.

Dynamic transition of current-driven single-skyrmion motion in a room-temperature chiral-lattice magnet

Driving and controlling single-skyrmion motion promises skyrmion-based spintronic applications. Recently progress has been made in moving skyrmionic bubbles in thin-film heterostructures and low-temperature chiral skyrmions in the FeGe helimagnet by electric current. Here, we report the motion tracking and control of a single skyrmion at room temperature in the chiral-lattice magnet Co9Zn9Mn2 using nanosecond current pulses. We have directly observed that the skyrmion Hall motion reverses its direction upon the reversal of skyrmion topological number using Lorentz transmission electron microscopy. Systematic measurements of the single-skyrmion trace as a function of electric current reveal a dynamic transition from the static pinned state to the linear flow motion via a creep event, in agreement with the theoretical prediction. We have clarified the role of skyrmion pinning and evaluated the intrinsic skyrmion Hall angle and the skyrmion velocity in the course of the dynamic transition. Our results pave a way to skyrmion applications in spintronic devices.

Application of a Single Porous Basket as a Pier Scour Countermeasure

This paper presents a study on bridge pier protection with a single porous basket (SPB) in clear-water experiments. The SPB is a type of combined flow-altering countermeasure. The SPB was installed at a distance ahead of the protected pier. After a series of tests, the results showed that appropriate installation of the SPB was able to effectively adjust the flow pattern to reduce the down-flow motion and horseshoe vortex ahead of the pier. Dominant factors for the pier protection—considered for all tests—included the distance between the basket and pier, submerged depth of the basket, basket length, pier diameter, basket diameter, hole size, porosity, and the flow approaching angle. After evaluating these parameters through laboratory tests, the results of protection were optimized. In optimal conditions, the SPB was able to provide maximum pier protection and decrease the maximum scour depth by as much as 75.53%.

The influence of a rocking-motion device built into classic cross-country roller-ski bindings on biomechanical, physiological and performance outcomes

This study aimed to determine whether the recently developed Flow Motion Technology® roller-ski prototype could improve indicators of performance during sub-maximal and maximal cross-country roller skiing. Thirteen national and international cross-country skiers completed 2 experimental trials: 1 with Flow Motion Technology® activated, allowing a rocking motion between the foot and ski binding, and 1 with the foot fixed in a traditional manner. Each trial included 2 sub-maximal bouts using the diagonal-stride and double-poling sub-techniques, as well as a double-poling maximal velocity test and a diagonal-stride 6-min time trial. There were no differences in performance between Flow Motion Technology® and traditional roller skiing during the maximal velocity test or the time trial. However, reductions in mean plantar force during sub-maximal diagonal stride (p = 0.011) and ankle range of motion during sub-maximal (p = 0.010) and maximal (p = 0.041) diagonal stride were observed with Flow Motion Technology® versus traditional roller skiing. This, together with a reduced minimum horizontal distance of the hips in front of the ankles during sub-maximal double poling (p = 0.001), indicated impaired technique with Flow Motion Technology®, which may have contributed to the trend for reduced gross efficiency during double poling with Flow Motion Technology® (pη2 = 0.214). Significant physiological differences included a reduced sub-maximal double poling respiratory exchange ratio (p = 0.03) and a greater maximal heart rate during the time trial (p = 0.014) with Flow Motion Technology®. We conclude that the application of Flow Motion Technology® requires further examination before use in training and competition.

Numerical analysis of hydrodynamics influenced by deformed bed due to near-bank vegetation patch

This study with a 2D hydro-morphological model analyzes hydrodynamics over flat and deformed beds with a near-bank vegetation patch. By varying the patch density, the generalized results show that the hydrodynamics over deformed beds differs a lot from those over flat beds. It is found that the deformed bed topography leads to an apparent decrease in longitudinal velocity and bed shear stress in the open region and longitudinal surface gradient for the entire vegetated reach. However, the transverse flow motion and transverse surface gradient in the region of the leading edge and trailing edge is enhanced or maintained, suggesting the strengthening of secondary flow motion. Interestingly, the deformed bed topography tends to alleviate the horizontal shear caused by the junction-interface horizontal coherent vortices, indicating that the turbulence-induced flow mixing is highly inhibited as the bed is deformed. The interior flow adjustment through the patch for the deformed bed requires a shorter distance, La, which is related to the vegetative drag length, (Cda)−1, with a logarithmic formula (La = 0.4ln[(Cda)−1] + b, with b = 3.83 and 4.03 for the deformed and flat beds). The tilting bed topographic effect in the open region accelerating the flow may account for the quick flow adjustment.

Resistance Analysis of Morphologies in Headwater Mountain Streams

River flow velocity is determined by the energy available for flow motion and the energy fraction lost by flow resistance. We compared the performance of different equations for the Darcy-Weisbach resistance coefficient (f) and empirical equations to predict flow velocity. The set of equations was tested using data from the Quinuas headwater mountain river in the Andean region. The data was collected in three Cascades, two Step-pools, and one Plane-bed covering a wide range of velocity magnitudes. The results reveal that nondimensional hydraulic geometry equations (NDHG) with a Nash-Sutcliffe efficiency index (EF) varying from 0.6–0.85 provide the most accurate velocity prediction. Furthermore, the study proposes a methodology applicable to all morphologies for defining the NDHG parameters using easily measured field data. The results show an improvement in predictability with EF values in the range of 0.81–0.86. Moreover, the methodology was tested against data from the literature, which was not divided into morphologies providing EF values of around 0.9. The authors encourage the application of the presented methodology to other reaches to obtain additional data about the NDHG parameters. Our findings suggest that those parameters could be related to reach characteristics (e.g., certain characteristic grain size), and in that case, the methodology could be useful in ungauged streams.

A Neural Network Time Dependent Hydrodynamic Force Model for Forced Two-Degree-Of-Freedom Sinusoidal Motion of a Circular Cylinder in a Free Stream

A data-driven hydrodynamic force model is developed to model the dynamic forces on an oscillating circular cylinder for flow conditions where Vortex-Induced-Vibrations (VIV) are known to occur. The model is developed for use in future control systems to improve VIV-based energy harvesting. The dynamic model is empirical, utilizing force measurements obtained for a large set of forced motion experiments, spanning a range of parametric values that prescribe the kinematics of the cylinder motion. The model includes the dynamics of a circular cylinder undergoing forced combined in-line and cross-flow motion in a free stream. The experiments were conducted in a fully automated towing tank where parameters of in-line amplitude of motion, cross-flow amplitude of motion, reduced velocity, and phase difference between in-line and cross-flow motion were varied over nearly 10,000 experiments. All experiments were carried out at a constant Reynolds number of 7620. A feed forward neural network is trained using the force database to develop a time dependent model of forces on the cylinder for given kinematic conditions. Selected outputs of the force model are presented, providing a basis for future synthesis in energy harvesting control applications.

Scaling of Droplet Breakup in High-Pressure Homogenizer Orifices. Part II: Visualization of the Turbulent Droplet Breakup

Emulsion formation is of great interest in the chemical and food industry and droplet breakup is the key process. Droplet breakup in a quiet or laminar flow is well understood, however, actual industrial processes are always in the turbulent flow regime, leading to more complex droplet breakup phenomena. Since high resolution optical measurements on microscopic scales are extremely difficult to perform, many aspects of the turbulent droplet breakup are physically unclear. To overcome this problem, scaled experimental setups (with scaling factors of 5 and 50) are used in conjunction with an original scale setup for reference. In addition to the geometric scaling, other non-dimensional numbers such as the Reynolds number, the viscosity ratio and the density ratio were kept constant. The scaling allows observation of the phenomena on macroscopic scales, whereby the objective is to show that the scaling approach makes it possible to directly transfer the findings from the macro- to the micro-/original scale. In this paper, which follows Part I where the flow fields were compared and found to be similar, it is shown by breakup visualizations that the turbulent droplet breakup process is similar on all scales. This makes it possible to transfer the results of detailed parameter variations investigated on the macro scale to the micro scale. The evaluation and analysis of the results imply that the droplet breakup is triggered and strongly influenced by the intensity and scales of the turbulent flow motion.