Perpendicular Velocity Component Research Articles

Numerical simulation of the vortex-induced vibrations of a 4:1 rectangular cylinder subjected to yaw uniform currents

The practical engineering structures are not always perpendicular to the wind but often in skew positions. This yawed configuration may lead to more complex three-dimensional flow characteristics and different vortex-induced vibration (VIV) responses. However, the related study is very inadequate. Therefore, comprehensive numerical investigations of the VIV characteristics of a 4:1 rectangular cylinder have been conducted using the three-dimensional (3D) Large-Eddy Simulation (LES) at five different yaw angles (β = 0°, 5°, 10°, 20°, 30°). An innovative method for creating 3D meshes was introduced, demonstrating a good agreement with the experimental results while significantly reducing the mesh number, and consequently the computation time. The VIV response, pressure distribution, aerodynamic force, flow field, and energy input/output were analyzed for various yaw angles. The results demonstrate that the periodic boundary conditions on the spanwise domain side is more reliable than those of the wall boundary conditions. The velocity component parallel to the cylinder axis can be ignored in the analysis of elastically-mounted rectangular rigid cylinders for β up to 30°. If only considering the perpendicular axis velocity component, the amplitudes and lock-in regions for β = 5°, 10°, 20°, and 30° are highly consistent with those of β = 0°, which have effectively validated the accuracy of Independence Principle in evaluating the VIV of a 4:1 rectangular cylinder.

Experimental and numerical study of a hollow droplet impacting on inclined solid surfaces

This study sheds light on the complex dynamics of hollow droplet impacts and highlights the unique behaviors that differentiate them from their dense counterparts. The impact dynamics of hollow droplets on surfaces at varying angles were investigated through a combination of experimental and numerical methods. Two-view imaging technique is used to capture the droplet flattening during the experimental study. A three-dimensional compressible solver is developed to model the droplet impact using the volume of fluid method to capture the liquid and gas interface. The study revealed two distinct behaviors when comparing the flattening of hollow droplets to that of dense droplets. First, a unique counter-jet formation was observed following the collision of a hollow droplet perpendicular to the surface, indicating an inherent characteristic of hollow droplet flattening. The length of this counter-jet was primarily influenced by the droplet velocity and liquid viscosity, with the perpendicular velocity component playing a key role in its size. Second, unlike dense droplets that recoil and form a dome shape upon impact on hydrophobic surfaces, hollow droplets form a donut shape due to disturbances caused by bubble rupture during spreading. These disturbances fragmented the liquid sheet, preventing the droplet from recoiling and resulting in a distinctive donut shape. On surfaces with different orientations, the hollow droplet exhibited two velocity components, where the normal component controls the counter-jet size while the tangential component induces tangential motion. The donut shape splat was also observed on surfaces with different orientations.

Mean-field transport equations and energy theorem for plasma edge turbulent transport

This paper establishes a mean-field equation set and an energy theorem to provide a theoretical basis in view of the development of self-consistent, physics-based turbulent transport models for mean-field transport codes. A rigorous averaging procedure identifies the exact form of the perpendicular turbulent fluxes which are modelled by ad hoc diffusive terms in mean-field transport codes, next to other closure terms which are not commonly considered. Earlier work suggested that the turbulent $E\times B$ particle and heat fluxes, which are thus identified to be important closure terms, can be modelled to reasonable accuracy using the kinetic energy in the $E\times B$ velocity fluctuations ( $k_{E}$ ). The related enstrophy led to further modelling improvements in an initial study, although further analysis is required. To support this modelling approach, transport equations are derived analytically for both quantities. In particular, an energy theorem is established in which the various source and sink terms of $k_{E}$ are shown to couple to mean-field and turbulent parallel kinetic energy, kinetic energy in the other perpendicular velocity components, the thermal energy and the magnetic energy. This provides expressions for the interchange, drift-wave and Reynolds stress terms amongst others. Note that most terms in these energy equations are in turn closure terms. It is suggested to evaluate these terms using reference data from detailed turbulence code simulations in future work.

Mass transfer enhancement in spacer-filled membrane channels by flow oscillation induced vortex shedding: Numerical study of the effect of amplitude

Techniques inducing unsteady flow have shown potential for promoting vortex shedding in membrane channels, affecting the boundary layer and increasing mass transfer. This work reports a CFD study of the effect of the characteristics of an oscillating flow (i.e., frequency and amplitude) on the occurrence of vortex shedding. The implications of imposing an oscillating flow are analysed by comparing mass transfer enhancement and required pumping power. Results show that the resonant frequency for the perpendicular velocity component does not maximise mass transfer, but it yields a significant increase. For Re = 300, a normalised amplitude of 0.01 appears as a balanced trade-off between mass transfer and pressure drop. The location of onset of vortex shedding in the channel moves upstream as the amplitude increases. Mass transfer reaches an upper limit as the amplitude increases, when the onset of vortex shedding reaches the first filament. The phenomena taking place in a 2D spacer-filled membrane channel, and their implications for real-world applications, are discussed.

Tracking Halo Orbits and Their Mass Evolution around Large-scale Filaments

We have explored the dynamical and mass evolution of halos driven by large-scale filaments using a dark-matter-only cosmological simulation with the help of a phase-space analysis. Since a nonnegligible number of galaxies are expected to fall into the cluster environment through large-scale filaments, tracking how halos move around large-scale filaments can provide a more comprehensive view on the evolution of cluster galaxies. Halos exhibit orbital motions around filaments, which emerge as specific trajectories in a phase space composed of halos’ perpendicular distance and velocity component with respect to filaments. These phase-space trajectories can be represented by three cases according to their current states. We parameterize the trajectories with halos’ initial position and velocity, maximum velocity, formation time, and time since first crossing, which are found to be correlated with each other. These correlations are explained well in the context of the large-scale structure formation. The mass evolution and dynamical properties of halos seem to be affected by the density of filaments, which can be shown from the fact that halos around denser filaments are more likely to lose their mass and be bound within large-scale filaments. Finally, we reproduce the mass segregation trend around filaments found in observations. The mass segregation has been developed because halos that formed earlier approached filaments earlier, grew efficiently, and ended up being more massive. We also found that dynamical friction helps to retain this segregation trend.

Numerical study of cosmic ray confinement through dust resonant drag instabilities

ABSTRACT We investigate the possibility of cosmic ray (CR) confinement by charged dust grains through resonant drag instabilities (RDIs). We perform magnetohydrodynamic particle-in-cell simulations of magnetized gas mixed with charged dust and cosmic rays, with the gyro-radii of dust and GeV CRs on ∼au scales fully resolved. As a first study, we focus on one type of RDI wherein charged grains drift super-Alfvénically, with Lorentz forces strongly dominating over drag forces. Dust grains are unstable to the RDIs and form concentrated columns and sheets, whose scale grows until saturating at the simulation box size. Initially perfectly streaming CRs are strongly scattered by RDI-excited Alfvén waves, with the growth rate of the CR perpendicular velocity components equaling the growth rate of magnetic field perturbations. These rates are well-predicted by analytic linear theory. CRs finally become isotropized and drift at least at ∼vA by unidirectional Alfvén waves excited by the RDIs, with a uniform distribution of the pitch angle cosine μ and a flat profile of the CR pitch angle diffusion coefficient Dμμ around μ = 0, without the ‘90○ pitch angle problem.’ With CR feedback on the gas included, Dμμ decreases by a factor of a few, indicating a lower CR scattering rate, because the backreaction on the RDI from the CR pressure adds extra wave damping, leading to lower quasi-steady-state scattering rates. Our study demonstrates that the dust-induced CR confinement can be very important under certain conditions, e.g. the dusty circumgalactic medium around quasars or superluminous galaxies.

Turbulence Models Studying the Airflow around a Greenhouse Based in a Wind Tunnel and Under Different Conditions

Turbulence phenomena created around a greenhouse due to different wind loads are key factors in its structural design and significantly affect the ventilation rates through its side and roof openings. Using the turbulence models of ANSYS FLUENT software to investigate the airflow around an arched-roof-greenhouse-shaped obstacle placed inside a wind tunnel was the aim of this study. Velocity and pressure areas around the obstacle were examined by selecting three different turbulence models (Standard, RNG and Realizable k–ε models) under three different airflow entry velocities (0.34, 1.00 and 10.00 m s−1) in the wind tunnel. All k–ε models showed that when the air velocity was intensified, the airflow was identified as turbulent. The horizontal velocity profile predicted more accurately the presence of vortices in contrast with the vector sum of the perpendicular velocity components. Vortices were formed upstream, above the roof and downstream of the obstacle, and the applied models showed that when airflow velocity increases, the size of the upstream vortex decreases. Finally, there was a strong indication from the modeling results that the vortex on the roof of the obstacle was an extension of the vortex that was created downstream.

Interaction of counterstreaming rotating electron-positron beams with inhomogeneous electron-ion plasma

The interaction occurring between two counterstreaming rotating electron-positron beams and an inhomogeneous magnetized electron-ion plasma is studied with the focus of research on either positrons or electrons propagating in the direction of the magnetic field. Using the Vlasov theory along with geometrical optics, the linear eikonal equation corresponds to the gradient drift wave which is extracted in the background plasma, taking into account the beam contribution. The results reveal that the gradient drift instability is experienced where the gradients of density and temperature of electrons stand in the opposite directions, and in addition, the gradients act as destabilization effects. Regarding the beam contribution, when the electron beams propagate in the direction of the magnetic field, the parallel and perpendicular components of velocity and the Langmuir frequency of the rotating beams can induce stabilization effects on the unstable inhomogeneous configuration. However, as a considerable achievement, the mentioned stabilization effects vanish for the perpendicular velocity component lower than a certain threshold value. In addition, the destabilization effects of the characteristic parameters of the counterstreaming beams are observed as well, when the positron beams propagate in the direction of the magnetic field.

Generation of EMIC Waves Observed by Van Allen Probes at Low L Shells

AbstractObservation of linearly polarized He+‐band electromagnetic ion cyclotron (EMIC) waves at low L shells is a new, and quite unexpected, result from the Van Allen Probes mission. Here we analyze the two EMIC wave events observed by Van Allen Probes at low L shells and put forward a new‐generation mechanism for the low‐L EMIC waves. Both events were observed at L ∼ 3 but one of them has a discrete spectrum near the O+ gyrofrequency and its second harmonic, whereas the second event has a broad spectrum between the O+ gyrofrequency and its second harmonic. For both events, the major conclusions of our analysis can be summarized as follows. (1) Only O+ causes EMIC wave generation, and instability is driven by the positive derivatives of distribution functions over perpendicular component of velocity. (2) The timing and frequencies of generated waves are in agreement with observations. The generated wave normal angles, however, are highly oblique being in strong disagreement with the minimum variance angles obtained from Fast Fourier transform. (3) The wave step analysis shows that a signal nonstationarity is not a major cause for disagreement between the minimum variance angles and theoretical predictions for normal angles. (4) A superposition of plane sine waves with the same frequency and normal angle but with different azimuthal angles for wave vector around the background magnetic field can reconcile the polarization properties of EMIC waves obtained from Fast Fourier transform and/or the wave step analysis with those predicted by the linear theory of EMIC waves.

Astrometric detection of intermediate-mass black holes at the Galactic Centre

We assess the astrometric detectability of intermediate-mass black holes populating the inner parsec of the Milky Way Galaxy. The presence of these objects induces dynamical effects on Sgr A* and the star S2, which could be detected by next generation astrometric instruments that enable micro-arcsecond astrometry. An allowed population of ten $10^4~M_{\odot}$ IMBHs within one parsec induces an angular shift of about 65 $\mu$as yr$^{-1}$ on the position of Sgr A*, corresponding to a perpendicular velocity component magnitude of 1.6 km s$^{-1}$. It also induces changes in the orbit of S2 that surpass those induced by general relativity but lie within observational constraints, generating a mean angular shift in periapse and apoapse of 62 $\mu$as and 970 $\mu$as respectively.

Evolution of Complex 3D Motions in Spicules

Abstract Ubiquitous transverse oscillations observed in spicular waveguides, identified as the kink wave-mode had previously been reported along with periodic structural distortions of the flux tubes, observed as cross-sectional width and associated photometric variations. Previous studies identified these perturbations as the observed signatures of concurrent kink and sausage wave-modes. High-resolution Hα imaging-spectroscopy data from the CRisp Imaging SpectroPolarimeter at the Swedish Solar Telescope are used to analyze the off-limb spicular structures. For the first time, the evolution of the resultant transverse displacement of the flux-tube structure, estimated from the perpendicular velocity components, is analyzed along with longitudinal, cross-sectional width, photometric, and azimuthal shear/torsion variations. The pulse-like nonlinear kink wave-mode shows strong coupling with these observables, with a period-doubling, -tripling aspect, supported by mutual phase relations concentrated around 0° and ± 180 ° . The three-dimensional ensemble of the observed dynamical components revealed complexities pertinent to the accurate identification and interpretation of, e.g., linear/nonlinear, coupled/uncoupled magnetohydrodynamical wave-modes in spicules.

Dynamic anisotropy in MHD turbulence induced by mean magnetic field

In this paper, we study the development of anisotropy in strong MHD turbulence in the presence of a large scale magnetic field B 0 by analyzing the results of direct numerical simulations. Our results show that the developed anisotropy among the different components of the velocity and magnetic field is a direct outcome of the inverse cascade of energy of the perpendicular velocity components u? and a forward cascade of the energy of the parallel component u k . The inverse cascade develops for a strong B0, where the flow exhibits a strong vortical structure by the suppression of fluctuations along the magnetic field. Both the inverse and the forward cascade are examined in detail by investigating the anisotropic energy spectra, the energy fluxes, and the shell to shell energy transfers among different scales.

Dynamics of the Rydberg state population of slow highly charged ions impinging a solid surface at arbitrary collision geometry

We consider the population dynamics of the intermediate Rydberg states of highly charged ions (core charge , principal quantum number interacting with solid surfaces at arbitrary collision geometry. The recently developed resonant two-state vector model for the grazing incidence (2012 J. Phys. B: At. Mol. Opt. Phys. 45 215202) is extended to the quasi-resonant case and arbitrary angle of incidence. According to the model, the population probabilities depend both on the projectile parallel and perpendicular velocity components, in a complementary way. A cascade neutralization process for ions, for , interacting with a conductive-surface is considered by taking into account the population dynamics. For an arbitrary collision geometry and given range of ionic velocities, a micro-staircase model for the simultaneous calculation of the kinetic energy gain and the charge state of the ion in front of the surface is proposed. The relevance of the obtained results for the explanation of the formation of nanostructures on solid surfaces by slow highly charged ions for normal incidence geometry is briefly discussed.

Wavelet phase analysis of two velocity components to infer the structure of interscale transfers in a turbulent boundary-layer

Scale-dependent phase analysis of velocity time series measured in a zero pressure gradient boundary layer shows that phase coupling between longitudinal and vertical velocity components is strong at both large and small scales, but minimal in the middle of the inertial regime. The same general pattern is observed at all vertical positions studied, but there is stronger phase coherence as the vertical coordinate, y, increases. The phase difference histograms evolve from a unimodal shape at small scales to the development of significant bimodality at the integral scale and above. The asymmetry in the off-diagonal couplings changes sign at the midpoint of the inertial regime, with the small scale relation consistent with intense ejections followed by a more prolonged sweep motion. These results may be interpreted in a manner that is consistent with the action of low speed streaks and hairpin vortices near the wall, with large scale motions further from the wall, the effect of which penetrates to smaller scales. Hence, a measure of phase coupling, when combined with a scale-by-scale decomposition of perpendicular velocity components, is a useful tool for investigating boundary-layer structure and inferring process from single-point measurements.

Spontaneous emission and spectral properties of radiation by relativistic electrons in a gyro-klystron and optical-klystron undulator.

In this paper spontaneous emission of radiation by relativistic electrons in a gyro-klystron is studied. The scheme consists of two solenoid sections separated by a dispersive section. In the dispersive section the electrons are made non-resonant with the radiation. The dispersive section transforms a small change of the velocity into changes of the phases of the electrons. This leads to enhanced radiation due to klystron-type modulation as compared with a conventional gyrotron-type device driven by cyclotron maser interaction. It is shown that the klystron-modulated spectrum depends on the dispersive field strength, finite perpendicular velocity component and length of the solenoids but is independent of the axial magnetic field strength. A simple scheme to design a gyro-klystron is discussed.

Proton velocity ring‐driven instabilities in the inner magnetosphere: Linear theory and particle‐in‐cell simulations

AbstractLinear dispersion theory and electromagnetic particle‐in‐cell (PIC) simulations are used to investigate linear growth and nonlinear saturation of the proton velocity ring‐driven instabilities, namely, ion Bernstein instability and Alfvén‐cyclotron instability, which lead to fast magnetosonic waves and electromagnetic ion cyclotron waves in the inner magnetosphere, respectively. The proton velocity distribution is assumed to consist of 10% of a ring distribution and 90% of a low‐temperature Maxwellian background. Here two cases with ring speeds vr/vA=1 and 2 (vA is the Alfvén speed) are examined in detail. For the two cases, linear theory predicts that the maximum growth rate γm of the Bernstein instability is 0.16Ωp and 0.19Ωp, respectively, and γm of the Alfvén‐cyclotron instability is 0.045Ωp and 0.15Ωp, respectively, where Ωp is the proton cyclotron frequency. Two‐dimensional PIC simulations are carried out for the two cases to examine the instability development and the corresponding evolution of the particle distributions. Initially, Bernstein waves develop and saturate with strong electrostatic fluctuations. Subsequently, electromagnetic Alfvén‐cyclotron waves grow and saturate. Despite their smaller growth rate, the saturation levels of the Alfvén‐cyclotron waves for both cases are larger than those of the Bernstein waves. Resonant interactions with the Bernstein waves lead to scattering of ring protons predominantly along the perpendicular velocity component (toward both decreasing and, at a lesser extent, increasing speeds) without substantial change of either the parallel temperature or the temperature anisotropy. Consequently, the Alfvén‐cyclotron instability can still grow. Furthermore, the free energy resulting from the pitch angle scattering by the Alfvén‐cyclotron waves is larger than the free energy resulting from the perpendicular energy scattering, thereby leading to the larger saturation level of the Alfvén‐cyclotron waves.

Spontaneous emission of radiation by relativistic electrons in a gyro-klystron

Abstract In this paper, we study spontaneous emission of radiation by relativistic electrons in a gyro-klystron. The scheme consists of two solenoid sections separated by a dispersive section. In the dispersive section the electrons are made non resonant with the radiation. The dispersive section transforms a small change of the velocity into changes of the phases of the electrons.This leads to enhanced radiation as compared to a conventional gyrotron type device driven by cyclotron maser interaction. It is shown that the klystron modulated spectrum depends on the dispersive field strength, finite perpendicular velocity component and length of the solenoids but do not depend on the axial magnetic field strength. The analysis is further extended to include the combined effects of the undulator aided gyrotron klystron radiation.

Slow mode structure in the nightside magnetosheath related to IMF draping

We apply a semianalytic magnetohydrodynamic approach to describe effects in the nightside magnetosheath related to accelerated magnetosheath flows caused by the draping of interplanetary magnetic field (IMF). Assuming a northward IMF direction, we show the development of slow mode fronts in the far tail (tailward of approximately −60 RmE). We find that accelerated flows north and south of the equator start to converge toward lower latitudes. The ensuing plasma compression gives rise to slow mode waves in the equatorial region which, further down the tail, evolve into slow mode shocks. These fronts propagating along the magnetic field lines are characterized by sharp increases of plasma density, pressure, and temperature and a decrease in the magnetic field strength. The magnetic pressure exhibits an anticorrelation with the plasma pressure, but the total pressure is fairly constant across the fronts. The field‐aligned plasma velocity component anticorrelates with the plasma density, while the perpendicular velocity component does not have sharp variations at the fronts. For northward IMF, these fronts appear near the equatorial region and then propagate to higher latitudes. This effect is not very sensitive to the particular shape of the magnetopause. Lowering the upstream Alfvén Mach number increases the strength of the slow mode waves, which also develop closer to Earth. We predict that this effect can be observed by space probes skimming the far tail.

Rotaphone, a Self-Calibrated Six-Degree-of-Freedom Seismic Sensor and Its Strong-Motion Records

Rotational seismology is attracting attention in the seismological community including seismic engineers interested in possible damages to man‐made constructions due to strong rotational ground motions. Mathematically, seismic rotation represents the curl of the seismic displacement u , ![Graphic][1] , with Cartesian components: ![Graphic][2] (1) Seismic rotation, due to S waves, is nonzero even in a homogeneous isotropic medium. The inhomogeneity and anisotropy of seismic structures generate more complex wavefield patterns and hence more significant seismic rotations. Moreover, a complex seismic source can contribute considerably to the rotation field, namely in a near‐source zone. In this paper we present a new six‐degree‐of‐freedom (6DOF) seismic sensor called Rotaphone and its strong‐motion records both from an anthropogenic source (blast) and from a tectonic event. By strong rotational motion we mean rotation rate of the order 10−4 rad/s and higher (according to a common convention accepted in the seismological community; see, e.g., Graizer, 2009). The sensor is portable, and easy to install and operate. It has a wide dynamic range of 120 dB; it is able to detect weak ground motions (from the order of 10−7 in SI units) as well as stronger motions (up to the order of 10−1 in SI units). The sensor seems to meet requirements of operative deployment for the needs of strong rotational ground‐motion studies. Rotaphone is a mechanical sensor system for recording ground velocity and determining the rotation rate by measuring the spatial velocity gradients at a point. It is based on differencing records from proximal geophones attached to a rigid frame. The first prototypes based on this principle were presented by Brokesova and Malek (2010) and Brokesova, Malek, and Kolinský (2012). Those instruments measured only the vertical‐axis rotation (torsion) and two perpendicular horizontal velocity components. The 6DOF instrument presented here represents an enhanced design of those … [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Liquid-crystal-enabled electrophoresis of spheres in a nematic medium with negative dielectric anisotropy

We describe electrophoresis of spherical dielectric particles in a uniformly aligned nematic medium with a negative dielectric anisotropy. A spherical particle that orients the liquid crystal (LC) perpendicularly to its surface moves under the application of a uniform direct current or alternating current electric field. The electric field causes no distortions of the LC director far away from the sphere. Electrophoresis in the nematic LC shows two types of nonlinearity in the velocity versus field dependence. The velocity component parallel to the applied electric field grows linearly with the field, but when the field is high enough, it also shows a cubic dependence. The most interesting is the second type of nonlinear electrophoresis that causes the sphere to move perpendicularly to the applied field. This perpendicular component of velocity is proportional to the square of the field. The effect exists only in an LC and disappears when the material is melted into an isotropic fluid. The quadratic effect is caused by the dipolar symmetry of director distortions around the sphere and is classified as an LC-enabled electrophoresis (LCEEP). The nonlinear electrophoretic mobility of particles in LCEEP offers a rich variety of control parameters to design three-dimensional trajectories of particles for microfluidic and optofluidic applications.