Component Of Phase Velocity Research Articles

Eulerian-Lagrangian Modelling of Turbulent Two-Phase Particle-Liquid Flow in a Stirred Vessel: CFD and Experiments Compared

A 3D Eulerian-Lagrangian CFD model is set up to simulate the turbulent flow and mixing of coarse dense particles in a standard batch vessel, mechanically agitated by a down-pumping pitched-blade turbine. Flow is investigated in the just-suspended regime and regimes much above. All simulations are fully and successfully validated by experimental Lagrangian measurements provided by positron emission particle tracking. Predicted distributions of the local 3D phase-velocity components, local particle slip velocities and local particle concentration show very good agreement with experiment. Lagrangian particle trajectories are exploited to infer detailed accurate information on particle circulation time and local residence time. The predicted two-phase flow number is in excellent agreement with experimental values under all flow regimes. Being accurate and robust, the numerical model can be extended to study even more complex particle-liquid suspensions of industrial relevance, and to provide much needed Lagrangian data to other data-driven modelling techniques.

Harmonics from a magic carpet

Abstract

Generation of negative group velocity Lamb waves by a moving laser source

Analytical existence conditions for negative group velocity (NGV) of an arbitrary mode Lamb waves in elastic plate was obtained. Experimentally, NGV waves were excited by means of a thermoelastic laser source moving across the sample plate with a controllable velocity. Such source is coupled with modes which phase velocity component parallel to the surface coincides with the source speed. Additional mode selection was performed by choosing a quasiperiodic shape of the laser spot, which specifies the range of the wavenumber k. This technique was successfully applied for generation of backward (NGV) waves. By tuning the speed of the source the propagation direction was switched from forward to reversal.

Whistler modes in highly nonuniform magnetic fields. III. Propagation near mirror and cusp fields

The properties of helicon modes in highly nonuniform magnetic fields are studied experimentally. The waves propagate in an essentially unbounded uniform laboratory plasma. Helicons with mode number m = 1 are excited with a magnetic loop with dipole moment across the dc magnetic field. The wave fields are measured with a three-component magnetic probe movable in three orthogonal directions so as to resolve the spatial and temporal wave properties. The ambient magnetic field has the topology of a mirror or a cusp, produced by the superposition of a uniform axial field B0 and the field of a current-carrying loop with the axis along B0. The novel finding is the reflection of whistlers by a strong mirror magnetic field. The reflection arises when the magnetic field changes on a scale length shorter than the whistler wavelength. The simplest explanation for the reflection mechanism is the strong gradient of the refractive index which depends on the density and magnetic field. More detailed observations show that the incident wave splits when the k vector makes an angle larger than 90° with respect to B0 which produces a parallel phase velocity component opposite to that of the incident wave. The reflection coefficient has been estimated to be close to unity. Interference between reflected and incident waves creates nodes in which the whistler mode becomes linearly polarized. When the magnetic field topology is that of a reversed field configuration (FRC), the incident wave is absorbed near the three-dimensional (3D) magnetic null point which prevents wave reflections. However, waves outside the separatrix are not absorbed and continue to propagate around the null point. When waves are excited inside the FRC, their polarization and helicon mode are reversed. Implications of these observations on research in space plasmas and helicon sources are pointed out.

Study of the surface wave vibrations in a functionally graded material layered structure: a WKB method

In this paper, a theoretical investigation is described of the propagation behaviour of Love-type waves in a functionally graded material layered structure. The Wentzel–Kramers–Brillouin (WKB) method was applied to obtain the analytical solution for the functionally graded material layer. The study was carried out to investigate the transference of Love-type waves in a functionally graded material layer bounded by a viscous fluid layer and a highly anisotropic half-space. Two cases were studied, one in which the material property variation of the functionally graded material layer was arbitrary and one in which the material property variation was taken as exponential. A variable separation method was employed to obtain the displacement components in both the viscous liquid and the highly anisotropic medium. Profound effects of material gradient coefficient, viscous coefficient, liquid mass density, initial stress parameter and wavelength on real and imaginary components of phase velocities were established. Some numerical calculations were also made. The considered model facilitates a theoretical foundation and practical application for the development of surface acoustic wave devices.

Satellite observations of wave disturbances caused by moving solar terminator

Wave disturbances caused by moving solar terminator were studied using in situ satellite measurements. Neutral species densities measured by low-latitude satellite Atmosphere Explorer-E in the altitude range of 250–400km were used for analysis. Wave disturbances of neutral species density with amplitudes of 2–4% were observed during few hours after passing the terminator, predominantly in time intervals of 6–9 LST and 20–23 LST. These disturbances were interpreted as the acoustic-gravity waves. Spatial scales of such waves range from few hundred to few thousand kilometers, major part of wave spectral power being concentrated in the horizontal wavelength range from 1000km to 1200km. It was shown that vertical and horizontal components of phase velocity of these waves coincide with vertical and horizontal components of terminator velocity, i.e. observed wave are synchronized with the terminator.

Numerical study on cuttings transport in vertical wells with eccentric drillpipe

Abstract In this paper, hydrodynamic of drilling mud and cuttings in a vertical well has been studied numerically. For solid–liquid two-phase flow simulation, the mixture model including continuity and momentum equations was used. To describe the rheological behavior of drilling mud, Herschel Bulky model was implemented. Then, a numerical code was developed to calculate flow field parameters (i.e. liquid phase velocity components, solid phase volume fraction, and solid phase velocity components in the annulus) and pressure drop. The results have been compared with available data in the literature and good agreement was found. Moving mud at entrance of the annulus and developed flow length were studied and pressure drop was calculated. Annular flow velocities in this study varied from 0.4 to 1.4 m/s. Then the effects of velocity, fluid type, pipe rotation, drilling cuttings and eccentricity on pressure drop were investigated. The pressure drop in the annulus increased while eccentricity and rotation of drill pipe increased. Rotation of the drill pipe shifts maximum fluid velocity location toward cornel space of the concentric annulus. After ensuring the accuracy of the simulation, the effects of drilling mud type, mud axial velocity, drilling rotation, fluid density and cuttings size on cuttings transport were studied. Finally, mud capacity for keeping cuttings was calculated in stasis. Then, with considering shape factor for cuttings, the velocity profile of cuttings, the mud capacity of the drilling flow and slip velocity were calculated. Slip velocity profile of cuttings in the annulus increased as cutting size increased. This indicates that cutting transport in the annulus enhances as velocity, mud density, viscosity and rotation of drill pipe increase. Eccentricity and cutting size have a negative effect on cutting transport.

Numerical modelling of velocity field and phase distribution in dense monodisperse solid–liquid suspensions under different regimes of agitation: CFD and PEPT experiments

Turbulent suspensions of monodisperse coarse glass particles of 1 and 3mm diameter in water were numerically simulated at their ‘just-suspended’ speed Njs and at speeds above it up to 2Njs, in a vessel agitated by a down-pumping pitched-blade turbine. The solid concentration was in the range 5.2–20wt%. The numerical results are compared to detailed 3-D distributions of the three local phase velocity components and solid concentration obtained by an accurate technique of positron emission particle tracking (PEPT). The predictions of flow number and mean velocity profiles for both phases are generally excellent both at Njs and above it. Predictions of the spatial solids distribution are good except near the base of the vessel and underneath the agitator where they are largely overestimated, however, they improve significantly with increasing solid concentration. At Njs and above it, the liquid velocity field is not significantly affected by the presence of solids up to ∼10wt%. At higher concentrations, however, some significant reductions in liquid velocity occur near the impeller and along the wall of the vessel. At Njs, there are wide variations in the spatial distribution of the inter-phase slip velocity. The largest total slip velocities are of considerable magnitude, on the order of ∼0.10utip. Increasing the agitation speed up to 2Njs, reduces the normalised slip velocities significantly. Results also indicate that there is no impact on the distributions of turbulent kinetic energy and Kolmogorov length scale. The eddy dissipation rate, however, is increasingly suppressed as solid concentration increases at Njs.

Probing Residual Vestibular Function With Galvanic Stimulation in Vestibular Loss Patients

Galvanic vestibular stimulation (GVS) was used to probe the residual vestibular function in unilateral vestibular loss. 39 control subjects, 60 patients areflexic to caloric tests, 21 patients with pre-operative acoustic neurinoma, and 26 patients with surgical deafferentation were tested with 4mA trapezoidal GVS. Horizontal slow phase velocity and vertical slow phase velocity components of the slow phase of induced nystagmus were recorded with videonystagmography. All patients had lower horizontal slow phase velocity compared with intact subject. In caloric areflexic patients, the intact ear responded similarly to controls. The lesioned ear was less excitable but could be inhibited as in controls. In patients with acoustic neurinoma, the intact ear was normally excitable but less sensitive to inhibition. The lesioned ear, although not excitable, could be inhibited. In patients with surgical deafferentation, the intact ear could be barely excited and not inhibited. The lesioned ear could not be excited but slightly inhibited. These results fit well with previous data in animal models after unilateral vestibular loss. Unilateral vestibular patients exhibit a progressive deterioration of the horizontal canal function, corresponding to 4 different patterns of GVS response. This study confirms that GVS is useful to probe evolution of the residual vestibular function in patients.

Reply to comment by R. Dhanya and S. Gurubaran on “Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system”

Reply to comment by R. Dhanya and S. Gurubaran on “Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system”

The Earth's hum: the excitation of Earth normal modes by ocean waves

SUMMARY The recent discovery that the seismic normal modes of the Earth are excited to a nearly constant level during seismically quiet days (‘the hum of the Earth’) has lead to much speculation as to what drives the observed background in the absence of large earthquakes. Other authors have shown that the hum cannot be explained by the many small earthquakes occurring each day and have also precluded excitation by ‘slow’ earthquakes as the source. The non-linear interaction of low frequency ocean waves (infragravity waves) generates high phase velocity components that excite the seismic normal modes of the Earth. I show the forcing of the normal modes of the Earth by infragravity waves interacting over the continental shelves and over the deep ocean basins is sufficient to explain the background seismic acceleration spectrum observed during seismically quiet days. Atmospheric turbulence is shown to be a negligible source of the Earth’s hum. Observations suggesting enhanced spectral levels for two seismic modes that couple strongly with atmospheric modes (0S27 and 0S39) have been cited as evidence that the source of the hum lies in the atmosphere. However, the wave interaction mechanism also couples energy into infrasound and thus may explain these observations. A calculation of the coupling between ocean waves and normal modes expected from ocean waves interacting over the shelves reproduces the vertical acceleration spectrum observed at quiet seismic sites from 2 to 40 mHz including the small ‘hump’ between 5 and 15 mHz and the observed rise at higher frequencies. The model diverges above 40 mHz because the ‘single frequency’ microseism peak is generated by a different mechanism. The shape of the hum spectrum is controlled primarily by the elastic properties of the Earth with the attenuation structure controlling mode amplitudes. A second calculation shows the excitation of Earth’s normal modes by infragravity waves interacting over the deep ocean basins can also explain the observed spectrum of the Earth’s hum. Infragravity waves are less energetic and the forcing mechanism weaker over deep water, but the integrative effect of forcing over a large area offsets the weaker forcing per unit area. It is likely that both pelagic and shelf ocean wave sources contribute significantly to the forcing of the Earth’s hum and both coastal and pelagic source regions have been identified using seismic techniques. Infragravity wave spectra in deep water over the Pacific basin vary little, thus explaining the small variation in the hum that is observed. However, time varying source regions are also observed in the hum, and may be best explained by intense near shore sources. The great similarity between the spectra predicted from continental shelf forcing and from forcing over the deep ocean basins precludes discriminating between coastal and pelagic sources based on the shape of the spectrum. A more complete knowledge of the climatology of infragravity waves coupled with better seismic observations are needed to resolve this question.

The Earth’s ‘hum’ is driven by ocean waves over the continental shelves

Observations show that the seismic normal modes of the Earth at frequencies near 10 mHz are excited at a nearly constant level in the absence of large earthquakes. This background level of excitation has been called the 'hum' of the Earth, and is equivalent to the maximum excitation from a magnitude 5.75 earthquake. Its origin is debated, with most studies attributing the forcing to atmospheric turbulence, analogous to the forcing of solar oscillations by solar turbulence. Some reports also predicted that turbulence might excite the planetary modes of Mars to detectable levels. Recent observations on Earth, however, suggest that the predominant excitation source lies under the oceans. Here I show that turbulence is a very weak source, and instead it is interacting ocean waves over the shallow continental shelves that drive the hum of the Earth. Ocean waves couple into seismic waves through the quadratic nonlinearity of the surface boundary condition, which couples pairs of slowly propagating ocean waves of similar frequency to a high phase velocity component at approximately double the frequency. This is the process by which ocean waves generate the well known 'microseism peak' that dominates the seismic spectrum near 140 mHz (refs 11, 12), but at hum frequencies, the mechanism differs significantly in frequency and depth dependence. A calculation of the coupling between ocean waves and seismic modes reproduces the seismic spectrum observed. Measurements of the temporal correlation between ocean wave data and seismic data have confirmed that ocean waves, rather than atmospheric turbulence, are driving the modes of the Earth.

Measurements of ocean surface waves and currents using l- and c-band along-track interferometric sar

Along-track interferometric synthetic aperture radar (ATI-SAR) is an active coherent imaging system, utilizing two antennas separated along the platform flight direction. The phase information of ATI-SAR from the Doppler shift of the backscattered signal represents the line-of-sight velocity of the water scatterers. While the advent of ATI-SAR provided us with a potentially powerful technique for ocean surface current and wave mapping, the surface current has not been measured exactly from the ATI-SAR velocity because the Doppler shift is not simply proportional to the component of the mean surface current. It also includes other types of contributions associated with the phase velocity of the Bragg waves and orbital motions of all ocean waves that are longer than Bragg waves. In this paper, we review how the phase difference measured by ATI-SAR is related to the mean Doppler frequency, and we develop a new and practically useful method to extract the surface current component utilizing simultaneously measured L- and C-band ATI-SAR data. Since the measured ATI-SAR velocity shows a different value at a different radar-frequency, we investigate the influence of Bragg-resonant waves and long ocean wave motions on the ATI-SAR velocity according to the radar frequency. The Bragg-wave phase velocity component, which is a significant source of error for extracting the surface current, can be effectively eliminated by using L- and C-band ATI-SAR. The method is applied to L- and C-band ATI-SAR measurements acquired at the Ulsan coast in the southeastern part of the Korean peninsula. The resulting ocean surface current vectors are compared with in situ measurements collected by recording current meter. We furthermore extract ocean surface wave information from the ATI-SAR phase image using a quasi-linear transform.

Characteristics of medium- and large-scale TIDs over Japan derived from OI 630-nm nightglow observation

A new optical instrument for studying upper atmospheric dynamics, called the Multicolor All-sky Imaging System (MAIS), has been developed. The MAIS can obtain all-sky images of airglow emission at two different wavelengths simultaneously with a time resolution of several minutes. Since December 1991, imaging observations with the MAIS have been conducted at the Zao observatory (38.09°N, 140.56°E). From these observations, two interesting events with wave structures have been detected in OI 630-nm nightglow images. The first event was observed on the night of June 2/3, 1992 during a geomagnetically quiet period. Simultaneous data of ionospheric parameters showed that they are caused by propagation of the medium-scale traveling ionospheric disturbance (TID). Phase velocity and horizontal wavelength determined from the image data are 45–100 m/s and ∼280 km, and the propagation direction is south-westward. The second event was observed on the night of February 27/28, 1992 during a geomagnetic storm. It is found that a large enhancement of OI 630-nm emission is caused by a propagation of the large-scale TID. Meridional components of phase velocities and wavelengths determined from ionospheric data are 305–695 m/s (southward) and 930–5250 km. The source of this large-scale TID appears to be auroral processes at high latitudes.

Modeling of Gas Turbine Fuel Nozzle Spray

Satisfactory performance of the gas turbine combustor relies on the careful design of various components, particularly the fuel injector. It is, therefore, essential to establish a fundamental basis for fuel injection modeling that involves various atomization processes. A two-dimensional fuel injection model has been formulated to simulate the airflow within and downstream of the atomizer and address the formation and breakup of the liquid sheet formed at the atomizer exit. The sheet breakup under the effects of airblast, fuel pressure, or the combined atomization mode of the airassist type is considered in the calculation. The model accounts for secondary breakup of drops and the stochastic Lagrangian treatment of spray. The calculation of spray evaporation addresses both droplet heat-up and steady-state mechanisms, and fuel vapor concentration is based on the partial pressure concept. An enhanced evaporation model has been developed that accounts for multicomponent, finite mass diffusivity and conductivity effects, and addresses near-critical evaporation. The presents investigation involved predictions of flow and spray characteristics of two distinctively different fuel atomizers under both nonreacting and reacting conditions. The predictions of the continuous phase velocity components and the spray mean drop sizes agree well with the detailed measurements obtained for the two atomizers, which indicates the model accounts for key aspects of atomization. The model also provides insight into ligament formation and breakup at the atomizer exit and the initial drop sizes formed in the atomizer near field region where measurements are difficult to obtain. The calculations of the reacting spray show the fuel-rich region occupied most of the spray volume with two-peak radial gas temperature profiles. The results also provided local concentrations of unburned hydrocarbon (UHC) and carbon monoxide (CO) in atomizer flowfield, information that could support the effort to reduce emission levels of gas turbine combustors.

Angular velocity detection by head movements orthogonal to the plane of rotation.

Sinusoidal oscillation of rhesus monkeys about a head-fixed, earth-horizontal axis while rotating at constant velocity about an earth-vertical axis generates a characteristic ocular nystagmus where the three-dimensional slow phase eye velocity is compensatory to the spatially and temporally changing head angular velocity vector. This includes the generation of a unidirectional nystagmus characterised by a "bias" slow phase velocity component, albeit of small gain (0.2-0.7), that persists for the duration of the combined two-axes stimulation and is compensatory to the constant velocity earth-vertical axis rotation. Specifically, there is a torsional bias velocity in supine position, a vertical bias velocity in ear down position and a horizontal bias velocity in upright position. Since the semicircular canals can not sense prolonged constant velocity rotation, the ocular bias velocity must be centrally constructed from canal afferent signals using head position information. Thus, optimal performance of the vestibular system as a three-dimensional rate sensor relies on afferent information from both the semicircular canals and the otolith organs.

A class of high‐m pulsations and its auroral radar signature

Bistatic Auroral Radar System radar events showing a repetitive equatorward (or radarward) traveling band signature in the range‐time‐intensity plots have been analyzed. The associated pulsations have many features in common with previously reported high‐m storm time Pc 5 pulsations. The periods, azimuthal wave numbers (m), Dst dependence, and polarization ellipses are similar. One difference is also seen. The pulsation phase velocity of our events often has a significant equatorward component in the ionosphere. However, of the three cases for which the phase velocity direction in the magnetospheric equatorial plane was determined, one had phase velocities close to azimuthal at both locations, one had an azimuthal phase velocity in the magnetosphere but an equatorward component of the phase velocity in the ionosphere, and the final event had equatorward or inward components of phase velocity at both locations. These events are very similar to the equatorward traveling bands events observed by the Sweden and Britain Auroral Radar Experiment system (Tian et al., 1991; Yeoman et al., 1992) except that our m values are distinctly larger. If coupling to energetic protons is involved in exciting these pulsations, this difference implies that the energy of the protons involved decreases with increasing L of the interaction location. Finally, our data do not lend themselves to the plasmapause field line resonance interpretation of Tian et al. (1991).

Very large phase velocities of non two‐stream, meter scale irregularities in the high latitude E region ionosphere

We present experimental evidence on a new category of meter wavelength radio auroral irregularities characterized by a unique Doppler spectrum signature. The new echo type has been identified with the Scandinavian Twin Auroral Radar Experiment radars which are capable of observing simultaneously full Doppler spectra from the same scattering volume along two widely different azimuths. The new echoes were seen only by the Finland radar which observes at large angles to the electrojet flow and at times when the Norway radar observed very stable type 1 echoes with phase velocities near the ion acoustic speed. The new spectrum, named type 5, is composed of three main frequency bands, two being symmetric about zero Doppler shift near ±450 m/s and a third always at very large phase velocities near 1000 m/s, this resulting to a characteristic asymmetry in the spectrum. The Doppler polarity of the large phase velocity component is always at negative shifts in the eastward electrojet and at positive shifts in the westward electrojet. These echoes, which are not strong and last from a few minutes to tens of minutes, were observed during strong but also moderate geomagnetic conditions but, apparently, always in the presence of large and stable electric fields. The evidence suggested that the new echoes cannot be due to direct generation by the two‐stream instability and, most likely, a secondary generation mechanism is involved. Although there are some similarities with 50 MHz type 4 spectral observations, type 5 echoes have unique characteristics which we believe are sufficient to justify their classification into a different echo category. We have no satisfactory theoretical explanation for the new echoes. That the fast moving irregularities have frequencies about two times the frequency of the simultaneously observed ion acoustic waves led us to suggest that a nonlinear resonant wave‐wave interaction process might be in operation under some appropriate conditions in the plasma.

An unambiguous determination of the propagation of a compressional Pc 5 wave

During a 3‐hour interval on August 21 and 22, 1978, a compressional Pc 5 event was observed by the ISEE‐1 magnetometer and medium energetic particle experiment instrument. During the event the ISEE spacecraft was inbound near the equator and near the dusk meridian at distances between 11 and 7 Re. The finite Larmor radii of the energetic protons allow us to determine unambiguously both the azimuthal and radial components of the phase velocity, and consequently the wave length and the azimuthal wave number. A 2nπ ambiguity in evaluating the phase velocity is removed by finding a consistent and physically reasonable solution for different energy channels. As the spacecraft approached the plasmapause, a quasi‐sinusoidal wave form having a 165‐s period was observed; it was found to be propagating azimuthally westward with a phase velocity of about 33 km/s and an azimuthal wave number of 60. In the outer magnetosphere, more irregular wave forms were observed. These waves also propagated azimuthally westward and may possibly have had a small earthward (radial) component of phase velocity. To examine the effect of heavy ions on the estimated wavelength, we have derived the perturbation of differential flux by calculating the first‐order perturbed phase space distribution of plasma consisting of two ion species. We found that, for the detector we used, the effect is very small even there is a significant fraction of heavy ions present. This is because for each energy channel, the detector response to the heavy ions is more sensitive to those with higher energy than those with the nominal proton energy, while the flux drops exponentially with the energy. In all cases the modulation amplitude of ion fluxes decreases with an increasing ratio of the ion Larmor radius to the wavelength.

The existence of Stoneley waves as a loss mechanism in plane wave reflection problems

In a previous paper [’’Influence of Stoneley waves on plane-wave reflection coefficients: Characteristics of bottom reflection loss,’’ K. E. Hawker, J. Acoust. Soc. Am. 64, 548 (1978)] it was shown that for certain configurations of layered fluids overlying a rigid (solid) substrate, the reflection loss would display narrow peaks of significant amplitude. It was suggested in that paper that these peaks were due to excitation of Stoneley waves at the fluid–solid interface. In the present paper this association is made more precise and definite. Through an extension of the classical theory of Stoneley waves to the case of inhomogeneous media, it is shown that the angles at which the reflection-loss peaks occur are precisely those angles for which the horizontal component of phase velocity in the fluid equals the Stoneley wave phase velocity. In addition, the near independence of the reflection loss peaks on frequency is explained quantitatively.