Large-scale Pulsations Research Articles

Density asymmetry and wind velocities in the orbital plane of the symbiotic binary EG Andromedae

Context. Non-dusty late-type giants without a corona and large-scale pulsations represent objects that do not fulfil the conditions under which standard mass-loss mechanisms can be applied efficiently. Despite the progress during the past decades, the driving mechanism of their winds is still unknown. Aims. One of the crucial constraints of aspiring wind-driving theories can be provided by the measured velocity and density fields of outflowing matter. The main goal of this work is to match the radial velocities of absorbing matter with a depth in the red giant (RG) atmosphere in the S-type symbiotic star EG And. Methods. We measured fluxes and radial velocities of ten Fe I absorption lines from spectroscopic observations with a resolution of ≈30 000. At selected orbital phases, we modelled their broadened profiles, including all significant broadening mechanisms. Results. The selected Fe I absorption lines at 5151–6469 Å originate at a radial distance ≈1.03 RG radii from its centre. The corresponding radial velocity is typically ≈1 km s−1, which represents a few percent of the terminal velocity of the RG wind. The high scatter of the radial velocities of several km s−1 in the narrow layer of the stellar atmosphere points to the complex nature of the near-surface wind mass flow. The average rotational velocity of 11 km s−1 implies that the rotation of the donor star can contribute to observed focusing the wind towards the orbital plane. The orbital variability of the absorbed flux indicates the highest column densities of the wind in the area between the binary components, even though the absorbing neutral material is geometrically more extended from the opposite side of the giant. This wind density asymmetry in the orbital plane region can be ascribed to gravitational focusing by the white dwarf companion. Conclusions. Our results suggest that both gravitational and rotational focusing contribute to the observed enhancement of the RG wind towards the orbital plane, which makes mass transfer by the stellar wind highly efficient.

Investigations of shock–boundary layer interaction dynamics using high-bandwidth pressure field imaging

The large-scale pulsations of shock-induced separation with length scale that significantly exceeds the incoming boundary layer thickness are investigated. The shock–boundary layer interaction (SBLI) unit is generated by an inward-turning axisymmetric compression ramp at an inflow Mach number of 2.5. A substantial region surrounding the centre azimuth exhibited mean and dynamic flow features that are consistent with two-dimensional separation. Two-dimensional highly resolved maps of surface pressure field are obtained using fast-response pressure-sensitive paint fluorescence imaging at 40 kHz repetition rate. The measurement domain covered significant regions of the incoming boundary layer through the relaxing boundary layer downstream of the reattachment as well as over 25 boundary layer thicknesses in the azimuthal direction. These measurements provide new insights into the spanwise coupling of the SBLI unit in addition to its inherent dynamics. The power spectral density (PSD) of the centreline pressure exhibits very good agreement with theoretical models and complementary measurements using fast-response pressure transducers, which served to validate the pressure field measurements. Detailed examination of the PSD reveals strong agreement with the literature, which includes the peak Strouhal number of the separation and reattachment shock motions as well as the downward frequency shift along the separation bubble. Furthermore, the pressure fluctuation maps reveal streamwise-elongated structures just downstream of the ramp leading edge that persist well downstream of the reattachment. A time sequence of conditional average pressure fluctuation maps is constructed surrounding isolated pressure excursions in the intermittent region. This sequence, along with two-point cross correlation analysis, provides critical information about the flow processes that drive the separation bubble pulsations in the SBLI units with large separation scales. Overall, the imbalance in the mass within the separation bubble appears to be the critical mechanism that drives the separation bubble pulsations. Furthermore, the pressure perturbations originating at azimuthally offset locations are also observed to influence the separation bubble dynamics.

Spatial scale of geomagnetic Pc5/Pi3 pulsations as a factor of their efficiency in generation of geomagnetically induced currents

Geomagnetically induced currents (GICs) in a quasi-meridional power transmission line on the Kola Peninsula are analyzed during the intervals of Pc5/Pi3 (frequency range from 1.5 to 5 mHz) pulsations recorded at the IMAGE magnetometer network. We have analyzed GIC in a transformer at the terminal station Vykhodnoy (68^{circ } N, 33^{circ } E) during the entire year of 2015, near the maximum of the 24th Solar cycle. To quantify the efficiency of GIC generation by geomagnetic pulsations, a ratio between power spectral densities of GIC and magnetic field variations is introduced. Upon examination of the geomagnetic pulsation efficiency in GIC generation, the emphasis is given to its dependence on frequency and spatial scale. To estimate pulsation spatial scales in latitudinal and longitudinal directions, the triangle of stations KEV-SOD-KIL has been used. Large-scale pulsations (with a high spectral coherence, low phase difference, and similar amplitudes at latitudinally separated stations) are found to be more effective in GIC generation than small-scale pulsations. The GIC response also depends on the pulsation scale across the electric power line.

Experimental investigation of two-phase flow evolution in a tight lattice bundle using wire-mesh sensor

Tight lattice fuel assemblies have significant advantages in power density and conversion ratios. However, the dynamic characteristics of two-phase flow in tight lattice bundles, which is very important to safety analysis, are still not clarified. This study presents an experimental investigation of two-phase flow evolution in a double sub-channels tight lattice bundle. The experimental channel was up-scaled (1:2.7) compared with the prototype fuel. Phase distributions at four different axial positions, z/Dh = 28.95, 57.90, 86.86, and 115.8, are measured with a high spatiotemporal resolution by a 16 × 32 wire-mesh sensor. The flow regime includes bubbly flow, cap-bubbly flow, and slug flow. In the bubbly flow, the large-scale pulsations across the gap of two sub-channels were observed, as same as the single-phase flow in a tight lattice bundle. The time-averaged void fraction shows a gap region peak in bubbly flow. In the cap-bubbly flow, the cap bubbles are confined to a single sub-channel due to the small gap. The cap bubbles are alternated arrangements in the two sub-channels and the large-scale pulsations across the gap are caused due to this special flow configuration. The time-averaged void fraction shows a core peak profile in a cap-bubbly flow. In the slug flow, the slug bubbles can across the small gap to span the two sub-channels and the time-averaged void fraction shows a core peak profile. Even though the time-averaged void fraction almost coincides at different axial positions, the phase interface structure shows a significantly different according to the virtual side view of void fraction. The accuracy of the drift-flux models has been evaluated with the experimental data. The Bestion's model underestimated the void fraction in the channel while the Chexal's model overestimated it. Clark's model gets the best prediction with an error of 10.1%.

Study on the Influence of Temperature on the Temporal and Spatial Distribution Characteristics of Natural Cavitating Flow around a Vehicle

Cavitation involves complex multiphase turbulence and has important research significance. In this study, the Schnerr–Sauer cavitation model was used to model cavitation, and the detached-eddy simulation (DES) method was used to calculate the unsteady natural cavitating flow. The predicted results are in good agreement with experimentally measured cavity evolution and pressure values, demonstrating the effectiveness of this numerical method. Low temperature causes changes in the properties of water. The density of water at 0° is 999.84 kg/m3 and the density of water at 25° is 997.04. Cavitation evolution and shedding are analyzed at temperatures of 0 °C and 25 °C. The results showed that lower temperature increased the frequency of cavitation and enhanced pressure pulsation. At the same time, low temperature also increases the frequency of cavity shedding and shortens the cycle. In addition, based on the Ω method, the difference between vortex dynamics at various temperatures was studied, and it was found that different cavity stages showed different vortex structure characteristics, and lower temperature would aggravate the change of wake vortex structure. At the same time, the analysis of the turbulence characteristics in the downstream of the cavity shows that the lower temperature reduces the velocity pulsation and reduces the turbulence integral scale. At the end of the model, large-scale pulsations are transformed into small-scale pulsations.

GS0525 微小空間によって接続された矩形断面管路内の乱流解析

Numerical analysis is carried out for turbulent flow in two rectangular channels connected by a small gap for the Reynolds number range from 3,300 to 580,000. The effect of the gap on the channel flow is visualized by the isocontours of the mean axial flow velocity and of the turbulence intensities. A large scale pulsation that oscillates in the gap that connects two rectangular channels is observed. The calculated results are compared with the experiments to confirm the validity of the numerical simulation. The present turbulent model relatively predicts the experimental data well. In addition, calculated results clearly show the generation of the large scale pulsation in the gap and the effect of Re.

Particle injections observed at the morning sector as a response to IMF turning

We report a detailed case study of substorm on January 6, 2008 in the interval 13–15 UT using data from four THEMIS satellites located in the morning sector magnetosphere eastward the onset location. The substorm of interest presents the ground-based magnetic disturbance consisted from the large-scale pulsations (4–5min) superposed on the substorm bay. One can distinguish at least two significant activations at different spatial regions. First activation, which follow after a short-living burst of the IMF Bz, developed westward of second one, which was sequent after the northward turning IMF Bz.We show the existence of fast magnetosonic mode after second activation. This mode was observed at 7.5RE in the morning sector at region of transition from dipole to tail-like configuration of the magnetic field. The increase of z-component of the magnetic field observed in magnetosphere during the non-diamagnetic structure is interpreted as an enhancement of westward ring (or partial-ring) current at closer to Earth distances. The appearance of the sub-keV plasma at ∼5.8RE (used as a tracer of substorm injection) supports this supposition.

A physical–mathematical model of the power circuit of a plasma torch

The article describes the results of development of a physical–mathematical model of a power source–arc system in plasma cutting which takes into account the instability of arcing at low currents (pilot arc), the process of transition of the arc to the working load and the non-stationary characteristics – the shunting mechanism of the DC arc, causing changes in the length of the arc and large-scale pulsations of current and voltage.

Numerical Analysis of Three Types of Cavitation Surge in Cascade

In the present study, numerical analysis of a cavitating three-blade cyclic flat-plate cascade was performed considering cavitation surge, which is a type of cavitation instability in pumping machinery. A numerical method employing a uniquely developed gas-liquid two-phase model was applied to solve the unsteady cavitating flow field, where compressibility is considered in the liquid phase of the model. From the numerical results, the surging oscillations by cavitation represented in the present cascade system can be classified into three types of cavitation surge based on the oscillation characteristics and the flow fields. In the first type, oscillation is composed of small-vortex cavitation and large scale pulsation, which correspond to “surge mode oscillation” whose frequency is not affected by cavity volume. The second type of oscillation is composed of sheet cavitation with a re-entrant jet, which corresponds to so-called “cavitation surge.” The final type of oscillation is subsynchronous rotating cavitation accompanied by pulsation, which is considered as superposition of system and local instability. In addition, the locking phenomenon of break-off frequency of cavitation in the surging oscillations and the mechanism of the pulsation phenomenon accompanied by re-entrant jet in the present cascade were investigated.

Pulsating stochastic flows accompanying microwave filament/supersonic shock layer interaction

The details of pulsating stochastic flows accompanying the interaction of a microwave filament (regarded as a heated rarefied channel) and an aerodynamic body in supersonic flow are examined numerically using the Euler equations. Symmetrical and asymmetrical filament locations relative to the aerodynamic body are considered. The flowfields are characterized by large scale pulsations and small scale stochastic fluctuations. The mechanisms of the formation of these flow structures are discussed. Two qualitatively different kinds of flowfields are observed depending on the magnitude of the filament radius, with domination of the pulsations of flow parameters or stochastic phenomena. Flow instabilities inherent to the problems under interest are described. The problems are considered in both plane and cylindrical configurations for a wide class of filament characteristics and freestream Mach numbers equal to 1.89 and 3.

Aerothermal Performance of Streamwise and Compound Angled Pulsating Film Cooling Jets

The quantification of aerothermal loss is carried out for streamwise and compound angled film cooling jets with and without large scale pulsation. This paper reports on the simultaneous measurements of the unsteady pressure and temperature field of streamwise and a 60 deg compound angled film cooling jet, both with a 30 deg surface angle over a flat plate with no pressure gradients. Turbine representative nondimensionals in terms of the geometry and operating conditions are studied. The main flow is heated more than the injected flow to have a temperature difference and hence a density ratio of 1.3, while the blowing ratio is maintained at 2. The entropy change, derived from pressure and temperature measurements, is calculated by using modified reference conditions to better reflect the losses in both the jet and the freestream. The effects of the periodic unsteadiness associated with rotating machinery are simulated by pulsating the jets. These effects are documented through time-resolved entropy change contours. Mass-averaged entropy and kinetic energy loss coefficients seem to be apt quantities for comparing the aerothermal performance of streamwise and compound angled injections. It is observed that the mass-averaged entropy loss of a streamwise jet doubles when it is pulsated, whereas that of a compound angled jet increases by around 50%. It may be conjectured from the measurements shown in this study that streamwise oriented jets suffer most of their entropy losses at the hole exit due to separation, whereas in compound angled jets, downstream thermal mixing between the jet and the freestream is the dominant mechanism.

Large-Scale Pulsation Detection by Means of Temperature Measurements

In the present work we explore the potential of time-resolved temperature measurements to obtain information on large-scale pulsations in a rod bundle geometry with axial flow. Large-scale flow pulsation is the phenomenon that dominates the turbulent mixing between the subchannels of rod bundles, which explains why it is of great importance for the design or assessment of nuclear fuel elements. The objective of the present work is to determine the characteristics of large-scale pulsations that can be used for the verification or validation of computational fluid dynamics code results. The method proposed is to generate a temperature gradient across the location of flow pulsations and to measure the time-varying temperature field downstream. Pulsation characteristic times, lengths, and traveling speed have been obtained. This study has been performed in a rod bundle similar to a nuclear fuel assembly and the results obtained are in good agreement with previous works on similar geometries. The technique can be applied to obtain additional large-scale structure information in test sections designed for thermal measurements, in situations where convection is dominated by these structures.

Boiling up of jets of superheated ethanol-water solutions

The stability and evolution of the shapes of jets of strongly superheated aqueous solutions of ethanol flowing out of a high-pressure chamber are experimentally investigated at various initial parameters. It has been found that a low-boiling liquid causes shifting to lower temperatures of the boundaries between various kinds of boiling-up jets, such as cylindrical, conical, and open. Large-scale pulsations of the jet shapes are observed in the temperature range from 454 to 484 K, where the jet completely breaks down. The conditions for the formation of large-scale low-frequency pulsations with a power spectrum diverging inversely proportional to the frequency f (1/f spectra) have been revealed. Such fluctuations prove that the flow is unstable and large-scale low-frequency ejections are possible.

Generation of small-scale structures in well-developed turbulence

The Navier-Stokes equation for incompressible liquid is considered in the limit of infinitely large Reynolds number. It is assumed that the flow instability leads to generation of steady-state large-scale pulsations. The excitation and evolution of the small-scale turbulence is investigated. It is shown that the developed small-scale pulsations are intermittent. The maximal amplitude of the vorticity fluctuations is reached along the vortex filaments. Basing on the obtained solution, the pair correlation function in the limit $r\to 0$ is calculated. It is shown that the function obeys the Kolmogorov law $r^{2/3}$.

Turbulent diffusion of passive impurities

It is shown that in turbulent flows with a divergent Lo\(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l} \)tsyanski\(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\smile}$}}{l} \) integral, the coefficient of turbulent diffusion may be determined by pulsations whose scales are comparable with those of the liquid itself rather than by the largest-scale turbulent vortices. This is a new physical mechanism which must be taken into account to explain the experimentally observed phenomenon of impurity “hyperdiffusion” in turbulent media. The contribution of large-scale pulsations increases substantially with the onset of turbulence regimes which are accompanied by the spontaneous generation of structures comparable with the dimensions of the liquid.

Comprehensive Modeling of Turbulent Flames With the Coherent Flame-Sheet Model—Part I: Buoyant Diffusion Flames

A modified version of the computational fluid dynamics code KIVA-II was used to model the transient behavior of buoyant turbulent diffusion flames burning in still air. Besides extensions to the range of permitted boundary conditions and the addition of buoyancy terms to the turbulence model, KIVA-II was augmented by a version of the coherent flame-sheet model, Tesner’s soot generation model, Magnussen’s soot oxidation model, and an implementation of the discrete transfer radiation model that included both banded and continuum radiation. The model captured many of the features of buoyant turbulent flames. Its predictions supported experimental observations regarding the presence and frequency of large-scale pulsations, and regarding axial distributions of temperature, velocity, and chemical species concentrations. The radial structure of the flame was less well represented. The axial radiative heat flux distribution from the flame highlighted deficiencies in the soot generation model, suggesting that a model of soot particle growth was required.

The expected pulsations of the heliosphere relevant to cosmic ray variations

The possible natural large-scale pulsations of the solar wind cavity are examined. The period of the pulsationsT∼2πR/a (R is the size of the cavity,a is the sound velocity in the interstellar space near the solar system) may vary from a year to tens of years. The relevant new type of cosmic ray variations is predicted.

Representation of interaction in the theory of turbulence

The concept that in a turbulent flow energy exchanges only take place of pulsations of near scales is the basis of macroscopic theory of local turbulence structure. Universality and similarity of small-scale statistical pulsations are inferred from the assumption that the energy exchange is of random character. In the Eulerian equations of motion, together with the interactions which implement the energy exchange between pulsations, there are fictitious interactions related to the transfer of pulsations of a given scalel by the pulsations of scalesl′ >>l. It was emphasized in [2, 3] that in the Eulerian description of turbulence the effect of transfer results in a strong statistical dependence of pulsations of different scales. Therefore, the universality and similarity of small-scale pulsations can be observed only in these variables in which there are no effects of pure transfer of some pulsations by the others. Qualitative considerations were therefore given in [1–3] on the need for describing small-scale pulsations in a reference system which is in motion at each point with all large-scale pulsations. It is shown in the present article that such description of small-scale pulsations can be implemented with the aid of transfer representation similar to the representation of interaction in the quantum field theory [4]. Representation of interaction is of intermediate position between the Lagrangian and Eulerian descriptions of turbulence, since a transfer of a packet as an entity can be described in variables which are Lagrangian only as regards large-scale motions. Another way of eliminating transfer interactions is based on the introduction of nonsolenoid velocity as in [5]. From the physical point of view, the method employed in this article seems to be more appropriate.