The small scale velocity field of a quiescent prominence

Solar Physics ◽  
1981 ◽  
Vol 70 (2) ◽  
pp. 315-324 ◽  
Author(s):  
O. Engvold
Keyword(s):  
Velocity Field ◽  
Small Scale ◽  
Quiescent Prominence

Buoyancy effects on large-scale motions in convective atmospheric boundary layers: implications for modulation of near-wall processes

2018 ◽  
Vol 856 ◽  
pp. 135-168 ◽  
Author(s):  
S. T. Salesky ◽  
W. Anderson
Keyword(s):  
Boundary Layer ◽  
Velocity Field ◽  
Boundary Layers ◽  
Vertical Velocity ◽  
Amplitude Modulation ◽  
Large Scale ◽  
Streamwise Velocity ◽  
Small Scale ◽  
Convective Conditions ◽  
Wide Range

A number of recent studies have demonstrated the existence of so-called large- and very-large-scale motions (LSM, VLSM) that occur in the logarithmic region of inertia-dominated wall-bounded turbulent flows. These regions exhibit significant streamwise coherence, and have been shown to modulate the amplitude and frequency of small-scale inner-layer fluctuations in smooth-wall turbulent boundary layers. In contrast, the extent to which analogous modulation occurs in inertia-dominated flows subjected to convective thermal stratification (low Richardson number) and Coriolis forcing (low Rossby number), has not been considered. And yet, these parameter values encompass a wide range of important environmental flows. In this article, we present evidence of amplitude modulation (AM) phenomena in the unstably stratified (i.e. convective) atmospheric boundary layer, and link changes in AM to changes in the topology of coherent structures with increasing instability. We perform a suite of large eddy simulations spanning weakly ($-z_{i}/L=3.1$) to highly convective ($-z_{i}/L=1082$) conditions (where$-z_{i}/L$is the bulk stability parameter formed from the boundary-layer depth$z_{i}$and the Obukhov length $L$) to investigate how AM is affected by buoyancy. Results demonstrate that as unstable stratification increases, the inclination angle of surface layer structures (as determined from the two-point correlation of streamwise velocity) increases from$\unicode[STIX]{x1D6FE}\approx 15^{\circ }$for weakly convective conditions to nearly vertical for highly convective conditions. As$-z_{i}/L$increases, LSMs in the streamwise velocity field transition from long, linear updrafts (or horizontal convective rolls) to open cellular patterns, analogous to turbulent Rayleigh–Bénard convection. These changes in the instantaneous velocity field are accompanied by a shift in the outer peak in the streamwise and vertical velocity spectra to smaller dimensionless wavelengths until the energy is concentrated at a single peak. The decoupling procedure proposed by Mathiset al.(J. Fluid Mech., vol. 628, 2009a, pp. 311–337) is used to investigate the extent to which amplitude modulation of small-scale turbulence occurs due to large-scale streamwise and vertical velocity fluctuations. As the spatial attributes of flow structures change from streamwise to vertically dominated, modulation by the large-scale streamwise velocity decreases monotonically. However, the modulating influence of the large-scale vertical velocity remains significant across the stability range considered. We report, finally, that amplitude modulation correlations are insensitive to the computational mesh resolution for flows forced by shear, buoyancy and Coriolis accelerations.

Periodic oscillations found in the velocity field of a quiescent prominence

Solar Physics ◽  
1986 ◽  
Vol 104 (2) ◽  
pp. 313-320 ◽  
Author(s):  
Tokio Tsubaki ◽  
Akitsugu Takeuchi
Keyword(s):  
Velocity Field ◽  
Quiescent Prominence ◽  
Periodic Oscillations

A movie of small-scale Doppler velocities in a quiescent prominence

2005 ◽  
pp. 240-240
Author(s):  
Serge Koutchmy ◽  
Jack Zirker ◽  
Lou B. Gilliam ◽  
Roy Coulter ◽  
Stephen Hegwer ◽  
...  
Keyword(s):  
Small Scale ◽  
Quiescent Prominence

scholarly journals Fractal Flux Tubes of the Solar Magnetic Field

1991 ◽  
Vol 130 ◽  
pp. 140-146 ◽  
Author(s):  
Alexander Ruzmaikin ◽  
Dmitry Sokoloff ◽  
Theodore Tarbell
Keyword(s):  
Magnetic Field ◽  
Velocity Field ◽  
Magnetic Structure ◽  
Solar Magnetic Field ◽  
Dynamo Model ◽  
Small Scale ◽  
Flux Tubes ◽  
Fractal Set ◽  
Fractal Distribution ◽  
Linear Resolution

Abstract The small-scale solar magnetic field exceeding a given threshold forms a fractal set. A dimension of this fractal is found from magnetograms with varying linear resolution. The dimension depends on the value of the threshold magnetic field (multifractality). A simple dynamo model explaining the origin of the fractal magnetic structure is considered. The dynamo produces a magnetic field in the form of flux tubes with a fractal distribution of magnetic field across the tube. The observed dimension gives a possibility of estimating a degree of structuredness of the solar velocity field.

scholarly journals Small-Scale Dissipative Processes in Stellar Atmospheres

1980 ◽  
Vol 5 ◽  
pp. 581-590
Author(s):  
J. W. Leibacher ◽  
R. F. Stein
Keyword(s):  
Velocity Field ◽  
Energy Flux ◽  
Thermal Energy ◽  
Solar Flares ◽  
Acoustic Waves ◽  
High Current Density ◽  
Stellar Atmospheres ◽  
Small Scale ◽  
High Current ◽  
Strong Magnetic Fields

AbstractThe outer atmospheres of stars must be heated by some non-thermal energy flux to produce chromospheres and coronae. We discuss processes which convert the non-thermal energy flux of organized, macroscopic motions into random, microscopic (thermal) motions. Recent advances in our description of the chromosphere velocity field suggest that the acoustic waves observed there transmit very little energy, and hence are probably incapable of heating the upper chromosphere and corona. The apparent failure of this long held mechanism and the growing appreciation of the importance of strong magnetic fields in the chromosphere and corona have led to hypotheses of heating by the dissipation of currents (both oscillatory and quasi-steady). This follows discoveries in laboratory and ionospheric plasmas and work on solar flares, that instabilities can concentrate currents into thin high current density filaments where they dissipate rapidly.

scholarly journals The response of the copepod Acartia tonsa to the hydrodynamic cues of small-scale, dissipative eddies in turbulence

2020 ◽  
pp. jeb.237297
Author(s):  
Dorsa Elmi ◽  
Donald R. Webster ◽  
David M. Fields
Keyword(s):  
Velocity Field ◽  
Behavioral Response ◽  
Swirling Flow ◽  
Three Dimensional ◽  
Acartia Tonsa ◽  
Small Scale ◽  
Swimming Velocity ◽  
Marine Copepod ◽  
Swimming Kinematics ◽  
Vortex Axis

This study quantifies the behavioral response of a marine copepod (Acartia tonsa) to individual, small-scale, dissipative vortices that are ubiquitous in turbulence. Vortex structures were created in the laboratory using a physical model of a Burgers vortex with characteristics corresponding to typical dissipative vortices that copepods are likely to encounter in the turbulent cascade. To examine the directional response of copepods, vortices were generated with the vortex axis aligned in either horizontal or vertical directions. Tomographic particle image velocimetry was used to measure the volumetric velocity field of the vortex. Three-dimensional copepod trajectories were digitally reconstructed and overlaid on the vortex flow field to quantify A. tonsa’s swimming kinematics relative to the velocity field and to provide insight to the copepod behavioral response to hydrodynamic cues. The data show significant changes in swimming kinematics and an increase in relative swimming velocity and hop frequency with increasing vortex strength. Furthermore, in moderate-to-strong vortices, A. tonsa moved at elevated speed in the same direction as the swirling flow and followed spiral trajectories around the vortex, which would retain the copepod within the feature and increase encounter rates with other similarly behaving Acartia. While changes in swimming kinematics depended on vortex intensity, orientation of the vortex axis showed minimal significant effect. Hop and escape jump densities were largest in the vortex core, which is spatially coincident with the peak in vorticity suggesting that vorticity is the hydrodynamic cue that evokes these behaviors.

Resolving Large- and Small-Scale Structures in the Swirl Flow of a Typical Land-Based Gas Turbine Combustor Single Nozzle Rig

2014 ◽  
Author(s):  
R. K. R. Katreddy ◽  
S. R. Chakravarthy
Keyword(s):  
Velocity Field ◽  
Gas Turbine ◽  
Large Scale ◽  
Recirculation Zone ◽  
Laser Diagnostics ◽  
Swirl Flow ◽  
Sudden Expansion ◽  
Small Scale ◽  
Gas Turbine Combustor ◽  
Scale Structures

The present study focuses on identifying and resolving large-scale energy containing structures and turbulent eddies in a typical gas turbine combustor single nozzle rig, using particle image velocimetry in cold flow. A generic fuel-air nozzle through a swirler is integrated with a sudden expansion square duct with optical access to perform laser diagnostics. Experiments are conducted to analyze the swirl flow field under starting and operating flow conditions. Three-component velocities are obtained in cross-sectional planes of Z/D = 0, 1.25, and 2.5 (normalized by the nozzle diameter), and two-component velocities are obtained in the mid-plane along the longitudinal (Z-) axis from Z/D = 0 to 2.5D. Velocity splitting is performed using spatial Gaussian smoothing with a kernel with filter width equal to integral scale is performed over the velocity fields to resolve the field of large-scale energy containing eddies. Proper orthogonal decomposition is performed over the large-scale velocity field, and the modes obtained indicate the existence of the precessing vortex core (PVC), formation of small scales Kelvin-Helmholtz (K-H) vortices for Z/D < 1.25D, and large-scale growing K-H structures in 1.25D < Z/D < 2.5D. Turbulent kinetic energy (TKE) is obtained from the turbulent velocity fluctuations below the integral length scale and is observed to be higher at the interface of the corner recirculation zone (CRZ) and central toroidal recirculation zone (CTRZ). Resolving the swirl velocity field obtained in the above manner into large-scale structures formed by the PVC, CTRZ, K-H vortices, CRZ, and small-scale turbulence field, indicates the clear distinction in rapid mixing zones and unsteady convective zones. The length-scales and zones of these structures within the swirl combustor are identified.

scholarly journals Mapping Small-Scale Horizontal Velocity Field in Panzhinan Waterway by Coastal Acoustic Tomography

Sensors ◽  
2020 ◽  
Vol 20 (19) ◽  
pp. 5717
Author(s):  
Haocai Huang ◽  
Xinyi Xie ◽  
Yong Guo ◽  
Hangzhou Wang
Keyword(s):  
Velocity Field ◽  
Travel Time ◽  
Sound Transmission ◽  
Volume Transport ◽  
Velocity Fields ◽  
Horizontal Velocity ◽  
Least Square ◽  
Small Scale ◽  
Acoustic Tomography ◽  
Mean Square Errors

Mapping small-scale high-precision velocity fields is of great significance to oceanic environment research. Coastal acoustic tomography (CAT) is a frontier technology used to observe large-scale velocity field in the horizontal slice. Nonetheless, it is difficult to observe the velocity field using the CAT in small-scale areas, specifically where the flow field is complex such as ocean ranch and artificial upwelling areas. This paper conducted a sound transmission experiment using four 50 kHz CAT systems in the Panzhinan waterway. Notably, sound transmission based on the round-robin method was recommended for small-scale CAT observation. The travel time between stations, obtained by correlation of raw data, was applied to reconstruct the horizontal velocity fields using Tapered Least Square inversion. The minimum net volume transport was 8.7 m3/s at 12:32, 1.63% of the total inflow volume transport indicating that the observational errors were acceptable. The relative errors of the range-average velocity calculated by differential travel time were 1.54% (path 2) and 0.92% (path 6), respectively. Moreover, the inversion velocity root-mean-square errors (RMSEs) were 0.5163, 0.1494, 0.2103, 0.2804 and 0.2817 m/s for paths 1, 2, 3, 4 and 6, respectively. The feasibility and acceptable accuracy of the CAT method in the small-scale velocity profiling measurement were validated. Furthermore, a three-dimensional (3-D) velocity field mapping should be performed with combined analysis in horizontal and vertical slices.

scholarly journals Intense velocity-shears and magnetic fields in diffuse molecular gas: from 10 pc to 5 mpc

2009 ◽  
Vol 5 (H15) ◽  
pp. 442-443
Author(s):  
Edith Falgarone ◽  
Pierre Hily-Blant
Keyword(s):  
Velocity Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Dust Emission ◽  
Large Scale Structure ◽  
Small Scale ◽  
Molecular Gas ◽  
Gas Velocity ◽  
Star Forming ◽  
Star Forming Regions

AbstractRegions of intense velocity-shears are identified on statistical grounds in nearby diffuse molecular gas: they form conspicuous thin (~ 0.03 pc) and parsec-long structures that do not bear the signatures of shocked gas. Several straight substructures, ~ 3 mpc thick, have been detected at different position-angles within one of them. Two exhibit the largest velocity-shears ever measured far from star forming regions, up to 780 kms−1pc−1. Their position-angles are found to be also those of 10-parsec striations in the I(100μm) dust emission of the large scale environment. The B field projections, where available in these fields, are parallel both to the parsec- and to one of the milliparsec-scale shears. These findings put in relation the small-scale intermittent facet of the gas velocity field and the large scale structure of the magnetic fields.

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