Line profiles and longitudinal velocity field in seeing limited small-scale atmospheric structures

2006 ◽  
pp. 172-177
Author(s):  
F. Falchi ◽  
R. Falciani ◽  
L. A. Smaldone
Keyword(s):  
Velocity Field ◽  
Longitudinal Velocity ◽  
Small Scale ◽  
Line Profiles

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.

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.

Velocity Field and Energy Dissipation in Viscoplastic Flow in Tubes of Non-Circular Cross-Section

2014 ◽  
Author(s):  
Mario F. Letelier ◽  
Dennis A. Siginer ◽  
Felipe Godoy
Keyword(s):  
Energy Dissipation ◽  
Velocity Field ◽  
Yield Stress ◽  
Cross Section ◽  
Mechanical Energy ◽  
Longitudinal Velocity ◽  
Circular Cross Section ◽  
Viscoplastic Flow ◽  
Entire Cross Section ◽  
Cross Sectional

An analytical method for determining the velocity field, shear stress and energy dissipation in viscoplastic flow in non-circular straight tubes is presented. Bingham’s model of fluid is used for the case of tubes with several cross-sectional contours that can be arbitrarily chosen through a shape factor imposed in the solution for the longitudinal velocity. The analysis is extended to steady flow in tubes in which the cross-section contour exhibits sharp corners. In these cases three flow zones are distinguished: stagnant, non-zero deformation, and plug zones. The method provides the expressions for determining the boundaries and characteristics of those three zones for a wide variety of cross-section shapes. In particular the dynamics of plug-zones for large values of the yield stress and for contours that markedly differ from circumferences is analyzed. Energy dissipation is determined throughout the entire cross-section, so that the effect of shape on mechanical energy loss is assessed in terms of the yield stress and viscosity of the fluid. Some general expressions that help understand energy dissipation mechanisms are derived by using natural coordinates for the velocity field and related variables. These results draw on several recent works from other researchers and the present authors, which have highlighted the significant difficulty of determining the zones of zero deformation in viscoplastic flow when the related solid boundaries are not elementary.

Small-scale statistics in high-resolution direct numerical simulation of turbulence: Reynolds number dependence of one-point velocity gradient statistics

2007 ◽  
Vol 592 ◽  
pp. 335-366 ◽  
Author(s):  
T. ISHIHARA ◽  
Y. KANEDA ◽  
M. YOKOKAWA ◽  
K. ITAKURA ◽  
A. UNO
Keyword(s):  
Reynolds Number ◽  
High Resolution ◽  
Longitudinal Velocity ◽  
Second Order ◽  
Distribution Functions ◽  
Small Scale ◽  
Kolmogorov Length Scale ◽  
Velocity Gradients ◽  
In Series ◽  
Derivatives Of

One-point statistics of velocity gradients and Eulerian and Lagrangian accelerations are studied by analysing the data from high-resolution direct numerical simulations (DNS) of turbulence in a periodic box, with up to 40963 grid points. The DNS consist of two series of runs; one is with kmaxη ~ 1 (Series 1) and the other is with kmaxη ~ 2 (Series 2), where kmax is the maximum wavenumber and η the Kolmogorov length scale. The maximum Taylor-microscale Reynolds number Rλ in Series 1 is about 1130, and it is about 675 in Series 2. Particular attention is paid to the possible Reynolds number (Re) dependence of the statistics. The visualization of the intense vorticity regions shows that the turbulence field at high Re consists of clusters of small intense vorticity regions, and their structure is to be distinguished from those of small eddies. The possible dependence on Re of the probability distribution functions of velocity gradients is analysed through the dependence on Rλ of the skewness and flatness factors (S and F). The DNS data suggest that the Rλ dependence of S and F of the longitudinal velocity gradients fit well with a simple power law: S ~ −0.32Rλ0.11 and F ~ 1.14Rλ0.34, in fairly good agreement with previous experimental data. They also suggest that all the fourth-order moments of velocity gradients scale with Rλ similarly to each other at Rλ > 100, in contrast to Rλ < 100. Regarding the statistics of time derivatives, the second-order time derivatives of turbulent velocities are more intermittent than the first-order ones for both the Eulerian and Lagrangian velocities, and the Lagrangian time derivatives of turbulent velocities are more intermittent than the Eulerian time derivatives, as would be expected. The flatness factor of the Lagrangian acceleration is as large as 90 at Rλ ≈ 430. The flatness factors of the Eulerian and Lagrangian accelerations increase with Rλ approximately proportional to RλαE and RλαL, respectively, where αE ≈ 0.5 and αL ≈ 1.0, while those of the second-order time derivatives of the Eulerian and Lagrangian velocities increases approximately proportional to RλβE and RλβL, respectively, where βE ≈ 1.5 and βL ≈ 3.0.

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 Observing convection in stellar atmospheres

2006 ◽  
Vol 2 (S239) ◽  
pp. 103-112
Author(s):  
John D. Landstreet
Keyword(s):  
Velocity Field ◽  
Stellar Atmosphere ◽  
Velocity Fields ◽  
Stellar Atmospheres ◽  
Line Profiles ◽  
Line Centre ◽  
Solar Granulation ◽  
Voigt Profile ◽  
Microturbulent Velocity ◽  
Granulation Pattern

AbstractConvection occurs in the visible photospheric layers of most stars having Te less than about 10000 K, and in some hotter stars. The solar granulation pattern is a symptom of this, as is the non-zero microturbulent velocity often required in abundance analysis to make both weak and strong lines yield the same abundance.In very sharp-lined stars, the presence of a non-thermal velocity field in the visible stellar atmosphere leads to several other effects which may be detected in spectral line profiles. These include radial velocities that vary systematically with equivalent width, distortions of the line profile as compared to a profile computed with a Voigt profile and rotational broadening (“macroturbulence”), and asymmetries with respect to the line centre (“bisector curvature”).Detection and interpretation of these effects, with the goal of obtaining empirical information about a velocity field present in the visible layers, requires comparison with calculated synthetic spectra which incorporate model velocity fields. Thus, this review will summarize some of the observational clues concerning photospheric velocity fields, as well as modelling aimed at interpreting these data.

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.

scholarly journals Small-scale Experimental Test Rig for Lateral Vehicle Control

2021 ◽  
Vol 65 (2) ◽  
pp. 163-170
Author(s):  
Illés Vörös ◽  
László Turányi ◽  
Balázs Várszegi ◽  
Dénes Takács
Keyword(s):  
Longitudinal Velocity ◽  
Cost Effective ◽  
Conveyor Belt ◽  
Scale Model ◽  
Small Scale ◽  
Control Methods ◽  
Steering Control ◽  
Test Rig ◽  
Test Environment ◽  
Analytical Stability

This paper presents the design and implementation of a small-scale hardware-in-the-loop test environment for lateral vehicle dynamics controllers. The test rig consists of a conveyor belt and a 1:10 scale model vehicle. The vehicle is anchored to the frame of the conveyor belt using a special fixture, which constrains only the longitudinal displacement of the car. Therefore, the longitudinal velocity of the vehicle is provided by the conveyor belt, while the steering is generated by the computational unit, where various control methods can be implemented. The test rig is equipped with sensors that provide accurate measurements of the position and orientation of the car, which can be used as feedback in the control algorithms. The paper includes a case study, where the analytical stability analysis of a lane-keeping controller is verified with experiments on the test rig. The proposed test environment provides a compact, cost effective and versatile framework for the testing of various steering control methods in a running vehicle, while maintaining the benefits of a controlled laboratory environment. The experimental setup can also be used for educational and demonstrational purposes.

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.

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