Structure Of Velocity Fields Research Articles

Decomposition of Turbulent Velocity Fields in an SI Engine

In this study, the turbulence filter, the phase averaging and the proper orthogonal decomposition methods are used to decompose experimentally measured turbulent velocity fields in an SI engine. The radial and circumferential turbulent velocity fields were measured using hot wire anemometer under motored conditions at different engine configurations. The decomposed results of each technique are compared with each other. In addition, the obtained organized and turbulence motions and their energy spectra are examined. Finally, coherent structures of velocity fields and their activities are investigated.

A resolvable subfilter-scale model specific to large-eddy simulation of under-resolved turbulence

Large-eddy simulation (LES) of boundary-layer flows has serious deficiencies near the surface when a viscous sublayer either does not exist (rough walls) or is not practical to resolve (high Reynolds numbers). In previous work, we have shown that the near-surface errors arise from the poor performance of algebraic subfilter-scale (SFS) models at the first several grid levels, where integral scales are necessarily under-resolved and the turbulence is highly anisotropic. In under-resolved turbulence, eddy viscosity and similarity SFS models create a spurious feedback loop between predicted resolved-scale (RS) velocity and modeled SFS acceleration, and are unable to simultaneously capture SFS acceleration and RS–SFS energy flux. To break the spurious coupling in a dynamically meaningful manner, we introduce a new modeling strategy in which the grid-resolved subfilter velocity is estimated from a separate dynamical equation containing the essential inertial interactions between SFS and RS velocity. This resolved SFS (RSFS) velocity is then used as a surrogate for the complete SFS velocity in the SFS stress tensor. We test the RSFS model by comparing LES of highly under-resolved anisotropic buoyancy-generated homogeneous turbulence with a corresponding direct numerical simulation (DNS). The new model successfully suppresses the spurious feedback loop between RS velocity and SFS acceleration, and greatly improves model predictions of the anisotropic structure of SFS acceleration and resolved velocity fields. Unlike algebraic models, the RSFS model accurately captures SFS acceleration intensity and RS–SFS energy flux, even during the nonequilibrium transient, and properly partitions SFS acceleration between SFS stress divergence and SFS pressure force.

Density, Velocity, and Magnetic Field Structure in Turbulent Molecular Cloud Models

We use three-dimensional (3D) numerical magnetohydrodynamic simulations to follow the evolution of cold, turbulent, gaseous systems with parameters chosen to represent conditions in giant molecular clouds (GMCs). We present results of three model cloud simulations in which the mean magnetic field strength is varied (B0 = 1.4-14 ?G for GMC parameters), but an identical initial turbulent velocity field is introduced. We describe the energy evolution, showing that (1) turbulence decays rapidly, with the turbulent energy reduced by a factor 2 after 0.4-0.8 flow crossing times (~2-4 Myr for GMC parameters), and (2) the magnetically supercritical cloud models gravitationally collapse after time ?6 Myr, while the magnetically subcritical cloud does not collapse. We compare density, velocity, and magnetic field structure in three sets of model snapshots with matched values of the Mach number ? 9,7,5. We show that the distributions of volume density and column density are both approximately log-normal, with mean mass-weighted volume density a factor 3-6 times the unperturbed value, but mean mass-weighted column density only a factor 1.1-1.4 times the unperturbed value. We introduce a spatial binning algorithm to investigate the dependence of kinetic quantities on spatial scale for regions of column density contrast (ROCs) on the plane of the sky. We show that the average velocity dispersion for the distribution of ROCs is only weakly correlated with scale, similar to mean size-line width distributions for clumps within GMCs. We find that ROCs are often superpositions of spatially unconnected regions that cannot easily be separated using velocity information; we argue that the same difficulty may affect observed GMC clumps. We suggest that it may be possible to deduce the mean 3D size-line width relation using the lower envelope of the 2D size-line width distribution. We analyze magnetic field structure and show that in the high-density regime n 103 cm-3, total magnetic field strengths increase with density with logarithmic slope ~1/3-2/3. We find that mean line-of-sight magnetic field strengths may vary widely across a projected cloud and are not positively correlated with column density. We compute simulated interstellar polarization maps at varying observer orientations and determine that the Chandrasekhar-Fermi formula multiplied by a factor ~0.5 yields a good estimate of the plane-of sky magnetic field strength, provided the dispersion in polarization angles is 25?.

On a small structure in velocity field within a contact region

In a contact region (contact surface in Euler terms) connecting the two uniform regions in a flow field such as caused by the sudden break of a membrane in a shock-tube, there exists a small structure in velocity field, which has been reported earlier by the calculation based on the Boltzmann equation of the BGK type. Probably, the accepted view would be that the velocity within this region is uniform. Here this velocity structure is investigated thoroughly based on not only the Boltzmann equation but also the Navier–Stokes equations. Actually the velocity field has a hump or a pit, the width of which is found to be the same as the thickness of the region. The magnitude of the hump or the pit decreases with time, inversely proportional to the square root of time. At the so-called tailoring condition at which the contact region has so far been thought not to manifest itself, actually it does exist; the velocity structure does not appear of course but the temperature and, hence, the density still have structures, although they are small in magnitude.

NUMERICAL SIMULATION OF TURBULENT CONVECTIVE HEAT TRANSFER IN SQUARE RIBBED DUCTS

This article reports on an investigation on numerical prediction of thermal characteristics of a certain type of duct. The ducts considered have rib turbulators to enhance the heat transfer rate. The calculation method consists of a low Re number turbulence model and two methods for determining the turbulent Reynolds stresses, namely, a simple eddy viscosity model (EVM) [1] and an explicit algebraic stress model (EASM) [2]. The model development is carried out to make the original EASM consistent with the low Re number k- epsilon turbulence model applied. A certain method is developed to deal with the decoupling of the velocity and Reynolds stress fields inthe collocated grid arrangement that is chosen in this study. The SIMPLEC algorithm handles the pressure-velocity coupling. The computations are performed with the assumption of fully developed periodic conditions. These models are used to predict the convective turbulent forced convection in different test cases and the results are compared with experiments. A ribbed duct with two ribs on opposite walls is chosen and the obtained results including the mean thermal characteristics of the considered duct are compared with an experimental correlation. Two further duct configurations, identical to an experimental setup, are then computed. These experimental cases are chosen because detailed thermal-hydraulic information is available and then local comparisons between the two prediction models and experimental results are possible. The calculated mean and local thermal-hydraulic values are compared with corresponding experimental data and the prediction capabilities of the two turbulence models (EVM and EASM) are discussed. Theresults show that the EASM has some superiority over the EVM in the prediction of the velocity field structure, but the mean thermal predictions are not very different. There are also some important features of the flow field, whichare not revealed by theEVM calculations. However, the required CPU times are considerably higher for the EASM case.

Horizontal Structure of the Upper Ocean Velocity and Density Fields in the Western Equatorial Pacific Warm Pool: Depth Range from 20 to 250 m*

Abstract Wavenumber spectra of velocity and density fields in the western equatorial Pacific warm pool on scales 6–120 km are estimated using the shipboard survey data collected during the TOGA Coupled Ocean–Atmosphere Response Experiment (COARE). The spectra are averaged over three depth intervals: 20–60, 60–110, and 110–250 m (corresponding to the Yoshida jet, the South Equatorial Current, and the southern edge of Equatorial Undercurrent). The velocity spectra are corrected for the mean flow shear advection, which is important under conditions of low gradient Richardson number (Ri). After that, both velocity and density spectra are consistent with an internal wave spectral model including a random component (equatorial version of the Garrett and Munk spectrum) and a tidal component (the Feng et al. tidal model). Tidal peaks, previously found by other COARE investigators as being prominent on the “moored” spectra (i.e., on the spectra derived from mooring data), appear to be much less significant on the ...

GROWTH OF TURBULENT FLOW IN A COMPOUND OPEN CHANNEL WHERE VEGETATION DENSITY CHANGES IN THE DOWNSTREAM DIRECTION

A turbulent velocity field was measured in a vegetated compound open channel flow where vegetation density changed abruptly from low to high and high to low in the downstream direction. The changing process of mean velocity, secondary currents and Reynolds stress fields were explained with three dimensional mixing structure caused by large horizontal eddies. As a result, three dimensional structure of turbulent flow, and relation between mean velocity field and turbulent structure were clarified experimentally in an open channel flow with abrupt change in the vegetation density in the downstream direction.

Regional, scale size, and interplanetary magnetic field variability of magnetic field and ion drift structures in the high‐latitude ionosphere

Using data from the Dynamics Explorer 2 satellite's ion drift meter and magnetometer, we have examined fluctuations in the high‐latitude velocity and magnetic field structure to better understand the coupling between the magnetosphere and ionosphere. Our study examines perturbations in the frequency range from 0.25 to 8 Hz, equivalent to static scale sizes between about 30 and 1 km. Seasonal variations in the perturbations at large‐scale sizes indicate that magnetosphere‐ionosphere coupling is dominated by static currents. These static currents display behavior consistent with a current driver in the auroral zone and a voltage driver in the cusp. In the frequency range 0.6 to 1.5 Hz the magnetosphere‐ionosphere coupling is significantly influenced by Alfven waves with the high‐frequency range extending up to 8 Hz in the cusp.

Heat-transfer intensification promoted by vortical structures in closed channel under magnetic field

The effect of large two-dimensional vortical structures on heat transfer in liquid metal flow in an annular channel under an axial uniform strong magnetic field has been experimentally investigated. Hot and cold channel walls were parallel to the field. Structures commensurable with the channel's cross-sectional dimension and stretched along the field were generated owing to flow around the stagnant zones in flow above and below the conducting cylindrical obstacles placed on the hot wall. It is shown that transversal heat transfer increases six times compared with free channel flow. Here, the hot-wall temperature decreased more than three times. In the presence of cylindrical obstacles in the channel, the increase in the power spent for fluid motion is not required to achieve the large Nusselt numbers. The mean and fluctuating velocities in the flow cross section were measured by using a potential probe. The Reynolds stresses and turbulent energy production for the flow were also obtained experimentally. The temperature distributions were measured by thermocouples. Local characteristics measured in the channel are used to provide analysis of flow structure and redistribution of velocity and temperature fields.

Velocity and magnetic field structures in a magnetohydrodynamic dynamo

Computer simulation of the magnetohydrodynamic dynamo in a rotating spherical shell geometry is performed. The convection velocity and the magnetic field structures are investigated in detail when the magnetic field is exponentially growing. It is found that the magnetic energy is strongly localized in space. The energy is generated when toroidal field lines are stretched by the fluid flows in columnar convection cells.

Fundamental Characteristics of Mixed Convective Laminar Flow in Rotating Curved Pipes. Scaling Law and Thermo-Fluid Structure.

The fluid flowing in rotating heated curved pipes is subjected to three body forces : centrifugal, Coriolis and buoyant forces. Fully developed laminar flow in rotating coiled pipes is investigated through similarity arguments and computational studies. The thermal boundary conditions at the wall are uniform wall heat flux axially and uniform wall temperature peripherally. Flow and heat transfer characteristics are elucidated for the case when they are governed by four parameters : Dean number, Prandtl number, body force ratio and buoyancy parameter. Detailed structures of velocity and temperature fields are shown for wide range of these parameters.

Boundary-layer relaxation after a separated region

With reference particularly to the work of Peter Bradshaw and his associates, some remarks are made about the recovery of previously distorted shear flows. It is emphasized that such recovery is usually extremely slow, and this is further illustrated by new measurements of the velocity field and turbulence structure in the relaxing flow downstream of a separated region. Data have been obtained for downstream distances ( x) up to about 20 times the length of the separated region ( x r), or about 75 times the flow thickness at reattachment. This is a significantly more extensive region than has been previously studied, and the data are more comprehensive than any previously available. It is shown that the recovery is even slower than previously surmized. Furthermore, the measurements demonstrate that the turbulence stresses eventually fall below standard boundary-layer values (at the same Reynolds number), although around reattachment they are very much higher, having values more akin to those in plane mixing layers. This undershoot is apparently a new finding and is argued to be a result of the influence of the outer part of the flow on the growing inner region. The usual log-law only begins to appear beyond x/ x r = 2.5. It effectively “sees” a turbulent outer region that recovers even more slowly than itself, and the response of the inner region therefore has similarities to the response of an ordinary boundary layer to free-stream turbulence. It is concluded that even current second-order (i.e., Reynolds stress) models may not capture the exquisitely slow decay of the strong, large eddy motions in the outer part of the flow and the subtleties of their influence on the inner region.

Cinematic particle image velocimetry of high-Reynolds-number turbulent free shear layer

The objective of this research was to study the time-evolving velocity field in a two-stream, turbulent, planar free shear layer using a cinematic partide image velocimetry technique. The water shear layer had a velocity ratio of 0.23 and a Reynolds number of 2.62 X 10 4 based on velocity thickness and velocity difference. The cinematic particle image velocimetry system employed an argon-ion laser, a scanning mirror, and a 35-mm movie camera. Experimental data obtained by this technique yielded a combined spatial and temporal evolution of the two-dimensional velocity and spanwise vorticity fields. The detailed velocity field structure of the shear layer was significantly different from previous lower Reynolds number flow visualizations in that the classical well-defined eddies and braids were replaced with complex three-dimensional agglomerated vortices of both signs. The velocity field evolution was also notably different from that of the passive scalar field, where the former exhibited stronger temporal variations and reduced spatial coherency. Temporal and spatial correlations yielded transverse distributions of convection velocities based on both streamwise velocity perturbations and vorticity.

Fundamental Characteristics of Mixed Convective Laminar Flow in Heated Curved Pipes.

The fluid flowing in heated curved pipes is subjected to centrifugal and buoyant forces. Fully developed laminar flow in horizontal, loosely coiled pipes is investigated through similarity arguments and computational studies. The thermal boundary condition at the wall is uniform wall heat flux axially and uniform wall temperature peripherally. Flow and heat transfer are characterized by three parameters: Dean number, Prandtl number and buoyancy parameter. Detailed structures of velocity and temperature fields are shown for a wide range of these parameters.

The second moments, spectra and correlation functions of velocity and temperature fluctuations in the gradient sublayer of a retarded boundary layer

The turbulent structure of velocity and temperature fields in moving equilibrium retarded boundary layers is analyzed. Most attention is given to ‘the gradient sublayer’, where, according to Ginevskii and Solodkin [ Prikl. Mat. Mech. (Appl. Math. Mech.) 22, 819–825 (1958)], Stratford [ J. Fluid Mech. 5, 1–16; 17–35 (1959)] and Perry et al. [ J. Fluid Mech. 25, 299–320 (1966)], the mean velocity and temperature profiles are described by the half-power and inverse half-power laws. Kader [ Dokl. Akad. Nauk U.S.S.R. 279, 323–327 (1984); Int. J. Heat Mass Trans. 34, 2837–2857 (1991)] deduced formulas for spectra and cospectra of velocity components and temperature in the gradient sublayer for the mesoscale range of wave numbers k by dimensional analysis and then compared them with available experimental data. It is shown that accurate determination of velocity variances and Reynolds stresses requires taking into account the contribution of large-scale turbulent disturbances corresponding to small values of k. It is not so for determination of the temperature variance and vertical heat flux evaluation. An analysis of low wave number parts of velocity and temperature spectra and cospectra is given, and its results are used to determine the correlation functions of turbulent fluctuations in the gradient sublayer. The formulas for one-point second-order moments (variances 〈 t 2〉, 〈 u 2) and 〈 v 2〉, temperature flux 〈 vt〉, and Reynolds stress 〈 −uv〉) in the gradient sublayer of quasi-equilibrium flows are also derived and compared with the available data. Comparison of calculated and experimental spectra of non-equilibrium retarded flows uncovers disagreement in the mesoscale wave number part of the spectra for vertical velocity and Reynolds stress fluctuations. At the same time longitudinal fluctuation spectra and one-point variance 〈 u 2) prove to be less sensitive to non-equilibrium conditions.

H? Doppler-velocity fields within sites of flares in a solar active region

In this paper, we analyze the relationship between photospheric magnetic fields and chromospheric velocity fields in a solar active region, especially evolving features of the chromospheric velocity field at preflare sites. It seems that flares are related to unusually distributed velocity field structures, and initial bright kernels and ribbons of the flares appear in the red-shifted areas (i.e., downward flow areas) close to the inversion line of Hβ Dopplergrams with steep gradients of the velocity fields, no matter whether the areas have simple magnetic structure or a weak magnetic field, or strong magnetic shear and complex structure of the magnetic fields. The data show that during several hours prior to the flares, while the velocity field evolves, the sites of the flare kernels (or ribbons) with red-shifted features come close to the inversion line of the velocity field. This result holds regardless of whether or not the flare sites are wholly located in blue-shifted areas (i.e., upward flow areas), or are far from the inversion line of the Doppler velocity field (V ∥ = 0 line), or are partly within red-shifted areas. There are two cases favourable for the occurrence of flares, one is that the gulf-like neutral lines of the magnetic field (B ∥ = 0 line) occur in the Hβ red-shifted areas, the other is that the gulf-like inversion lines of the Hβ Doppler velocity field (V ∥ = 0 line) occur in the unipolar magnetic areas. These observational facts indicate that the velocity field and magnetic field have the same effect on the process of flare energy accumulation and release.

Structure of temperature and velocity fields in the Si melt of a Czochralksi crystal growth system

Non-axisymmetric temperature and velocity profiles attributed to crucible rotation in the Si melt of a Czochralski (Cz) system were studied numerically by using three-dimensional (3D) numerical simulation and experimentally by measuring temperature. Two types of non-axisymmetric temperature and velocity profiles were found to exist in the melt. The type observed at low rotation rates occurs under conditions in which Rayleigh-Bénard instability occurs, and the other type of non-axisymmetric flow observed at high rotation rates occurs under conditions corresponding to the occurrence of the baroclinic instability in the rotating annulus. The 3D structure of the temperature field was studied experimentally to confirm the possibility of baroclinic instability in the melt. The predominant trend was a tilt of the temperature field in the same direction as crucible rotation. It was found that the temperature field tilted along the azimuthal direction under conditions in which baroclinic instability occurs.

Dynamics of the Galactic Globular Cluster NGC 3201

B,V CCD frames have been used to derive surface brightness profiles for NGC 3201 out to ~18 arcmin. A total of 857 radial velocities with median precision ~1 km/s for 399 member giants have been used to trace the velocity dispersion profile out to 32' (the approximate tidal radius from fits of single-mass, isotropic King-Michie models to the cluster surface brightness profiles). The median difference in radial velocity for stars on either side of an imaginary axis moved through the cluster in 1 degree steps shows a significant maximum amplitude of 1.22+/-0.25 km/s. We discuss possible explanations of this result, including: (1) cluster rotation; (2) preferential stripping of stars on prograde orbits near the limiting radius; (3) the projection of the cluster space velocity onto the plane of the sky and (4) a slight drift in the velocity zero point. It is difficult to identify the primary cause of the observed velocity field structure unambiguously, and we suspect that all of the above processes may play a role. The B,V surface brightness profiles and radial velocities have been modeled with single- & multi-mass King-Michie models and nonparametric techniques. The density and M/L profiles show good agreement over 1.5<R<10 pc, and both approaches suggest a steady rise in M/L with distance from the cluster center. Due to the low cluster luminosity, we are unable to place useful constraints on the anisotropy of the velocity dispersion profile, though the global mass-to-light ratio is well-constrained by the models as ~2.0 +/-0.2 for the multi-mass and nonparametric models, compared to ~ 1.65 +/-0.15 for models having equal-mass stars. Our best-fit, multi-mass models have mass function slopes of x~0.75 +/-0.25, consistent with findings that mass function depends on the position relative to the potential of the Galaxy.

Mass transfer characteristics for a rotating cup-like cylinder

A numerical study is made of flow and mass transfer characteristics for a cup-like cylinder, which rotates steadily about its own central longitudinal axis. This study simulates the earlier mass transfer experiment of Sparrow and Chaboki, which provided only the averaged value of Sherwood number. Comprehensive numerical solutions have been obtained for the Navier-Stokes equations over an extended range of Reynolds numbers. Numerically-constructed flow visualizations exhibit the structures of three-component velocity and concentration fields. The patterns of meridional flows, which are directly responsible for convective transport, are analyzed. The distributions of azimuthal flow are illustrated. Plots of the local Sherwood number at the inner surface of the cup are given. Physically plausible descriptions are presented Gf the local mass transfer characteristics for both cases of a transferring base endwall and a non transferring base endwall. The numerical results of the cavity-average Sherwood number are consistent with the previous experimental data.

Infrared spectroscopy of Be/X-ray binaries

We present infrared spectra of 4 Be/X-ray binaries in the K band, and 4 spectra in the J, H and K bands of 2 more sources. The HI IR emission lines are useful determinators of the conditions in the inner regions of the circumstellar disk about the Be star, due to optical depth effects. These are preliminary results, and hope to be followed up by high resolution echelle spectra, where we wish to estimate the velocity field, temperature and density structure of the circumstellar material.