Random Turbulent Motions Research Articles

Redshift-space distortions of the H i 21-cm intensity mapping signal due to the internal motions within galaxies

ABSTRACTThe H i 21-cm intensity mapping signal experiences redshift-space distortions due to the motion of the galaxies which contain the H i as well as the motions of the H i gas within the galaxies. A detailed modelling is essential if this signal is to be used for precision cosmology. Considering dark-matter-only simulations where the H i is assumed to reside in galaxies which are associated with haloes, in this work we introduce a technique to incorporate the H i motions within the galaxies. This is achieved through a line profile which accounts for both the rotational and random (thermal and turbulent) motions of the H i within galaxies. The functional form of the double-horned line profiles used here is motivated by observations of z = 0 spiral galaxies. Analyzing the simulated 21-cm power spectrum over the redshift range 1 ≤ z ≤ 6 we find that the H i motions within galaxies make a significant contribution that is manifested as an enhancement in the Finger of God (FoG) effect which can be modelled reasonably well through a Lorentzian damping profile with a single free parameter σp. The value of σp is significantly enhanced if motions within the galaxies are included. This is particularly important at z > 3 where σp is dominated by the internal motions and a measurement of the FoG effect here could provide a handle on the line profiles of high-redshift galaxies.

The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially resolved local star-forming galaxies

We investigate the energy sources of random turbulent motions of ionised gas from H$\alpha$ emission in eight local star-forming galaxies from the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey. These galaxies satisfy strict pure star-forming selection criteria to avoid contamination from active galactic nuclei (AGN) or strong shocks/outflows. Using the relatively high spatial and spectral resolution of SAMI, we find that -- on sub-kpc scales our galaxies display a flat distribution of ionised gas velocity dispersion as a function of star formation rate (SFR) surface density. A major fraction of our SAMI galaxies shows higher velocity dispersion than predictions by feedback-driven models, especially at the low SFR surface density end. Our results suggest that additional sources beyond star formation feedback contribute to driving random motions of the interstellar medium (ISM) in star-forming galaxies. We speculate that gravity, galactic shear, and/or magnetorotational instability (MRI) may be additional driving sources of turbulence in these galaxies.

On the genesis and evolution of barchan dunes: morphodynamics

Barchan dunes are crescent-shaped formations of sand that can dominate both desert and subaqueous landscapes when the supply of sand is scarce. Because of the complexity and scale of the underlying phenomena, the mechanisms governing the entire process from the genesis to the long-term evolution of barchan fields still remain to be better understood. Herein, we attempt to present a description of this process in a subaqueous environment by employing a large-eddy simulation approach that couples turbulent flow and sand-bed morphodynamics. We show that the seeds of the emergent structure in barchan fields are random turbulent flow motions near the initially flat bed. We also provide high-resolution insights into phenomena such as barchan migration and merging, and show how transverse sand waves are formed and migrate over the barchan horns. Furthermore, the transverse sand waves over the barchan horns are shown to be the seeds of the newly born barchans at the end points of the two horns of a barchan through the process known as calving. To show this, we examine the celerity, wavelength and amplitude of the transverse sand waves over the barchan as they approach the end of its horn. The celerity and wavelength of these transverse sand waves are shown to be the defining factors in determining the frequency of the calving process. The amplitude of the newly born barchans (through calving) is also shown to be associated with the amplitude of the transverse waves near the end of the horn. The simulation data also show that the wavelength of the newly born barchans (the distance between individual dunes) is closely related to that of the transverse sand waves over their maternal barchan. Finally, we use the simulation results to discuss past conclusions derived from theory, conventional models and field observations.

Tip-vortex instability and turbulent mixing in wind-turbine wakes

Kinetic-energy transport and turbulence production within the shear layer of a horizontal-axis wind-turbine wake are investigated with respect to their influence on the tip-vortex pairwise instability, the so-called leapfrogging instability. The study quantifies the effect of near-wake instability and tip-vortex breakdown on the process of mean-flow kinetic-energy transport within the far wake of the wind turbine, in turn affecting the wake re-energising process. Experiments are conducted in an open-jet wind tunnel with a wind-turbine model of 60 cm diameter at a diameter-based Reynolds number range$\mathit{Re}_{D}=150\,000{-}230\,000$. The velocity fields in meridian planes encompassing a large portion of the wake past the rotor are measured both in the unconditioned and the phase-locked mode by means of stereoscopic particle image velocimetry. The detailed topology and development of the tip-vortex interactions are discussed prior to a statistical analysis based on the triple decomposition of the turbulent flow fields. The study emphasises the role of the pairing instability as a precursor to the onset of three-dimensional vortex distortion and breakdown, leading to increased turbulent mixing and kinetic-energy transport across the shear layer. Quadrant analysis further elucidates the role of sweep and ejection events within the two identified mixing regimes. Prior to the onset of the instability, vortices shed from the blade appear to inhibit turbulent mixing of the expanding wake. The second region is dominated by the leapfrogging instability, with a sudden increase of the net entrainment of kinetic energy. Downstream of the latter, random turbulent motion characterises the flow, with a significant increase of turbulent kinetic-energy production. In this scenario, the leapfrogging mechanism is recognised as the triggering event that accelerates the onset of efficient turbulent mixing followed by the beginning of the wake re-energising process.

Agglomeration of Ni-rich hydroxide crystals in Taylor vortex flow

The agglomeration of Ni-rich hydroxide crystals (Ni0.9Co0.05Mn0.05)(OH)2 was studied in a continuous Couette-Taylor (CT) crystallizer. Due to the excellent mixing and periodic fluid motion of the Taylor vortex flow in the CT crystallizer, the formation of spherical and uniform agglomerate particles of Ni-rich crystals was promoted as the rotation speed and the mean residence time increased. Thus, for a rotation speed of 1500rpm and mean residence time of 60min, with a high feed concentration of 3.0mol/L produced a high tap-density of agglomerate particles of 2.13g/cm3, which is higher than any previously reported values. These results were due to the excellent micromixing effects of Taylor vortex in the CT crystallizer. In addition, the long mean residence time contributed to the formation of spherical particles due to the extended exposure of the agglomerates to the fluid shear. Furthermore, the formation of spherical particles was optimized at a pH of 12.0 and required a high concentration of ammonia due to the high composition of Ni ions in the Ni-rich hydroxide crystals and the short mean residence time in the continuous CT crystallizer. Moreover, when comparing the crystallization in a continuous MSMPR crystallizer, which required a long mean residence time of 12h for a high tap-density of above 2.0g/cm3, the continuous CT crystallizer was clearly much more effective for the formation of uniform spherical particles due to the more efficient hydrodynamic fluid motion of the Taylor vortex in the CT crystallizer in contrast to the random turbulent eddy motion in the MSMPR crystallizer.

Three-Dimensional Turbulent Vortex Shedding From a Surface-Mounted Square Cylinder: Predictions With Large-Eddy Simulations and URANS

The paper reports on the prediction of the turbulent flow field around a three-dimensional, surface mounted, square-sectioned cylinder at Reynolds numbers in the range 104–105. The effects of turbulence are accounted for in two different ways: by performing large-eddy simulations (LES) with a Smagorinsky model for the subgrid-scale motions and by solving the unsteady form of the Reynolds-averaged Navier–Stokes equations (URANS) together with a turbulence model to determine the resulting Reynolds stresses. The turbulence model used is a two-equation, eddy-viscosity closure that incorporates a term designed to account for the interactions between the organized mean-flow periodicity and the random turbulent motions. Comparisons with experimental data show that the two approaches yield results that are generally comparable and in good accord with the experimental data. The main conclusion of this work is that the URANS approach, which is considerably less demanding in terms of computer resources than LES, can reliably be used for the prediction of unsteady separated flows provided that the effects of organized mean-flow unsteadiness on the turbulence are properly accounted for in the turbulence model.

UNDERSTANDING GALAXY OUTFLOWS AS THE PRODUCT OF UNSTABLE TURBULENT SUPPORT

The interstellar medium is a multiphase gas in which turbulent support is as important as thermal pressure. Sustaining this configuration requires both continuous turbulent stirring and continuous radiative cooling to match the decay of turbulent energy. While this equilibrium can persist for small turbulent velocities, if the one-dimensional velocity dispersion is larger than approximately 35 km/s, the gas moves into an unstable regime that leads to rapid heating. I study the implications of this turbulent runaway, showing that it causes a hot gas outflow to form in all galaxies with a gas surface density above approximately 50 solar masses/pc^2 corresponding to a star formation rate per unit area of 0.1$ solar masses/yr/kpc^2. For galaxies with escape velocities above 200 km/s, the sonic point of this hot outflow should lie interior to the region containing cold gas and stars, while for galaxies with smaller escape velocities, the sonic point should lie outside this region. This leads to efficient cold cloud acceleration in higher mass galaxies, while in lower mass galaxies, clouds may be ejected by random turbulent motions rather than accelerated by the wind. Finally, I show that energy balance cannot be achieved at all for turbulent media above a surface density of approximately 10^5 solar masses/pc^2.

Prediction of the effects of vortex shedding on UV disinfection efficiency

Research Article| May 01 2011 Prediction of the effects of vortex shedding on UV disinfection efficiency B. A. Younis; B. A. Younis 1Department of Civil & Environmental Engineering, University of California – Davis, Davis, CA 95616, USA E-mail: bayounis@ucdavis.edu Search for other works by this author on: This Site PubMed Google Scholar T. H. Yang T. H. Yang 1Department of Civil & Environmental Engineering, University of California – Davis, Davis, CA 95616, USA Search for other works by this author on: This Site PubMed Google Scholar Journal of Water Supply: Research and Technology-Aqua (2011) 60 (3): 147–158. https://doi.org/10.2166/aqua.2011.008 Article history Received: February 22 2010 Accepted: September 29 2010 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Cite Icon Cite Permissions Search Site Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAll JournalsThis Journal Search Advanced Search Citation B. A. Younis, T. H. Yang; Prediction of the effects of vortex shedding on UV disinfection efficiency. Journal of Water Supply: Research and Technology-Aqua 1 May 2011; 60 (3): 147–158. doi: https://doi.org/10.2166/aqua.2011.008 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex An assessment is made of the importance of vortex shedding which occurs from UV lamps on the efficacy of disinfection in UV channels. The focus is on the high Reynolds-number regime where turbulent flow conditions prevail and where there exists a strong interaction between the periodic mean-flow unsteadiness and the random turbulent motions. Simulations are performed of the flow around four circular lamps with axes perpendicular to the flow and which are arranged in a diamond configuration. Turbulence closure is achieved using a modified version of the k − ɛ model which takes into account the modification of the turbulence energy spectrum due to vortex shedding. The disinfection efficiency is estimated using a Lagrangian approach in which the trajectories of massless particles are tracked to estimate their residence time and the dose received. The modified turbulence model shows that the occurrence of vortex shedding produces wide variation in the particles trajectories and, consequently, in the UV dose received. These and other results strongly suggest that the effects of vortex shedding on disinfection are very important and thus must be accounted for if the uncertainties inherent in the use of computational fluid dynamics (CFD) for the design of UV treatment channels are to be reduced. CFD, disinfection, Lagrangian dose estimation, turbulence modeling, UV, vortex shedding This content is only available as a PDF. © IWA Publishing 2011 You do not currently have access to this content.

A stochastic jump diffusion particle‐tracking model (SJD‐PTM) for sediment transport in open channel flows

A stochastic jump diffusion (SJD) process is used to describe movement of sediment particles in open channel flows. The stochastic jump diffusion particle‐tracking model (SJD‐PTM) is governed by the stochastic differential equation (SDE). The SDE consists of three main terms including a mean drift motion term, a Wiener process representing random turbulent motion, and a Poisson process describing the abrupt movement of particles caused by probabilistic occurrences of the extreme flow perturbations. The proposed SJD‐PTM can characterize the probabilistic properties of sediment particle movement by simulating the most probable trajectory and ensemble variances of particle movement associated with both flow turbulence and random occurrences of extreme flow perturbations. For the particle movement in response to extreme flow occurrences, we have introduced the particle relaxation time (temporal lag) to quantify the delayed response of sediment particles to large flow perturbations because of the particle inertia effect. The particle relaxation time is derived from a consideration of the forces exerted on particles under the effect of flow accelerations. The particle relaxation time is demonstrated to be dependent on particle size and density as well as the particle Reynolds number. The impact of drag and added mass forces on the ensemble mean and variance of particle trajectory is also evaluated. The particle relaxation time is found to be a factor that diminishes the impact of the extreme flow perturbations. It is shown that heavier and larger particles normally have a larger particle relaxation time, a decreased particle jump magnitude and a smaller ensemble variance of particle trajectories in the occurrences of extreme flow perturbations. It is concluded that the proposed SJD‐PTM with the particle relaxation time can better describe the movement of sediment particles of nonnegligible size in response to extreme flow perturbations. The ability to quantify the variances of particle movement more comprehensively is one of the major advantages associated with the SJD‐PTM for longer‐term sediment transport modeling and predictions. The proposed SJD‐PTM is verified against two distinct experimental data.

OUTFLOW-DRIVEN TURBULENCE IN MOLECULAR CLOUDS

In this paper we explore the relationship between protostellar outflows and turbulence in molecular clouds. Using 3-D numerical simulations we focus on the hydrodynamics of multiple outflows interacting within a parsec scale volume. We explore the extent to which transient outflows injecting directed energy and momentum into a sub-volume of a molecular cloud can be converted into random turbulent motions. We show that turbulence can readily be sustained by these interactions and show that it is possible to broadly characterize an effective driving scale of the outflows. We compare the velocity spectrum obtained in our studies to that of isotropically forced hydrodynamic turbulence finding that in outflow driven turbulence a power law is indeed achieved. However we find a steeper spectrum (beta ~ 3) is obtained in outflow driven turbulence models than in isotropically forced simulations (beta ~ 2). We discuss possible physical mechanisms responsible for these results as well and their implications for turbulence in molecular clouds where outflows will act in concert with other processes such as gravitational collapse.

ROTATION AND TURBULENCE OF THE HOT INTRACLUSTER MEDIUM IN GALAXY CLUSTERS

Cosmological simulations of galaxy clusters typically find that the weight of a cluster at a given radius is not balanced entirely by the thermal gas pressure of the hot intracluster medium (ICM), with theoretical studies emphasizing the role of random turbulent motions to provide the necessary additional pressure support. Using a set of high-resolution, hydrodynamical simulations of galaxy clusters that include radiative cooling and star formation and are formed in a cold dark matter (CDM) universe, we find instead that in the most relaxed clusters rotational support exceeds that from random turbulent motions for radii 0.1–0.5 r500, while at larger radii, out to 0.8 r500, they remain comparable. We also find that the X-ray images of the ICM flatten prominently over a wide radial range, 0.1–0.4 r500. When compared with the average ellipticity profile of the observed X-ray images computed for nine relaxed nearby clusters, we find that the observed clusters are much rounder than the relaxed CDM clusters within ≈0.4 r500. Moreover, while the observed clusters display an average ellipticity profile that does not vary significantly with radius, the ellipticity of the relaxed CDM clusters declines markedly with increasing radius, suggesting that the ICM of the observed clusters rotates less rapidly than that of the relaxed CDM clusters out to ≈0.6 r500. When these results are compared with those obtained from a simulation without radiative cooling, we find a cluster ellipticity profile in much better agreement with the observations, implying that overcooling has a substantial impact on the gasdynamics and morphology out to larger radii than previously recognized. It also suggests that the 10–20% systematic errors from nonthermal gas pressure support reported for simulated cluster masses, obtained from fitting simulated X-ray data over large radial ranges within r500, may need to be revised downward. These results demonstrate the utility of X-ray ellipticity profiles as a probe of ICM rotation and overcooling which should be used to constrain future cosmological cluster simulations.

The Absence of Adiabatic Contraction of the Radial Dark Matter Profile in the Galaxy Cluster A2589

We present an X-ray analysis of the radial mass profile of the radio-quiet galaxy cluster A2589 between 0.015 and 0.25rvir, using an XMM-Newton observation. Except for a ≈16 kpc shift of the X-ray center of the R = 45-60 kpc annulus, A2589 possesses a remarkably symmetrical X-ray image and is therefore an exceptional candidate for precision studies of its mass profile by applying hydrostatic equilibrium. The total gravitating matter profile is well described by the NFW model with cvir = 6.1 ± 0.3 and Mvir = 3.3 ± 0.3 × 1014 M☉ (rvir = 1.74 ± 0.05 Mpc), in excellent agreement with ΛCDM. When the mass of the hot intracluster medium is subtracted from the gravitating matter profile, the NFW model fitted to the resulting dark matter (DM) profile produces essentially the same result. However, when accounting for the stellar mass (M*) of the cD galaxy, the NFW fit to the DM profile substantially degrades in the central r ~ 50 kpc for reasonable values of M*/LV. Modifying the NFW DM halo by adiabatic contraction arising from the early condensation of stellar baryons in the cD galaxy further degrades the fit. The fit is improved substantially with a Sersic-like model recently suggested by high-resolution N-body simulations but with an inverse Sersic index, α ~ 0.5, that is a factor of ~3 higher than predicted. We argue that neither random turbulent motions nor magnetic fields can provide sufficient nonthermal pressure support to reconcile the XMM-Newton mass profile with adiabatic contraction of a CDM halo, assuming reasonable values of M*/LV. Our results support the scenario in which, at least for galaxy clusters, processes during halo formation counteract adiabatic contraction so that the total gravitating mass in the core approximately follows the NFW profile.

Turbulence levels in a flume compared to the field: Implications for larval settlement studies

The settlement stage of shellfish larvae is important in determining distribution patterns of adults. In order to reach the substratum, the larvae need to cross the benthic boundary layer near the sediment surface. Suspended larvae reach the substratum by a combination of passive sinking, turbulent advection and possibly active swimming. Subsequently the larvae need a certain amount of time in contact with the substratum to attach themselves to the bed. Flumes are commonly used experimental set-ups to study larval settlement in flowing water. Since turbulence plays an important role in the vertical transport of larvae and in the probability of resuspension, such experiments must take turbulence scaling into account. This study compares turbulence characteristics in the flume at the NIOO-CEME with field conditions encountered by intertidal bivalve larvae during their settlement stage. Turbulence in the flume was manipulated throughout the water column using a grid and in the benthic boundary layer by introducing roughness structures. Hydrodynamic parameters such as shear velocity ( u ⁎), roughness height (z 0), turbulence intensity (TI) and Reynolds stress were determined. We used the dimensionless Rouse number to scale the ratio between advective motion and random (turbulent) motion. Relative particle concentrations (Rouse distribution) near the bottom, as calculated theoretically for the flume under normal conditions and in the field, show that the flume is biased towards sinking. In the Oosterschelde, where flow is tidally driven, the ratio between advective motion and random (turbulent) motion for flow velocities around 0.35 m s − 1 is balanced. In this situation a small change in directional movement (e.g. by swimming) may have an effect on settlement success of larvae. The grid has a modifying effect on mixing throughout the water column, but close to the bottom turbulence intensities are similar to the situation without a grid. Turbulence introduced by bottom roughness has a different effect from that introduced by a grid. Turbulence introduced by bottom roughness has only minor effects on mixing higher up in the water column, but due to the wake-flow has dramatic effects on near-bed turbulence. We discuss the effects of turbulence generated at the bottom on larval settlement as a case study. Both grids and bottom roughness used to induce turbulence in the flume are valuable tools in scaling turbulence parameters for quantitative research on larval settlement.

Loops, Drips, and Walls in the Galactic Chimney GSH 277+00+36

We present new high-resolution H I images of the Galactic chimney GSH 277+00+36. The chimney is at a distance of ~6.5 kpc, is more than 600 pc in diameter, and extends at least 1 kpc above and below the Galactic midplane. Using the Australia Telescope Compact Array and the Parkes Radiotelescope as part of the Southern Galactic Plane Survey, we have imaged the H I associated with this chimney, with a spatial resolution of ~6 pc. These are among the highest spatial resolution images of an H I chimney. We find very narrow well-defined shell walls, a remarkably empty interior, and complex small-scale structures. The shell walls show a very steep reduction in emission at the interior edge and a more gradual decline toward the exterior. We suggest that this structure is characteristic of compression and may be used to distinguish stellar by-product shells from shell-like structures resulting from random turbulent motions. The shell and chimney walls also exhibit a great deal of small-scale structure, which we discuss in the context of hydrodynamic instabilities. We find that these structures are primarily cold gas with narrow line widths in the range 1.5-2.5 km s-1.

Harmonic Perturbations in Turbulent Wakes

A theoretical model of harmonic perturbations in far turbulent wakes is considered. The proposed model is based on the triple decomposition method. It is assumed that the instantaneous velocities and pressures consist of three distinctive components: the mean (time-averaged), the coherent (phase-averaged), and the random (turbulent) motion. The interaction between incoherent turbulent fluctuations and large-scale coherent disturbances is incorporated by means of a Newtonian eddy viscosity model. For high-amplitude perturbations, the nonlinear feedback to the mean flow is taken into account by means of the coherent Reynolds stresses. The equations for the mean flow are coupled with the linearized equations for the disturbances, taking into account the mean flow nonparallel effects. The model resolves uncertainties noted in previous theories and provides a correct comparison with available experimental data. The effect of the harmonic perturbations on the turbulent wake growth at high amplitudes is investigated as well

On harmonic perturbations in a turbulent mixing layer

A theoretical model of harmonic perturbations in a turbulent mixing layer is proposed. The model is based on the triple decomposition method. It is assumed that the instantaneous velocities and pressure consist of three distinctive components: the mean (time average), the coherent (phase average), and the random (turbulent) motion. The interaction between incoherent turbulent fluctuations and large-scale coherent disturbances is incorporated by the Newtonian eddy viscosity model. A slight divergence of the flow is also taken into account, and the results are compared with experimental data. For a high amplitude of the perturbations, the nonlinear feedback to the mean flow is taken into account by means of the coherent Reynolds stresses. The results reveal the possibility of a negative spreading rate of the mixing layer. A simultaneous consideration of the mean flow divergence and nonlinear self-interaction results in Landau-like amplitude equations. It is observed that the nonlinear term in the amplitude equation is not significant at the levels of amplitude considered. The velocity disturbance profiles of the second harmonic are also presented and, at low-level amplitude, they are in good agreement with experiments.

Some aspects of turbulent flow structure in large alluvial rivers

This paper reports different aspects of turbulent flow structure using the ship-board time-series velocity data collected by an Acoustic Doppler Current Profiler (ADCP) in the large multi-thread Jamuna River in Bangladesh. The measurements were made during different river stages in various bed-form environments ranging from mega-ripples to dunes. Based on the analysis of the downstream flow velocity obtained from the three subdivisions of the water column, different turbulence parameters such as frequency spectra and intensities were found. Spectral analysis indicates a mixed picture of random turbulent motion and periodicity. The maximum period observed was about 18 min and the minimum was about 11.5 s. Analysis of the frequency of occurrence of all the grouped events shows that about 70% of the turbulence have their origins in the river-bed, the rest are horizontal eddies. It is shown that a 15 min averaging time is required to estimate the higher order moments such as the turbulence intensity. The vertical distribution of' the ratio of turbulence intensity and bed friction velocity indicates the presence of two segments separated by a peak: a growing segment limited within a height of 5 to 10% above the bed and an exponentially decaying segment above. The turbulence intensity is 7 to 10% of the local downstream velocity in the free-stream region but increases to about 11 to 23% in the wall region.

Stochastic theory of compressible turbulent fluid transport

We develop a stochastic model for the turbulent transport of passive scalars based on the Fokker–Planck equation for the probability density distribution of the displacements of infinitesimal fluid parcels (“particles”) in random turbulent motion. Such a theory is the microscopic basis behind semiempirical models of turbulent diffusion which apparently have been developed only for incompressible flow. Here, we specifically develop the theory so that it applies to compressible flow. We then apply it to the particular case of stratified mesoscale turbulent transport of tracers in the ocean, and we find that it generalizes the recent parametrization of Gent and McWilliams [J. Phys. Oceanogr. 20, 150 (1990)].

Computational simulation of turbulent signal loss in 2D time-of-flight magnetic resonance angiograms.

Time-of-flight magnetic resonance (MR) angiography is currently limited in the evaluation of arterial stenoses by flow-induced signal loss. This signal loss has been attributed to phase dispersion and to phase misregistration. We have developed a fluid mechanics model of 2D time-of-flight MR angiograms to study the amount of signal loss caused by random turbulence. The simulations were created by stochastic analysis of particle pathlines determined by computational fluid dynamics for turbulent flow. The images obtained by the model compare well to actual MR images of flow in stenoses. By selectively removing the random turbulent motion in the simulation, it can be seen that random phase dispersion is the dominant mechanism of signal loss. Phase misregistration and mean flow phase dispersion act as secondary effects. The MR simulation model recreates accurately the variation of signal loss over a range of echo times. The model can be used further to explore and design new pulse sequences. For example, the current study showed that high slew rate gradient waveforms can significantly reduce poststenotic signal loss. In conclusion, computational modeling of MR angiography can be a useful approach for the analysis of MRA signal loss and the design of improved pulse sequences.

An adaptive turbulence filter for decomposition of organized turbulent flows

A new decomposition has been developed in which turbulent processes in shear flows may be represented as a combination of organized and more random turbulent motions. Each component is modeled as a summation of its characteristic eddies, of strength that varies in time and space as a function of the entire process. The contribution of all turbulent eddies of the more random component are estimated with an adaptive turbulence filter, which recognizes this component as the orthogonal partner to organized motion, with a power density spectrum of appropriate shape. The decomposition recovers organized motion from time and space series of data in a physically meaningful way, and can be used to characterize interaction between coherent and more random motions. It also provides an estimate for the turbulence in shear flows that are too complex for a meaningful average motion to be identified.