arXiv subjects

H. Mouri

Publications and source records attributed to H. Mouri.

At least 37 records · Page 2Linked to original sources

arXiv2008

Fluctuations of statistics among subregions of a turbulence velocity field

To study subregions of a turbulence velocity field, a long record of velocity data of grid turbulence is divided into smaller segments. For each segment, we calculate statistics such as the mean rate of energy dissipation and the mean energy at each scale. Their values significantly fluctuate, in lognormal distributions at least as a good approximation. Each segment is not under equilibrium between the mean rate of energy dissipation and the mean rate of energy transfer that determines the mean energy. These two rates still correlate among segments when their length exceeds the correlation length. Also between the mean rate of energy dissipation and the mean total energy, there is a correlation characterized by the Reynolds number for the whole record, implying that the large-scale flow affects each of the segments.

physics.flu-dyn↗

arXiv2008

Vortex Tubes in Turbulence Velocity Fields at High Reynolds Numbers

The elementary structures of turbulence, i.e., vortex tubes, are studied using velocity data obtained in laboratory experiments for boundary layers and duct flows at microscale Reynolds numbers 332-1934. While past experimental studies focused on intense vortex tubes, the present study focuses on all vortex tubes with various intensities. We obtain the mean velocity profile. The radius scales with the Kolmogorov length. The circulation velocity scales with the Kolmogorov velocity, in contrast to the case of intense vortex tubes alone where the circulation velocity scales with the rms velocity fluctuation. Since these scaling laws are independent of the configuration for turbulence production, they appear to be universal at high Reynolds numbers.

physics.flu-dyn↗

arXiv2007

Hierarchical Object Formation in the Peculiar Velocity Field

Using the initial peculiar velocity field, we analytically study the hierarchical formation of gravitationally bound objects. The field is smoothed over a scale that corresponds to the mass of a given class of objects. Through the Zel'dovich approximation, the smoothed field determines how the objects cluster together to form a new class of more massive objects. The standard cosmological parameters lead to the evolution of primordial clouds with 10^6 M(sun) -> galaxies with 10^12 M(sun) -> clusters of galaxies with 10^15 M(sun) -> superclusters of galaxies with 10^16 M(sun). The epochs obtained for the formation of these classes of objects are consistent with observations.

astro-ph↗

arXiv2007

Laboratory experiments for intense vortical structures in turbulence velocity fields

Vortical structures of turbulence, i.e., vortex tubes and sheets, are studied using one-dimensional velocity data obtained in laboratory experiments for duct flows and boundary layers at microscale Reynolds numbers from 332 to 1934. We study the mean velocity profile of intense vortical structures. The contribution from vortex tubes is dominant. The radius scales with the Kolmogorov length. The circulation velocity scales with the rms velocity fluctuation. We also study the spatial distribution of intense vortical structures. The distribution is self-similar over small scales and is random over large scales. Since these features are independent of the microscale Reynolds number and of the configuration for turbulence production, they appear to be universal.

physics.flu-dyn↗

arXiv2006

Downsizing of Star-Forming Galaxies by Gravitational Processes

There is observed a trend that a lower mass galaxy forms stars at a later epoch. This downsizing of star-forming galaxies has been attributed to hydrodynamical or radiative feedback processes that regulate star formation. However, here we explain the downsizing by gravitational processes alone, in the bottom-up scenario where galaxies evolve from subgalactic-scale objects. Within a region of the initial density field that is to evolve into a lower mass galaxy, subgalactic-scale fluctuation is of a smaller amplitude. The formation of subgalactic-scale objects, i.e., gravitational collapse of the subgalactic-scale fluctuation, and the subsequent onset of star formation accordingly occur at a later epoch for a lower mass galaxy. As a function of galaxy mass, we calculate the peak epoch of formation of subgalactic-scale objects. The peak epoch is consistent with the peak epoch of star formation derived from observations. [abridged]

astro-ph↗

arXiv2005

Why is the Zel'dovich Approximation so Accurate?

Why does the Zel'dovich approximation (ZA) work well to describe gravitational collapse in the universe? This problem is examined by focusing on its dependence on the dimensionality of the collapse. The ZA is known to be exact for a one-dimensional collapse. We show that the ZA becomes progressively more accurate in the order of three-, two-, and one-dimensional collapses. Furthermore, using models for spheroidal collapse, we show that the ZA remains accurate in all collapses, which become progressively lower dimensional with the passage of time. That is, the ZA is accurate because the essence of the gravitational collapse is incorporated in the ZA.

astro-ph↗

arXiv2005

On Landau's prediction for large-scale fluctuation of turbulence energy dissipation

Kolmogorov's theory for turbulence in 1941 is based on a hypothesis that small-scale statistics are uniquely determined by the kinematic viscosity and the mean rate of energy dissipation. Landau remarked that the local rate of energy dissipation should fluctuate in space over large scales and hence should affect small-scale statistics. Experimentally, we confirm the significance of this large-scale fluctuation, which is comparable to the mean rate of energy dissipation at the typical scale for energy-containing eddies. The significance is independent of the Reynolds number and the configuration for turbulence production. With an increase of scale r above the scale of largest energy-containing eddies, the fluctuation becomes to have the scaling r^-1/2 and becomes close to Gaussian. We also confirm that the large-scale fluctuation affects small-scale statistics.

physics.flu-dyn↗

arXiv2004

Vortex Tubes in Turbulence Velocity Fields at Reynolds Numbers 300-1300

The most elementary structures of turbulence, i.e., vortex tubes, are studied using velocity data obtained in a laboratory experiment for boundary layers with microscale Reynolds numbers 295-1258. We conduct conditional averaging for enhancements of a small-scale velocity increment and obtain the typical velocity profile for vortex tubes. Their radii are of the order of the Kolmogorov length. Their circulation velocities are of the order of the root-mean-square velocity fluctuation. We also obtain the distribution of the interval between successive enhancements of the velocity increment as the measure of the spatial distribution of vortex tubes. They tend to cluster together below about the integral length and more significantly below about the Taylor microscale. These properties are independent of the Reynolds number and are hence expected to be universal.

physics.flu-dyn↗

arXiv2004

Galaxy Interaction and Starburst-Seyfert Connection

Galaxy interactions are studied in terms of the starburst-Seyfert connection. The starburst requires a high rate of gas supply. Since the efficiency for supplying the gas is high in a galaxy interaction, although the companion is not necessarily discernible, Seyfert galaxies with circumnuclear starbursts are expected to be interacting. Since the large amounts of circumnuclear gas and dust obscure the broad-line region, they are expected to be observed as Seyfert 2. The active galactic nucleus itself does not require a high rate of gas supply. Seyfert galaxies without circumnuclear starbursts are not necessarily expected to be interacting even at the highest luminosities. They are not necessarily expected to evolve from Seyfert galaxies with circumnuclear starbursts. We derive these and other theoretical expectations and confirm them with statistics on observational data of magnitude-limited samples of Seyfert galaxies.

astro-ph↗

arXiv2003

Probability density function of turbulent velocity fluctuations in rough-wall boundary layer

The probability density function of single-point velocity fluctuations in turbulence is studied systematically using Fourier coefficients in the energy-containing range. In ideal turbulence where energy-containing motions are random and independent, the Fourier coefficients tend to Gaussian and independent of each other. Velocity fluctuations accordingly tend to Gaussian. However, if energy-containing motions are intermittent or contaminated with bounded-amplitude motions such as wavy wakes, the Fourier coefficients tend to non-Gaussian and dependent of each other. Velocity fluctuations accordingly tend to non-Gaussian. These situations are found in our experiment of a rough-wall boundary layer.

physics.flu-dyn↗

arXiv2003

Negative skewness of radial pairwise velocity in the quasi-nonlinear regime: Zel'dovich approximation

According to N-body numerical simulations, the radial pairwise velocities of galaxies have negative skewness in the quasi-nonlinear regime. To understand its origin, we calculate the probability distribution function of the radial pairwise velocity using the Zel'dovich approximation, i.e., an analytical approximation for gravitational clustering. The calculated probability distribution function is in good agreement with the result of N-body simulations. Thus the negative skewness originates in relative motions of galaxies in the clustering process that the infall dominates over the expansion.

astro-ph↗

arXiv2002

Vortex tubes in velocity fields of laboratory isotropic turbulence: dependence on the Reynolds number

The streamwise and transverse velocities are measured simultaneously in isotropic grid turbulence at relatively high Reynolds numbers, Re(lambda) = 110-330. Using a conditional averaging technique, we extract typical intermittency patterns, which are consistent with velocity profiles of a model for a vortex tube, i.e., Burgers vortex. The radii of the vortex tubes are several of the Kolmogorov length regardless of the Reynolds number. Using the distribution of an interval between successive enhancements of a small-scale velocity increment, we study the spatial distribution of vortex tubes. The vortex tubes tend to cluster together. This tendency is increasingly significant with the Reynolds number. Using statistics of velocity increments, we also study the energetical importance of vortex tubes as a function of the scale. The vortex tubes are important over the background flow at small scales especially below the Taylor microscale. At a fixed scale, the importance is increasingly significant with the Reynolds number.

physics.flu-dyn↗

arXiv2002

Orbital Decay and Tidal Disruption of a Star Cluster: Analytical Calculation

The orbital decay and tidal disruption of a star cluster in a galaxy is studied in an analytical manner. Owing to dynamical friction, the star cluster spirals in toward the center of the galaxy. Simultaneously, the galactic tidal field strips stars from the outskirts of the star cluster. Under an assumption that the star cluster undergoes a self-similar evolution, we obtain the condition and timescale for the star cluster to reach the galaxy center before its disruption. The result is used to discuss the fate of so-called intermediate-mass black holes with >10^3 M(sun) found recently in young star clusters of starburst galaxies and also the mass function of globular clusters in galaxies.

astro-ph↗

arXiv2002

Mass Segregation in Star Clusters: Analytic Estimation of the Timescale

Mass segregation in a star cluster is studied in an analytical manner. We consider a two-component cluster, which consists of two types of stars with different masses. Plummer's model is used for the initial condition. We trace the overall behaviors of the probability distribution functions of the two components and obtain the timescale of mass segregation as a simple function of the cluster parameters. The result is used to discuss the origin of a black hole with mass of > 1000 M(sun) found in the starburst galaxy M82.

astro-ph↗

arXiv2002

Probability density function of turbulent velocity fluctuation

The probability density function (PDF) of velocity fluctuations is studied experimentally for grid turbulence in a systematical manner. At small distances from the grid, where the turbulence is still developing, the PDF is sub-Gaussian. At intermediate distances, where the turbulence is fully developed, the PDF is Gaussian. At large distances, where the turbulence has decayed, the PDF is hyper-Gaussian. The Fourier transforms of the velocity fluctuations always have Gaussian PDFs. At intermediate distances from the grid, the Fourier transforms are statistically independent of each other. This is the necessary and sufficient condition for Gaussianity of the velocity fluctuations. At small and large distances, the Fourier transforms are dependent.

physics.flu-dyn↗

arXiv2002

Runaway Merging of Black Holes: Analytical Constraint on the Timescale

Following the discovery of a black hole (BH) with a mass of 10^3-10^6 M(sun) in a starburst galaxy M82, we study formation of such a BH via successive merging of stellar-mass BHs within a star cluster. The merging has a runaway characteristic. This is because massive BHs sink into the cluster core and have a high number density, and because the merging probability is higher for more massive BHs. We use the Smoluchowski equation to study analytically the evolution of the BH mass distribution. Under favorable conditions, which are expected for some star clusters in starburst galaxies, the timescale of the runaway merging is at most of order 10^7 yr. This is short enough to account for the presence of a BH heavier than 10^3 M(sun) in an ongoing starburst region.

astro-ph↗

arXiv2001

Wavelet Analysis of Vortex Tubes in Experimental Turbulence

This report proposes a new method to study vortex tubes in one-dimensional velocity data of experimental turbulence. Vortex tubes are detected as local maxima on the scale-space plot of wavelet transforms of the velocity data. Then it is possible to extract a typical velocity pattern. The result for data obtained in a wind tunnel is consistent with those of three-dimensional direct numerical simulations.

physics.flu-dyn↗

arXiv2001

Far-Infrared Census of Starburst-Seyfert Connection

Far-infrared flux densities are newly extracted from the IRAS database for the RSA and CfA complete samples of Seyfert galaxies. These data are used to classify the Seyfert galaxies into those where the far-infrared continuum emission is dominated by the active galactic nucleus (AGN), circumnuclear starburst, or host galaxy. While AGN-dominant objects consist of comparable numbers of Seyfert 1 and 2 galaxies, starburst- and host-dominant objects consist preferentially of Seyfert 2 galaxies. Thus, in addition to the dusty torus, the circumnuclear starburst region and host galaxy are important in hiding the broad-line region. Morphologically, starburst-dominant Seyfert galaxies are of later types and more strongly interacting than AGN-dominant Seyfert galaxies. In a later-type galaxy, the AGN central engine has a lower Eddington luminosity, and the gaseous content is higher. The gas is efficiently supplied to the starburst via a galaxy-galaxy interaction. Morphologies of host-dominant Seyfert galaxies are of various types. Since starbursts in Seyfert galaxies are older than those in classical starburst galaxies, we propose an evolution from starburst to starburst-dominant Seyfert to host-dominant Seyfert for a late-type galaxy. An evolution from AGN-dominant Seyfert to host-dominant Seyfert is proposed for an early-type galaxy. These sequences have durations of a few x 10^8 yr and occur repeatedly within a galaxy during its evolution from a late type to an early type.

astro-ph↗