Small-Scale Properties of HI in Nearby Galaxies

1991 ◽  
pp. 213-222
Author(s):  
Robert Braun
Keyword(s):  
Small Scale ◽  
Nearby Galaxies ◽  
Scale Properties

scholarly journals Small-scale Properties of HI in Nearby Galaxies

1991 ◽  
Vol 144 ◽  
pp. 213-222
Author(s):  
Robert Braun
Keyword(s):  
Energy Deposition ◽  
Neutral Hydrogen ◽  
Shell Structures ◽  
Great Promise ◽  
Small Scale ◽  
Hard Constraints ◽  
Absorption Studies ◽  
External Galaxies ◽  
Nearby Galaxies ◽  
Scale Properties

The instrumental requirements and current observational results are outlined for the study of resolved neutral hydrogen structures in both emission and absorption in external galaxies. Neutral super-shell structures exterior to ionized shells around OB associations seem to be a common phenomenon. Further analysis of existing data should allow quantification of the degree and rate of energy deposition in the ISM by massive stars. HI absorption studies offer great promise in constraining the physical properties of the neutral ISM, including an estimate of the gas pressure. Together these data will provide hard constraints on the existence and prevalence of chimney formation, and hence disk – halo interactions in a variety of galactic environments.

Small-scale anisotropy in turbulent boundary layers

2016 ◽  
Vol 804 ◽  
pp. 5-23 ◽  
Author(s):  
Alain Pumir ◽  
Haitao Xu ◽  
Eric D. Siggia
Keyword(s):  
Reynolds Number ◽  
Channel Flow ◽  
Isotropic Turbulence ◽  
Turbulent Channel Flow ◽  
Statistical Properties ◽  
Simultaneous Action ◽  
Large Reynolds Number ◽  
Small Scale ◽  
Velocity Fluctuations ◽  
Scale Properties

In a channel flow, the velocity fluctuations are inhomogeneous and anisotropic. Yet, the small-scale properties of the flow are expected to behave in an isotropic manner in the very-large-Reynolds-number limit. We consider the statistical properties of small-scale velocity fluctuations in a turbulent channel flow at moderately high Reynolds number ($Re_{\unicode[STIX]{x1D70F}}\approx 1000$), using the Johns Hopkins University Turbulence Database. Away from the wall, in the logarithmic layer, the skewness of the normal derivative of the streamwise velocity fluctuation is approximately constant, of order 1, while the Reynolds number based on the Taylor scale is $R_{\unicode[STIX]{x1D706}}\approx 150$. This defines a small-scale anisotropy that is stronger than in turbulent homogeneous shear flows at comparable values of $R_{\unicode[STIX]{x1D706}}$. In contrast, the vorticity–strain correlations that characterize homogeneous isotropic turbulence are nearly unchanged in channel flow even though they do vary with distance from the wall with an exponent that can be inferred from the local dissipation. Our results demonstrate that the statistical properties of the fluctuating velocity gradient in turbulent channel flow are characterized, on one hand, by observables that are insensitive to the anisotropy, and behave as in homogeneous isotropic flows, and on the other hand by quantities that are much more sensitive to the anisotropy. How this seemingly contradictory situation emerges from the simultaneous action of the flux of energy to small scales and the transport of momentum away from the wall remains to be elucidated.

scholarly journals Non-local dispersion and the reassessment of Richardson's t3-scaling law

2021 ◽  
Vol 932 ◽  
Author(s):  
G.E. Elsinga ◽  
T. Ishihara ◽  
J.C.R. Hunt
Keyword(s):  
Large Scale ◽  
Scaling Law ◽  
Taylor Dispersion ◽  
Small Time ◽  
Inertial Range ◽  
Small Scale ◽  
Mean Square ◽  
Dispersion Regime ◽  
Non Local ◽  
Scale Properties

The Richardson-scaling law states that the mean square separation of a fluid particle pair grows according to t3 within the inertial range and at intermediate times. The theories predicting this scaling regime assume that the pair separation is within the inertial range and that the dispersion is local, which means that only eddies at the scale of the separation contribute. These assumptions ignore the structural organization of the turbulent flow into large-scale shear layers, where the intense small-scale motions are bounded by the large-scale energetic motions. Therefore, the large scales contribute to the velocity difference across the small-scale structures. It is shown that, indeed, the pair dispersion inside these layers is highly non-local and approaches Taylor dispersion in a way that is fundamentally different from the Richardson-scaling law. Also, the layer's contribution to the overall mean square separation remains significant as the Reynolds number increases. This calls into question the validity of the theoretical assumptions. Moreover, a literature survey reveals that, so far, t3 scaling is not observed for initial separations within the inertial range. We propose that the intermediate pair dispersion regime is a transition region that connects the initial Batchelor- with the final Taylor-dispersion regime. Such a simple interpretation is shown to be consistent with observations and is able to explain why t3 scaling is found only for one specific initial separation outside the inertial range. Moreover, the model incorporates the observed non-local contribution to the dispersion, because it requires only small-time-scale properties and large-scale properties.

scholarly journals A geostatistical analysis of multiscale metallicity variations in galaxies [I]: Introduction and comparison of high-resolution metallicity maps to an analytic metal transport model

2021 ◽  
Author(s):  
Benjamin Metha ◽  
Michele Trenti ◽  
Tingjin Chu
Keyword(s):  
Large Scale ◽  
Transport Model ◽  
Small Scale ◽  
H Ii Regions ◽  
Integral Field Spectroscopy ◽  
Physical Scale ◽  
Interstellar Media ◽  
Nearby Galaxies ◽  
The Mean ◽  
Linear Regressions

Abstract Thanks to recent advances in integral field spectroscopy (IFS), modern surveys of nearby galaxies are capable of resolving metallicity maps of H ii regions down to scales of ∼50pc. However, statistical analysis of these metallicity maps has seldom gone beyond fitting basic linear regressions and comparing parameters to global galaxy properties. In this paper (the first of a series), we introduce techniques from spatial statistics that are well suited for detailed analysis of both small- and large-scale metallicity variations within the interstellar media (ISMs) of local galaxies. As a first application, we compare the observed structure of small-scale metallicity fluctuations within 7 local galaxies observed by the PHANGS collaboration to predictions from a stochastic, physically motivated, analytical model developed by Krumholz & Ting. We show that while the theoretical model underestimates the amount of correlated scatter in the galactic metallicity distributions by 3 − 4 orders of magnitude, it provides good estimates of the physical scale of metallicity correlations. We conclude that the ISM of local spiral galaxies is far from homogeneous, with regions of size ∼1 kpc showing significant departures from the mean metallicity at each galactocentric radius.

scholarly journals Estimation of parameters in a distributed precipitation-runoff model for Norway

2003 ◽  
Vol 7 (3) ◽  
pp. 304-316 ◽  
Author(s):  
S. Beldring ◽  
K. Engeland ◽  
L. A. Roald ◽  
N. R. Sælthun ◽  
A. Voksø
Keyword(s):  
Land Surface ◽  
Distributed Model ◽  
Small Scale ◽  
Model Parameters ◽  
Estimation Of Parameters ◽  
Ungauged Catchments ◽  
Time Step ◽  
Landscape Characteristics ◽  
Average Annual Runoff ◽  
Scale Properties

Abstract. A distributed version of the HBV-model using 1 km2 grid cells and daily time step was used to simulate runoff from the entire land surface of Norway for the period 1961-1990. The model was sensitive to changes in small scale properties of the land surface and the climatic input data, through explicit representation of differences between model elements, and by implicit consideration of sub-grid variations in moisture status. A geographically transferable set of model parameters was determined by a multi-criteria calibration strategy, which simultaneously minimised the residuals between model simulated and observed runoff from 141 Norwegian catchments located in areas with different runoff regimes and landscape characteristics. Model discretisation units with identical landscape classification were assigned similar parameter values. Model performance was evaluated by simulating discharge from 43 independent catchments. Finally, a river routing procedure using a kinematic wave approximation to open channel flow was introduced in the model, and discharges from three additional catchments were calculated and compared with observations. The model was used to produce a map of average annual runoff for Norway for the period 1961-1990. Keywords: distributed model, multi-criteria calibration, global parameters, ungauged catchments.

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