419 research outputs found
The Turbulence Power Spectrum in Optically Thick Interstellar Clouds
The Fourier power spectrum is one of the most widely used statistical tools
to analyze the nature of magnetohydrodynamic turbulence in the interstellar
medium. Lazarian & Pogosyan (2004) predicted that the spectral slope should
saturate to -3 for an optically thick medium and many observations exist in
support of their prediction. However, there have not been any numerical studies
to-date testing these results. We analyze the spatial power spectrum of MHD
simulations with a wide range of sonic and Alfv\'enic Mach numbers, which
include radiative transfer effects of the CO transition. We confirm
numerically the predictions of Lazarian & Pogosyan (2004) that the spectral
slope of line intensity maps of an optically thick medium saturates to -3.
Furthermore, for very optically thin supersonic CO gas, where the density or CO
abundance values are too low to excite emission in all but the densest shock
compressed gas, we find that the spectral slope is shallower than expected from
the column density. Finally, we find that mixed optically thin/thick CO gas,
which has average optical depths on order of unity, shows mixed behavior: for
super-Alfv\'enic turbulence, the integrated intensity power spectral slopes
generally follow the same trend with sonic Mach number as the true column
density power spectrum slopes. However, for sub-Alfv\'enic turbulence the
spectral slopes are steeper with values near -3 which are similar to the very
optically thick regime.Comment: accepted to Ap
State forest resources of the peninsula and their management - past, present and future
Utilisation of forest resources of Tasman Peninsula began during the convict period of Port Arthur settlement and continues today. The Tasmanian Forestry Commission manages
State Forests of the peninsula for muiltiple uses including wood production, conservation,
recreation, landscape and water
Minimum energy route optimisation of a quad-copter UAV with landing incentivisation
Recent advancements in the technology surrounding UAVs have expanded the possibility of incorporating them into current logistical solutions. In order to accurately assess their capabilities, it is important that minimum energy trajectories can be generated to increase the travel range of a UAV as well as its possible number of visited locations. However, in current formulations of the optimisation problem, UAV dynamics do not incorporate a contact force on the ground. This results in hover-to-hover trajectories where the duration of the journey is exactly equal to an arrival time which is set as one of the problem's parameters. Those solutions are likely to be energetically sub-optimal if an unnecessarily large value of arrival time is chosen. This paper introduces landing capability by modifying gravitational acceleration in the dynamics using a sigmoid function which approaches zero at the destination. In this way, the trip can be conducted in a shorter amount of time if it results in lower energy consumption. The new model is compared against an example from the literature, where the corresponding solution results in a reduction of the travel time and energy consumption by approximately 80%. It is also applied to a real-world example where it is demonstrated that a UAV can provide energy savings if it replaces a van completing a delivery in the Solent region of the UK
Density Studies of MHD Interstellar Turbulence: Statistical Moments, Correlations and Bispectrum
We present a number of statistical tools for obtaining studying turbulence in
molecular clouds and diffuse interstellar medium. For our tests we used of
three-dimensional 512 cube compressible MHD isothermal simulations performed
for different sonic and Alfvenic Mach numbers. We introduce the bispectrum, a
new tool for statistical studies of the interstellar medium which, unlike an
ordinary power spectrum of turbulence, preserves the phase information of the
stochastic field. We show that the bispectra of the 3D stochastic density field
and of column densities, available from observations, are similar. We use the
bispectrum technique to define the role of non-linear wave-wave interactions in
the turbulent energy cascade. We also obtained the bispectrum function for
density and column densities with varying magnetic field strength. Larger
values of sonic Mach number result in increased correlations for modes with
different wavenumbers. This effect becomes more evident with increasing
magnetic field intensity. In addition to the bispectrum, we calculated the 3rd
and 4th statistical moments of density and column density, namely, skewness and
kurtosis, respectively. We found a strong dependence of skewness and kurtosis
with the sonic Mach number. In particular, as this number increases, so does
the asymmetry of the density distribution. We also studied the correlations of
2D column density with dispersion of velocities and magnetic field, as well as
the correlations of 3D density with magentic and kinetic energy and Alfven Mach
number for comparison. Our results show that column density is linearly
correlated with magnetic field for high sonic Mach number. This trend is
independent of the turbulent kinetic energy and can be used to characterize
inhomogeneities of physical properties in low density clumps in the ISM.Comment: apj emulated, 18 pages, 11 figures. Accepted to Astrophysical Journa
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