3,651 research outputs found
Optimal control theory investigation of proprotor/wing response to vertical gust
Optimal control theory is used to design linear state variable feedback to improve the dynamic characteristics of a rotor and cantilever wing representing the tilting proprotor aircraft in cruise flight. The response to a vertical gust and system damping are used as criteria for the open and closed loop performance. The improvement in the dynamic characteristics achievable is examined for a gimballed rotor and for a hingeless rotor design. Several features of the design process are examined, including: (1) using only the wing or only the rotor dynamics in the control system design; (2) the use of a wing flap as well as the rotor controls for inputs; (3) and the performance of the system designed for one velocity at other forward speeds
Direct Measurement of Effective Magnetic Diffusivity in Turbulent Flow of Liquid Sodium
The first direct measurements of effective magnetic diffusivity in turbulent
flow of electro-conductive fluids (the so-called beta-effect) under magnetic
Reynolds number Rm >> 1 are reported. The measurements are performed in a
nonstationary turbulent flow of liquid sodium, generated in a closed toroidal
channel. The peak level of the Reynolds number reached Re \approx 3 10^6, which
corresponds to the magnetic Reynolds number Rm \approx 30. The magnetic
diffusivity of the liquid metal was determined by measuring the phase shift
between the induced and the applied magnetic fields. The maximal deviation of
magnetic diffusivity from its basic (laminar) value reaches about 50% .Comment: 5 pages, 6 figuser, accepted in PR
On the effects of turbulence on a screw dynamo
In an experiment in the Institute of Continuous Media Mechanics in Perm
(Russia) an non--stationary screw dynamo is intended to be realized with a
helical flow of liquid sodium in a torus. The flow is necessarily turbulent,
that is, may be considered as a mean flow and a superimposed turbulence. In
this paper the induction processes of the turbulence are investigated within
the framework of mean--field electrodynamics. They imply of course a part which
leads to an enhanced dissipation of the mean magnetic field. As a consequence
of the helical mean flow there are also helical structures in the turbulence.
They lead to some kind of --effect, which might basically support the
screw dynamo. The peculiarity of this --effect explains measurements
made at a smaller version of the device envisaged for the dynamo experiment.
The helical structures of the turbulence lead also to other effects, which in
combination with a rotational shear are potentially capable of dynamo action. A
part of them can basically support the screw dynamo. Under the conditions of
the experiment all induction effects of the turbulence prove to be rather weak
in comparison to that of the main flow. Numerical solutions of the mean--field
induction equation show that all the induction effects of the turbulence
together let the screw dynamo threshold slightly, at most by one per cent,
rise. The numerical results give also some insights into the action of the
individual induction effects of the turbulence.Comment: 15 pages, 7 figures, in GAFD prin
Connection between slow and fast dynamics of molecular liquids around the glass transition
The mean-square displacement (MSD) was measured by neutron scattering at
various temperatures and pressures for a number of molecular glass-forming
liquids. The MSD is invariant along the glass-transition line at the pressure
studied, thus establishing an ``intrinsic'' Lindemann criterion for any given
liquid. A one-to-one connection between the MSD's temperature dependence and
the liquid's fragility is found when the MSD is evaluated on a time scale of
approximately 4 nanoseconds, but does not hold when the MSD is evaluated at
shorter times. The findings are discussed in terms of the elastic model and the
role of relaxations, and the correlations between slow and fast dynamics are
addressed.Comment: accepted by Phys Rev E (2010
Energy transfer in two-dimensional magnetohydrodynamic turbulence: formalism and numerical results
The basic entity of nonlinear interaction in Navier-Stokes and the
Magnetohydrodynamic (MHD) equations is a wavenumber triad ({\bf k,p,q})
satisfying . The expression for the combined energy transfer
from two of these wavenumbers to the third wavenumber is known. In this paper
we introduce the idea of an effective energy transfer between a pair of modes
by the mediation of the third mode, and find an expression for it. Then we
apply this formalism to compute the energy transfer in the quasi-steady-state
of two-dimensional MHD turbulence with large-scale kinetic forcing. The
computation of energy fluxes and the energy transfer between different
wavenumber shells is done using the data generated by the pseudo-spectral
direct numerical simulation. The picture of energy flux that emerges is quite
complex---there is a forward cascade of magnetic energy, an inverse cascade of
kinetic energy, a flux of energy from the kinetic to the magnetic field, and a
reverse flux which transfers the energy back to the kinetic from the magnetic.
The energy transfer between different wavenumber shells is also complex---local
and nonlocal transfers often possess opposing features, i.e., energy transfer
between some wavenumber shells occurs from kinetic to magnetic, and between
other wavenumber shells this transfer is reversed. The net transfer of energy
is from kinetic to magnetic. The results obtained from the studies of flux and
shell-to-shell energy transfer are consistent with each other.Comment: 27 pages REVTEX; 14 ps figure
Influence of density and temperature on the microscopic structure and the segmental relaxation of polybutadiene
We investigate the influence of temperature and density on the local structure and the dynamics of polybutadiene by controlling both hydrostatic pressure and temperature in polarized neutron diffraction experiments on deuterated polybutadiene and in inelastic incoherent scattering experiments on protonated polybutadiene. We observe that the static structure factor S(Q) does not change along macroscopic isochores. This behavior is contrary to the relaxations observed on the nanosecond and picosecond time scales and viewed by the dynamic incoherent scattering law S(Q,omega), which differ strongly along the same thermodynamic path. We conclude that the static behavior, i.e., S(Q), is dominated by macroscopic density changes, similar to the vibrational excitations in the meV range. However, the relaxation dynamics is more sensitive to thermal energy changes. This is confirmed by the finding that lines of identical relaxation behavior (in time, shape, and Q dependence), isochrones on the 10(-9) sec time scale, clearly cross the constant density lines in the (P,T) plane. Concerning S(Q), we can reasonably relate the variation of the main-peak position to the average neighbor chain distance and deduce crude microscopic thermal expansion and compressibility coefficients. In the low-Q regime, the observed pressure and temperature variation of S(Q) exceeds the compressibility contribution and suggests the existence of additional scattering, which might originate from structural correlations arising at higher temperature and low pressure
Hyperfine interaction and electronic spin fluctuation study on SrLaFeCoO (x = 0, 1, 2) by high-resolution back-scattering neutron spectroscopy
The study of hyperfine interaction by high-resolution inelastic neutron
scattering is not very well known compared to the other competing techniques
viz. NMR, M\"ossbauer, PACS etc. Also the study is limited mostly to
magnetically ordered systems. Here we report such study on
SrLaFeCoO (x = 0, 1, 2) of which first (SrFeCoO with x
= 0) has a canonical spin spin glass, the second (SrLaFeCoO with x = 1) has
a so-called magnetic glass and the third (LaFeCoO with x = 2) has a
magnetically ordered ground state. Our present study revealed clear inelastic
signal for SrLaFeCoO, possibly also inelastic signal for SrFeCoO
below the spin freezing temperatures but no inelastic signal at all
for for the magnetically ordered LaFeCoO in the neutron scattering
spectra. The broadened inelastic signals observed suggest hyperfine field
distribution in the two disordered magnetic glassy systems and no signal for
the third compound suggests no or very small hyperfine field at the Co nucleus
due to Co electronic moment. For the two magnetic glassy system apart from the
hyperfine signal due only to Co, we also observed electronic spin fluctuations
probably from both Fe and Co electronic moments. \end{abstract
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