21,248 research outputs found
Transonic rotor noise: Theoretical and experimental comparisons
Two complementary methods of describing the high speed rotor noise problem are discussed. The first method uses the second order transonic potential equation to define and characterize the nature of the aerodynamic and acoustic fields and to explain the appearance of radiating shock waves. The second employs the Ffowcs Williams and Hawkings equation to successfully calculate the acoustic far field. Good agreement between theoretical and experimental waveforms is shown for transonic hover tip Mach numbers from 0.8 to 0.9
Acoustically swept rotor
Impulsive noise reduction is provided in a rotor blade by acoustically sweeping the chord line from root to tip so that the acoustic radiation resulting from the summation of potential singularities used to model the flow about the blade tend to cancel for all times at an observation point in the acoustic far field
Aeroacoustic research: An Army perspective
A short perspective of the Army aeroacoustic research program is presented that emphasizes rotary wing, aerodynamically generated noise. Exciting breakthroughs in experimental techniques and facilities are reviewed which are helping build a detailed understanding of helicopter external noise. Army and joint Army/NASA supported research programs in acoustics which promise to reduce the noise of future helicopters without severe performance penalties are included
Helicopter impulsive noise: Theoretical and experimental status
The theoretical and experimental status of helicopter impulsive noise is reviewed. The two major source mechanisms of helicopter impulsive noise are addressed: high-speed impulsive noise and blade-vortex interaction impulsive noise. A thorough physical explanation of both generating mechanism is presented together with model and full-scale measurements of the phenomena. Current theoretical prediction methods are compared with experimental findings of isolated rotor tests. The noise generating mechanism of high speed impulsive noise are fairly well understood - theory and experiment compare nicely over Mach number ranges typical of today's helicopters. For the case of blade-vortex interaction noise, understanding of noise generating mechanisms and theoretical comparison with experiment are less satisfactory. Several methods for improving theory-experiment are suggested
Kinetic Vlasov Simulations of collisionless magnetic Reconnection
A fully kinetic Vlasov simulation of the Geospace Environment Modeling (GEM)
Magnetic Reconnection Challenge is presented. Good agreement is found with
previous kinetic simulations using particle in cell (PIC) codes, confirming
both the PIC and the Vlasov code. In the latter the complete distribution
functions () are discretised on a numerical grid in phase space.
In contrast to PIC simulations, the Vlasov code does not suffer from numerical
noise and allows a more detailed investigation of the distribution functions.
The role of the different contributions of Ohm's law are compared by
calculating each of the terms from the moments of the . The important role
of the off--diagonal elements of the electron pressure tensor could be
confirmed. The inductive electric field at the X--Line is found to be dominated
by the non--gyrotropic electron pressure, while the bulk electron inertia is of
minor importance. Detailed analysis of the electron distribution function
within the diffusion region reveals the kinetic origin of the non--gyrotropic
terms
Leptogenesis after Chaotic Sneutrino Inflation and the Supersymmetry Breaking Scale
We discuss resonant leptogenesis arising from the decays of two
nearly-degenerate right-handed neutrinos, identified as the inflaton and
stabiliser superfields in a model of chaotic sneutrino inflation. We compare an
analytical estimate of the baryon asymmetry in the Boltzmann
approximation to a numerical solution of the full density matrix equations, and
find that the analytical result fails to capture the correct physics in certain
regions of parameter space. The observed baryon asymmetry can be realised for a
breaking of the mass degeneracy as small as . The
origin of such a small mass splitting is explained by considering supersymmetry
(SUSY) breaking in supergravity, which requires a constant in the
superpotential of the order of the gravitino mass to cancel the
cosmological constant. This yields additional terms in the (s)neutrino mass
matrices, lifting the degeneracy and linking to the SUSY breaking
scale. We find that achieving the correct baryon asymmetry requires a gravitino
mass TeV.Comment: v2: 25 pages, 4 figures; version published in NPB, minor corrections.
v1: 24 pages, 4 figure
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