A new class of galactic discrete gamma ray sources: Chaotic winds of massive stars

We propose a new class of galactic discrete gamma-ray sources, the chaotic, high mass-loss-rate winds from luminous early-type stars. Early-type stellar winds are highly unstable due to intrinsic line-driven instabilities, and so are permeated by numerous strong shocks. These shocks can accelerate a small fraction of thermal electrons and ions to relativistic energies via the first-order Fermi mechanism. A power-law-like photon spectrum extending from keV to above 10 MeV energies is produced by inverse Compton scattering of the extremely abundant stellar UV photons by the relativistic electrons. In addition, a typical pi(sup 0)-decay gamma-ray spectrum is generated by proton-ion interactions in the densest part of the winds

Low energy gamma ray emission from the Cygnus OB2 association

According to our newly developed model of gamma-ray emission from chaotic early-type stellar winds, we predict the combined gamma-ray flux from the circumstellar winds of many very luminous early-type stars in the Cyg OB2 association can be detectable by the Energetic Gamma Ray Experiment Telescope (EGRET) (and maybe also by OSSE) on CGRO. Due to different radiation mechanisms, the gamma-ray spectrum from stellar winds can be quite different from that of CYG X-3; this spectral difference and the time-variation of Cyg X-3 flux will help to distinguish the gamma-ray components from different sources in this small region, which is spatially unresolvable by CGRO

Effect of rotor stiffness and lift offset on the aeroacoustics of a coaxial rotor in level flight

The acoustic characteristics of a twin contra-rotating coaxial rotor configuration with significant flapwise stiffness are investigated in steady forward flight. The Vorticity Transport Model is used to simulate the aerodynamics of the rotor system and the acoustic field is determined using the Ffowcs Williams-Hawkings equation implemented using the Farassat-1A formulation. Increasing the hub stiffness alters the strengths of the blade vortex interactions, particularly those between the upper and lower rotors, and affects the intensity and directivity of the blade vortex interaction noise produced by the system. The inter-rotor blade vortex interaction on the advancing side of the lower rotor is the principal source of the most intensively focused noise that is generated by a conventionally articulated coaxial rotor system. For stiffened coaxial rotors, this particular inter-rotor blade vortex interaction is weakened as a result of a broad redistribution in lateral loading, yielding a reduction in the intensity of the noise that is produced by this interaction. The spanwise distribution of loading on the rotors of a stiffened coaxial system can be modified further by altering the lateral partition of lift (or lift offset). It is shown that decreasing the lift offset has the effect of counteracting the redistribution of loading due to flapwise stiffness and hence increases the blade vortex interaction noise as well as the power consumed by the rotor. Conversely, a reduction in both the power consumption and the blade vortex interaction noise is observed if the lift offset is increased, with the maximum benefit of lift offset being achieved at high speed. The computational results suggest that the noise from the dominant inter-rotor blade vortex interaction can be ameliorated through the use of lift offset control on stiffened coaxial systems, to the extent that the noise produced by this interaction can be made to be comparable to that produced by the other, weaker interactions between the two rotors of the system

Interactional aerodynamics and acoustics of a propeller-augmented compound coaxial helicopter

The aerodynamic and acoustic characteristics of a generic hingeless coaxial helicopter with a tail-mounted propulsor and stabiliser have been simulated using Brown's Vorticity Transport Model. This has been done to investigate the ability of models of this type to capture the aerodynamic interactions that are generated between the various components of realistic, complex helicopter configurations. Simulations reveal the aerodynamic environment of the coaxial main rotor of the configuration to be dominated by internal interactions that lead to high vibration and noise. The wake of the main rotor is predicted to interact strongly with the tailplane, particularly at low forward speed, to produce a strong nose-up pitching moment that must be countered by significant longitudinal cyclic input to the main rotor. The wake from the main rotor is ingested directly into the tail propulsor over a broad range of forward speeds, where it produces significant vibratory excitation of the system as well as broadband noise. The numerical calculations also suggest the possibility that poor scheduling of the partition of the propulsive force between the main rotor and propulsor as a function of forward speed may yield a situation where the propulsor produces little thrust but high vibration as a result of this interaction. Although many of the predicted effects might be ameliorated or eliminated entirely by more careful or considered design, the model captures many of the aerodynamic interactions, and the resultant effects on the loading on the system, that might be expected to characterise the dynamics of such a vehicle. It is suggested that the use of such numerical techniques might eventually allow the various aeromechanical problems that often beset new designs to be circumvented - hopefully well before they manifest on the prototype or production aircraft