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At least 307 records · Page 17

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NASA NTRS1988

Computer modeling of test particle acceleration at oblique shocks

The present evaluation of the basic techniques and illustrative results of charged particle-modeling numerical codes suitable for particle acceleration at oblique, fast-mode collisionless shocks emphasizes the treatment of ions as test particles, calculating particle dynamics through numerical integration along exact phase-space orbits. Attention is given to the acceleration of particles at planar, infinitessimally thin shocks, as well as to plasma simulations in which low-energy ions are injected and accelerated at quasi-perpendicular shocks with internal structure.

Decker, Robert B.↗

NASA NTRS1989

Experimental and numerical investigation of an oblique shock wave/turbulent boundary layer interaction with continuous suction

An numerical and experimental investigation has been conducted into the interaction of an incident oblique shock wave with a turbulent boundary layer, for the cases of a rough plate and a porous plate with suction, at a nominal Mach number of 2.5 and flow deflection angles of 0, 4, 6, and 8 deg. Attention is given to the pitot pressure profiles, wall static pressures, and porous plate local bleed distributions measured for the two plates. Suction is found to increase the strength of the incident shock required to separate the boundary layer; for all shock strengths tested, separation is completely eliminated.

Benhachmi, Driss↗

NASA NTRS1989

Fokker-Planck electron diffusion caused by an obliquely propagating electromagnetic wave packet of narrow bandwidth

The relativistic motion of electrons in an intense electromagnetic wave packet propagating obliquely to a uniform magnetic field is analytically studied on the basis of the Fokker-Planck-Kolmogorov (FPK) approach. The wavepacket consists of circularly polarized electron-cyclotron waves. The dynamical system in question is shown to be reducible to one with three degrees of freedom. Within the framework of the Hamiltonian analysis the nonlinear diffusion tensor is derived, and it is shown that this tensor can be separated into zeroth-, first-, and second-order parts with respect to the relative bandwidth. The zeroth-order part describes diffusive acceleration along lines of constant unperturbed Hamiltonian. The second-order part, which corresponds to the longest time scale, describes diffusion across those lines. A possible transport theory is outlined on the basis of this separation of the time scales.

Hizanidis, Kyriakos↗

NASA NTRS1989

Electron acceleration by an obliquely propagating electromagnetic wave in the regime of validity of the Fokker-Planck-Kolmogorov approach

The relativistic motion of an ensemble of electrons in an intense monochromatic electromagnetic wave propagating obliquely in a uniform external magnetic field is studied. The problem is formulated from the viewpoint of Hamiltonian theory and the Fokker-Planck-Kolmogorov approach analyzed by Hizanidis (1989), leading to a one-dimensional diffusive acceleration along paths of constant zeroth-order generalized Hamiltonian. For values of the wave amplitude and the propagating angle inside the analytically predicted stochastic region, the numerical results suggest that the diffusion probes proceeds in stages. In the first stage, the electrons are accelerated to relatively high energies by sampling the first few overlapping resonances one by one. During that stage, the ensemble-average square deviation of the variable involved scales quadratically with time. During the second stage, they scale linearly with time. For much longer times, deviation from linear scaling slowly sets in.

Hizanidis, Kyriakos↗

NASA NTRS1989

Nonlinear evolution of oblique waves on compressible shear layers

The effects of critical-layer nonlinearity on spatially growing oblique instability waves on compressible shear layers between two parallel streams are considered. The analysis shows that mean temperature nonuniformities cause nonlinearity to occur at much smaller amplitudes than it does when the flow is isothermal. The nonlinear instability wave growth rate effects are described by an integrodifferential equation which bears some resemblance to the Landau equation, in that it involves a cubic-type nonlinearity. The numerical solutions to this equation are worked out and discussed in some detail. Inviscid solutions always end in a singularity at a finite downstream distance, but viscosity can eliminate this singularity for certain parameter ranges.

Goldstein, M. E.↗

NASA NTRS1989

Nonlinear evolution of interacting oblique waves on two-dimensional shear layers

The effects of critical layer nonlinearity are considered on spatially growing oblique instability waves on nominally two-dimensional shear layers between parallel streams. The analysis shows that three-dimensional effects cause nonlinearity to occur at much smaller amplitudes than it does in two-dimensional flows. The nonlinear instability wave amplitude is determined by an integro-differential equation with cubic type nonlinearity. The numerical solutions to this equation are worked out and discussed in some detail. The numerical solutions always end in a singularity at a finite downstream distance.

Goldstein, M. E.↗

NASA NTRS1990

Bipolar supernova remnants and the obliquity dependence of shock acceleration

The diffusive shock acceleration mechanism proposed to explain the bipolarity observed in the synchrotron radio emission of young adiabatically expanding shell SNRs is investigated by means of numerical simulations. The theoretical basis of the SNR models and the numerical computation methods are explained, and the results are presented in graphs and synthetic radio maps and discussed in detail. It is found that the efficiency of the acceleration process depends on the obliquity angle theta(Bn) between the shock normal and the uniform magnetic field: models with theta(Bn) of about 90 deg can reproduce the observed azimuthal intensity ratios in most cases, but models with theta(Bn) near 0 deg cannot.

Fulbright, Michael S.↗

NASA NTRS1990

Flow separation in oblique shock wave turbulent boundary layer interactions

Available experimental data on flow separation in shock wave turbulent boundary layer interactions are reviewed at hypersonic and supersonic speeds in two dimensional and axisymmetric interactions. The conditions leading to flow separation and the subsequent changes in the flow are discussed. The effects of the Mach number, Reynolds number, surface cooling and the methods of detecting separation are discussed. Pertinent experimental data for the separated flow characteristics in turbulent boundary layer oblique shock wave interactions are also presented and discussed. Empirical correlations for separated flows are also reviewed.

Hamed, A.↗

NASA NTRS1990

A numerical simulation of climate changes during the obliquity cycle on Mars

A one-dimensional seasonal energy balance climate model of the Martian surface is developed. The model shows the importance of using short-period diurnal and seasonal variations of solar irradiance instead of yearly-averaged quantities. The roles of meridional heat transport and greenhouse warming are shown to be important. The possible existence of hysteresis cycles in the formation and sublimation of permanent deposits during the course of the obliquity cycle is demonstrated.

Francois, L. M.↗

NASA NTRS1991

Application of modern control design methodology to oblique wing research aircraft

A Linear Quadratic Regulator synthesis technique was used to design an explicit model following control system for the Oblique Wing Research Aircraft (OWRA). The forward path model (Maneuver Command Generator) was designed to incorporate the desired flying qualities and response decoupling. The LQR synthesis was based on the use of generalized controls, and it was structured to provide a proportional/integral error regulator with feedforward compensation. An unexpected consequence of this design approach was the ability to decouple the control synthesis into separate longitudinal and lateral directional designs. Longitudinal and lateral directional control laws were generated for each of the nine design flight conditions, and gain scheduling requirements were addressed. A fully coupled 6 degree of freedom open loop model of the OWRA along with the longitudinal and lateral directional control laws was used to assess the closed loop performance of the design. Evaluations were performed for each of the nine design flight conditions.

Vincent, James H.↗

NASA NTRS1990

The sonic boom of an oblique flying wing

An analysis of sonic boom characteristics of an oblique flying wing is presented. The wing, represented by a slewed lift and area-distribution as well as a panel geometry, promises a reduction of sonic boom signature. For every azimuth angle these distributions are represented by an equivalent body. The near-field pressure signature is determined by using the Whitham F-function with a correction to account for nonlinear wave propagation. The geometric asymmetry leads to an asymmetrical sonic boom beneath the flight track with bow shocks between 1.0 and 1.5 PSF. Due to favorable volume-lift interference the aft shock has only half the amplitude of the bow shock. A fast numerical method is described to calculate the perceived loudness.

Kroo, Ilan↗

NASA NTRS1991

Pitch angle scattering of energetic particles by oblique whistler waves

First order cyclotron or Landau resonant pitch angle scattering of electrons by oblique whistler waves propagating at large angles to the ambient field are found to be at least as large as that due to parallel propagating waves. Commonly observed precipitation of more than 40 keV electrons in association with ducted whistlers may thus be accompanied by substantial fluxes of lower energy (10 eV-40 keV) electrons precipitated by the nonducted components.

Inan, U. S.↗

NASA NTRS1991

Transverse conductivity of a relativistic plasma in oblique electric and magnetic fields

Resistive tearing in a primary candidate for flares occurring in stressed magnetic fields. Its possible application to the strongly magnetized environments (Hz about 10 to the 12th G) near the surface of neutron stars, particularly as a mechanism for generating the plasma heating and particle acceleration leading to gamma-ray bursts, has motivated a quantum treatment of this process, which requires knowledge of the electrical conductivity sigma of a relativistic gas in a new domain (i.e., that of a low-density n/e/) plasma in oblique electric and magnetic fields. This paper discusses the mathematical formalism for calculating sigma and present numerical results for a wide range of parameter values. The results indicate that sigma depends very strongly on both the applied electric and magnetic fields.

Melia, Fulvio↗

NASA NTRS1991

Numerical studies of electron dynamics in oblique quasi-perpendicular collisionless shock waves

Linear and nonlinear electron damping of the whistler precursor wave train to low Mach number quasi-perpendicular oblique shocks is studied using a one-dimensional electromagnetic plasma simulation code with particle electrons and ions. In some parameter regimes, electrons are observed to trap along the magnetic field lines in the potential of the whistler precursor wave train. This trapping can lead to significant electron heating in front of the shock for low beta(e). Use of a 64-processor hypercube concurrent computer has enabled long runs using realistic mass ratios in the full particle in-cell code and thus simulate shock parameter regimes and phenomena not previously studied numerically.

Liewer, P. C.↗

NASA NTRS1991

A planar oblique wave model for the Euler equations

A numerical method in which any transition between neighboring states in a multidimensional flowfield is reconstructed by a pattern of oblique waves. The wave pattern is obtained from a superposition of model waves which arise in a local linearization based on Rankine-Hugoniot averaging. The strengths and orientations of these waves are derived from a minimum path length rule applied in state space. Two test cases with comparisons to other methods in planar flow problems are presented to demonstrate the strengths of the method, and to point out its weaknesses.

Parpia, Ijaz H.↗

NASA NTRS1992

Oblique-Flying-Wing Supersonic Transport Airplane

Oblique-flying-wing supersonic airplane proposed as possible alternative to B747B (or equivalent). Tranports passengers and cargo as fast as twice speed of sound at same cost as current subsonic transports. Flies at same holding speeds as present supersonic transports but requires only half takeoff distance.

Van Der Velden, Alexander J. M.↗

NASA NTRS1992

Decoupling Control For Oblique-Wing Airplane

Technical memorandum proposes procedure for mathematical synthesis of control signals proportional to commanded deflections of rudder, elevators, and ailerons of oblique-wing airplane. Procedure designed to generate signals compensating for aerodynamical and inertial cross-coupling among yawing, pitching, and rolling motions. Control interactions decoupled by feedforward and feedback.

Kempel, Robert W.↗

NASA NTRS1992

Transonic Wind-Tunnel Test Of An Oblique Wing

Report describes transonic wind-tunnel tests of oblique, pivotable wing fitted to 0.087-scale model of F-8 airplane. Purpose of tests to study performance and stability characteristics. Conducted to determine whether placement of pivot at higher position above fuselage than previously results in less side force and yawing moment than observed in previous tests at low speeds.

Kennelly, R. A.↗