6,711 research outputs found
A numerical method for the prediction of high-speed boundary-layer transition using linear theory
A method is described of estimating the location of transition in an arbitrary laminar boundary layer on the basis of linear stability theory. After an examination of experimental evidence for the relation between linear stability theory and transition, a discussion is given of the three essential elements of a transition calculation: (1) the interaction of the external disturbances with the boundary layer; (2) the growth of the disturbances in the boundary layer; and (3) a transition criterion. The computer program which carried out these three calculations is described. The program is first tested by calculating the effect of free-stream turbulence on the transition of the Blasius boundary layer, and is then applied to the problem of transition in a supersonic wind tunnel. The effects of unit Reynolds number and Mach number on the transition of an insulated flat-plate boundary layer are calculated on the basis of experimental data on the intensity and spectrum of free-stream disturbances. Reasonable agreement with experiment is obtained in the Mach number range from 2 to 4.5
Numerical Investigation of Second Mode Attenuation over Carbon/Carbon Surfaces on a Sharp Slender Cone
We have carried out axisymmetric numerical simulations of a spatially
developing hypersonic boundary layer over a sharp 7-half-angle cone
at inspired by the experimental investigations by Wagner (2015).
Simulations are first performed with impermeable (or solid) walls with a
one-time broadband pulse excitation applied upstream to determine the most
convectively-amplified frequencies resulting in the range 260kHz -- 400kHz,
consistent with experimental observations of second-mode instability waves.
Subsequently, we introduce harmonic disturbances via continuous periodic
suction and blowing at 270kHz and 350kHz. For each of these forcing frequencies
complex impedance boundary conditions (IBC), modeling the acoustic response of
two different carbon/carbon (C/C) ultrasonically absorptive porous surfaces,
are applied at the wall. The IBCs are derived as an output of a pore-scale
aeroacoustic analysis -- the inverse Helmholtz Solver (iHS) -- which is able to
return the broadband real and imaginary components of the surface-averaged
impedance. The introduction of the IBCs in all cases leads to a significant
attenuation of the harmonically-forced second-mode wave. In particular, we
observe a higher attenuation rate of the introduced waves with frequency of
350kHz in comparison with 270kHz, and, along with the iHS impedance results, we
establish that the C/C surfaces absorb acoustic energy more effectively at
higher frequencies.Comment: AIAA-SciTech 201
Absolute and convective instabilities of an inviscid compressible mixing layer: Theory and applications
This study aims to examine the effect of compressibility on unbounded and parallel shear flow linear instabilities. This analysis is of interest for industrial, geophysical, and astrophysical flows. We focus on the stability of a wavepacket as opposed to previous single-mode stability studies. We consider the notions of absolute and convective instabilities first used to describe plasma instabilities. The compressible-flow modal theory predicts instability whatever the Mach number. Spatial and temporal growth rates and Reynolds stresses nevertheless become strongly reduced at high Mach numbers. The evolution of disturbances is driven by Kelvin -Helmholtz instability that weakens in supersonic flows. We wish to examine the occurrence of absolute instability, necessary for the appearance of turbulent motions in an inviscid and compressible two-dimensional mixing layer at an arbitrary Mach number subject to a three-dimensional disturbance. The mixing layer is defined by a parametric family of mean-velocity and temperature profiles. The eigenvalue problem is solved with the help of a spectral method. We ascertain the effects of the distribution of temperature and velocity in the mixing layer on the transition between convective and absolute instabilities. It appears that, in most cases, absolute instability is always possible at high Mach numbers provided that the ratio of slow-stream temperature over fast-stream temperature may be less than a critical maximal value but the temporal growth rate present in the absolutely unstable zone remains small and tends to zero at high Mach numbers. The transition toward a supersonic turbulent regime is therefore unlikely to be possible in the linear theory. Absolute instability can be also present among low-Mach-number coflowing mixing layers provided that this same temperature ratio may be small, but nevertheless, higher than a critical minimal value. Temperature distribution within the mixing layer also has an effect on the growth rate, this diminishes when the slow stream is heated. These results are applied to the dynamics of mixing layers in the interstellar medium and to the dynamics of the heliopause, frontier between the interstellar medium, and the solar wind. (C) 2009 American Institute of Physics
Multi-Frequency Study of the B3-VLA Sample II. The Database
We present total flux densities of 1049 radio sources in the frequency range
from 151 MHz to 10.6 GHz. These sources belong to the B3-VLA sample, which is
complete down to 100 mJy at 408 MHz. The data constitute a homogeneous spectral
database for a large sample of radio sources, 50 times fainter than the 3C
catalogue, and will be used to perform a spectral ageing analysis, which is one
of the critical points in understanding the physics and evolution of
extragalactic radio sources.Comment: 14 pages, 3 figures, accepted for publication in Astronomy &
Astrophysics Supplement Series, gzipped postscript file also available at
http://multivac.jb.man.ac.uk:8000/ceres/papers/papers.html or
http://gladia.astro.rug.nl:8000/ceres/papers/papers.htm
Design evaluation criteria for commercial STOL transports
Handling qualities criteria and operational performance margins have been determined for the landing phase of commercial short-takeoff-and-landing airplanes. The requirements are the result of a literature survey, analysis of areas found to be inadequately covered by current criteria, and a subsequent piloted simulator investigation of critical criteria requiring substantiation. Three complete simulator models were used, each describing the characteristics of a different high-lift system, the externally blown flap, the augmentor flap, and the internally blown flap. The proposed criteria are presented with substantiating discussions from currently available data or directly from the results of this simulation work where it is applicable
A wind-tunnel investigation of sonic-boom pressure distributions of bodies of revolution at Mach 2.96, 3.83, and 4.63
Measurements of sonic boom pressure distribution of bodies of revolution at Mach 2.96, 3.83, and 4.63 in Unitary Plan wind tunne
Transition and laminar instability
The linear stability theory was applied to the problem of boundary layer transition in incompressible flow. The theory was put into a form suitable for three-dimensional boundary layers; both the temporal and spatial theories were examined; and a generalized Gaster relation for three-dimensional boundary layers was derived. Numerical examples include the stability characteristics of Falkner-Skan boundary layers, the accuracy of the two-dimensional Gaster relation for these boundary layers, and the magnitude and direction of the group velocity for oblique waves in the Blasius boundary layer. Available experiments which bear on the validity of stability theory and its relation to transition are reviewed and the stability theory is applied to transition prediction. The amplitude method is described in which the wide band disturbance amplitude in the boundary layer is estimated from stability theory and an interaction relation for the initial amplitude density of the most unstable frequency
Mechanics of Boundary Layer Transition. Part 5: Boundary Layer Stability theory in incompressible and compressible flow
The fundamentals of stability theory, its chief results, and the physical mechanisms at work are presented. The stability theory of the laminar boundary determines whether a small disturbance introduced into the boundary layer will amplify or damp. If the disturbance damps, the boundary layer remains laminar. If the disturbance amplifies, and by a sufficient amount, then transition to turbulence eventually takes place. The stability theory establishes those states of the boundary layer which are most likely to lead to transition, identifys those frequencies which are the most dangerous, and indicates how the external parameters can best be changed to avoid transition
Factorizing Numbers with the Gauss Sum Technique: NMR Implementations
Several physics-based algorithms for factorizing large number were recently
published. A notable recent one by Schleich et al. uses Gauss sums for
distinguishing between factors and non-factors. We demonstrate two NMR
techniques that evaluate Gauss sums and thus implement their algorithm. The
first one is based on differential excitation of a single spin magnetization by
a cascade of RF pulses. The second method is based on spatial averaging and
selective refocusing of magnetization for Gauss sums corresponding to factors.
All factors of 16637 and 52882363 are successfully obtained.Comment: 4 pages, 4 figures; Abstract and Conclusion are slightly modified.
References added and formatted with Bibte
The giant radio galaxy 8C0821+695 and its environment
We present new VLA and Effelsberg observations of the radio galaxy
8C0821+695. We have obtained detailed images in total intensity and
polarization of this 2 Mpc sized giant. The magnetic field has a configuration
predominantly parallel to the source main axis. We observe Faraday rotation at
low frequencies, most probably produced by an ionized medium external to the
radio source. The spectral index distribution is that typical of FR II radio
galaxies, with spectral indices gradually steepening from the source extremes
towards the core. Modeling the spectrum in the lobes using standard synchrotron
loss models yields the spectral age of the source and the mean velocity of the
jet-head with respect to the lobe material. The existence of a possible
backflow in the lobe is considered to relate spectral with dynamical
determinations of the age and the velocity with respect to the external medium.
Through a very simple model, we obtain a physical characterization of the jets
and the external medium in which the radio galaxy expands. The results in
8C0821+695 are consistent with a relativistic jet nourishing the lobes which
expand in a hot, low density halo. We infer a deceleration of the source
expansion velocity which we explain through a progressive increase in the
hot-spot size.Comment: 11 pages; 8 figures; accepted in A&
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