2,580 research outputs found
Analytical and experimental studies of shock interference heating in hypersonic flows
An analytical and experimental study is presented of the aerodynamic heating resulting from six types of shock interference patterns encountered in high speed flow. Centerline measurements of pressure and heat transfer distributions on basic bodies were obtained in four wind tunnels for Mach numbers from 6 to 20, specific heat ratios from 1.27 to 1.67, and free stream Reynolds numbers from 3 million to 25.6 million per meter. Peak heating and peak pressures up to 17 and 7.5 times stagnation values, respectively, were measured. In general, results obtained from semiempirical methods developed for each of the six types of interference agreed with the experimental peaks
Modifications to the nozzle test chamber to extend nozzle static-test capability
The nozzle test chamber was modified to provide a high-pressure-ratio nozzle static-test capability. Experiments were conducted to determine the range of the ratio of nozzle total pressure to chamber pressure and to make direct nozzle thrust measurements using a three-component strain-gage force balance. Pressure ratios from 3 to 285 were measured with several axisymmetric nozzles at a nozzle total pressure of 15 to 190 psia. Devices for measuring system mass flow were calibrated using standard axisymmetric convergent choked nozzles. System mass-flow rates up to 10 lbm/sec are measured. The measured thrust results of these nozzles are in good agreement with one-dimensional theoretical predictions for convergent nozzles
Off-center-line shock interference heating patterns on basic shapes in hypersonic flows
The results of an experimental study of off-center-line shock-interference heating on basic shapes at hypersonic speeds are presented. The study covered three types of shock-interference patterns over a range of nominal Mach numbers (6 to 20), specific heat ratios (1.40 and 1.67), free-stream Reynolds numbers(8 million to 26 million per meter), and impinging shock strengths. Heat-transfer rates higher than stagnation levels were measured over much of the off-center-line model surface. Peak heating up to 17 times the stagnation heating was measured
Force testing manual for the Langley 20-inch Mach 6 tunnel
Data reduction and procedures for conducting force tests in a 20 inch Mach 6 tunnel are described. A discussion of pretest and testing phases are included. Items that are to be checked during model design and construction are outlined as well as safety requirements, starting loads tests, instructions for data acquisition and model installation. Measurement of balance and model misalignment and instructions for calibrating the angle of attack screen are covered. Procedures for making reference pressure, attitude tare, and data runs are included. The 20 inch tunnel force program is examined, and a description of data recording system input and load contrast sheets is given. An appendix presents a description, operating characteristics, and Mach number calibration of the tunnel, as well as tunnel characteristics
Noise reduction in a Mach 5 wind tunnel with a rectangular rod-wall sound shield
A rod wall sound shield was tested over a range of Reynolds numbers of 0.5 x 10 to the 7th power to 8.0 x 10 to the 7th power per meter. The model consisted of a rectangular array of longitudinal rods with boundary-layer suction through gaps between the rods. Suitable measurement techniques were used to determine properties of the flow and acoustic disturbance in the shield and transition in the rod boundary layers. Measurements indicated that for a Reynolds number of 1.5 x 10 to the 9th power the noise in the shielded region was significantly reduced, but only when the flow is mostly laminar on the rods. Actual nozzle input noise measured on the nozzle centerline before reflection at the shield walls was attenuated only slightly even when the rod boundary layer were laminar. At a lower Reynolds number, nozzle input noise at noise levels in the shield were still too high for application to a quiet tunnel. At Reynolds numbers above 2.0 x 10 the the 7th power per meter, measured noise levels were generally higher than nozzle input levels, probably due to transition in the rod boundary layers. The small attenuation of nozzle input noise at intermediate Reynolds numbers for laminar rod layers at the acoustic origins is apparently due to high frequencies of noise
Electron-Acoustic Phonon Energy Loss Rate in Multi-Component Electron Systems with Symmetric and Asymmetric Coupling Constants
We consider electron-phonon (\textit{e-ph}) energy loss rate in 3D and 2D
multi-component electron systems in semiconductors. We allow general asymmetry
in the \textit{e-ph} coupling constants (matrix elements), i.e., we allow that
the coupling depends on the electron sub-system index. We derive a
multi-component \textit{e-ph}power loss formula, which takes into account the
asymmetric coupling and links the total \textit{e-ph} energy loss rate to the
density response matrix of the total electron system. We write the density
response matrix within mean field approximation, which leads to coexistence of\
symmetric energy loss rate and asymmetric energy loss rate
with total energy loss rate at temperature
. The symmetric component F_{S}(T) F_{S}(T)\propto T^{n_{S}}n_{S}F_{A}(T). Screening strongly
reduces the symmetric coupling, but the asymmetric coupling is unscreened,
provided that the inter-sub-system Coulomb interactions are strong. The lack of
screening enhances and the total energy loss rate .
Especially, in the strong screening limit we find . A
canonical example of strongly asymmetric \textit{e-ph} matrix elements is the
deformation potential coupling in many-valley semiconductors.Comment: v2: Typos corrected. Some notations changed. Section III.C is
embedded in Section III.B. Paper accepted to PR
The Potential Energy Landscape and Mechanisms of Diffusion in Liquids
The mechanism of diffusion in supercooled liquids is investigated from the
potential energy landscape point of view, with emphasis on the crossover from
high- to low-T dynamics. Molecular dynamics simulations with a time dependent
mapping to the associated local mininum or inherent structure (IS) are
performed on unit-density Lennard-Jones (LJ). New dynamical quantities
introduced include r2_{is}(t), the mean-square displacement (MSD) within a
basin of attraction of an IS, R2(t), the MSD of the IS itself, and g_{loc}(t)
the mean waiting time in a cooperative region. At intermediate T, r2_{is}(t)
posesses an interval of linear t-dependence allowing calculation of an
intrabasin diffusion constant D_{is}. Near T_{c} diffusion is intrabasin
dominated with D = D_{is}. Below T_{c} the local waiting time tau_{loc} exceeds
the time, tau_{pl}, needed for the system to explore the basin, indicating the
action of barriers. The distinction between motion among the IS below T_{c} and
saddle, or border dynamics above T_{c} is discussed.Comment: submitted to pr
Inherent-Structure Dynamics and Diffusion in Liquids
The self-diffusion constant D is expressed in terms of transitions among the
local minima of the potential (inherent structure, IS) and their correlations.
The formulae are evaluated and tested against simulation in the supercooled,
unit-density Lennard-Jones liquid. The approximation of uncorrelated
IS-transition (IST) vectors, D_{0}, greatly exceeds D in the upper temperature
range, but merges with simulation at reduced T ~ 0.50. Since uncorrelated IST
are associated with a hopping mechanism, the condition D ~ D_{0} provides a new
way to identify the crossover to hopping. The results suggest that theories of
diffusion in deeply supercooled liquids may be based on weakly correlated IST.Comment: submitted to PR
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