266 research outputs found
Study of nonequilibrium two-phase flow of a gas-particle mixture Technical note no. 2
Two-phase nonequilibrium flow of particle suspensions in gaseous mediu
A theoretical and flight test study of pressure fluctuations under a turbulent boundary layer. Part 2: Flight test study
The study of pressure fluctuations under a turbulent boundary layer was undertaken with the objective of extending previous work to lower frequencies. Wind tunnel and flight test measurements are invalid at low frequencies because of extraneous acoustic noises and free stream turbulence. A glider was instrumented and used as a test bed to carry microphones into a smooth flow free of acoustic noise. Hodgson had previously measured the spectrum of boundary layer noise on a glider wing. These tests showed a drop off at low frequencies that could not be reproduced in any other facility. The measurements were made on the forward fuselage of a glider where the boundary layer could develop naturally and have some length in a zero pressure gradient before the measurements were made. Two different sets of measurements were made
The effect of a row of Helmholtz resonators on the turbulence in a boundary layer
Detailed acoustic measurements were made of the resonator's response as the free stream speed was varied from 30 mph to 155 mph. The average sound pressure level (SPL) and peak frequency response clearly show the existence of strong tuning between the boundary layer and the resonator for the Helmholtz (f sub 0) mode as well as for the first standing wave (f sub 1) mode. The narrow speed range for tuning and the gap between the strong tuning for the Helmholtz mode and the first standing wave mode is evident. The frequency at which the peak SPL response occurred at each speed is given. The peak response for the Helmholtz mode occurred at a free stream velocity of 26 m/s (at which RE sub theta = 6,560) with a resonant frequency of 570 Hz and a sound pressure level of 141 dB. The peak response for the first standing wave occurs at the maximum wind tunnel speed of 70 m/s (at which Re sub theta = 14,900) Hz with a resonant frequency of 1,890 Hz and a sound pressure level of 154 dB. At resonance the microphones do not maintain a constant SPL, but rather the latter fluctuates in a seemingly random fashion. The values presented here are time averaged rms values. Of interest was the occurrence of phase locking between adjacent resonators witha phase lag of 180 deg. This report covers the period from 5 May to 30 September on the subject project
A bilateral shear layer between two parallel Couette flows
We consider a shear layer of a kind not previously studied to our knowledge.
Contrary to the classical free shear layer, the width of the shear zone does
not vary in the streamwise direction but rather exhibits a lateral variation.
Based on some simplifying assumptions, an analytic solution has been derived
for the new shear layer. These assumptions have been justified by a comparison
with numerical solutions of the full Navier-Stokes equations, which accord with
the analytical solution to better than 1% in the entire domain. An explicit
formula is found for the width of the shear zone as a function of wall-normal
coordinate. This width is independent of wall velocities in the laminar regime.
Preliminary results for a co-current laminar-turbulent shear layer in the same
geometry are also presented. Shear-layer instabilities were then developed and
resulted in an unsteady mixing zone at the interface between the two co-current
streams.Comment: 6 pages, 7 figures. Accepted for publication in Phys. Rev.
Generalization of the JTZ model to open plane wakes
The JTZ model [C. Jung, T. T\'el and E. Ziemniak, Chaos {\bf 3}, (1993) 555],
as a theoretical model of a plane wake behind a circular cylinder in a narrow
channel at a moderate Reynolds number, has previously been employed to analyze
phenomena of chaotic scattering. It is extended here to describe an open plane
wake without the confined narrow channel by incorporating a double row of
shedding vortices into the intermediate and far wake. The extended JTZ model is
found in qualitative agreement with both direct numerical simulations and
experimental results in describing streamlines and vorticity contours. To
further validate its applications to particle transport processes, the
interaction between small spherical particles and vortices in an extended JTZ
model flow is studied. It is shown that the particle size has significant
influences on the features of particle trajectories, which have two
characteristic patterns: one is rotating around the vortex centers and the
other accumulating in the exterior of vortices. Numerical results based on the
extended JTZ model are found in qualitative agreement with experimental ones in
the normal range of particle sizes.Comment: 21 pages, 4 figures, 1 tabl
Rotational dynamics and heating of trapped nanovaterite particles
We synthesize, optically trap, and rotate individual nanovaterite crystals with a mean particle radius of 423 nm. Rotation rates of up to 4.9 kHz in heavy water are recorded. Laser-induced heating due to residual absorption of the nanovaterite particle results in the superlinear behavior of the rotation rate as a function of trap power. A finite element method based on the Navier-Stokes model for the system allows us to determine the residual optical absorption coefficient for a trapped nanovaterite particle. This is further confirmed by the theoretical model. Our data show that the translational Stokes drag force and rotational Stokes drag torque need to be modified with appropriate correction factors to account for the power dissipated by the nanoparticle
Statistical Properties of Turbulence: An Overview
We present an introductory overview of several challenging problems in the
statistical characterisation of turbulence. We provide examples from fluid
turbulence in three and two dimensions, from the turbulent advection of passive
scalars, turbulence in the one-dimensional Burgers equation, and fluid
turbulence in the presence of polymer additives.Comment: 34 pages, 31 figure
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