362 research outputs found
Acoustic analysis of the propfan
A review of propeller noise prediction technology is presented. Two methods for the prediction of the noise from conventional and advanced propellers in forward flight are described. These methods are based on different time domain formulations. Brief descriptions of the computer algorithms based on these formulations are given. The output of the programs (the acoustic pressure signature) was Fourier analyzed to get the acoustic pressure spectrum. The main difference between the two programs is that one can handle propellers with supersonic tip speed while the other is for subsonic tip speed propellers. Comparisons of the calculated and measured acoustic data for a conventional and an advanced propeller show good agreement in general
Nanoflows through disordered media: a joint Lattice Boltzmann and Molecular Dynamics investigation
We investigate nanoflows through dilute disordered media by means of joint
lattice Boltzmann (LB) and molecular dynamics (MD) simulations -- when the size
of the obstacles is comparable to the size of the flowing particles -- for
randomly located spheres and for a correlated particle-gel. In both cases at
sufficiently low solid fraction, , LB and MD provide similar values
of the permeability. However, for , MD shows that molecular size
effects lead to a decrease of the permeability, as compared to the
Navier-Stokes predictions. For gels, the simulations highlights a surplus of
permeability, which can be accommodated within a rescaling of the effective
radius of the gel monomers.Comment: 4 pages, 4 figure
Lattice Boltzmann models for non-ideal fluids with arrested phase-separation
The effects of mid-range repulsion in Lattice Boltzmann models on the
coalescence/breakup behaviour of single-component, non-ideal fluids are
investigated. It is found that mid-range repulsive interactions allow the
formation of spray-like, multi-droplet configurations, with droplet size
directly related to the strength of the repulsive interaction. The simulations
show that just a tiny ten-percent of mid-range repulsive pseudo-energy can
boost the surface/volume ratio of the phase- separated fluid by nearly two
orders of magnitude. Drawing upon a formal analogy with magnetic Ising systems,
a pseudo-potential energy is defined, which is found to behave like a
quasi-conserved quantity for most of the time-evolution. This offers a useful
quantitative indicator of the stability of the various configurations, thus
helping the task of their interpretation and classification. The present
approach appears to be a promising tool for the computational modelling of
complex flow phenomena, such as atomization, spray formation and
micro-emulsions, break-up phenomena and possibly glassy-like systems as well.Comment: 12 pages, 9 figure
On the design and test of a low noise propeller
An extensive review of noise and performance of general aviation propellers was performed. Research was done in three areas: The acoustic and aerodynamic theory of general aviation propellers, wind tunnel tests of three one-quarter scale models of general aviation propellers, and flight test of two low noise propellers. The design and testing of the second propeller is reviewed. The general aerodynamic considerations needed to design a new propeller are described. The design point analysis of low noise propellers is reviewed. The predicted and measured noise levels are compared
Limits on the prediction of helicopter rotor noise using thickness and loading sources: Validation of helicopter noise prediction techniques
The techniques of helicopter rotor noise prediction attempt to describe precisely the details of the noise field and remove the empiricisms and restrictions inherent in previous methods. These techniques require detailed inputs of the rotor geometry, operating conditions, and blade surface pressure distribution. The Farassat noise prediction techniques was studied, and high speed helicopter noise prediction using more detailed representations of the thickness and loading noise sources was investigated. These predictions were based on the measured blade surface pressures on an AH-1G rotor and compared to the measured sound field. Although refinements in the representation of the thickness and loading noise sources improve the calculation, there are still discrepancies between the measured and predicted sound field. Analysis of the blade surface pressure data indicates shocks on the blades, which are probably responsible for these discrepancies
Statistics of precursors to fingering processes
We present an analysis of the statistical properties of hydrodynamic field
fluctuations which reveal the existence of precursors to fingering processes.
These precursors are found to exhibit power law distributions, and these power
laws are shown to follow from spatial -Gaussian structures which are
solutions to the generalized non-linear diffusion equation.Comment: 7 pages incl. 5 figs; tp appear in Europhysics Letter
Channel Flow of a Tensorial Shear-Thinning Maxwell Model: Lattice Boltzmann Simulations
We introduce a nonlinear generalized tensorial Maxwell-type constitutive
equation to describe shear-thinning glass-forming fluids, motivated by a recent
microscopic approach to the nonlinear rheology of colloidal suspensions. The
model captures a nonvanishing dynamical yield stress at the glass transition
and incorporates normal-stress differences. A modified lattice-Boltzmann (LB)
simulation scheme is presented that includes non-Newtonian contributions to the
stress tensor and deals with flow-induced pressure differences. We test this
scheme in pressure-driven 2D Poiseuille flow of the nonlinear generalized
Maxwell fluid. In the steady state, comparison with an analytical solution
shows good agreement. The transient dynamics after startup and cessation of the
pressure gradient are studied; the simulation reproduces a finite stopping time
for the cessation flow of the yield-stress fluid in agreement with previous
analytical estimates
Efficient simulation of non-crossing fibers and chains in a hydrodynamic solvent
An efficient simulation method is presented for Brownian fiber suspensions,
which includes both uncrossability of the fibers and hydrodynamic interactions
between the fibers mediated by a mesoscopic solvent. To conserve hydrodynamics,
collisions between the fibers are treated such that momentum and energy are
conserved locally. The choice of simulation parameters is rationalised on the
basis of dimensionless numbers expressing the relative strength of different
physical processes. The method is applied to suspensions of semiflexible fibers
with a contour length equal to the persistence length, and a mesh size to
contour length ratio ranging from 0.055 to 0.32. For such fibers the effects of
hydrodynamic interactions are observable, but relatively small. The
non-crossing constraint, on the other hand, is very important and leads to
hindered displacements of the fibers, with an effective tube diameter in
agreement with recent theoretical predictions. The simulation technique opens
the way to study the effect of viscous effects and hydrodynamic interactions in
microrheology experiments where the response of an actively driven probe bead
in a fiber suspension is measured.Comment: 12 pages, 2 tables, 5 figure
Validation of helicopter noise prediction techniques
The current techniques of helicopter rotor noise prediction attempt to describe the details of the noise field precisely and remove the empiricisms and restrictions inherent in previous methods. These techniques require detailed inputs of the rotor geometry, operating conditions, and blade surface pressure distribution. The purpose of this paper is to review those techniques in general and the Farassat/Nystrom analysis in particular. The predictions of the Farassat/Nystrom noise computer program, using both measured and calculated blade surface pressure data, are compared to measured noise level data. This study is based on a contract from NASA to Bolt Beranek and Newman Inc. with measured data from the AH-1G Helicopter Operational Loads Survey flight test program supplied by Bell Helicopter Textron
Combining Molecular Dynamics with Lattice-Boltzmann: A Hybrid Method for the Simulation of (Charged) Colloidal Systems
We present a hybrid method for the simulation of colloidal systems, that
combines molecular dynamics (MD) with the Lattice-Boltzmann (LB) scheme. The LB
method is used as a model for the solvent in order to take into account the
hydrodynamic mass and momentum transport through the solvent. The colloidal
particles are propagated via MD and they are coupled to the LB fluid by viscous
forces. With respect to the LB fluid, the colloids are represented by uniformly
distributed points on a sphere. Each such point (with a velocity V(r) at any
off-lattice position r is interacting with the neighboring eight LB nodes by a
frictional force F=\xi_0(V(r)-u(r)) with \xi_0 being a friction force and u(r)
being the velocity of the fluid at the position r. Thermal fluctuations are
introduced in the framework of fluctuating hydrodynamics. This coupling scheme
has been proposed recently for polymer systems by Ahlrichs and D"unweg [J.
Chem. Phys. 111, 8225 (1999)]. We investigate several properties of a single
colloidal particle in a LB fluid, namely the effective Stokes friction and long
time tails in the autocorrelation functions for the translational and
rotational velocity. Moreover, a charged colloidal system is considered
consisting of a macroion, counterions and coions that are coupled to a LB
fluid. We study the behavior of the ions in a constant electric field. In
particular, an estimate of the effective charge of the macroion is yielded from
the number of counterions that move with the macroion in the direction of the
electric field.Comment: 37 pages, 12 figure
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