342 research outputs found
Distribution of Acoustic Power Spectra for an Isolated Helicopter Fuselage
The broadband aerodynamic noise can be studied, assuming isotropic flow, turbulence and decay. Proudman’s approach allows practical calculations of noise based on CFD solutions of RANS or URANS equations at the stage of post processing and analysis of the solution. Another aspect is the broadband acoustic spectrum and the distribution of acoustic power over a range of frequencies. The acoustic energy spectrum distribution in isotropic turbulence is non monotonic and has a maximum at a certain value of Strouhal number. In the present work the value of acoustic power peak frequency is determined using a prescribed form of acoustic energy spectrum distribution presented in papers by S. Sarkar and M. Y. Hussaini and by G. M. Lilley. CFD modelling of the flow around isolated helicopter fuselage model was considered using the HMB CFD code and the RANS equations
Statistics of extinction and survival in Lotka-Volterra systems
We analyze purely competitive many-species Lotka-Volterra systems with random
interaction matrices, focusing the attention on statistical properties of their
asymptotic states. Generic features of the evolution are outlined from a
semiquantitative analysis of the phase-space structure, and extensive numerical
simulations are performed to study the statistics of the extinctions. We find
that the number of surviving species depends strongly on the statistical
properties of the interaction matrix, and that the probability of survival is
weakly correlated to specific initial conditions.Comment: Previous version had error in authors. 11 pages, including 5 figure
Aerodynamic optimization of helicopter rear fuselage
An optimization process for the rear helicopter fuselage is presented using Genetic Algorithms and Kriging surrogate models. Shape parameterization is carried out with the super ellipse technique employed for the well-known ROBIN fuselage. The simulations were based on the RANS equations solved using the HMB CFD code. It is shown that a decrease of fuselage drag around 2.5% is possible without compromising the structure and the functionality of the design. Combined with an optimization of the helicopter skids, benefits of up to 4.6% were possible. The demonstrated method can be applied to fuselages of any shape during the initial design phase
Hierarchy problem and the cosmological constant in a five-dimensional Brans-Dicke brane world model
We discuss a new solution, admitting the existence of dS_{4} branes, in
five-dimensional Brans-Dicke theory. It is shown that, due to a special form of
a bulk scalar field potential, for certain values of the model parameters the
effective cosmological constant can be made small on the brane, where the
hierarchy problem of gravitational interaction is solved. We also discuss new
stabilization mechanism which is based on the use of auxiliary fields.Comment: 11 page
Controlling spatiotemporal chaos in oscillatory reaction-diffusion systems by time-delay autosynchronization
Diffusion-induced turbulence in spatially extended oscillatory media near a
supercritical Hopf bifurcation can be controlled by applying global time-delay
autosynchronization. We consider the complex Ginzburg-Landau equation in the
Benjamin-Feir unstable regime and analytically investigate the stability of
uniform oscillations depending on the feedback parameters. We show that a
noninvasive stabilization of uniform oscillations is not possible in this type
of systems. The synchronization diagram in the plane spanned by the feedback
parameters is derived. Numerical simulations confirm the analytical results and
give additional information on the spatiotemporal dynamics of the system close
to complete synchronization.Comment: 19 pages, 10 figures submitted to Physica
Mutual synchronization and clustering in randomly coupled chaotic dynamical networks
We introduce and study systems of randomly coupled maps (RCM) where the
relevant parameter is the degree of connectivity in the system. Global
(almost-) synchronized states are found (equivalent to the synchronization
observed in globally coupled maps) until a certain critical threshold for the
connectivity is reached. We further show that not only the average
connectivity, but also the architecture of the couplings is responsible for the
cluster structure observed. We analyse the different phases of the system and
use various correlation measures in order to detect ordered non-synchronized
states. Finally, it is shown that the system displays a dynamical hierarchical
clustering which allows the definition of emerging graphs.Comment: 13 pages, to appear in Phys. Rev.
Differential constraints compatible with linearized equations
Differential constraints compatible with the linearized equations of partial
differential equations are examined. Recursion operators are obtained by
integrating the differential constraints
Kang-Redner Anomaly in Cluster-Cluster Aggregation
The large time, small mass, asymptotic behavior of the average mass
distribution \pb is studied in a -dimensional system of diffusing
aggregating particles for . By means of both a renormalization
group computation as well as a direct re-summation of leading terms in the
small reaction-rate expansion of the average mass distribution, it is shown
that \pb \sim \frac{1}{t^d} (\frac{m^{1/d}}{\sqrt{t}})^{e_{KR}} for , where and . In two
dimensions, it is shown that \pb \sim \frac{\ln(m) \ln(t)}{t^2} for . Numerical simulations in two dimensions supporting the analytical
results are also presented.Comment: 11 pages, 6 figures, Revtex
Fronts dynamics in the presence of spatio-temporal structured noises
Front dynamics modeled by a reaction-diffusion equation are studied under the
influence of spatio-temporal structured noises. An effective deterministic
model is analytical derived where the noise parameters, intensity, correlation
time and correlation length appear explicitely. The different effects of these
parameters are discussed for the Ginzburg-Landau and Schl\"ogl models. We
obtain an analytical expression for the front velocity as a function of the
noise parameters. Numerical simulations results are in a good agreement with
the theoretical predictions.Comment: 11 pages, 6 figures; REVTEX; to be published in Phys.Rev.E, july 200
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