347 research outputs found
Front dynamics in turbulent media
A study of a stable front propagating in a turbulent medium is presented. The
front is generated through a reaction-diffusion equation, and the turbulent
medium is statistically modeled using a Langevin equation. Numerical
simulations indicate the presence of two different dynamical regimes. These
regimes appear when the turbulent flow either wrinkles a still rather sharp
propagating interfase or broadens it. Specific dependences of the propagating
velocities on stirring intensities appropriate to each case are found and
fitted when possible according to theoretically predicted laws. Different
turbulent spectra are considered.Comment: 8 pages, REVTEX, 6 postscript figures included. To appear in Phys.
Fluids (1997
Langevin approach to generate synthetic turbulence
We present an analytical scheme, easily implemented numerically, to generate
synthetic Gaussian turbulent flows by using a linear Langevin equation, where
the noise term acts as a stochastic stirring force. The characteristic
parameters of the velocity field are well introduced, in particular the
kinematic viscosity and the spectrum of energy. As an application, the
diffusion of a passive scalar is studied for two different energy spectra.
Numerical results are compared favorably with analytical calculations.Comment: 7 pages, REVTEX, 6 figures. To appear in Physics of Fluids (April
1997
Run-to-run control with Bayesian optimization for soft landing of short-stroke reluctance actuators
There is great interest in minimizing the impact forces of reluctance actuators during commutations, in order to reduce contact bouncing, acoustic noise and mechanical wear. In this regard, a run-to-run control algorithm is proposed to decrease the contact velocity, by exploiting the repetitive operations of these devices. The complete control is presented, with special focus on the optimization method and the input definition. The search method is based on Bayesian optimization, and several additions are introduced for its application in run-to-run control, e.g. the removal of stored points and the definition of a new acquisition function. Additionally, methods for the input parametrization and dimension reduction are presented. For analysis, Monte Carlo simulations are performed using a dynamic model of a commercial solenoid valve, comparing the proposed search method with two alternatives. Furthermore, the control strategy is validated through experimental testing, using several devices from the same ensemble of solenoid valves. IEE
Evolution of pH during in-situ leaching in small concrete cavities
Small amounts (0.4 cc) of neutral water placed in small cylindrical cavities (5 mm diameter) in concrete exposed to 100% relative humidity first developed a pH comparable to that of a saturated Ca(OH)2 solution. The pH then increased over a period of days-weeks toward a higher terminal value. A micro pH electrode arrangement was used. This behavior was observed in samples of 12 different concrete mix designs, including some with pozzolanic additions. The average terminal cavity pH closely approached that of expressed pore water from the same concretes. A simplified mathematical model reproduced the experimentally observed behavior. The model assumed inward diffusional transport of the pH-determining species in the surrounding concrete pore solution. The experimental results were consistent with the model predictions when using diffusion parameters on the order of those previously reported for alkali cations in concrete. The cavity size, cavity water content, and exposure to atmospheric CO2 should be minimized when attempting to obtain cavity pH values approaching those of the surrounding pore water
Reluctance actuator characterization via FEM simulations and experimental tests
Modeling the reluctance of an electromagnetic actuator is a critical step to analyze its dynamics and design model-based controllers. On the one hand, analytical expressions based on either theoretical or empirical models often lack accuracy due to model inconsistencies. On the other, numerical methods are much more precise but require exact information about the system geometry, materials and winding configuration. In this paper we present a new method that brings together the good properties of the finite element method and of system identification techniques to obtain an accurate description of the reluctance and its derivative. Since the method is designed to identify the unknown parameters of the system, it is particularly well suited for modeling existing commercial devices. An application on a safety valve used in gas lines is included to illustrate the method and a discussion on the results shows the advantages of our proposal
Multi-robot Implicit Control of Massive Herds
This paper solves the problem of herding countless evaders by means of a few
robots. The objective is to steer all the evaders towards a desired tracking
reference while avoiding escapes. The problem is very challenging due to the
highly complex repulsive evaders' dynamics and the underdetermined states to
control. We propose a solution that is based on Implicit Control and a novel
dynamic assignment strategy to select the evaders to be directly controlled.
The former is a general technique that explicitly computes control inputs even
in highly complex input-nonaffine dynamics. The latter is built upon a
convex-hull dynamic clustering inspired by the Voronoi tessellation problem.
The combination of both allows to choose the best evaders to directly control,
while the others are indirectly controlled by exploiting the repulsive
interactions among them. Simulations show that massive herds can be herd
throughout complex patterns by means of a few herders.Comment: E. Sebastian, E. Montijano and C. Sagues,"Multi-robot Implicit
Control of Massive Herds'', Fifth Iberian Robotics Conference (ROBOT22), 202
Self-heterodyne detection for SNR improvement and distributed phase-shift measurements in BOTDA
In this paper we present a Brillouin optical time domain analysis (BOTDA) sensor that takes advantage of the enhanced characteristics obtained employing self heterodyne optical detection combined with synchronous demodulation. By employing this technique we increase the sensitivity of the sensor and demonstrate experimentally a 12.35 dB enhancement in the SNR compared to conventional direct-detection systems. This detection scheme also enables distributed measurements of the Brillouin phase shift in an optical fiber, which can lead to enhanced BOTDA schemes
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