Bekki-Nozaki Amplitude Holes in Hydrothermal Nonlinear Waves

We present and analyze experimental results on the dynamics of hydrothermal waves occuring in a laterally-heated fluid layer. We argue that the large-scale modulations of the waves are governed by a one-dimensional complex Ginzburg-Landau equation (CGLE). We determine quantitatively all the coefficients of this amplitude equation using the localized amplitude holes observed in the experiment, which we show to be well described as Bekki-Nozaki hole solutions of the CGLE.Comment: 4 pages, uses RevTeX, 9 EPS figure

Diseño, control e implementación de un simulador de un robot Scara de 3 grados de libertad con Matlab/Simulink

[ES] En este trabajo se analizará el diseño de un robot Scara para el transporte de paquetes de mascarillas quirúrgicas desde una cinta transportadora a la caja correspondiente para su envío. Para ello, se detallarán las especificaciones técnicas de la tarea a resolver, que condicionarán la elección de los motores de articulación del robot SCARA, así como sus dimensiones y materiales. Tras la fase de diseño mecánica, el trabajo presenta un estudio comparativo de varios métodos de control. En concreto, se diseñarán y compararán controladores realizados mediante diferentes métodos (PID, PID con filtro de derivada y realimentación de estados sin y con observador) para finalmente escoger y utilizar el método que genere controladores más robustos frente a perturbaciones y ruido. Finalmente, la viabilidad del diseño de robot propuesto se realiza mediante la simulación en Simulink de la tarea de Pick & Place de mascarillas quirúrgicas.[EN] This work will analyze the design of a Scara robot for transporting packages of surgical masks from a conveyor belt to the corresponding box for shipment. For this, the technical specifications of the task to be solved will be detailed, which will determine the choice of the SCARA robot articulation motors, as well as their dimensions and materials. After the mechanical design phase, the work presents a comparative study of several control methods. Specifically, controllers will be designed and compared performed using different methods (PID, PID with derivative filter and feedback of states without and with observer) to finally choose and use the method that generates more robust drivers against disturbances and noise. Finally, the feasibility of the proposed robot design is made by simulating the Pick & Place task of surgical masks in Simulink.Burguete Alventosa, J. (2021). Diseño, control e implementación de un simulador de un robot Scara de 3 grados de libertad con Matlab/Simulink. Universitat Politècnica de València. http://hdl.handle.net/10251/170914TFG

Conservative numerical boundary conditions

4 Pags., 2 Figs. Trabajo originalmente presentado en International Workshop on Numerical Modelling of Hydrodynamic for Water Resources (Zaragoza. 2007)A recent technique for the numerical boundary conditions in 1D shallow water flow models is able to preserve the good properties of a conservative scheme used for the interior points. The implementation in a conservative scheme is shown in a test cases with exact solution and applied to the simulation of a real river flood wave leading to very satisfactory results.Peer reviewe

Inverse cascades sustained by the transfer rate of angular momentum in a 3D turbulent flow

The existence of energy cascades as signatures of conserved magnitudes is one of the universal characteristics of turbulent flows. In homogeneous 3D turbulence, the energy conservation produces a direct cascade from large to small scales, although in 2D, it produces an inverse cascade pointing towards small wave numbers. In this Letter, we present the first evidence of an inverse cascade in a fully developed 3D experimental turbulent flow where the conserved magnitude is the angular momentum. Two counterrotating flows collide in a central region where very large fluctuations are produced, generating a turbulent drag that transfers the external torque between different fluid layers

Experimental Characterisation of Flow Near Bridges for the Validation of a One-Dimensional Hydraulic Simulation Model

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Subcritical instabilities in a convective fluid layer under a quasi-one-dimensional heating.

The study and characterization of the diversity of spatiotemporal patterns generated when a rectangular layer of fluid is locally heated beneath its free surface is presented. We focus on the instability of a stationary cellular pattern of wave number ks which undergoes a globally subcritical transition to traveling waves by paritybreaking symmetry. The experimental results show how the emerging traveling mode 2ks /3 switches on a resonant triad ks ,ks /2,2ks /3 within the cellular pattern yielding a “mixed” pattern. The nature of this transition is described quantitatively in terms of the evolution of the fundamental modes by complex demodulation techniques. The Bénard-Marangoni convection accounts for the different dynamics depending on the depth of the fluid layer and on the vertical temperature difference. The existence of a hysteresis cycle has been evaluated quantitatively. When the bifurcation to traveling waves is measured in the vicinity of the codimension-2 bifurcation point, we measure a decrease of the subcritical interval in which the traveling mode becomes unstable. From the traveling wave state the system undergoes a global secondary bifurcation to an alternating pattern which doubles the wavelength ks /2 of the primary cellular pattern; this result compares well with theoretical predictions P. Coullet and G. Iooss, Phys. Rev. Lett. 64, 866 1990 . In this cascade of bifurcations towards a defect dynamics, bistability due to the subcritical behavior of our system is the reason for the coexistence of two different modulated patterns connected by a front. These fronts are stationary for a finite interval of the control parameters

Instabilities of conducting fluid layers in cylindrical cells under the external forcing of weak magnetic fluids

In this work, we analyze recent results concerning the instabilities created in a layer of liquid metal by the action of time-dependent magnetic fields. The experimental setup allows the characterization of different patterns very close to the threshold. For very low frequencies of the forcing field, the axisymmetric fluid layer destabilizes with different azimuthal wavenumbers. An improved analysis allows the characterization of different patterns for interaction parameter values as low as 5 · 10 −3

HYSTERESIS AND VORTICES DYNAMICS IN A TURBULENT FLOW

Recent results about the slow dynamics present in a fully developed turbulent flow are reported. In a previous paper [de la Torre & Burguete, 2007] we showed that the mean velocity field in a turbulent flow bifurcates subcritically breaking some symmetries of the problem and becomes time-dependent because of equatorial vortices moving with a precession movement. This subcriticality produces a bistable regime, whose main characteristics were successfully reproduced using a three-well potential model with additive noise. In this paper we present the characterization of the hysteresis region, not previously observed, in this bifurcation. This hysteresis appears only for an extremely small range of parameters

Impact of time-dependent nonaxisymmetric velocity perturbations on dynamo action of von Kármán-like flows

We present numerical simulations of the kinematic induction equation in order to examine the dynamo efficiency of an axisymmetric von K´arm´an–like flow subject to time-dependent nonaxisymmetric velocity perturbations. The numerical model is based on the setup of the French von K´arm´an-sodium dynamo (VKS) and on the flow measurements from a water experiment conducted at the University of Navarra in Pamplona, Spain. The principal experimental observations that are modeled in our simulations are nonaxisymmetric vortexlike structures which perform an azimuthal drift motion in the equatorial plane. Our simulations show that the interactions of these periodic flow perturbations with the fundamental drift of the magnetic eigenmode (including the special case of nondrifting fields) essentially determine the temporal behavior of the dynamo state.We find two distinct regimes of dynamo action that depend on the (prescribed) drift frequency of an (m = 2) vortexlike flow perturbation. For comparatively slowly drifting vortices we observe a narrow window with enhanced growth rates and a drift of the magnetic eigenmode that is synchronized with the perturbation drift. The resonance-like enhancement of the growth rates takes place when the vortex drift frequency roughly equals the drift frequency of the magnetic eigenmode in the unperturbed system. Outside of this small window, the field generation is hampered compared to the unperturbed case, and the field amplitude of the magnetic eigenmode is modulated with approximately twice the vortex drift frequency. The abrupt transition between the resonant regime and the modulated regime is identified as a spectral exceptional point where eigenvalues (growth rates and frequencies) and eigenfunctions of two previously independent modes collapse. In the actual configuration the drift frequencies of the velocity perturbations that are observed in the water experiment are much larger than the fundamental drift frequency of the magnetic eigenmode that is obtained from our numerical simulations. Hence, we conclude that the fulfillment of the resonance condition might be unlikely in present day dynamo experiments. However, a possibility to increase the dynamo efficiency in the VKS experiment might be realized by an application of holes or fingers on the outer boundary in the equatorial plane. These mechanical distortions provoke an anchorage of the vortices at fixed positions thus allowing an adjustment of the temporal behavior of the nonaxisymmetric flow perturbations