1,720,977 research outputs found
Modelling the transitional wall-jet by using a dynamical system
No full textA low-dimensional dynamical system has been used to model the transition process in a wall-jet. Numerical simulations of non-linear dynamics have been pursued considering the dominant interaction among triadic resonant modes. The analysed time evolutions involve the most unstable linear mode and other two-dimensional or three-dimensional modes; the dependence of the transition behaviours from different type of resonant interaction has been pointed out. For a local self-similar evolution of the mean flow, the spreading rate of the length scale has been deduced from the mean kinetic energy flux equation. The obtained results show that the development of large scale coherent structures enhances the momentum diffusion
The influence of the measurement errors on the determination of the degrees of freedom of dynamic systems
No full textExperimental and theoretical investigation on the sloshing of a two-liquid system with free surface
No full textIn this paper a theoretical and experimental investigation is performed on the sloshing of a two-liquid system with both separation and free surface. The experimental configuration consists of an oscillating tank filled with two layers of immiscible liquids. The mathematical model is obtained by applying the Lagrangian variational approach to the potential formulation of the fluid motion, and a dynamical system which describes the dynamics of motion is derived. In order to account for the damping of the motion, generalized dissipative forces are considered. For this purpose, the logarithmic decrement coefficients are estimated by means of a wavelet analysis performed on the experimental free oscillations of the fluid system. Numerical integration of the mathematical model gives results which are in a fair agreement with the experimental results. (c) 2005 American Institute of Physics
Triadic resonant modes: dynamical model and truncation criterion
No full textIn the present paper we propose a three-dimensional model describing the local evolution of any two- or three-dimensional disturbance superimposed on a basic shear flow. A numerical investigation, using a truncated form of the evolution model, was performed to select resonant tree-dimensional disturbance in a transitional cylinder wake. A truncation criterion of non-linear triadic resonance, which is a generalisation of the standard linear-resonance condition, allowed us to locate some triadic sets of linear eigenmodes. Numerical integration of the corresponding low-dimensional dynamical systems showed interesting results that are in qualitative agreement with those performed by Williamson and Prasad [j. Fluid Mech. 256 (1993) 269]. This seems to validate the truncation criterion which, together with the theoretical model, proved to be an appropriate tool for describing the local evolution of large-scale structures in shear flows.
Numerical simulation of a cylinder far wake
No full textIn this paper a low dimensional dynamical system is used to model the local evolution of a disturbance superimposed on a basic shear flow. In particular, nonlinear interactions between planar waves and oblique waves which travel phase-locked in the far wake of a cylinder are simulated. At this aim a low-dimensional dynamical system is derived by imposing the respect of a nonlinear truncation condition together with the phase-locked condition. The numerical results are compared with some recent experimental results obtained analysing the dynamics of a cylinder far wake. Different experimental conditions were modelled by dynamical systems defined by one or more coupled triadic systems. The energy spectral distribution of the numerical time histories of the streamwise velocity component well reproduce the experimental dynamics of the cylider far-wake. Also the numerical reconstruction of the streaklines shows a good agreement with the visualized flow field.
Interactions of phase-locked waves in the far wake of a cylinder
No full textIn this paper a low dimensional dynamical system is used to model the local evolution of a disturbance superimposed on a basic shear flows. In particular, nonlinear interactions between planar waves and oblique waves which travel phase-locked in the far wake of a cylinder are simulated. At this aim a low dimensional dynamical system is derived by imposing the respect of a nonlinear truncation criterion together with the phase-locked condition. The numerical results are compared with some recent experimental results obtained analysing the dynamics of a cylinder far wake. Different experimental conditions were modelled by dynamical systems defined by one or more coupled triadic systems. The energy spectral distribution of the numerical time histories of the streamwise velocity component well reproduce the experimental dynamics of the cylinder far wake. Also the numerical reconstruction of the streaklines shows a good agreement with the visualized flow field
Background-noise effects on modelling transitional shear flows
No full textPresented in this article is an experimental and numerical study aimed at analysing the wake dynamics downstream of a cylinder in an unsteady flow. The experimental study was carried out by measuring the mean velocity field with a laser Doppler anemometer, visualizing the fluid field by means of laser induced fluorescence, and performing a statistical analysis of the instantaneous velocity signals using Fourier and wavelet transforms. The visualized images were processed to determine the mean-statistical characteristics of large-scale structures in the wake. Experimental results provided the essential information for the numerical investigations of the temporal evolution of large-scale structures. A theoretical model based on the theory of dynamical systems with few degrees of freedom was adopted. The effects induced by the unsteady nature of the incoming flow were simulated. Experimental and numerical results were in excellent agreement, underlining the validity of the methods of analysis and simulation proposed
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