[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Preface Part I. Turbulence: 1. Introduction 2. Coherent structures 3. Proper orthogonal decomposition 4. Galerkin projection Part II. Dynamical Systems: 5. Qualitative theory 6. Symmetry 7. One-dimensional 'turbulence' 8. Randomly perturbed systems Part III. 9. Low-dimensional Models: 10. Behaviour of the models Part IV. Other Applications and Related Work: 11. Some other fluid problems 12. Review: prospects for rigor Bibliography.

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Turbulence, Coherent Structures, Dynamical Systems and Symmetry

The understanding of fluid turbulence has considerably progressed in recent years. The application of the methods of statistical mechanics to the description of the motion of fluid particles, i.e., to the Lagrangian dynamics, has led to a new quantitative theory of intermittency in turbulent transport. The first analytical description of anomalous scaling laws in turbulence has been obtained. The underlying physical mechanism reveals the role of statistical integrals of motion in nonequilibrium systems. For turbulent transport, the statistical conservation laws are hidden in the evolution of groups of fluid particles and arise from the competition between the expansion of a group and the change of its geometry. By breaking the scale-invariance symmetry, the statistically conserved quantities lead to the observed anomalous scaling of transported fields. Lagrangian methods also shed new light on some practical issues, such as mixing and turbulent magnetic dynamo.

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Particles and fields in fluid turbulence

To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

https://iopscience.iop.org/article/10.1016/0169-5983(92)90023-P/pdf

New Insights into Large Eddy Simulation

Analysis of influence of cavity content on flow pulsations

The theory of cavitation in an ideal fluid is utilized to design hydrofoils that have a significant increase of lift to drag ratio for a regime of partially cavitating flows. Our recently reported experiments with natural cavitation have confirmed the existence of such an increase within a certain range of cavitation number and angle of attack for the specially designed hydrofoil designated as OK-2003. For applications of such a design to engineering, it would be necessary to keep the cavitation number within this favorable range and ventilation looks to be the most promising tool for control of cavitating flows. Therefore, comparative water tunnel tests have been carried out for both natural and ventilated cavitation of the OK-2003. The general similarity between the two kinds of partial cavitation for the developed low-drag hydrofoil is proven. When validating theory with the aid of water tunnel experiments, a general issue of how to make a comparison between natural cavitation and ventilated cavitation was encountered. This issue is the difficulty to determine the pressure within partial cavities. During natural cavitation the cavity pressure can deviate from vapor pressure due to the effects of dissolved gas and possibly other water quality effects. Direct pressure measurements within the partial cavity have proved to be unstable due to the unsteadiness of the cavity. The unsteadiness effect becomes more dominant as cavitation number is increased and the cavity becomes smaller. There is a point where the measured cavity pressure becomes unusable. In the case of ventilated cavitation, the interaction of the airflow with the surface of relatively thin cavities can be significant. Finally, it was experimentally determined that different dynamics of cavity pulsation are inherent to natural and ventilated cavitation.Copyright © 2005 by ASME

Comparison of Hydrofoil Drag Reduction by Natural and Ventilated Partial Cavitation

Although cavitation inception in jets has been studied extensively, little is known about the more complex problem of a jet flow interacting with an outer flow behind a moving body. This problem is studied experimentally by considering inception behind an axisymmetric body driven by a waterjet. Tests were carried out for various water tunnel velocities and jet speeds such that jet velocity ratio U J /U could be varied in the range of 0―2. Distinctly different cavitation patterns were observed at zero jet velocity (when cavitation appeared in spiral vortices in such flows) and at various jet velocity ratios (when cavitation appeared around the jet in such flows). A simple superposition analysis, utilizing particle imaging velocimetry (PIV) measurements, is able to qualitatively predict the experimental result. On the basis of these observations, a numerical prediction of cavitation inception number based on viscous-inviscid interaction concept is suggested.

Cavitation Inception in the Wake of a Jet-Driven Body

On axial deformation of ventilated supercavities in closed-wall tunnel experiments

A complete physical model of ventilated supercavitation is not well established. Efforts documented display the ability, with a finite volume, locally homogeneous approach, to simulate supercavitating flows and obtain good agreement with experiments. Several modeling requirements appear critical, especially in physical hysteretic conditions or configurations. The hysteresis presented is due to obstruction of the flow with a solid object. The modeling approach taken correctly captures a full hysteresis loop and the corresponding dimensionless ventilation rate to cavity pressure (CQdelta) relationship. This correspondence supports the suggestion that the main mechanism of cavity gas entrainment is via shear layers attached to the cavity walls. With such validated solutions, additional insight into the flow within the cavity is gained.

Roger E. A. Arndt

Papers

Analysis of influence of cavity content on flow pulsations

Comparison of Hydrofoil Drag Reduction by Natural and Ventilated Partial Cavitation

Cavitation Inception in the Wake of a Jet-Driven Body

On axial deformation of ventilated supercavities in closed-wall tunnel experiments

Computational Investigations of Air Entrainment, Hysteresis, and Loading for Large-Scale, Buoyant Cavities