[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.

/pdf/particles-and-fields-in-fluid-turbulence-f80f8gqw2g.pdf

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

Skin friction drag reduction through the use of riblets has been a topic of intensive research during the last decades. Main efforts have been placed on both numerical (mainly DNS) and experimental (wind tunnel and fluid channel) approaches. In spite of the valuable efforts to date, the fundamental mechanisms that induce drag reduction are not well established and, in particular, the potential benefits associated with the use of riblets in wind turbines are unclear. In this research, wind tunnel experiments were performed to quantify the drag reduction in a wind turbine airfoil using V-groove riblet structures. A full-scale 2.5MW wind turbine airfoil section (typical of the 75-80% blade span), was placed in the freestream flow of the wind tunnel at the Saint Anthony Falls Laboratory (SAFL), University of Minnesota. Different sizes of V-groove riblets were tested at different angles of attack at full scale Reynolds number. Force sensors were used to measure lift and drag. Momentum deficit was also measured in the wake of the airfoil to determine the net drag. Flat plate experiments were also performed to determine the distribution of the skin friction coefficient in a developing boundary layer. The experimental measurements will be used to develop and test the performance of near-wall boundary conditions for the velocity field that can account for the effects of riblet roughness in the context of RANS and hybrid RANS/LES models. The numerical simulations along with the laboratory experiments will be used in the future to determine the best riblet configuration and guide the application and testing of riblets in a 2.5MW wind turbine under real-life wind conditions.

http://enu.kz/repository/2011/AIAA-2011-351.pdf

On the skin friction drag reduction in large wind turbines using sharp V-grooved riblets

A wind tunnel experiment was performed to study turbulence processes within a model wind turbine array of 3 by 8 model wind turbines of alternating sizes placed aligned with the mean flow. The model wind farm was placed in a boundary layer developed over both smooth and rough surfaces under neutrally stratified conditions. Turbulence statistics, TKE budget terms, and the spectral structure of the turbulence generated within and above the wind farm reveal relevant information about the processes modulating the turbulent energy transfer from the boundary layer to the turbines. The results of the experiment suggest that heterogeneity in turbine size within a wind farm introduce complex flow interactions not seen in a homogeneous farm, and may have positive effects on turbulent loading on the turbines and turbulent exchange with the atmosphere. In general, large scale motions are heavily dampened behind the first row of turbines but a portion of such structures are generated far inside the wind farm, and the scale of the most energetic eddy motions was relatively consistent at different elevations. Overall, the experiment revealed the possibility that heterogeneity of wind turbine size within wind farms have the potential to change the overall potential to harvest energy from the wind, and alter the economics of a project.

Turbulence within variable-size wind turbine arrays

Turbulence intensity measurements were made in three different-size jets at various Mach numbers and the aerodynamic jet axis, defined as the locus of points in the flow at which the turbulence intensity is minimum, was determined. The deviations from the geometrical jet axis were on the order of 2-4% of the nozzle diameter, and the pure jet axis had the form of a helical wave. A similar asymmetry in the radiated noise was also noted. The measured circumferential distribution of rms fluctuating velocity signals confirmed the asymmetry of even a circular jet.

Asymmetry of a circular jet observed in near and far fields

SUMMARY A study is being carried out at Saint Anthony Falls Laboratory (SAFL) of the University of Minnesota to develop the necessary algorithms to determine the velocity deficit and bubble size distribution in the bubbly wake behind a ventilated or cavitating hydrofoil. This is done by utilizing background illumination of the bubbly wake. A simplified experiment, to validate the proposed bubble detection algorithm, was carried out in a fish tank. Compressed air was injected into a porous media, located in the bottom of the tank, which broke up the air into bubbles that rose to the surface. Shadows of the bubbles, cast on a bright background, were collected by a high-speed camera. For processing, a reference image, taken when no bubbles were present in the flow, was subtracted from the images to reduce background noise. Once this was done, the image was segmented using an edge detection technique. The Canny algorithm was determined to be the best suited. Applying the proper thresholds ensured that only bubbles in the plane of interest were considered during analysis. Once bubble edges were detected, a curvature profile method was employed to distinguish individual bubbles within a cluster of highly overlapping bubbles. The utilized algorithm was made to detect partly overlapping bubbles and reconstruct the missing parts. The movement of recognized individual bubbles was tracked on a two dimensional plane within a flow volume. In order to obtain quantitative results, the wake of a ventilated NACA0015 hydrofoil was investigated by applying a shadowgraphy technique and the described bubble detection algorithm. There experiments were conducted in the SAFL high speed cavitation tunnel.

Recognition of highly overlapping bubbles in the wake of a ventilated hydrofoil

ACRES AMERICAN INCORPORATED
Buffalo, New York

ELECTRIC POWER RESEARCH INSTITUTE
Palo Alto, California

Roger E. A. Arndt

Papers

On the skin friction drag reduction in large wind turbines using sharp V-grooved riblets

Turbulence within variable-size wind turbine arrays

Asymmetry of a circular jet observed in near and far fields

Recognition of highly overlapping bubbles in the wake of a ventilated hydrofoil

Assessment of design considerations and Field Repair procedures to mitigate cavitation pitting in Hydraulic turbines