The significance of coherent flow structures for the turbulent mixing ...

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5 Investigation of the xy-plane diameter, surrounded by a dark background. The loss of light associated with this setting was compensated by combining the output beams of four independent Nd:YAG laser cavities (BMI model 5013 DNS 10), each approximately 255 mJ output energy per pulse ×XØ È¿Â0Û at nm (see figure 4.2 on page 52) and by triggering the system in such a way that always two differently polarised pulses leave the laser-heads simultaneously. It is clear that always a delay of the order of the pulse length must exist between the two pulses to avoid laser induced damage of the frequency doubler crystal due to the high peak intensity, which might occur when the two beams interfere. Usually, this is automatically realized because the time accuracy of the PIV triggering systems is not suited to superimpose exactly two ÆÁÌÈ m long light pulses (È ns pulse duration assumed) travelling with speed of Â†ÄÐÆ¾zy m/s. The ¾ÁÃß mm light-sheet was formed by three appropriate cylindrical and spherical lenses mounted in an optical bench and directed into the test-section by using a properly coated mirror, see section 4.6. To ensure that the measurement plane is aligned with respect to the stream-wise wall-normal plane, the cylindrical lens was rotated until the light-sheet reflection at the ceiling was parallel with a mark indicating the centreline of the wind-tunnel and the mirror was tilted in such a way that the weak light-sheet reflections, coming from the ceiling- and floor-window of the test-section, coincide on the mirror with the light-sheet coming from the optical system. The tracer particles generated for this experiment were delivered from a smoke generator which produces high concentrations of monodisperse poly-ethylene-glycol particles with a mean diameter of Û@Õ m according to figure 2.4 on page 18 or [47]. To obtain ideal conditions for accurate PIV measurements, the closed circuit wind-tunnel was completely seeded and continuously operated until no seeding inhomogeneities could be observed at all. During this procedure all diffuse wall reflections, caused by the interaction of the laser-light with tracer particles sticked on the wall, were removed with appropriate liquids and pressurised air while the wind-tunnel was running at a moderate speed. The operation of the wind-tunnel is essential, and should never be stopped before the experiment is finished in order to avoid any settling of the tracer particles which might deteriorate the performance of the measurements. As these particles follow the flow precisely, no settling of the particles or other diffuse reflections could be observed on the acquired data while the experiment was running. Only a tiny bright line of the wall reflection (Â pixel wide) was visible due to the glass / air interface. After the experiment, this line was detected by using correlation algorithms in order to determine the position of the wall with sub-pixel accuracy. For the evaluation of the stereo-scopic images the second order warping technique, outlined in section 3.2, was applied along with the calibration validation procedure to ensure that the interrogation spots from each of a pair of stereo-scopic images correspond exactly to the same region of the flow, see section 3.3. The interrogation of the data was performed with the FFT-based free shape cross-correlation and for the peak detection with sub-pixel accuracy the two-dimensional Gaussian fit was applied. This peak finding method is less sensitive to sub-pixel displacements compared with the three point Gaussian peak fit, see figure 2.13 and 2.14. For the calculation of the velocity Â0ÛÓÄœÂ0Û vectors pixel ß¿ËFÄ Æß and pixel interrogation windows were selected. Because of the increased spatial resolution of the stretched windows in wall-normal direction spatial averaging effects could be easily estimated from the performed evaluation. It should be noted that an aspect ratio of 4 between the stream-wise and wall-normal dimensions of the measurement volume is relatively moderate regarding the typical values which are applied in numerical flow simulations. The evaluation of the data was performed in parallel on an eight processor high performance SGI computer, and a raid sys- 74
{ : { L Ø { 9 D { 9 D T W ð 9 ó { L Ø 9 ó W 9 { { { { { 5.3 Statistical properties of the flow tem was available for the storage of the data. The number of obviously incorrect vectors is
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The significance of coherent flow s
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Contents 1 Introduction 5 2 Particl
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1 Introduction One of the fascinati
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As order can be always observed whe
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were not considered in detail in th
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varying signal can be transformed i
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2 Particle Image Velocimetry The tr
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2.1 Principles It is of fundamental
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’ ’“ ” ”• Ž Ž Ž Ž
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¹ and 2.2 Generation of appropriat
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2.2 Generation of appropriate trace
Page 23 and 24: 2.3 Registration of the particle im
Page 25 and 26: 2.3 Registration of the particle im
Page 27 and 28: ë ë 2.3 Registration of the parti
Page 29 and 30: ýÿþ¡ û û ¢ ¢ £ôþ¡ î¥
Page 31 and 32: or = ö D Q ù ù ?xö ù Y[Z]\ & &
Page 33 and 34: ñ † † † † † † † †
Page 35 and 36: ˜ E ˜ T ˜ T 2.4 Particle image
Page 37 and 38: 3 Stereo-scopic Particle Image Velo
Page 39 and 40: › B › B › J ù ž & 3.1 Princ
Page 41 and 42: Æ e¼c’„ïG ù # #dc’e › B
Page 43 and 44: Ð È Ñ ù Ñ ù Ð È Ð È Ñ ù
Page 45 and 46: 3.2 Evaluation of stereo-scopic ima
Page 47 and 48: ú ¦ 3.3 Calibration validation by
Page 49 and 50: 4 Multiplane Stereo Particle Image
Page 51 and 52: £ è ¤ ó 4.2 Four-pulse-laser S
Page 53 and 54: £ ¤ £ £ 4.2 Four-pulse-laser Sy
Page 55 and 56: ( ì ( æ ( ì è ð ( ( ( ( ( ( ì
Page 57 and 58: æ ( | | | | | | | ì æ õ æ õ
Page 59 and 60: 4.5 Simplified recording system 4.5
Page 61 and 62: 4.7 Monochromatic aberrations refle
Page 63 and 64: º¹ n 4.7 Monochromatic aberration
Page 65 and 66: 4.8 Feasibility study experiments i
Page 67 and 68: 4.8 Feasibility study 10 20 30 40 5
Page 69 and 70: é¥î ëÝïñð€òôó 5 Invest
Page 71 and 72: I L § § § : ? ; õ : I I I W
Page 73: 5.2 Experimental set-up α4 α3 α2
Page 77 and 78: : T ž ž ò 5.3 Statistical pro
Page 79 and 80: ¾ ¾ 5.3.2 Spatial auto- and cross
Page 81 and 82: ½ Ë 5.3 Statistical properties of
Page 83 and 84: ½ Ë 5.3 Statistical properties of
Page 85 and 86: ô ó ñ ½ ñ ô ó 5.3 Statistica
Page 87 and 88: æ å ä ã ã ã ½ Ë ä æ å ½
Page 89 and 90: æ æ å ä À Â ½ 5.3 Statistica
Page 91 and 92: ½ 5.4 Properties of coherent veloc
Page 93 and 94: » % %QÄ ¾ Ó turbulent velocit
Page 95 and 96: * Ó , Ó * Á , Â 5.4 Properties
Page 97 and 98: ! 6 Investigation of the xz-plane O
Page 99 and 100: 6.1 Experimental set-up the (virtua
Page 101 and 102: ß ß 6.2 Statistical properties
Page 103 and 104: g ˆ g ˆ 6.2 Statistical propertie
Page 105 and 106: — ¯ 6.2 Statistical properties o
Page 107 and 108: å å ç æ æ ã ã ã å å è æ
Page 109 and 110: 6.2.3 Spatial cross-correlation fun
Page 111 and 112: 6.2 Statistical properties of the b
Page 113 and 114: 6.2 Statistical properties of the b
Page 115 and 116: performed at and the second at
Page 117 and 118: 6.3 Spatio-temporal buffer layer st
Page 119 and 120: M M æ æ æ æ æ æ ã ã ã H M
Page 121 and 122: V W V 6.4 Properties of coherent ve
Page 123 and 124: 6.4 Properties of coherent velocity
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6.4 Properties of coherent velocity
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d d 6.4 Properties of coherent velo
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'§ 'Î ª ¿ ¦ '§ ')( 7 Investi
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% { s s s s 7.1 Experimental set-up
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‘ — s 7.2 Statistical propertie
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7.2 Statistical properties of the l
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¿ 7.2.2 Spatial cross-correlations
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¿ 7.2 Statistical properties of th
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ô ô 7.3 Spatio-temporal correlati
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7.3 Spatio-temporal correlations wi
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¨ ¨ ¨ 7.3 Spati
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£ 7.4 Spatio-temporal correlations
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¿ 7.5 Properties of coherent veloc
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¿ 8 Summary The first part of this
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aperture, it is shown how to elimin
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¡ ¿ tures keep their spatial orga
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Bibliography [1] ACARLAR MS, SMITH
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[30] HINSCH K (1995) Three-dimensio
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[60] KNEUBÜHL FK, SIGRIST MW (1995
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[93] SMITH CR, METZLER SP (1983) Th
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Z k l l l l l { l R q ‡ ˆ ’
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Ø á a Õ Œ v momentum thickness,
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Index Kolmogorov micro-scale . . .
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Acknowledgement First of all I woul
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Lebenslauf von Christian Joachim K
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