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Chapter 6 - Analysis of the test results 6.6 Analysis of the flow field The presence of the macro-roughness influences the scour process by modifying the velocity distribution in the bend. It is well known that the velocity is higher on the outside of the bend and that there is a secondary circulation also. Both factors combine to give a large scour on the outside of the bend. A brief description of the flow field was already given in the paragraph discussing the preliminary tests (§ 5.2.1/2)e). The analysis of the main tests is presented here. a) Tangential velocities The velocities in tangential direction (along the channel axis) are somewhat modified in the bend. Instead of a classical “log”-velocity profiles as it can be seen on top of Table 6.6 1 undergoes the following modifications in a curved channel. The maximum velocity is no longer found close to the free water surface but close to the bed surface, particularly in the scour holes; this can be seen very well in cross-section 40° in the second profile from the left. The highest tangential velocities ( ) are now found in the outer half of the cross-section. V θ B01−Q210−F06 0 0 100 X 200 800 300 600 400 0 200 400 600 800 1000 Distance to outer bank (mm) 400 With macro-roughness, the previously mentioned increase of the water depth in the inlet reach (§ 6.2.2) is well visible. But this has no influence on the shape of the velocity profile in this zone. In the bend and especially in the scour holes, the near bed velocity is reduced and the maximum velocity shifts towards the free water surface (Table 6.6 at 40°, first two profiles at the outer bank). Appendix 11.1 shows that the highest velocities are located in a straight reach next to the free water surface. In the bend, this zone of maximum velocity first shifts towards the outer side wall and then down the wall towards the channel ground. At the maximum scour location, high (tangential) velocities can be observed near the bottom (Figure 6.22). 100 200 300 400 Figure 6.22: Tangential velocities at 40° without macro-roughness (left) and with ribs spaced every 2° (right); view in the downstream direction (see also Appendix 11.4) X = max. velocity With macro-roughness (Appendix 11.4), the zone of maximum velocity remains all over the bend close to the surface for the ribs spaced every 4°. Introducing additional ribs (every 2°) the maximum velocity moves towards the channel ground, but at a significant distance from the outer wall (about 20 cm, that is about the average flow depth). For the smallest rib-spacing, the high-velocity zone gets even closer to the ground, but again at about 20 cm from the outer side wall. Distance to free water surface (mm) 1200 1000 X B03−Q210−F06 0 200 400 600 800 100 200 Distance to outer bank (mm) 1400 1200 1000 800 600 400 1. Table 6.6 gives the raw velocity profiles. It shows that the first 2 cm from the free water surface are strongly influenced by the measurement, since the velocity probe had to touch the fluid. page 138 / November 9, 2002 Wall roughness effects on flow and scouring
Analysis of the flow field XS without macro-roughness with ribs every 4° 10° 0 0 50 50 100 100 150 150 200 200 40° 0 0 50 50 100 100 150 150 200 200 250 250 300 300 55° 350 0 350 0 50 50 100 100 150 150 200 200 250 250 85° 300 0 300 0 50 50 100 100 150 150 200 200 250 300 250 300 2 m/s 350 350 Table 6.6: Mean velocity profiles and standard deviation in tangential direction (along the channel axis) View in the downstream direction. Left profile at 90 cm from the outer bank, right profile at the inner bank. Radial distance between two profiles 180 cm (every 2nd measured profile was left out). On the vertical axis the distance to the free water surface is given (in mm). EPFL Ph.D thesis 2632 - Daniel S. Hersberger November 9, 2002 / page 139
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WALL ROUGHNESS EFFECTS ON FLOW AND
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Abstract By applying vertical ribs
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Résumé En disposant des nervures
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Zusammenfassung • Ein markanter S
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Acknowledgments page x / November 9
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Table of contents 4. Smart & Jäggi
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Table of contents 7.6 Summary and c
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Chapter 1 - Introduction 1.1 Contex
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Chapter 1 - Introduction page 4 / N
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Chapter 2 - State of the art 2.1 Fl
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Chapter 2 - State of the art 2.2.2
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Chapter 2 - State of the art Theref
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Chapter 2 - State of the art resolu
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Chapter 2 - State of the art comple
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Chapter 2 - State of the art ORLAND
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Chapter 2 - State of the art page 2
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Chapter 3 - Theoretical considerati
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Chapter 8 - Summary, conclusions an
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Notations K S [m 1/3 /s] roughness
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Notations bed b bf c cr d e g w o o
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References de Vriend H.J. (1981). S
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References Meyer-Peter, E., Favre,
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References Williams, R. (1899). Flu
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References by chapter Keulegan (193
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References by chapter page 214 / No
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Index of Authors Hoffmanns 13 Hoffm
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Index of Authors page 218 / Novembe
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Index of Figures Fig.4.19: Frame po
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Index of Figures 22] 183 Fig.7.15:
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Index of Tables Table 7.3: Table of
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Index of Keywords dimensional analy
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Index of Keywords sediment sampling
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Index of Keywords parameter 77 Pete
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Publications (continued) 1997 Hersb
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