4. Conclusions

Figure 16 shows the ratio to the number of events occurring/contributing in Q2

and Q4 for H = 0–3 and for two different Reynolds numbers. The ratio to the number of events occurring/contributing to Q2 and Q4 shows different trends for the threshold value of H = 0 (Figure 16a and b) compared to the threshold value of H=2–3 (Figure 16c–h). This is very unlike to the ratio of the Reynolds shear stress contributions of Q2/Q4 as shown in Figure 15. The NQ2/NQ4 ratio is near unity at the location very close to the bed indicating almost equal occurrence of ejection and sweep events as one can note from Figure 16a and b. The NQ2/NQ4 ratio decreases from near unity to minimum at around mid-depth of the flow (y/d 0.5) as one progress from the bed and towards the free surface which is an indication of relatively reduced ejection events compared to the sweep events. Moving farther away from bed (y > 0.5d) and towards the free surface, the ratio of NQ2/NQ4 ratio is keep on increasing again and reaches to near unity indicating almost equal occurrence of ejection and sweep events. Figure 16c–h show a trend different from Figure 16a and b. As one progress from the bed towards the free surface, there is an increment of 30 over fold for the value of NQ2/NQ4 at around y 0.5d, indicating substantial increase of ejection events. As one can also note from Figure 16 that there is little dependency on bed conditions of smooth and rough for H = 0 on the

Boundary Layer Flows - Theory, Applications and Numerical Methods

ratio of number of events in Q2 and Q4.

Figure 16.

74

Ratio of number of different quadrant events for flow over different bed condition.

The purpose of the present study [1] is to explain how the roughness and Reynolds number affect flow characteristics in an open channel flow (OCF). Tests were conducted with four different types of bed surface conditions and at two different Reynolds number for each and every bed surface. Instantaneous velocity components are used to analyze the streamwise mean velocity, turbulence intensity in both streamwise and vertical direction, Reynolds shear stress including shear stress correlation and higher-order moments including vertical flux of the turbulent kinetic energy. In order to extract the magnitude of the Reynolds shear stress related to turbulent bursting events quadrant decomposition was used. The main findings are summarized as follows:


layer, the turbulent mixing properties should be essentially the same for the flow over smooth and rough walls which was initially proposed by [33] and generalized by [34].

12.The ratio to the Reynolds shear stress contributions of Q2/Q4 is near unity at the location very close to the bed and location close to the free surface indicating identical strength of sweep and ejection. With the exception of near bed and near free surface, relatively stronger ejection events compared to the sweep events can be seen for throughout the flow depth and the strength of the ejection events increases many fold with increase of the

Roughness Effects on Turbulence Characteristics in an Open Channel Flow

13.The ratio to the number of events occurring/contributing in Q2 and Q4 is near unity at the location very close to the bed and location close to the free surface indicating almost equal occurrence of sweep and ejection events. With the exception of near bed and near free surface, relatively reduced ejection events compared to the sweep events can be seen for throughout the flow depth for H = 0 but shows substantial increase of ejection events compared to

Civil Engineering Technology, Rochester Institute of Technology, Rochester,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

threshold value of H.

DOI: http://dx.doi.org/10.5772/intechopen.85990

the sweep events for H > 0.

Author details

Abdullah Faruque

New York, USA

77

\*Address all correspondence to: aafite@rit.edu

provided the original work is properly cited.


Roughness Effects on Turbulence Characteristics in an Open Channel Flow DOI: http://dx.doi.org/10.5772/intechopen.85990

