2.2 Test conditions to study the effect of roughness

One hydraulically smooth and three characteristically different rough surfaces are used in this study to capture and understand the open channel flow characteristics. Figure 2a shows the hydraulically smooth bed condition made up by a polished aluminum plate spanning full width of the flume. Sand composed of uniform particles with gradation characteristics as shown in Table 1 is used to create the three different rough surfaces. Four different types of bed surface conditions were used in this study. Figure 2b shows the 'distributed roughness' rough surface, Figure 2c shows the 'continuous roughness' rough surface and Figure 3 shows the 'natural sand bed' rough surface. A 18 mm wide sand strip is glued on top of the polished aluminum plate spanning full width of the flume alternate by a 19 mm wide smooth strip to generate the distributed roughness. The same sand grain is glued on top of the entire polished aluminum plate spanning full width of the flume to generate the continuous roughness. Natural sand bed condition is consist of 3.7 m long 200 mm thick uniform sand of the same characteristics

spanning full width of the flume. Special care had been taken in maintaining the flow condition in such a way that there were no sand movement in any period of time of running the test. As a precautionary measure of accidental sand movement and sand entering into the pipe/pump system causing damage to the pump, a sand

Plan view of different fixed bed condition. (a) Hydraulically smooth surface, (b) Distributed roughness surface,

Roughness Effects on Turbulence Characteristics in an Open Channel Flow

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

2.46 1.91 1.34 1.26 1.24 1.00

Two different flow Reynolds numbers (Re = Uavgd/ν ≈ 47,500 and 31,000) correspondence to two different Froude numbers (Fr = Uavg/(gd)0.5 ≈ 0.40 and 0.24) respectively are used for each four bed surface conditions. The parameters used for Reynolds and Froude number calculations are the average streamwise velocity (Uavg), nominal depth of flow (d), kinematic viscosity of the fluid (ν) and gravitational acceleration g. The flow conditions are maintained to be subcritical (i.e., Froude numbers less than unity) and choose the flow Reynolds numbers accordingly. The variation of water surface elevation were measured for the test section and there are less than 1 mm variation of surface water for a streamwise distance of 600 mm proves that pressure gradient is negligible. In order of conditioning the flow, two sets of flow straighteners are placed at the beginning and end

trap is constructed at the end of the flume.

d50 (mm) d95/d5 d95/d50 d84/d50 <sup>σ</sup><sup>g</sup> <sup>¼</sup> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d84=d<sup>16</sup> p

Figure 2.

Cz <sup>¼</sup> <sup>d</sup><sup>2</sup>

Table 1.

Figure 3.

53

Section of natural sand bed.

<sup>30</sup>=ð Þ d60d<sup>10</sup>

(c) Continuous roughness surface.

Gradation measurements of the sand.

Figure 1. Schematic of the open channel flume and experimental setup.

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

Figure 2.

2. Experimental setup

2.1 Open channel flume

Figure 1.

52

Schematic of the open channel flume and experimental setup.

A 9-m long open channel flume at the University of Windsor with a rectangular cross-section dimension of 1100 mm 920 mm is used to perform the experiment. Figure 1 shows the schematic of the experimental setup with open channel flume. A squire cross-section dimension of 1.2 m and depth of 3 m header tank is placed at the beginning of the flume. The depth of flow for this series of experiments are kept to 100 mm, eventually achieving the aspect ratio (width/depth ratio of flow, b/d) of 11. Choice of this aspect ratio is based on the expectation that the generation of the secondary current will be minimum and the flow can be a representation of twodimensional flow [7]. Two centrifugal pumps of 15 horsepower capacity each are used to recirculate the water. Tempered transparent glass materials are used to build the sidewalls and bottom of the flume and will enable the LDA (laser Doppler anemometer) to measure the instantaneous velocity. There were many previous studies [20–21] confirmed the quality of the flow of this permanent facility. The flume has an adjustable slope mechanism at the bottom but was kept horizontal for this series of test. 720 and 450 GPM are the two constant flow rate used for the tests.

One hydraulically smooth and three characteristically different rough surfaces are used in this study to capture and understand the open channel flow characteristics. Figure 2a shows the hydraulically smooth bed condition made up by a polished aluminum plate spanning full width of the flume. Sand composed of uniform particles with gradation characteristics as shown in Table 1 is used to create the three different rough surfaces. Four different types of bed surface conditions were used in this study. Figure 2b shows the 'distributed roughness' rough surface, Figure 2c shows the 'continuous roughness' rough surface and Figure 3 shows the 'natural sand bed' rough surface. A 18 mm wide sand strip is glued on top of the polished aluminum plate spanning full width of the flume alternate by a 19 mm wide smooth strip to generate the distributed roughness. The same sand grain is glued on top of the entire polished aluminum plate spanning full width of the flume to generate the continuous roughness. Natural sand bed condition is consist of 3.7 m long 200 mm thick uniform sand of the same characteristics

2.2 Test conditions to study the effect of roughness

Boundary Layer Flows - Theory, Applications and Numerical Methods

Plan view of different fixed bed condition. (a) Hydraulically smooth surface, (b) Distributed roughness surface, (c) Continuous roughness surface.


#### Table 1.

Gradation measurements of the sand.

Figure 3. Section of natural sand bed.

spanning full width of the flume. Special care had been taken in maintaining the flow condition in such a way that there were no sand movement in any period of time of running the test. As a precautionary measure of accidental sand movement and sand entering into the pipe/pump system causing damage to the pump, a sand trap is constructed at the end of the flume.

Two different flow Reynolds numbers (Re = Uavgd/ν ≈ 47,500 and 31,000) correspondence to two different Froude numbers (Fr = Uavg/(gd)0.5 ≈ 0.40 and 0.24) respectively are used for each four bed surface conditions. The parameters used for Reynolds and Froude number calculations are the average streamwise velocity (Uavg), nominal depth of flow (d), kinematic viscosity of the fluid (ν) and gravitational acceleration g. The flow conditions are maintained to be subcritical (i.e., Froude numbers less than unity) and choose the flow Reynolds numbers accordingly. The variation of water surface elevation were measured for the test section and there are less than 1 mm variation of surface water for a streamwise distance of 600 mm proves that pressure gradient is negligible. In order of conditioning the flow, two sets of flow straighteners are placed at the beginning and end of the flume. A turbulent boundary layer presence is ensured by tripping the flow using a 3 mm diameter rod at the upstream of the measurement section as shown in Figure 1. Shape factor of the boundary layer (the ratio of displacement to momentum thickness) for the flow over smooth bed is found for this case as 1.3 and flow can be considered as fully developed turbulent flow [22]. The instantaneous velocity measurement is carried out on top of the 60th sand strip for the flow over distributed roughness bed. To minimize the effect of secondary current, measurements for all flow test conditions are carried out along the flume centerline. Preliminary tests for all bed conditions are carried out to confirm that the flow condition is fully developed. Table 2 presents the summary of various test conditions.

3. Results and discussions

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

3.1 Mean velocity profiles

with the increment of Reynolds stress.

3.1.2 Inner coordinates

55

3.1.1 Outer coordinates

The purpose of the present study 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 (an impermeable smooth bed, impermeable rough bed, permeable sand bed and an impermeable bed with distributed roughness) and at two different Reynolds number (Re = 47,500 and 31,000) for each and every bed surface. Instantaneous velocity components are

Roughness Effects on Turbulence Characteristics in an Open Channel Flow

used to analyze the streamwise mean velocity, turbulence intensity in both

Reynolds shear stress from the turbulent bursting events.

streamwise and vertical direction, Reynolds shear stress including shear stress correlation and higher-order moments including vertical flux of the turbulent kinetic energy. Quadrant decomposition was also used to extract the magnitude of the

Figure 4 shows the variation of streamwise component of the velocity with respect to the depth of flow in outer coordinates. The mean velocity (U) is nondimensionalize by the maximum velocity (Ue) and the wall normal distance (y) is non-dimensionalize by the maximum flow depth (d). As one can see in the inset in Figure 4 that the velocity profiles of every flow conditions show a slight dip in the outer region where the location of maximum velocity happened to be occurred below the free surface with dU/d∂y is negative in the location close to the free surface. Velocity dip is different with different rough bed conditions with flow over natural sand bed showing the biggest dip followed by distributed roughness and continuous roughness bed. However, the flow over smooth surface shows the dip higher than the flow over distributed roughness and continuous roughness bed. Effect of bed roughness is very evident at the location close to the bed with velocity profile for the flow over smooth wall is fuller compared the flow over different rough beds. The same phenomenon was also observed by [15] and blamed it to the increment of surface drug due to the effect bed roughness. Comparing the effect of various type of bed roughness on the streamwise velocity component as one can see from Figure 4a that distributed roughness profile has the biggest deviation from smooth bed profile with continuous roughness and natural sand bed shows identical deviation. The variation of streamwise component of the velocity with respect to the depth of flow in outer coordinates with respect to the lower Reynolds number is shown in Figure 4b. The velocity profile characteristics are very similar for the lower Reynolds number flow compared to the flow for higher Reynolds number with the exception of flow over natural sand bed, which shows much higher deviation than flow over the bed of continuous roughness. One can correlate this with the interchange of fluid and momentum across the boundary, which is permeable like the flow over the bed of natural sand. The subsequent momentum/energy loss due to the effect of infiltration and corresponding differences on mean velocity reduces

Figure 5 shows the variation of streamwise component of the velocity with respect to the depth of flow in inner coordinates. The Clauser method was used to calculate the friction velocity for flow over smooth and rough bed conditions by
