3.2.1 Streamwise turbulence intensity

The distribution of the streamwise component of the turbulence intensity for flow over both smooth and rough beds is shown in Figure 6. Computed quantities

Figure 6. Streamwise turbulence intensity for flow over different bed condition.

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

can bring additional uncertainties in relation to the scaling parameters and to avoid any additional uncertainties, the streamwise turbulent intensity (u) is nondimensionalize by the maximum velocity (Ue) and the wall normal distance (y) is non-dimensionalize by the maximum flow depth (d). Magnitude of the streamwise component of the turbulence intensity reaches to the maximum at the location very close to the bed irrespective of the bed condition as one can note from Figure 6a. The location of maximum streamwise component of the turbulence intensity is different with different bed conditions. The location of the peak for the flow over smooth bed is very close to the bed at y/d 0, whereas the peak for the flow over rough surfaces varies with the different type of roughness. As one can note from Figure 6a that the distributed roughness shows the highest peak compared to the flow over continuous roughness and flow over natural sand bed. The location of the peak for the flow over rough beds are also varied depending on the type of roughness. The location of the peak for the flow over distributed roughness is at around y/d 0.08 whereas the location of the peak for the flow over continuous roughness and natural sand bed have occurred at the same location of y/d 0.04 which is a distance closer to the bed compared to the flow over distributed roughness. Immediately after reaching the peak the streamwise component of the turbulence intensity for flow over both smooth and rough beds reduces but the trend of reduction is very different for the flow over smooth bed compared to the flow over rough surfaces. There is a sharp drop of the magnitude of the streamwise component of the turbulence intensity for the smooth bed before a more constant drop towards the free surface and reaching a near constant value at y/d 0.5. For the flow over rough surfaces the drop of the value towards the free surface after the peak is linear and attains a near constant value but the location and magnitude of constant value is different for different rough surfaces (distributed roughness does not attain constant value but variation near free surface is minimal). The location of a near constant value for the flow over continuous roughness and natural sand bed is at the same level of y/d 0.62. The streamwise component of the turbulence intensity near the free surface also shows the effect of roughness with natural sand bed shows the highest intensity followed by the distributed roughness with flow over continuous roughness is the lowest. The effect of roughness on the distribution of the streamwise component of the turbulence intensity is very evident throughout the flow depth with distributed roughness shows the highest deviation followed by natural sand bed and continuous roughness compared to the smooth surfaces with the exception at the location very close to the bed. Although the sand grain used to create all three bed roughness is of the same gradation characteristics but the geometry of the roughness formation is different causing the differences in the distribution of the streamwise component of the turbulence intensity.

The variation of streamwise component of the turbulence intensity with respect to the depth of flow for the flow conditions to the lower Reynolds number is shown in Figure 6b. The streamwise component of the turbulence intensity 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 distributed roughness bed, which shows much higher deviation than flow over the smooth bed at the lower Reynolds number. In lower Reynolds number flow, the differences in streamwise component of the turbulence intensity for continuous roughness and natural sand bed is negligible.

#### 3.2.2 Vertical turbulence intensity

The distribution of the vertical component of the turbulence intensity for flow over both smooth and rough beds is shown in Figure 7. Significant effect of

magnitude of friction resistance is seen to be higher for the flow over a permeable bed compared to the flow over an impermeable bed. Dissipation of energy happened in the transition zone of the porous permeable medium with added loss of energy due to interchange of fluid and momentum across the permeable boundary translated back into the main flow. They commented that the net effect of com-

The distribution of the streamwise component of the turbulence intensity for flow over both smooth and rough beds is shown in Figure 6. Computed quantities

bined energy loss might be responsible for the higher resistance.

Boundary Layer Flows - Theory, Applications and Numerical Methods

3.2 Turbulence intensity

Figure 6.

58

Streamwise turbulence intensity for flow over different bed condition.

3.2.1 Streamwise turbulence intensity

number with the exception that there are almost no effect of roughness on the vertical

The distribution of the Reynolds shear stress in outer variables for flow over both smooth and rough beds is shown in Figure 8. Magnitude of the Reynolds shear stress reaches to the maximum at the location very close to the bed (y/d < 0.2) irrespective of the bed condition as one can note from Figure 8a. Effect of roughness on the Reynolds shear stress is very evident for lower two third of the depth of flow with the effect tapered off at the location closer to the surface. The peak for the flow over rough surfaces varies with the different type of roughness. As one can

component of the turbulence intensity for the location closer to the surface.

Roughness Effects on Turbulence Characteristics in an Open Channel Flow

3.3 Reynolds shear stress

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

Figure 8.

61

Reynolds shear stress distribution for flow over different bed condition.

Figure 7. Vertical turbulence intensity for flow over different bed condition.

roughness can be seen for lower two third of the depth of flow with the effect tapered off at the location closer to the surface. Comparing the effect of various type of bed roughness on the vertical component of the turbulence intensity as one can see from Figure 7a that distributed roughness profile has the biggest deviation from smooth bed profile with continuous roughness and natural sand bed shows identical deviation for most the depth of the flow. For the location closer to the surface, the flow over natural sand bed shows higher magnitude of the vertical component of the turbulence intensity compared to any other surfaces. The variation of the vertical component of the turbulence intensity for flow with respect to the lower Reynolds number is shown in Figure 7b. The profile characteristics are very similar for the lower Reynolds number flow compared to the flow for higher Reynolds Roughness Effects on Turbulence Characteristics in an Open Channel Flow DOI: http://dx.doi.org/10.5772/intechopen.85990

number with the exception that there are almost no effect of roughness on the vertical component of the turbulence intensity for the location closer to the surface.
