Relative importance of mean velocity and bed shear on biofilm accumulation in open-channel flows

1995 ◽  
Vol 32 (8) ◽  
pp. 193-198 ◽  
Author(s):  
Y. L. Lau
Keyword(s):  
Biofilm Formation ◽  
Average Velocity ◽  
Open Channel ◽  
Channel Flows ◽  
Shear Stresses ◽  
Relative Importance ◽  
Mean Velocity ◽  
Biofilm Growth ◽  
Roughness Elements ◽  
Set Up

Experiments on biofilm growth were carried out to investigate whether bottom shear stress or average velocity is more appropriate as a parameter for investigating the effect of flow on biofilm formation in channel flows. The tests were conducted in two identical channels located side by side, using the same water supply. By having different bottom slopes or roughness elements, or both, tests were set up in which the flows in the two channels had equal velocities but different bottom shear stresses or equal bottom shear stresses but different velocities. Porcelain balls were used as bottom roughness elements and the accumulation of biofilm on the balls was monitored. Comparisons of the rates of biofilm accumulation indicate that the average velocity is the more important parameter.

Turbulent Boundary Layers in Low Reynolds Number Shallow Open Channel Flows

1999 ◽  
Vol 121 (3) ◽  
pp. 684-689 ◽  
Author(s):  
Ram Balachandar ◽  
Shyam S. Ramachandran
Keyword(s):  
Reynolds Number ◽  
Boundary Layers ◽  
Open Channel ◽  
Reynolds Numbers ◽  
Skin Friction Coefficient ◽  
Turbulent Boundary Layers ◽  
Channel Flows ◽  
Open Channel Flows ◽  
Mean Velocity ◽  
Low Reynolds

The results of an experimental investigation of turbulent boundary layers in shallow open channel flows at low Reynolds numbers are presented. The study was aimed at extending the database toward lower values of Reynolds number. The data presented are primarily concerned with the longitudinal mean velocity, turbulent-velocity fluctuations, boundary layer shape parameter and skin friction coefficient for Reynolds numbers based on the momentum thickness (Reθ) ranging from 180 to 480. In this range, the results of the present investigation in shallow open channel flows indicate a lack of dependence of the von Karman constant κ on Reynolds number. The extent to which the mean velocity data overlaps with the log-law decreases with decreasing Reθ. The variation of the strength of the wake with Reθ is different from the trend proposed earlier by Coles.

scholarly journals Measuring the turbulent characteristics in an open channel using the PIV method

2013 ◽  
Vol 14 (3) ◽  
pp. 378-385
Keyword(s):  
Open Channel ◽  
Flow Characteristics ◽  
Turbulent Shear ◽  
Shear Stresses ◽  
Mean Velocity ◽  
Vegetation Height ◽  
Hydraulic Behaviour ◽  
Turbulent Characteristics ◽  
Wide Range ◽  
Vegetation Region

Investigation of open channel flows is very important for a wide range of applications, including restoration and enhancement of river aquatic systems. As a result, the scientific community has focused on providing further insights on the flow characteristics in vegetated channels. Vegetation may be submerged or emerged, rigid or flexible with high or low density. For rigid vegetation, the hydraulic behaviour of the channel is similar to the behaviour of a channel with macro-roughness which could be caused by the presence of geometrical elements (e.g. cylinders, cubes). For flexible vegetation, both the flexibility of the vegetation and the hydrodynamic of the flow contribute to the generation of several formations such as erect, gently swaying, and prone. In this study, the characteristics of turbulent flow in an open channel were studied experimentally using Particle Image Velocimetry (PIV). This method assumes that the particles of a fluid faithfully follow the flow dynamics, hence the motion of these seeding particles are used to calculate velocity information of the flow. The experiments were conducted for both impermeable and permeable beds in a channel of 6.5m length, 7.5 cm width and 25 cm height. Two grass-like vegetation types of different height (2 and 6 cm) were used to represent permeable beds. These conditions are typical of flows encountered in sediment transport problems. Hydraulic characteristics such as distributions of velocities, turbulent intensities and Reynolds stress are investigated at a fine resolution using the PIV. Velocity is measured above the vegetation at different heights. Results show that velocity over the vegetation region is a function of the vegetation height and the total flow depth; velocity decreases as the vegetation height increases. In addition, we show that velocities above the vegetation region are much lower than velocities above an impermeable bed. This is due to the turbulent shear stresses and the existence of turbulence in the vegetation region, which reduce the mean velocity above the vegetation region. In addition, results show a region of zero velocity; between 3 and 6 cm and 1 and 2 cm for a 6 cm and 2 cm vegetation. This result shows that 50% of the vegetation behaves like an impermeable bed.

Flow Formed by Spanwise Gaps Between Roughness Elements

1985 ◽  
Vol 107 (3) ◽  
pp. 385-391
Author(s):  
Earl Logan ◽  
Shu Ho Lin ◽  
Obaidul Islam
Keyword(s):  
Free Stream ◽  
Roughness Element ◽  
Stream Velocity ◽  
Shear Stresses ◽  
Wall Region ◽  
Mean Velocity ◽  
Wake Flows ◽  
Gap Flow ◽  
Roughness Elements ◽  
Made In

Measurements of the three mean velocity components and the three Reynolds shear stresses were made in the region downstream of gaps between wall-mounted roughness elements of square cross section and high aspect ratio in a thick turbulent boundary layer. The effect of small and large gaps was studied in a wind tunnel at a Reynolds number of 3600, based on obstacle height and free-stream velocity. The small gap produces retardation of the gap flow as with a two-dimensional roughness element, but a definite interaction between gap and wake flows is observed. The interaction is more intense for the large gap than for the small. Both gaps generate a secondary crossflow which moves fluid away from the centerline in the wall region and toward the centerline in the outer (y > 1.5H) region.

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