3. Results and discussions

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

Boundary Layer Flows - Theory, Applications and Numerical Methods

Velocity measurements were done using A commercial two-component fiberoptic LDA system (Dantec Inc.) which is powered by a 300-mW Argon-Ion laser. Details of this is avoided for brevity because using the same system in several previous studies [20, 23, 9]. A Bragg cell and a focusing lens of 500 mm with beam spacing of 38 mm are the optical elements of the LDA system. A large amount of data collected (10,000 validated samples at each and every measurement location) to minimize the uncertainty of the data collection. The data rate varied widely based on the location of the measurement and ranges from 4 Hz to 65 Hz. The water used in the test is seeded with hollow spheres with density of 1.13 g/cc with mean particle size of 12 microns after filtering the water for many days and it is done prior to the start of the measurement. The seeded particles can stuck on the flume side wall and can cause extraneous scattered light distributed throughout the illuminating beams. The glass side wall around the measurement region were cleaned before

each set of measurement to avoid the erroneous data collection due to the scattered light. Due to the measurement location at the flume centerline, two scattered beams of the present two-component LDA system measuring the vertical component of the velocity cannot reach at very close to the bed or very close to the free surface but measurement of streamwise one-component velocity were carried out for full depth of flow. Following the footsteps of other researchers

pursuit to collect two dimensional velocity data closer to the wall, the LDA probe for the present tests was tilted 2o towards the bottom wall to capture data for two-

Test Bed condition d (mm) Re Fr Smooth bed 100 47,500 0.40 100 31,000 0.24 Distributed roughness 100 47,500 0.40 100 31,000 0.24 Continuous roughness 100 47,500 0.40 100 31,000 0.24 Natural sand bed 100 47,500 0.40 100 31,000 0.24

, respectively, in their

[16, 6] who have successfully tilted the probe by 3<sup>o</sup> and 2o

Summary of test conditions to study the effect of roughness.

component velocity measurements at near proximity of the wall.

conditions.

Table 2.

54

2.3 The laser Doppler anemometry

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 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. Quadrant decomposition was also used to extract the magnitude of the Reynolds shear stress from the turbulent bursting events.
