2.3 The laser Doppler anemometry

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 [16, 6] who have successfully tilted the probe by 3<sup>o</sup> and 2o , respectively, in their 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 twocomponent velocity measurements at near proximity of the wall.


#### Table 2.

Summary of test conditions to study the effect of roughness.
