Laser Doppler velocimetry (LDV) systems have been used for a number of years in field and laboratory applications. The normal LDV system consists of a two-color, four-beam system for obtaining two-component velocity measurements. These systems perform well in open flow but are not capable of obtaining velocity measurements along surfaces unless an angle is added to the LDV system probe. But adding an angle to the probe alignment introduces a small error into the measured velocities. The error can be as high as 1.38 percent of the vertical component for a system with a 50-mm beam spacing and a 600-mm focal length.
Through the use of physical models, the Army Corps of Engineers WES has been involved in designing fish bypass screening systems for Corps projects on the Snake and Columbia Rivers in the northwest United States for 13 years (Figure 1).
Velocities along the screen surface dictate whether a juvenile salmon will be guided around the turbines without being injured. These velocities must be accurate and repeatable.
To insure a precise measurement, WES uses a two-color, three-beam arrangement to perform measurements along a screen surface (Figure 2). With this system, both colors are present in one shared beam as well as each individually colored beam. The mutual beam is at a 90-degree angle to the other two, which changes the beam spacing from 50 mm to 35.35 mm. In a three-beam system, the beam arrangement can be rotated to align the hypotenuse of the arrangement along the screen surface, and the velocity magnitude can be resolved by determining how the arrangement has been rotated from the reference plane. The system can also easily obtain velocities with the colors at right angles to the mutual beam oriented to an x-y plane in the physical model.
The disadvantage of using a three-beam LDV system is that less power is delivered to the measuring volume, resulting in a noisier signal. In most cases, this can be resolved by using a higher-power laser or by adding more seeding material to the water flow. WES uses a 4-W argon laser attached through fiber optics to a fiber probe that collects light in the backscatter mode. For difficult measurements, seeding material is added to a column of water located at a safe distance upstream of the point at which measurements are recorded. Successful measurements have been recorded along screen surfaces through 550 mm of water and three sheets of 0.013-m-thick plexiglass.
Along with precise measurements, precisely controlled measuring positions are also important. To complement the three-beam LDV system, WES uses computer-controlled traversing systems to control the position of the laser probe to within 0.1 mm, thus assuring any difference in the flow field is due to a physical change in the model.
This work was done at the U.S. Army Engineers Waterways Experiment Station, Vicksburg, MS.
For more information about the application of two-color, three-beam LDV systems, contact Bob Davidson, Research Hydraulic Engineer, at (601) 634-3052; fax (601) 634-4158.