Design, Construction, and Use of a High-Resolution, Two-Component Lda (Laser-Doppler Anemometry).

The study of wall bound turbulent flows at high Reynolds numbers has shown that the scale of turbulence near the wall, is too small to allow accurate velocity measurements with conventional instrumentation. The objective of this study was to advance the use of Laser-Doppler anemometry, LDA, in the investigation of small scale turbulent structure. In order to resolve this structure in the region near the wall, a LDA system with a high-resolution is necessary. In this investigation, a novel LDA system was designed, constructed, and placed in operation. This system had a measurement volume of (TURN)40 (mu)m or about two viscous lengths. This small size of the measurement volume was produced by a wide angle for two of the incident beams (60 degrees), and by collecting the scattered radiation partially in the side scatter mode. Measurements were performed in a fully developed turbulent water channel, seeded with TiO(,2) particles, and data were obtained at a rate of (TURN)2000 per sec., and stored in digital form with the aid of a computer. Measurements were made of two velocity components (one in the stream direction, and the other inclined 45 degrees in a plane normal to the wall) using two Doppler burst processors interfaced to a computer. The two velocity components readings had to be obtained within a preselected time interval, less than the average transit time of a particle through the measuring volume. This mode of operation permitted the non-intrusive measurement of Reynolds stress using scattering particles within a measuring volume of a scale less than 40 (mu)m. Measurements and calculations of the mean flow velocity components (u and v), the Reynolds stress, and the statistical moments of the two velocity components in the channel flow were made at a Reynolds number of 14,700 based on the maximum velocity and the channel half width. Measurements as close to the wall as y('+) (TURNEQ) 25 ((DELTA)y('+) = (+OR-) 3) are reported here. Measurements closer to the wall were not possible due to noise in the signal. This noise is believed to have been introduced by scattered radiation from the clear plastic window through which the laser beams entered the water channel. Changes in the water channel facility are being made to reduce the thickness of this plastic window and therefore allow measurements closer to the wall.PhDAerospace engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/159492/1/8324123.pd