Book Description

Particle-image velocimetry (PIV) is used to measure instantaneous velocity fields in the streamwise wall-normal plane of a zero-pressure-gradient turbulent boundary layer. In total, 340 PIV realizations of the boundary layer were obtained at Reynolds numbers based on momentum thickness of Ree = 930, 2370 and 6845. Each realization contains approximately 10000 two-component velocity vectors which are accurate to within 0.4 to 1.0% of the free-stream velocity. The PIV results are used to examine coherent structures throughout the boundary layer and determine their Reynolds-number dependencies. Coherent structures have been an important part of turbulent boundary-layer research for more than thirty years. By understanding coherent structures, their Reynolds-number dependencies and their dynamical characteristics, engineers and scientists will be better able to control and predict turbulent flows. A 20 m long low-turbulence boundary-layer research facility was developed for the measurement of turbulent boundary layers using particle-image velocimetry. Hot-film anemometer measurements show that the turbulence intensities at the test section inlet are about 0.16%, at a free-stream velocity of 6 m/s. A PIV interrogation and vector-validation system was developed to conduct the PIV measurements efficiently. The interrogation system utilizes eight i860 array processors in parallel to achieve a peak aggregate performance of 640 Mflops. Using 128 x 128 pixel cross correlations, it processes 100 vectors per second. Two-dimensional maps of in-plane velocity vectors, spanwise vorticity, Reynolds stress, and streamwise and wall-normal velocity contours show the instantaneous structures in the overlap and outer regions of the boundary layer. Details of large-scale motions protruding into the free stream are clearly shown in the outer region. The overlap region consists of shear layers oriented at 45$spcirc$ from the wall and are associated with transverse vortical elements, which are located above and slightly downstream of the shear layers. Comparisons between measured coherent structures and theories of Theodorsen, Townsend and Perry et al. are given. Two-dimensional spatial correlation functions of streamwise velocity, wall-normal velocity, spanwise vorticity and Reynolds stress are used to examine the statistically relevant structure of the boundary layer and its Reynolds-number dependency. Stochastic estimation is used to estimate conditional eddies throughout the boundary layer and reveal their Reynolds-number dependencies.