Study of Turbulent Structure with Hot-Wires Smaller Than the Viscous Length.

This study is intended as a step toward a better underst and ing of the small scale structure in the wall region of a turbulent boundary layer. To achieve this goal, the smallest reported hot-wire U-probes (typical dimensions: = 25 (mu)m, d = 0.5 (mu)m) have been constructed. In the new wind tunnel facility in which these probes were used, the dimensionless length scale of these probes, based on wall parameters, is u(,(tau))/(upsilon) = 0.3. Theoretical analyses of the static and dynamic response of small aspect ratio wires (operated in the constant-current mode) show that, though the hot-wires are uniformly less sensitive to velocity fluctuations, their frequency response is not affected. The analyses indicate that, for the small probes used in this study, the frequency response is uniform to 10 kHz, which is more than adequate for the present work. Measurements of the mean velocity profile and streamwise turbulence intensity profile are reported with the small U-probes. Comparison of the turbulence intensity data with larger probes shows that the smaller probes measure more energy due to their better spatial resolution. The data measured by the small probes are also analyzed to determine the burst frequency at various distances from the wall and at two Reynolds numbers. The results show that the burst frequency scaled with respect to the wall parameters is less a function of Reynolds number than the burst frequency scaled with respect to the outer flow variables. To obtain more quantitative information regarding the turbulence structure, measurements are reported with two small U-probes. From the correlation coefficient deduced from the measured data, it can be concluded that the turbulence structure increases in size, in both the spanwise and transverse direction, as the distance from the wall increases. Data for two probes less than two viscous lengths apart are analyzed to reveal the small scale features during a burst and in the absence of a burst. The main conclusions that can be drawn are that (1) intermittent small-scale shear layers occur more frequently when bursts are present than when they are not present and (2) on occasions, the velocity gradient of these small-scale shear layers is as large as the mean velocity gradient at the wall.Ph.D.Aerospace engineeringUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/157831/1/8017364.pd