A theoretical and flight test study of pressure fluctuations under a turbulent boundary layer. Part 2: Flight test study

The study of pressure fluctuations under a turbulent boundary layer was undertaken with the objective of extending previous work to lower frequencies. Wind tunnel and flight test measurements are invalid at low frequencies because of extraneous acoustic noises and free stream turbulence. A glider was instrumented and used as a test bed to carry microphones into a smooth flow free of acoustic noise. Hodgson had previously measured the spectrum of boundary layer noise on a glider wing. These tests showed a drop off at low frequencies that could not be reproduced in any other facility. The measurements were made on the forward fuselage of a glider where the boundary layer could develop naturally and have some length in a zero pressure gradient before the measurements were made. Two different sets of measurements were made

Individual Realization Laser-Doppler Technique Applied to Turbulent Channel Flow

Measurements of the mean and turbulence intensity of the streamwise velocity component in a fully developed, two-dimensional channel flow of water are presented. The measurements were made with the individual realization laser Doppler technique and emphasize the near-wall region of the flow. A dual-scatter optical arrangement was used which employs 90° scattering and yields a probe volume whose length normal to the wall is 0.0075 inches. A correction has been made to the data that accounts for the statistical biasing which occurs in the individual realization technique. The corrected data demonstrate that the individual realization technique can yield accurate velocity estimates in the near-wall region where turbulent fluctuations are large

Laser Doppler velocimetry studies of early turbulence

Laser Doppler anemometry is employed to measure the mean velocity and turbulence intensity in the early turbulence regime of the flow of a dilute polymer solution through a small-diameter tube. (This regime is initiated at an onset Reynolds number in the subcritical range, above which the polymer additive increases drag, and continues through the transitional Reynolds number range, where a continuous transition to turbulent flow with reduced drag is observed.) When the onset flow rate for early turbulence is exceeded, the mean velocity profile becomes progressively flatter with increasing flow rate. That is, a gradual transition from a laminar profile at onset to a more turbulent-like profile takes place. Turbulence intensities at the tube centerline are about one half those of a fully developed turbulent flow. The inference is drawn from these results that the onset of early turbulence corresponds to the flow becoming hydrodynamically unstable due to viscoelastic effects introduced by the polymer additive and that the character of the flow is intermediate between laminar and fully developed turbulent flow. Copyright © 1977 American Institute of Physics