AN EXPERIMENTAL STUDY OF TOLLMIEN-SCHLICHTING WAVE CANCELLATION
Abstract
On modern aircraft friction accounts for about one-half the total drag. Methods of boundary layer control currently being developed to maintain laminar flow are complex and do not reduce the drag sufficiently. A method has been proposed which would maintain laminar flow by cancelling boundary layer disturbance waves in the initial stages of growth by interference with artificially created waves of the proper amplitude and phase. In order to prove the validity of the fundamental concept of boundary layer control by wave cancellation an experiment was conducted to determine if the simplest possible disturbance, i.e. a Tollmien-Schlichting wave, could be cancelled by interference with an artificially generated disturbance. The experiment was conducted on a flat plate with essentially zero pressure gradient in a low turbulence water channel. The disturbance wave and the cancellation wave were introduced artificially by electromagnetically driven wave generators located 59.5 inches and 83.5 inches, respectively, from the leading edge of the flat plate. The nominal flow velocity was 0.5 to 0.6 feet per second. Both wave generators operated at the same frequency which ranged from 0.5 to 0.6 Hertz. Two-dimensional flow was maintained over a region 3 inches to each side of the centerline of the flat plate which was 14 feet long. The disturbances were detected with a hot film anemometer. The linear stability theory shows that 90% cancellation would be obtained if the amplitudes of the two waves are within (+OR-)5% of each other with the phase adjusted to 180(DEGREES) (+OR-) 18(DEGREES). In the experiment, cancellation of as high as 90% was shown to be possible, although, generally, absolute amplitudes of the perturbation velocity much below 0.1% of the freestream velocity were seldom obtained. From these results it is concluded that wave cancellation as a concept of boundary layer control is feasible. Considerable additional research and development are required to define practical configurations. Recommendations for future research are given.
Degree
Ph.D.
Subject Area
Aerospace materials
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