An Experimental Investigation of Artificially Induced Transition to Turbulence on a Body of Revolution

Transition to turbulence in a boundary layer continues to be a topic of considerable interest to both aerodynamicists and hydrodynamicists. The present study focuses on the particular aspect of the problem pertaining to transition zone characteristics induced by a circular trip wire located upstream of the point of instability on a blunt nosed body of revolution. The actual transition to turbulence occurs in a region of adverse pressure gradient. The study was made on an eight-inch diameter body of revolution which had a blunt nose faired to a cylindrical afterbody with a modified ellipsoid contour. The velocity field was measured with hot-wire sensors and analyzed using analog techniques to determine velocity, intermittency, spatial correlation, and spectral characteristics. Transition to turbulence was found to be very sensitive to trip wire diameter and location and to the free-stream Reynolds number. Certain of the high amplitude instability waves induced by the trip mechanism were observed to be selectively amplified into transition in a manner similar to that predicted by stability theory. Secondary instabilities were observed to contribute to the transition process. Cross spectral densities were measured for the bursts moving downstream to determine if any narrowband range of frequencies were present which could contribute to far-field noise radiated from the boundary layer. A one-third octave filter was used to determine spectral velocity fluctuation concentrations of the bursts at a location within the transition region and compared to similar concentrations at a fully turbulent location. (Author)