The behaviour of naturally oscillating three-dimensional flow around a cylinder

Abstract

Split-film electrochemical transducer theory is briefly outlined so as to show its ability to separate statistical properties of each component of the surface velocity gradient in regions as different as a boundary layer, a separated zone and a three-dimensional separation line. A lot of measurements of the flow field near a cylinder which are not yet available elsewhere are carried out everywhere on the cross-section of a yawed cylinder. A direct verification of the independence principle and Wild's thermal analogy in separated zones is executed. The periodic and random parts of each component of the surface velocity gradient are separated. Cross-spectral analysis between any pair of points shows that the periodic surface flow is coherent upon any cross-section and in a large region in the spanwise direction. The stationary wave system is investigated in space and time. The reconstitution, instant by instant, of the averaged integral wall streamlines leads to an understanding of the synchronization of the natural oscillations. The four main stagnation or separation lines quickly move from one extreme position to the other, and at certain times contrary vortices stretching in the direction of the generators can appear. In the rearward stagnation region random small-scale fluctuations are probably turbulent, but in the intermediate region high-level random fluctuations in the spanwise direction are much more coherent and probably contribute to keeping the flow in phase at large distances.