The Structure of Turbulent Velocity Fields.

Abstract

Abstract : A systematic study of two-dimensional, turbulent, small-deficit wakes investigated the structure and universality of their self-preserving states. Various wake generators, including circular cylinders, a symmetrical airfoil, a flat plate, and an assortment of screens of varying solidity, were studied for a wide range of downstream distances. The drag coefficient of most generators, and therefore the momentum thickness, was tailored to be identical, permitting comparison at identical Reynolds numbers and aspect ratios. The flat plate and airfoil and had a small, trailing-edge flap which could be externally driven to introduce forced sinuous oscillations into the wake. Results indicate that the normalized characteristics velocity and length scales depend on the initial conditions, while the shape of the normalized mean velocity profile is independent of initial conditions or nature of the generator. The normalized distributions of the longitudinal turbulence intensity, however, are dependent on the initial conditions. Linear inviscid stability theory, in which the divergence of the mean flow was taken into account, predicts quite well the amplification and transverse distributions of amplitudes and phases of externally imposed sinuous waves on a fully developed turbulent wake generated by a flat plate. There is a strong indication that the large structures observed in the unforced wake are related to the two-dimensional instability modes and therefore can be modelled by linear stability theory. Interaction of the two possible modes of instability may be responsible for the vortex street-type pattern observed visually in the small-deficit, turbulent wake.