MOTION FIELDS GENERATED BY THE OSCILLATORY MOTION OF A CIRCULAR-CYLINDER IN A LINEARLY STRATIFIED FLUID

The motion field generated by either the horizontal or the vertical fi nite-amplitude oscillation of a long right circular cylinder in a line arly stratified fluid is investigated by a series of laboratory experi ments; the oscillation frequencies considered cover a range both less than and greater than the buoyancy frequency. The governing parameters are shown to be the oscillation amplitude to cylinder diameter ratio a/D, the oscillation frequency to buoyancy frequency ratio omega/N, an d the viscous to buoyancy time-scale ratio T-vb = ND2/v (or the Stokes number beta = omega D-2/V), where v is the kinematic viscosity of the fluid. Flow regime diagrams are developed as functions of a/D and ome ga/N for a fixed T-vb. The characteristic flow types for the horizonta l oscillation case include weakly detached flow, localized mixing, lay ering, and single intrusion, whereas the vertical oscillation leads to weakly detached flow, localized mixing, and single and double intrusi ons. Both oscillation directions lead to propagating internal waves wh en omega/N < 1; the direction of the group velocity is observed to be given by the angle sin(-1)(omega/N) to the horizontal axis, in accord with linear theory. A nondiffusive numerical model is developed to inv estigate the flow behavior during the initial stages of the cylinder o scillations. The large-scale motion features obtained from the numeric al model for the early flow development of the various flow regimes ar e found to compare well with those of the laboratory experiments. (C) Elsevier Science Inc., 1997.