TURBULENCE-INDUCED RECTIFIED FLOWS IN ROTATING FLUIDS

Laboratory experiments dealing with Reynolds stress gradients in shear -free turbulence in homogeneous rotating fluids were conducted to bett er understand associated physical phenomena. The study was motivated b y possible applications to the oceanic environment where such Reynolds stress gradients are ubiquitous (e.g. in the vicinity of the continen tal shelf break, where turbulence decays away from the boundary). The turbulence was generated by vertical oscillations of a circular shaft with O-ring surface roughness elements; the oscillation axis coincided with the axis of symmetry of the cylindrical test cell. In the absenc e of background rotation, the turbulence is strong in the immediate vi cinity of the shaft surface and decays with the radial distance, r. Th e turbulence in the boundary layer is such that u(r) similar to u(thet a) similar to w, where u(r), u(theta), w are the radial, azimuthal and vertical r.m.s. velocity components, respectively. These velocity com ponents are found to be proportional to S omega, where S and omega are the stroke and frequency of the shaft oscillations, respectively, i.e . much the same as for the case of oscillating-grid turbulence, which has been studied extensively. When background rotation is present, the steady-state turbulent intensity close to the shaft is similar to tha t of the non-rotating experiments. Away from the shaft, in the central portion of the test cell, large-scale motions containing randomly dis tributed cyclonic and anticyclonic vortices are developed owing to sma ll local Rossby numbers. In the vicinity of the shaft, a rectified ant icyclonic flow U-theta is observed. The magnitude of U-theta is found to be proportional to the characteristic r.m.s. turbulence velocity u, but independent of the rate of background rotation. Consideration of the equations of motion shows that mean flows should not be expected i f background rotation is absent. With rotation, however, the equations indicate that the turbulent stresses can initiate, further develop an d then maintain a mean anticyclonic (rectified) flow around the cylind er; the azimuthal momentum equation is shown to play a critical role i n the generation of the mean anticyclonic flow.