Laboratory experiments dealing with Reynolds stress gradients in shear-free turbulence in homogeneous rotating fluids were conducted to better understand associated physical phenomena. The study was motivated by possible applications to the oceanic environment where such Reynolds stress gradients are ubiquitous (e.g. in the vicinity of the continental 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 absence of background rotation, the turbulence is strong in the immediate vicinity of the shaft surface and decays with the radial distance, r. The turbulence in the boundary layer is such that ur∼uθ∼w, where ur, uθ, w are the radial, azimuthal and vertical r.m.s. velocity components, respectively. These velocity components are found to be proportional to Sω, where S and ω 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 that of the non-rotating experiments. Away from the shaft, in the central portion of the test cell, large-scale motions containing randomly distributed cyclonic and anticyclonic vortices are developed owing to small local Rossby numbers. In the vicinity of the shaft, a rectified anticyclonic flow Uθ is observed. The magnitude of Uθ 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 if background rotation is absent. With rotation, however, the equations indicate that the turbulent stresses can initiate, further develop and then maintain a mean anticyclonic (rectified) flow around the cylinder; the azimuthal momentum equation is shown to play a critical role in the generation of the mean anticyclonic flow.