PHYSICAL REVIEW E
The cylindrical wall boundary layer of a closed cylinder split in two halves at the equator is studied experimentally. When these two parts rotate in exact corotation the internal flow is essentially in solid-body rotation at the angular velocity of both halves. When a slight difference between the rotation frequencies is established a secondary flow is created due to the differential rotation between both sides and restricted to the boundary layer. This behavior of the boundary layer is compared with theoretical and numerical results finding the ¿sandwich¿ structure of a Stewartson boundary layer. Time-dependent waves are observed near the cylindrical wall. Their behavior for different values of the control parameters are presented. Finally, a global recirculation mode is also found due to a symmetry-breaking induced between sides that appears because of a slight misalignment of the experimental setup, whose characteristics are compatible with the behavior of a precessing cylinder.
PHYSICS OF FLUIDS
The optimal response of the Batchelor vortex is studied by considering the time-harmonically forced problem with frequency omega. High variance levels are sustained in this system under periodic forcing. The optimal response is largest when the input frequency is null in the axisymmetric case (m= 0). In addition, the axial flow does not play a relevant part in determining the optimal response. When considering helical modes vertical bar m vertical bar = 1, perturbations are excited through a resonance mechanism at moderate and large wavelengths. At smaller wavelengths, a large response is excited by steady forcing. Regarding the axial flow, the response is largest when the axial velocity intensity is near to zero. For perturbations with larger azimuthal wavenumbers vertical bar m vertical bar > 1, the magnitude of the response is smaller than those for helical modes. Therefore, studying the response for vertical bar m vertical bar > 1 is of no interest. Published by AIP Publishing.