Rotation of a pulsed jet, or plume, in a rotating flow:: A source of helicity for an α-ω astrophysical dynamo

A fluid-flow experiment was performed in water to investigate how an expanding pulsed jet rotates when injected off-axis into a rotating annular flow. The pulsed jet simulates a large-scale rising plume in a stellar or accretion disk environment. In the experiment, the pulsed jet was injected parallel to, but radially displaced from, the axis of rotation. The jet was observed to counterrotate Deltaphisimilar topi/2 to pi rad relative to the rotating frame before dispersing into the background flow. The counterrotation of an expanding pulsed jet in a rotating frame is the key result of the experiment. The counterrotation of the jet occurs because of the increased moment of inertia due to its expansion and conserved angular momentum. Rapid turbulent entrainment of the pulsed jet with the background flow during radial divergence when striking the end wall limits the net rotation of the jet. Shear within the differentially rotating background flow enhances the net rotation of the jet. Naturally occurring buoyant plumes in rotating flows should exhibit this same coherent nature, i.e., the same direction of rotation, the same vertical motion, and the same finite angle of rotation for every plume. This should make pulsed jets, or plumes, when occurring in a conducting medium, nearly ideal for producing helicity for the alpha-omega dynamo. In the experiment, Couette flow was established in water between two coaxial cylinders with an outer radius R-0=15 cm, an inner radius R-1=7.5 cm, and height Z=10 cm. The Reynolds number of the experiment was Resimilar or equal to10(5) in order to simulate the behavior of turbulent entrainment at high Reynolds number. The differential rotation of the background flow was varied from constant rotation, dOmega/dR=0, to Omega proportional to 1/R. The flow was made visible by pulsed hydrogen electrolysis from a tungsten wire and by dispersed guanidine in water. The flow was imaged using a digital video camcorder. These measurements are a precursor to the design and development of an alpha-omega dynamo experiment using liquid sodium, in turn with the purpose of simulating positive dynamo gain in a naturally occurring astrophysical flow.