Geophysical and Astrophysical Fluid Dynamics

Abstract:

Results from an experimental study of vortices in a rotating shear layer will be presented. Through the rotation of circular sections in the base and lid of a circular tank, a vertical shear layer is created in the fluid interior. In supercritical conditions, the flow is in the form of a regular string of two-dimensional, vertically uniform, vortices along the now wavy shear layer. Once established, the vortices are very stable flow structures that persist as long as the shear is maintained. Under most conditions the vortices were steady, but quasi-periodic and chaotic flows were also observed. The data from the experiments are in the form of maps of the instantaneous horizontal velocity field obtained by a particle tracking technique similar to Particle Image Velocimetry (PIV). The data would be useful validate both the spatial and temporal behaviour of numerical models.

Abstract:

We consider several protostellar systems in which either a precessing jet or at least two misaligned jets have been observed. We assume that the precession of jets is caused by tidal interactions in noncoplanar binary systems. For Cep E, V1331 Cyg, and RNO 15-FIR, the inferred orbital separations and disk radii are in the range 4-160 AU and 1-80 AU, respectively, consistent with those expected for pre-main-sequence stars. Furthermore, we assume or use the fact that the source of misaligned outflows is a binary and evaluate the length scale over which the jets should precess as a result of tidal interactions. For T Tau, HH1 VLA 1/2, and HH 24 SVS63, it may be possible to detect a bending of the jets rather than "wiggling." In HH 111 IRS and L1551 IRS5, wiggling may be detected on the current observed scale. Our results are consistent with the existence of noncoplanar binary systems in which tidal interactions induce jets to precess.

Abstract:

The Proper Orthogonal Decomposition (POD) is a procedure to compute an orthogonal basis from a time series of spatial fields. This basis is optimal among all linear decompositions, in the sense that for a given number of modes, the projection of the original signal onto the subspace will contain the most variance on average. This algorithm is applied to streamfunction fields derived from measurements of the flow in the thermally forced rotating annulus experiment. Results of this analysis are presented, and a method to derive low-dimensional models of the flow by projecting the equations of motion onto these empirical eigenfunctions is discussed.