Search

We have exploited the temperature dependence of sound velocity to measure the thermal fields in transparent and opaque fluids. A chamber containing glycerol undergoing Rayleigh–Benard convection was probed with an ultrasound transducer operating in the pulse-echo mode. The times-of-flight for the ultrasound pulse to traverse the fluid at several transducer locations were converted into a temperature profile that is in qualitative agreement with simultaneous thermochromic liquid crystal visualization of the flow pattern. Temperature profiles in a mercury-filled stainless steel chamber have also been obtained, both for quiescent and turbulent flows, thereby validating the ultrasound thermometry concept for opaque fluids as well.

Nonlinear mode interactions and pattern formation occurring along the primary stability boundary for counterrotating eccentric cylinders have been studied experimentally. Counterrotating concentric cylinders produce either Taylor vortex flow or spiral vortices with various azimuthal wave numbers. Eccentric cylinders break the rotational symmetry or the base flow, introducing a delay or the onset or the primary instability and changes in the locations or the bicritical points separating patterns with differing azimuthal wave number. A theoretical model for this system is in qualitative agreement with the observations

The transition to spatiotemporal chaos in the neighborhood of a codimension-two point observed in the Taylor-Dean system is investigated. The initial instability is to stationary Dean vortex flow, which becomes unstable to a drift instability, followed by a wavelength-halving instability. This sequence of transitions may be explained in terms of the interaction between the first and second spatial harmonics of the basic pattern

We study the phase dynamics of turbulent Taylor vortex flow by applying a forced modulation to the upper boundary of a large-aspect-ratio system of concentric cylinders. The experiment shows that the axial phase variable of the turbulent Taylor vortex flow is a deterministic variable and its dynamics can be described by a simple diffusion model. The resultant diffusion coefficient of the turbulent Taylor vortex flow is about an order of magnitude larger than that of laminar/regular Taylor vortex flow. We argue that the approach of phase dynamics can also be applied to turbulent flows as long as there is, on sufficiently long-time scales, a coherent structure discernable in the system.

Space-time diagrams are used to analyze the dynamics of the liquid-crystal smectic C director in two-dimensional shear. We find a steady-state ring pattern only persists if there are no broken rings. A pair of defects, disclinations, is created when a ring breaks. With broken rings, the pattern dynamics is a sensitive balance between phase winding by shearing and unwinding by defect motion. Because the film is a 2-d elastic medium, low-frequency elastic oscillations occur in the ring pattern in the absence of shear. Using dimensional analysis and measurements of the in-plane orientational diffusion constant, estimates are obtained for an in-plane elastic constant and rotational viscosity that are consistent with bulk measurements.

Flow between two horizontal coaxial cylinders with a partially filled gap is investigated for the case when the primary instability forms a pattern of traveling inclined rolls which is simply periodic in space and time. Slightly above the onset, there is a second instability which results in a periodic modulation of the rolls with an axial wavelength of ∼3 rolls and a lower frequency. Both the traveling inclined rolls and the ∼3-roll-modulation pattern exhibit long-wavelength, low-frequency phase modulations and associated defects

The phase dynamics of wavy vortex flow is studied by applying a forced modulation to the upper boundary of a large-aspect-ratio concentric cylinders system. The experimental results are consistent with a model based on coupled phase equations. We find that the perturbations propagate as traveling waves parallel with the axis of the system when there is a strong coupling between the axial and azimuthal phase variables, and diffusively for weak coupling. The propagating wave speed is measured for a range of Reynolds numbers.

The resonant response of a stack of disks driven into torsional oscillation within a container of rotating superfluid helium has been observed. It is shown that the oscillation modes excited are related to Tkachenko waves, that is, vortex displacement waves in the vortex array propagating in a direction transverse to the vortex lines. In particular, the resonances occur at peaks in the vortex wave density of states.

Hysteretic spiral turbulence is a remarkable phenomenon of coexistence of turbulent and laminar domains in Taylor-Couette flow. We observe and measure for the first time a nonuniform pitch in long geometries and its dependence on boundary conditions at the cylinder ends, and we explain these results within the framework of phase dynamics. We also discuss the influence of secondary flow on the azimuthal width of the spiral.