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.

We have developed a new type of target for intense laser-matter experiments that offers significant advantages over those currently in use. The targets consist of a liquid crystal film freely suspended within a metal frame. They can be formed rapidly on-demand with thicknesses ranging from nanometers to micrometers, where the particular value is determined by the liquid crystal temperature and initial volume as well as by the frame geometry. The liquid crystal used for this work, 8CB (4'-octyl-4-cyanobiphenyl), has a vapor pressure below $10^{-6}$ Torr, so films made at atmospheric pressure maintain their initial thickness after pumping to high vacuum. Additionally, the volume per film is such that each target costs significantly less than one cent to produce. The mechanism of film formation and relevant physics of liquid crystals are described, as well as ion acceleration data from the first shots on liquid crystal film targets at the Ohio State University Scarlet laser facility., Comment: 5 pages, 4 figures

Liquid crystal films are variable thickness, planar targets for ultra-intense laser matter experiments such as ion acceleration. Their target qualities also make them ideal for high-power laser optics such as plasma mirrors and waveplates. By controlling parameters of film formation, thickness can be varied on-demand from 10 nm to above 50 μm, enabling real-time optimization of laser interactions. Presented here are results using a device that draws films from a bulk liquid crystal source volume with any thickness in the aforementioned range. Films form within 2 μm of the same location each time, well within the Rayleigh range of even tight F/# systems, thus removing the necessity for realignment between shots. The repetition rate of the device exceeds 0.1 Hz for sub-100 nm films, facilitating higher repetition rate operation of modern laser facilities.

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

We have explored flow states of the Taylor-Couette system in the high radius ratio limit (0.99) with only the inner cylinder rotating. The usual Taylor vortices and wavy vortices were found, but the transition to higher states is interrupted by the emergence of intermittent short-wavelength bursts. This flow state is distinct from previously observed bursting phenomena found at lower radius ratios, while it is similar in appearance to states found in inclined layer convection.

The presence of a density maximum in water near 4 °C significantly modifies the nature and onset conditions of convective flows due to imposed temperature differences. In the present study, vertical temperature gradients are imposed upon a horizontal, rectangular layer of water, with the top and bottom surfaces maintained above and below the maximum density temperature, respectively. In such an arrangement, convection beginning in the lower, unstable portion of the layer (as small as 1/3 of the layer height) may penetrate into the upper, stable region. The resulting convection patterns are visualized using schlieren or shadowgraph techniques along multiple visual axes. The measured onset conditions and observed patterns are discussed in the context of preceding predictions and experimental observations in similar penetrative systems. As expected from the non-Boussinesq nature of water in this temperature range, convection sets in at temperature differences below those predicted by linear stability theory when the unstable portion of the layer is sufficiently small. The conduction-convection transition is also hysteretic in nature. At onset, the convection pattern consists of parallel, transverse rolls due to the boundary conditions of the fluid chamber. When the unstable portion of the layer is significantly less than half of the fluid layer height, the convective motion is found to penetrate only partway into the upper stable region, within which weakly counter-rotating motions are driven. At higher Rayleigh numbers, the fluid undergoes secondary transitions to either hexagonal cellular or longitudinal roll states which are visualized for the first time. Pattern heights and wavenumbers were measured in some instances, establishing qualitative (in general) and quantitative (over some parameter ranges) agreement with linear theory.

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 structure of longitudinal rolls of a fluid with a low Prandtl number in a differentially heated shallow cavity is considered. Computational results, carried out by a pseudo-spectral method, include the calculation of velocity and temperature fields. The secondary variables including energy, transfer rates from the base flow, transfer from the buoyancy field, and dissipation fields, give a physical picture of the flow. The experimental investigation verified the existence of the longitudinal rolls, with wavelengths and oscillation frequencies in reasonable agreement with theory. However, they occurred at much larger Grashof number than expected, which suggests a need for a secondary stability study for transverse stationary cells.

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.

The flow of a fluid between concentric rotating cylinders constitutes one of the simplest situations for studying the formation of nonequilibrium structures, both static and time-dependent, that one could imagine. In spite of this simplicity it exhibits an amazingly rich range of possible states, only some of which are well understood. In this contribution we will give a brief review of some recent experimental observations, and note some of the areas in which theoretical insight exists and where it is lacking.

We have conducted a flow-visualization and spectral survey of the flow patterns in a circular Couette system with both cylinders rotating [1]. The ranges of occurrence of previously reported flows and some newly observed flows have been determined as functions of the inner- and outer-cylinder Reynolds numbers (Ri and Ro, respectively) for a system with radius ratio 0.883 and aspect ratio 30 (the upper and lower boundaries were attached to the outer cylinder). The flow transitions were located by fixing Ro and slowly increasing Ri; by decreasing Ri several of these transitions were found to have hysteresis. Pattern velocities have been measured for some cases for flows between both counter- and co-rotating cylinders. For example, the turbulent spiral found between counter-rotating cylinders was found to move at an angular velocity of about 0.5Ωο, independent of Ri, for low outer cylinder speeds (Ωο is the angular velocity of the outer cylinder). The following new flows were found with counter-rotating cylinders: (1) wavy spirals (waves on laminar spiral vortices), (2) modulated wavy vortices (two waves on Taylor vortices) at very low Ri, and (3) V-shaped turbulence (a sometimes long- lived transient state of competing right- and left-handed spiral turbulence). We have also characterized several previously reported flows including spiral turbulence, intermittency, and laminar spiral vortices. The time-dependent flows found for co-rotating cylinders consist of periodic, multiply periodic or nonperiodic waves on Taylor vortices [2]. These flows include wavy-inflow and outflow boundary states, wavelets (both boundaries have waves, but they move at different velocities), twists (rope-like patterns internal to the vortices), and combinations of these. Interestingly, the wavy-inflow and outflow boundary states were found to have an axial periodicity extending over two Taylor vortex pairs, twice the axial wavelength of the usual wavy vortex flow.

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.

A spectrum 34'' wide along the major axis of the nucleus and bulge of M31 has been obtained with about 1 A resolution in the region of the H and K lines using the Princeton SEC-vidicon television sensor. Four unresolved absorption lines appear superposed on the broad stellar K line, and at least the strongest one is present at the H line. The heliocentric radial velocities of -30, -300, -560, and -750 km s/sup -1/ suggest that the four components are due to interstellar absorption in our Galaxy, similar absorption in M31, and two clouds ejected from the M31 nucleus at -260 and -450 km s/sup -1/. In addition, measurement of the equivalent widths of the stellar H and K lines showed that they increase from the bulge to the nucleus by about 40 percent. (AIP)