Bubble Mode Research Articles

Numerical Studies of Photon Bubble Instability in a Magnetized, Radiation‐dominated Atmosphere

An initially static, plane-parallel, radiation pressure supported exponential atmosphere in a strong magnetic field is found to be unstable to the growth of buoyant low-density regions or in linear stability analysis. Here we present a series of numerical studies of the photon bubbles in such an atmosphere carried out using a quasi-two-dimensional radiation hydrodynamical code. When a single-mode perturbation is applied to the atmosphere, we find that the growth of these bubbles is in good agreement with the linear theory. When the evolution becomes nonlinear, the growth of the photon bubbles is found to be an efficient mechanism of energy transport. The presence of the low-density regions helps to increase the photon diffusion speed by a factor of several, while the buoyancy of the bubbles serves to transport energy via advection. Multimode studies, consisting of a perturbation with the linear combination of two single modes and a random perturbation, suggest that there is a tendency toward merger of the photon bubbles in their transport properties. The small wavenumber modes eventually dominate in radiation pressure, while the large wavenumber modes, although having higher growth rates, also saturate more quickly, and their contribution to the energy transport can be truncated. A grid study and the multimode calculations indicate that the energy transport through the atmosphere is well represented as long as the photon bubble mode with optical depth of ~10 over a wavelength is well resolved. Possible applications of the photon bubble instability includes the settling layer in the accretion column of a neutron star undergoing super-Eddington accretion. The enhanced energy transport may manifest itself in the emergent spectrum from the accreting pulsars and in the short-time variability in the light curves.

The effect of bubbling regime on gas and liquid phase mixing in bubble column reactors

An experimental study has been aimed at linking the extent of axial mixing in gas and liquid phases in tall bubble column reactors with the respective bubbling regimes (homogeneous, transition, heterogeneous) encountered in such reactors and with corresponding macro-scale flow patterns of both phases. The experimental programme, carried out in a bubble column reactor of 0.14 m in diameter and 4.1 m in height, included determination of gas and liquid phase residence time distribution (RTD) characteristics for different modes of primary gas dispersion and flow visualisation experiments aimed at identification of macro-scale flow patterns in the reactor. The experimental results proved an essential effect of gas dispersion mode (bubbling regime) on the extent of gas and liquid phase mixing in the reactor. The respective dependences of Pe G and Pe L on the superficial gas velocity, obtained with the distributing plate generating sequentially homogeneous, transition and heterogeneous bubbling regimes, indicated very sensitively the transition from the homogeneous to the heterogeneous bubbling mode (the onset of the transition bubbling regime) occurring at u 0G = 0.04 m s −1 with the appearance of local macroeddies in the bed. The comparison of our mixing data with corresponding dependences of gas holdup and k La L on the superficial gas velocity revealed the existence of direct links between the variations of RTD and mass transfer characteristics induced by the changes of gas dispersion (bubbling) mode. The results of the analysis of variances of gas phase RTD curves justified (in agreement with corresponding Pe G data) the plug flow approximation for the gas phase in the homogeneous bubbling regime and indicated the convective mechanism of gas mixing within the whole region of heterogeneous bubbling conditions. Analogically, the data for the liquid phase yielded the convective mechanism of liquid mixing within the whole range of experimental conditions. The mixing data obtained for both phases in the heterogeneous bubbling regime were adequately described by the model of consecutive circulation cells with the backflow between adjacent cells, proposed on the basis of the flow visualisation experiments as a realistic simplified representation of the macro-scale flow structures in heterogeneous bubble beds.

Comparisons of experimental and numerical results for axisymmetric vortex breakdown in pipes

Experimental data for the bubble mode of breakdown in pipe flows is compared with calculations of the axisymmetric form of the Navier-Stokes equations. The location of breakdown in the calculations is found to be extremely sensitive to the inlet conditions; however, generally good agreement between the experimental and numerical breakdown locations is obtained. Detailed comparisons of the velocity distributions show that the bubble form of breakdown is well approximated by the axisymmetric Navier-Stokes equations for the flow upstream of the middle of the recirculation region. In the bubble tail, larger discrepancies exist between the experiment and calculation suggesting that the bubble form of breakdown is unstable to non-axisymmetric disturbances in the tail. Also, high Reynolds number bubble structures are found to exhibit periodic vortex ring shedding from the tail of the breakdown bubble.

In-Canopy Application Mode and Soil Surface Modification for Corn

A center-pivot sprinkler was equipped with low pressure spray nozzles, Senninger Quad Spray, mounted on drop tubes near the soil surface. Water was applied in the LEPA (low energy precision application) bubble andflat spray modes under the last span with various soil surface modification (SSM) treatments during the 1990 and 1991 seasons. The SSH treatments included no SSH, basin till, subsoil, and implanted reservoir. Field slope ranged from 0.3 to 6.9%. Individual irrigation amounts were 19 to 25 mm (0.75 to 1.0 in.). The bubble mode performed well, as long as reservoir tillage was used and field slope was less than I to 2%. Reservoir tillage was effective in maintaining soil water content and corn yield, but the flat spray mode was more effective. Linear coefficients of the yield function as affected by field slope were -1.48 Mg ha-1 slope %-1 (-23.6 bu a .1 slope %-I) for the bubble mode and -0.73 Mg ha-1 slope %-I (-11.7 bu a-1 slope 101ci-I)for theflat spray mode.

Pressure and temperature dependence of the surface tension in the system natural gas/water principles of investigation and the first precise experimental data for pure methane/water at 25°C up to 46.8 MPa

The surface tension in the system methane/water has been measured with the pendent drop method (rising bubble mode) at 25°C for pressures between 0.5 and 46.8 MPa. Precise data with a slightly pressure dependent absolute error of 0.3–0.4% are presented. Principles of the investigation, precision and execution of the experiments have been examined widely. Based on a profile parameter a simple correlation is presented which allows the estimation of the uncertainty of surface and interfacial tension data for any pendent drop/rising bubble profile. Comparison of the achieved data is done with the few published results available. The surface tension decreases for moderate pressures distinctly more with increasing pressure but reveals clearly higher values for higher pressures than previously known. The concentration of methane in the surface phase tends quickly to a saturation value as pressure is increased. A first interpretation of the achieved surface tension data shows that the surface concentration of methane is increased by nearly two orders of magnitude compared with the methane concentration in the aqueous volume phase, calculated from solubility.

Periodic Solutions in Bubble Mode Resonance

AbstractA theory of periodic solutions for magnetostatic modes in a hexagonal bubble lattice based upon the application of the quasiperiodic Weierstrass ζ‐function and its even derivatives as matrix potential in common with new boundary conditions on the elementary cell is introduced. Periodic solutions for magnetostatic fields described by doubly periodic elliptic functions P(z) are obtained. The magnetic field structure for all four branches is investigated and is constructed for the low‐frequency branch ω = ω1.

Gas holdup in a three-phase fluidized-bed bioreactor

The effects of system parameters on the gas holdup in a three-phase fluidized-bed bioreactor were examined. A valve technique was used to measure the average gas holdup in the reactor. Gas holdup was found to be strongly affected by fluid superficial velocities, bead size and density, electrolyte concentration, type of gas sparger, and temperature. Quantitative and qualitative observations were made regarding the differences between conventional glass-particle systems and low-density gel-particle systems. Most notably, the degree of bubble coalescence and mode of fluidization were highly dependent on each system.

Formation studies of field-reversed configurations in a slow field-reversed theta pinch

Field-reversed configurations (FRC’s) have been successfully formed in a field-reversed theta pinch in a regime where there was little or no radial shock heating. It has been observed that even with the elimination of radial shock heating there remains sufficient resistive and compressional heating to burn through any radiation barriers caused by the low Z contaminates introduced during formation. The equilibrium behavior of the FRC was not influenced by the loss of radial shock heating other than yielding a lower equilibrium flux for a given initial reverse flux. With the slower reversal of the axial field, it was possible to exert considerable control over the formation dynamics through the use of end control coils. Two modes of formation have been identified wherein either the equilibrium flux was maximized (slingshot mode), or, plasma heating was maximized and formation dynamics were minimized (bubble mode).

FERROMAGNETIC RESONANCE OF CYLINDRICAL DOMAIN LATTICE

The recent experimental work on the ferromagnetic resonance of cylindrcal domain lattice at action of arbitrary magnetic field is reported. The device used in the experiments is a microwave microstrip transmission line placed between the poles of a electromagnet. Based on the experimental results, the present theories on the bubble mode resonance is analysed and some significant results are achieved.

Dynamic centering of liquid shells

The moderate-amplitude axisymmetric oscillations of an inviscid liquid shell surrounding an incompressible gas bubble are calculated by a multiple-time-scale expansion for initial deformations composed of two-lobed perturbations of the shell and a displacement of the bubble from the center of mass of the liquid. Two types of small-amplitude motion are identified and lead to very different nonlinear dynamic interactions, as described by the results valid up to second order in the amplitude of the initial deformation. In the ‘‘bubble mode,’’ the oscillations of the captive bubble and the liquid shell are exactly in phase and the bubble vibrates about its initial eccentric location. The bubble moves toward the center of the drop when the shell is perturbed into a ‘‘sloshing mode’’ of oscillation where both interfaces move out of phase. These results explain the centering of liquid shells observed in several experiments.

Recording mechanisms in antireflection trilayer structures

Any recording mechanism which produces a high-reflectivity spot by disrupting the interference condition which results in the initially low reflectivity of the trilayer structure is a potential mode of recording and data storage. Experimental results are presented for deep pit and bubble modes of recording. In the deep pit mode both the absorber and dielectric layers are melted so that the pit penetrates to the reflector layer. In the bubble mode, outgassing of the dielectric layer results in a micron-sized raised blister or bubble in the absorber layer. The bubble mode, in particular, offers the potential for high sensitivity and high SNR recording characteristics without the use of low melting point absorber layer materials.

Blocking in horizontal nozzles during injection of gas into liquid cast iron

An X-ray fluoroscopy technique has been used to study bubble formation and blocking in horizontal nozzles during the injection of nitrogen gas into liquid cast iron. Bubble frequency and mode of formation were found to be insensitive to the gas flowrate over the range investigated. Blocking is believed to be associated with the periodic collapse of bubbles at the nozzle exit, causing metal droplets to be propelled into the nozzle tube. Nozzles designed to reduce blocking were examined.

Bubble mode of flow of a gas-liquid mixture in a vertical pipe

The results of the measurement of the local frictional stress at the wall in an ascending two-phase stream using the electrochemical method are presented.

Bubble mode resonance

Recent experimental and theoretical work on the ferromagnetic resonance absorption in unsaturated samples of uniaxial materials containing mutually interacting bubbles is reported. The experiments were carried out on magnetoplumbite platelets 5 to 36 μm thick in the frequency range 16 to 50 GHz with fields both parallel and normal to the easy axis. The originally single absorption line for saturated samples splits into a number of lines the positions of which are practically independent of thickness dependent, however, on the magnetization suggesting magnetostatic modes. Interacting equiphase bubble sublattices precessing in different modes are proposed to explain these bubble mode resonances (BMR).