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Rayleigh-Taylor mixing: confinement by stratification and geometry
Get PDFRayleigh-Taylor instability has been an area of active research in fluid dynamics for
the last twenty years, but relatively little attention has been paid to the dynamics
of problems where Rayleigh-Taylor instability plays a role, but is only one component of a more complex system. Here, Rayleigh-Taylor instability between miscible fluids is examined in situations where it is confined by various means: by geometric
restriction, by penetration into a stable linear stratification, and by impingement
on a stable density interface. Water-based experiments are modelled using a variety
of techniques, ranging from simple hand calculation of energy exchange to full
three-dimensional numerical simulation. Since there are well known difficulties in
modelling unconfined Rayleigh-Taylor instability, the confined test cases have been sequenced to begin with dynamically simple benchmark systems on which existing
modelling approaches perform well, then they progress to more complex systems and
explore the limitations of the various models. Some work on the phenomenology of
turbulent mixing is also presented, including a new experimental technique that allows mixed fluid to be visualised directly, and an analysis of energy transport and mixing efficiency in variable density flows dominated by mixing
Analysis of Turbulent Free Convection in a Rectangular Rayleigh-Benard Cell
No full textThe present study compares results of direct numerical simulations of thermal convection within a rectangular geometry with Rayleigh-Benard convection in thin fluid layers and cylindrical geometries. It is shown that boundary layer thicknesses show similar tendencies in the rectangular geometry and a thin fluid layer, but the quasi two-dimensional geometry of the container delays the onset of convection significantly. Analysis of the thermal dissipation rates indicates that there are three distinct regimes, of the small scale contribution grows rap-idly as Rayleigh number is increasing, whereas the large scale contribution remains almost constant
Moist Rayleigh-Benard Convection
No full textWe report the observations from turbulent thermal Rayleigh-Benard convection experiment with a two-phase liquid-vapor binary mixture. Evaporation/condensation processes in a turbulent convection are accompanied by the formation of cloud like structures above the liquid-vapor interface. We also found that a liquid film condensation on the underside of the top plate results in regular hexagonal patterns of falling droplets
The shear-driven Rayleigh problem for generalised Newtonian fluids
No full textWe consider a variant of the classical ‘Rayleigh problem’ (‘Stokes’s first problem’) in which a semi-infinite region of initially quiescent fluid is mobilised by a shear stress applied suddenly to its boundary. We show that self-similar solutions for the fluid velocity are available for any generalised Newtonian fluid, regardless of its constitutive law. We demonstrate how these solutions may be used to provide insight into some generic questions about the behaviour of unsteady, non-Newtonian boundary layers, and in particular the effect of shear thinning or thickening on the thickness of a boundary layer
Large scale circulation in turbulent Rayleigh-Bénard convection
No full textStatistical properties of turbulent Rayleigh-Bénard convection (RBC) are investigated experimentally in a cylindrical cell of aspect ratio one. We specifically analyze the large scale circulation of RBC based on measurements of temperature fluctuations by small Ge sensors placed inside the cell. The resulting dependencies of Reynolds numbers on Rayleigh number up to 2x10^13 are compared to available theoretical and experimental results for similar geometries
Precise BER Formulas for Asynchronous QPSK-Modulated DS-CDMA Systems Using Random Quaternary Spreading Over Rayleigh Channels
No full textPrecise bit-error-ratio (BER) analysis of an asynchronous QPSK-modulated direct-sequence code-division multiple-access system using random quaternary spreading sequences for transmission over Rayleigh channels is performed based on the characteristic-function approach. Its accuracy is verified by our numerical simulation results and also compared with those of the Gaussian approximation. Index Terms—Asynchronous direct-sequence code-division multiple-access (DS-CDMA), bit-error-ratio (BER), precise, QPSK, quarternary spreading, Rayleigh
Performance Upper Bound of Adaptive QAM in Slow Rayleigh-Fading Environments
No full textThe performance of adaptive modulation schemes that employ either no transmission, BPSK, QPSK, 16 Square QAM and 64 Square QAM is characterised for transmissions over Rayleigh channels. The optimum value for the switching levels between the Axed modulation schemes is found. This is achieved by ,optimising the BER and throughput for wireless computer data and speech systems. Using the optimised switching thresholds, the system performance is also evaluated over Rician channels
Analysis of Rayleigh wave interactions for surface crack characterization
Get PDFElectromagnetic acoustic transducers (EMATs) have been used in pitch-catch mode for a better characterization of surface cracks in metals. The system, which combines the measurement of Rayleigh wave velocity in the in-plane and out-of-plane directions, has been used to understand the interaction of Rayleigh waves with inclined surface cracks. This shows a stronger and more prominent enhancement pattern when compared to the enhancement caused by cracks which are normal to the surface. In addition, measurements in the far-field are combined with the near-field enhancement measurement, with a view to characterize the cracks. An algorithm for characterizing surface cracks is presented. A finite element method model has been computed to simulate the experiment, and the cause of the prominent enhancement in the inclined cracks is explained
geodynamics/Rayleigh: Rayleigh Version 0.9.0
No full text<p>Initial public release of Rayleigh.</p>
Rayleigh scattering temperature measurements in a swirl stabilized burner
No full textRayleigh scattering temperature measurements were obtained in a turbulent reactive swirling coaxial jet discharged from a swirl-stabilized burner along the jet-flame centerline. They are reported up to 10 fuel nozzle diameters downstream of the burner exit at a Reynolds number of 29000. The effect of swirl numbers (S=0.3, 0.58, 1.07) on the temperature fields, the power spectral density of temperature fluctuations and on the probability density functions of the temperature fluctuations was determined
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