Numerical study into the morphology and formation mechanisms of three dimensional particle structures in vibrated cylindrical cavities with various heating conditions

The present analysis extends the author’s earlier work (Lappa, Phys. Fluids, 26, 093301, 2014) on the properties of patterns formed by the spontaneous accumulation and ordering of solid particles in certain types of flow. It is shown that under certain conditions, when subjected to vibrations to induce natural flow, non-isothermal fluids with dispersed solid particles are characterized by intervals of solid-pattern-forming behaviour due to particle rearrangements preceded by intervals in which no recognizable structures of solid matter can be detected. The dynamics of these systems are highly nonlinear in nature. Because this new family of particle attractors is known to exhibit strong sensitivity to the “symmetry properties” of the considered vibrated system and related geometrical constraints, the present study attempts to clarify the related dynamics in a geometry with curved walls (cylindrical enclosure). In particular, by assuming vibrations always directed perpendicularly to the imposed temperature gradient, we show that the morphology, spatial extension (percentage of physical volume occupied), “separation” (spatial distance) and mechanisms responsible for the formation of the resulting particle structures change significantly according to whether the temperature gradient is parallel or perpendicular to the symmetry axis of the cylinder. This indicates that the “physics” is not invariant with respect to 90 rotations in space of the specific forcing considered (direction of the imposed temperature gradient and associated perpendicular vibrations). Additional insights into the problem are obtained by assessing separately the influence played by the time-averaged (mean) and oscillatory effects. According to the numerical results, the intriguing diversity of particle agglomerates results from the different role/importance played by (curved or straight) boundaries in constraining particles and, from the different structure and topology of the resulting macroscopic (large-scale) thermovibrational flow oscillating in time at the same acceleration frequency of the imposed vibrations

Thermally-driven flows in polymeric liquids

Within the vast array of problems related to the processing of plastic materials and polymeric liquids, the main focus of the present chapter is the illustration of the typical dynamics of thermally-driven flows in these fluids, i.e. ‘natural’ fluid motion induced by gravity or surface-tension effects. Experimental struggles with understanding the basic mechanisms of these fundamental types of convection in viscoelastic fluids have been mitigated over recent years by the development of relevant models and adequate numerical techniques. On the one hand, this progress has produced new fundamental knowledge; on the other hand, it has led to new questions, both general and system-specific (a relevant example being represented by the intriguing concept of “elastic turbulence”). This field continues to burgeon and bring surprises to this day. A synthetic review of the salient outcomes of past and recent studies is reported in this chapter (articulated in focused subsections) together with a critical discussion of some accompanying necessary definitions and paradigms used to reveal the intimate essence of these liquids. For the convenience of the reader, however, advanced formal discussions and mathematical arguments are stripped down to their essentials. In the same spirit, the use of jargon is kept at minimum and much of the available space is devoted to the description (both qualitative and quantitative) of the considered convective phenomena, the related thresholds in terms of characteristic numbers and the underlying “physics”

On the role of thermal boundary conditions in typical problems of buoyancy convection : a combined experimental-numerical analysis

Buoyancy flows of thermal origin and related heat transfer problems are central in a variety of disciplines and technological applications. In the present study the classical case of a cavity heated from one side and cooled from the other (internal size 4 cm, filled with water) is tackled both experimentally and numerically for different circumstances (horizontal and inclined temperature gradients). The main objective is to fill a gap, namely, the surprising lack of knowledge relating to the role played by the effective heat loss taking place through the walls delimiting the fluid domain from above and from below and along the spanwise direction in influencing the instabilities of these flows and their progression towards chaos. We explore the response of such systems with respect to several parameters, including the inclination of the cavity with respect to gravity, the average temperature of the fluid, the applied temperature difference, the dependence of fluid properties on temperature and the intensity of heat transfer to the ambient. Experiments are supported by dedicated numerical simulations based on the Navier Stokes and energy equations in their time-dependent and non-linear formulation (solved by means of the PISO method with a collocated-grid approach). It is shown that a kaleidoscope of states is possible depending on the considered conditions. The results reveal the counter-intuitive triadic relationship among heat loss through non-thermally active walls, the hierarchy of bifurcations displayed by the system and the prevailing two-dimensional or three-dimensional nature of the flow

On the competition of transverse and longitudinal modes of Marangoni convection in a three-dimensional layer of viscoelastic fluid

Within the vast array of applications encompassed by viscoelastic fluids, some lack of knowledge seems to affect the non-linear behavior of Marangoni convection when its typical initial unicellular and steady state is taken over by more complex flow configurations. These still hide a not-fully-understood competition of complex and diverse physical mechanisms that determine the prevailing macroscopic behavior. In the present study, relevant insights are sought from consideration of the classical differentially heated rectangular layer of liquid with adiabatic bottom and top free surface. It is shown that, for increasing values of the Marangoni number and/or the elasticity parameter, this problem offers a multifaceted spectrum of different outcomes depending on the non-trivial interplay established between two distinct categories of disturbances (transverse and longitudinal). These are studied using a diversity of model types in which some processes are on or off to discern selectively their effect in the laminar state and their contribution to the evolution of the system towards chaos. The characteristic marks by which the ensuing elastic turbulence can be distinguished from the companion Kolmogorov counterpart are highlighted through analysis of the emerging scaling laws in the velocity spectrum and the sensitivity of these to the intensity of the driving force and the considered elasticity level. It is shown that these two forms of turbulence can coexist in the considered problem

On the nature, formation and diversity of particulate coherent structures in microgravity conditions and their relevance to materials science and problems of astrophysical interest

Different phenomena related to the spontaneous accumulation of solid particles dispersed in a fluid medium in microgravity conditions are discussed, with an emphasis on recent discoveries and potential links with the general field of astrophysical fluid-dynamics on the one hand, and with terrestrial applications in the field of materials science on the other hand. With special attention to the typical physical forces at play in such an environment, namely, surface-tension gradients, oscillatory residual gravity components, inertial disturbances and forces of an electrostatic nature, specific experimental and numerical examples are presented to provide inputs for an increased understanding of the underlying cause-and-effect relationships. Studying these systems can be seen as a matter of understanding how macroscopic scenarios arise from the cooperative behaviour of sub-parts or competing mechanisms (nonlinearities and interdependencies on various spatial and temporal scales). Through a critical assessment of the properties displayed by the resulting structures (which appear in the form of one-dimensional circuits formed by aligned particles, planar accumulation surfaces, three-dimensional compact structures resembling “quadrics”, micro-crystallites or fractal aggregates), we discuss a possible classification of the related particle attractors in the space of parameters according to the prevailing effect

Irreversibilities in a triple diffusive flow in various porous cavities

Entropy generation minimization approach is a very good method allowing to analyze the engineering systems to exclude technical failure. The present study deals with computational analysis of triple diffusive flow, energy transference and entropy production in different porous cavities from square to triangular through trapezoidal shape. The formulated boundary-value problem has been worked out using the finite element technique and non-primitive variables. The developed computational code has been verified using numerical results of other researchers. Analysis of entropy production due to energy and mass transport, motion friction, and porous material has been performed for different chamber's shapes. Entropy generation analysis in chambers of various geometries under the triple-diffusive flow is a novelty of the present research, where different entropy production mechanisms have been scrutinized for one complex problem. It has been ascertained that average total entropy generation strength raises with buoyancy ratios, Lewis and Rayleigh numbers, but it has the minimum value for the square chamber in comparison with triangular and trapezoidal shapes. Moreover, obtained results characterize a neglecting influence of motion friction on the total entropy generation

Flow topology and bifurcations of buoyancy and mixed convection in an elongated channel with an abrupt section variation

A forward-facing step (FFS) located half-way in an elongated duct subjected to a horizontal pressure gradient and a vertical temperature difference is considered as a simplified geometrical model to investigate numerically typical problems of internal non-isothermal flow in the presence of blunt obstacles. The sensitivity of this system to thermal buoyancy for each considered rate of fluid injection (measured through the related Richardson number, Ri) is explored by varying parametrically the corresponding Rayleigh number (Ra) over a large interval of orders of magnitude (up to the onset of chaos) and assuming two alternate paradigms for the bottom of the considered channel, namely an adiabatic or kept-at-constant temperature (hot) boundary. Through this conceptual framework, a kaleidoscope of situations are revealed in the (Ri, Ra) space, differing in terms of flow patterning behavior, thermal plume generation phenomena, intensity of heat exchange at the walls and bifurcation scenario. In particular, while for the isothermal floor case a higher Ri leads to an increase in the value of the Rayleigh number needed for transition to time-dependent flow, the corresponding trend becomes non-monotonic if the same boundary is thermally insulated. In such a case the Nusselt number (Nu) calculated for the horizontal surface of the step is always smaller than the equivalent Nu evaluated for the vertical side. The latter is significantly lowered when the hot-floor condition is assumed

Microgravity science and applications bibliography, 1989 revision

This edition of the Microgravity Science and Applications (MSA) Bibliography is a compilation of government reports, contractor reports, conference proceedings, and journal articles dealing with flight experiments utilizing a low gravity environment to elucidate and control various processes, or with ground based activities that provide supported research. It encompasses literature published but not cited in the 1988 Revision and that literature which has been published in the past year. Subdivisions of the Bibliography include: electronic materials, metals, alloys, and composites; fluids, interfaces, and transport; glasses and ceramics; biotechnology; combustion science; experimental technology, facilities, and instrumentation. Also included are publications from the European, Soviet, and Japanese programs