Thermovibrationally-driven flows and particle accumulation in microgravity environments

In the present work, thermovibrationally-driven flows and ensuing particle accumulation phenomena are studied in the context of a microgravity environment. The problem is addressed numerically through solution of the governing equations for fluid flow and particle transport (Eulerian-Lagrangian one-way coupled approach). This work follows a logical approach with cases of increasing complexity being analysed as the discussion progresses, in particular, first the properties of this type of fluid flow are investigated in the single-phase (pure fluid) situation considering both two-dimensional (2D) and three-dimensional (3D) geometries. Then the multiphase problem, resulting from the addition of solid particles, is examined. The role played by the direction of the vibrations with respect to the temperature gradient is also investigated. Starting from the situation with concurrent vibrations and temperature difference (parallel case), it is shown that complexity of this situation essentially stems from the properties that are inherited from the corresponding case with steady gravity, i.e., the standard Rayleigh–B´enard convection. The need to overcome a threshold to induce convection from an initial quiescent state, together with the opposite tendency of acceleration to damp fluid motion when its sign is reversed, causes a variety of possible solutions that can display synchronous, non-synchronous, time-periodic, and multi-frequency responses. Moreover, as the constraint of two-dimensionality is removed, the intrinsically three-dimensional nature of the problem and its sensitivity to the thermal boundary conditions can have a remarkable influence on the multiplicity of emerging solutions and the system temporal response even if a geometry as simple as a cubic enclosure is considered. If solid particles are added to the fluid, the hallmark of the phenomena occurring in this case is an endless squeezing and expansion of particle formations along the direction of the temperature gradient. A kaleidoscope of previously unknown solutions is also reported for the situation with vibrations perpendicular to the temperature gradient (perpendicular case) giving emphasis to some still poorly known aspects such as the complex nature of the textural transitions undergone by the time-averaged flow as the Gershuni number is increased. Chaotic states are enabled when larger frequencies of vibration are considered. When particles are added in these cases, while clusters with a perfect (very regular and stationary) morphology emerge in laminar flow, when thermovibrational flow is chaotic the topology of the structures is relatively irregular and time-dependent. Nevertheless, precise trends and relationships can be established if specific problem ‘statistics’ are connected to the behaviour of the temporally evolving structures. Finally, cases are considered where the imposed temperature gradient is not unidirectional, i.e. the direction of such a gradient is allowed to change inside the fluid. The relationship between the multiplicity (N) of the loci of particle attraction and the inhomogeneities in the temperature field is studied. It is shown that N can exceed the limit N = 2 found in earlier studies and that a zoo of new particle accumulation structures show up, whose ranges of existence depend on the amplitude and frequency of vibrational acceleration, the particle Stokes number, the orientation of vibrations, and the number of inversions in the direction of the temperature gradient. Some experimental activities conducted to support the so-called PARTICLE VIBRATION microgravity experiment are also presented.In the present work, thermovibrationally-driven flows and ensuing particle accumulation phenomena are studied in the context of a microgravity environment. The problem is addressed numerically through solution of the governing equations for fluid flow and particle transport (Eulerian-Lagrangian one-way coupled approach). This work follows a logical approach with cases of increasing complexity being analysed as the discussion progresses, in particular, first the properties of this type of fluid flow are investigated in the single-phase (pure fluid) situation considering both two-dimensional (2D) and three-dimensional (3D) geometries. Then the multiphase problem, resulting from the addition of solid particles, is examined. The role played by the direction of the vibrations with respect to the temperature gradient is also investigated. Starting from the situation with concurrent vibrations and temperature difference (parallel case), it is shown that complexity of this situation essentially stems from the properties that are inherited from the corresponding case with steady gravity, i.e., the standard Rayleigh–B´enard convection. The need to overcome a threshold to induce convection from an initial quiescent state, together with the opposite tendency of acceleration to damp fluid motion when its sign is reversed, causes a variety of possible solutions that can display synchronous, non-synchronous, time-periodic, and multi-frequency responses. Moreover, as the constraint of two-dimensionality is removed, the intrinsically three-dimensional nature of the problem and its sensitivity to the thermal boundary conditions can have a remarkable influence on the multiplicity of emerging solutions and the system temporal response even if a geometry as simple as a cubic enclosure is considered. If solid particles are added to the fluid, the hallmark of the phenomena occurring in this case is an endless squeezing and expansion of particle formations along the direction of the temperature gradient. A kaleidoscope of previously unknown solutions is also reported for the situation with vibrations perpendicular to the temperature gradient (perpendicular case) giving emphasis to some still poorly known aspects such as the complex nature of the textural transitions undergone by the time-averaged flow as the Gershuni number is increased. Chaotic states are enabled when larger frequencies of vibration are considered. When particles are added in these cases, while clusters with a perfect (very regular and stationary) morphology emerge in laminar flow, when thermovibrational flow is chaotic the topology of the structures is relatively irregular and time-dependent. Nevertheless, precise trends and relationships can be established if specific problem ‘statistics’ are connected to the behaviour of the temporally evolving structures. Finally, cases are considered where the imposed temperature gradient is not unidirectional, i.e. the direction of such a gradient is allowed to change inside the fluid. The relationship between the multiplicity (N) of the loci of particle attraction and the inhomogeneities in the temperature field is studied. It is shown that N can exceed the limit N = 2 found in earlier studies and that a zoo of new particle accumulation structures show up, whose ranges of existence depend on the amplitude and frequency of vibrational acceleration, the particle Stokes number, the orientation of vibrations, and the number of inversions in the direction of the temperature gradient. Some experimental activities conducted to support the so-called PARTICLE VIBRATION microgravity experiment are also presented