VORTEX - Grupo de Mecânica dos Fluidos de Escoamentos Complexos

Mecânica de suspensões magnéticas diluídas (2009)

Autor: Hugo Leonnardo Gomides do Couto

Orientador: Francisco Ricardo Cunha

This work concerns with an investigation of the dynamics of dilute suspensions composed by magnetic particles, considering microstructural and macroscopic aspects of this particulate systems. From the microstructure, it is known that magnetostatic attraction may lead to formation of aggregates in magnetic suspensions. In this way, numerical simulations have been developed, based on the relative trajectories of magnetic particles in a dilute sedimenting suspension for computing the dimensionless rate at which aggregates are formed. The numerical results show that hydrodynamic interactions exhibit a strong influence in the process of magnetic particle aggregation. Furthermore, the collisions may result simply in a break of particle relative trajectory time-reversibility. After summing over all possible encounters, the transverse self-diffusion and down-gradient diffusion coefficients that describe the cross-flow migration of the particles are calculated. Within the same context, a general three-dimensional boundary integral formulation for magnetic free surface in viscous flows at low Reynolds numbers has been developed. Combining the reciprocal theorem for a magnetic fluid and the fundamental solution of a creeping flow we obtain the integral representation of the flow in terms of hydrodynamic and magnetic potentials. The proposed boundary integral equations has been developed in order to simulate the full time-dependent low Reynolds number distortion and orientation of a three-dimensional ferrofluid droplet under action of shearing motions and magnetic fields and, consequently, the rheology of magnetic emulsions. In a macroscopic point of view, a dilute magnetic suspension is said to be stable, i.e. particle-doublets do not evolve in time, being treated as homogeneous continuum material. Considering this assumption, a theoretical analysis is developed in order to investigate the thermoconvective motion of a magnetic fluid under an applied magnetic field in a narrow rectangular cavity. A scaling analysis of the momentum and energy equations have been performed, in order to relate the heat transfer taxes with the key parameters of the thermomagnetic phenomenon.


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