RESUMO
We analyze the mixing properties of a floating stirrer driven electromagnetically in a thin layer of electrolyte, consisting of two free-floating magnets with opposite polarities connected by a rigid coupling. The magnetic rotor is set in circular motion using Lorentz forces created due to the interaction of the magnetic field of the rotor with dc currents actuated in logic sequence. We identify a coherent structure similar to a tripolar vortex whose central vortex rotates in the same direction of the rotor promoting chaotic mixing of the fluid in the laminar regime (Re=45). Dyed water visualization and particle image velocimetry were performed to characterize experimentally the mixing and flow dynamics at the surface of the electrolyte layer. A quasitwo-dimensional numerical simulation based on the immersed boundary method, which incorporates the fluid-solid interaction and reproduces the experimental observations satisfactorily, was carried out. Optimal mixing conditions are determined through the exponential growth of the material interfaces, which are established mainly by varying the distance separating the magnets of the rotor.
RESUMO
This paper presents a numerical analysis of the transient heat transfer problem arising when a functionally graded material is subjected to a fixed temperature difference. Varying the gradation of the system, the thermal performance of the material is assessed both in time-dependent and steady-state conditions by means of temperature profiles and entropy production. One of the main contributions of this paper is the analysis of the system in the transient, from which it is found that the entropy production has a non-monotonic behaviour since maximum and minimum values of this physical quantity could be identified by varying the grading profile of the material. The latter allows to propose an optimization criterion for functionally graded materials which consists of the identification of spatial regions where temperature gradients are large and find thermal conductivity profiles that attenuate those gradients, thus reducing the thermal stresses present inside the material.