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- Resumen es exacto " The present thesis analyses different aspects of the dynamics of particle suspensions subjected to vertical vibration. In the working conditions, the particles, heavier than the liquid in which they are dispersed (carrier fluid), form a bed at the bottom of the container (substrate). The layer of liquid over the bed of particles is also in contact with the side walls of the container and with air (free surface)
In order to study the interaction between the carrier liquid, the particles and the container, we use a progressive approach in terms of the complexity of the system under study, which first considers the case of a layer of pure liquid over a rough and permeable substrate that imitates a consolidated bed of particles, and then analyses the case of an unconsolidated bed, in which, in principle, the dynamics of the particles and the liquid are coupled. Visualisation and photometric techniques were used to determine the nature of the wavelike flow structures in the liquid layer, their correlation with the patterns observed in the particle bed, and the conditions associated with the initiation of movement and eventual resuspension of the particles.
In the first experimental model, metal meshes of different textures were used as substrates, which were used in two ways: unmodified (open), or sealed with an impermeable paint. The variation in the texture and the sealing of the meshes allowed the roughness and permeability of the substrate to be altered, studying their influence on the threshold acceleration r c, and on the wavelength Aw, of Faraday’s instability which is presented in the form of waves in the supernatant liquid layer. As a main result, it was found that r c is greater in the presence of a mesh than for a smooth substrate, notably for both open and sealed meshes. The results are compared with predictions from a linear theory for a smooth substrate. An effective thickness of the liquid layer was estimated, which depends on the characteristics of the meshes.
The other two experimental models sought to study the interaction between waves on the surface of the supernatant liquid and the unconsolidated bed of particles, with the technological interest associated with the liquid template technique. Both studied complementary aspects:
The second, using a cylindrical vessel and zenithal observation, made it possible to determine, the nature and characteristics of the waves in the supernatant liquid layer and their relationship with the patterns formed in the particle bed. In the experimental conditions used, the waves in the liquid are capillary waves, of a harmonic nature. Their characteristic wavelength, Aw, determined by the vibration frequency f , shows correlation with that of the particle bed, Ap, which appears as a succession of ridges and valleys in the radial direction, forming concentric rings. We analysed how the roughness of the bed depends on the number of Shields 0 , which quantifies the shear acting on the particle bed, finding a quadratic dependence, with a certain threshold value
The third model made it possible to observe, in cut view, of a narrow vessel (quasi-2D configuration). Above the critical acceleration r c, and depending on the excitation frequency f , Faraday waves appear in the liquid layer overlying the particle bed.
The dependence of r c on the vibration frequency f and on the thickness of the supernatant liquid layer Ah was studied. On the other hand, three regimes (a,b,c) of particle bed behaviour as Ah decreases were characterised, in which:
(a) Particles are at rest, forming a plane bed
(b) Particles mobilise due to their interaction with the waves on the surface of the liquid, generating a periodic roughness in the bed.
(c) Particles begin to resuspend.
The transition between these regimes depends on f , but not on the a radius of the particle. For regime (b), the length Aw and height hw of the waves in the liquid were correlated with the length Ap and the characteristic height of the particle bed A hp.
The obtained results provide elements to characterise the interaction between the particles, the flow structures in the carrier liquid and the vessel. Also, they constitute a contribution regarding the perspectives of technological applications, such as those associated with the liquid template technique.
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