51 research outputs found
A robust numerical strategy for finding surface waves in flows of non-Newtonian liquids
Gravity-driven flows of liquid films are frequent in nature and industry,
such as in landslides, lava flow, cooling of nuclear reactors, and coating
processes. In many of these cases, the liquid is non-Newtonian and has
particular characteristics. In this paper, we analyze numerically the temporal
stability of films of non-Newtonian liquids falling by gravity, on the onset of
instability. The liquid flows over an incline, where surface waves appear under
certain conditions, and we do not fix a priori its rheological behavior. For
that, we made used of the Carreau-Yasuda model without assigning specific
values to its constants, and we compute general stability solutions. The
numerical strategy is based on expansions of Chebyshev polynomials for
discretizing the Orr-Sommerfeld equation and boundary conditions, and a
Galerkin method for solving the generalized eigenvalue problem. In addition, an
Inverse Iteration method was implemented to increase accuracy and improve
computational time. The result is a robust and light numerical tool capable of
finding the critical conditions for different types of fluids, which we use to
analyze some key fluids. We show that the outputs of the general code match
previous solutions obtained for specific computations. Besides increasing our
knowledge on surface-wave instabilities in non-Newtonian liquids, our findings
provide a new tool for obtaining comprehensive solutions on the onset of
instability.Comment: This manuscript version is made available under the CC-BY-NC-ND 4.0
license http://creativecommons.org/licenses/by-nc-nd/4.0
Bidisperse micro fluidized beds: Effect of bed inclination on mixing
Micro fluidized beds are basically suspensions of solid particles by an
ascending fluid in a mm-scale tube, with applications in chemical and
pharmaceutical processes involving powders. Although in many applications beds
are polydisperse, previous works considered only monodisperse beds aligned in
the vertical direction. However, introducing an inclination with respect to
gravity leads to different bed patterns and mixing levels, which can be
beneficial for some applications. In this paper, we investigate experimentally
the behavior of micro gas-solid beds consisting of bidisperse mixtures under
different inclinations. In our experiments, mono and bidisperse beds are filmed
with a high-speed camera and the images are processed for obtaining
measurements at both the bed and grain scales. We show that the degree of
segregation is larger for vertical beds, but mixing varies non-monotonically
with inclination, with an optimal angle of 30--50 with
respect to gravity. By computing the mean and fluctuation velocities of grains,
we reveal that the mixing layer results from the competition between
segregation by kinetic sieving and circulation promoted by the fluid flow. We
also observe worse fluidization as the angle relative to gravity increases,
accounting then for the non-monotonic behavior. Our results bring new insights
into mixing and segregation in polydisperse beds, which can be explored for
processing powders in industry.Comment: This article appeared in Phys. Fluids 36, 013303 (2024) and may be
found at https://doi.org/10.1063/5.017915
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