Scielo RSS <![CDATA[Latin American applied research]]> vol. 34 num. 4 lang. es <![CDATA[SciELO Logo]]> <![CDATA[Under the southern cross: Stories around Humboldt and Bonpland's trip to the New Continent]]> <![CDATA[Humboldt y Colón: A 200 años del retorno de Humboldt de América]]> <![CDATA[Minimum gas flow rate in a countercurrent isothermal gas stripper]]> It is presented an analytical expression for the minimum gas flowrate required for the design of an isothermal countercurrent gas stripper, when Henry's law (H > 1) applies and the gas and liquid streams are concentrated in the soluble gas of the binary gas mixture. This solution allows for a faster and more accurate result, for (GB)min, than the graphical procedure presently in use. <![CDATA[Natural frequencies of thin rectangular plates with partial intermediate supports]]> In the present study, a methodology to find natural frequencies with arbitrary precision of thin rectangular plates on linear intermediate supports and mixed boundary conditions is presented. This means that the edges are total or partially supported, clamped or free, or any combination of these. The layout, number and place of linear intermediate supports are arbitrary, which allows for the analysis of a wide range of cases that include intermediate supports of different kinds: simple and multiple, straight and curved, complete (the ends coincide with the plate edges) and partial (at least one of the ends is not coincident with the plate edges). In the case of curved linear supports, the curve can be open or closed. The generalized solution is obtained using the Whole Element Method. A continuous and a discrete model of equidistant points are studied both for intermediate supports and clamped edges. In all cases, both a systematic approach to the solution and the theoretical basis, which ensures the arbitrary precision of the results, should be emphasized. In order to illustrate the accuracy and efficiency of the method described, numerical results are presented for several problems and comparison is made with previously published results in some cases and in some others with the Finite Element Method. These numerical results may be of interest to design engineers and researchers who conduct vibration studies. <![CDATA[Preparation and characterization of porous blocks of synthetic hydroxyapatite]]> Porous structures were conformed by pressing and heating of hydroxyapatite powders. Samples were obtained in the form of blocks with 47 % porosity. An interval of pore sizes interconnected among 50-120 mm was obtained. The diametrile tensile stress of evaluated samples oscillated between 4 and 21 MPa, according to what has been reported for porous ceramics. The diffraction studies of x-ray of heated samples indicated an increase of hydroxyapatite crystallinity. The microstructure was studied by scanning electronic microscopy. Statistical techniques were employed to determine the certainty of the answer. <![CDATA[Cosserat continua-based micro plane modelling. Theory and numerical analysis]]> The well-known micro plane theory is extended to account for micro rotations and couple stresses in the framework of the micro polar Cosserat continua. The main purpose is to obtain reliable macroscopic constitutive equations and models for engineering materials like concrete and other composites based on available and precise information of their complex microstructure. The proposed macroscopic descriptions account also for anisotropic material response behavior by means of the well-developed micro plane concept applied within a micro polar continuum setting. For the formulation of the micro polar-based micro plane theory a thermodynamically consistent approach is considered, whereby the main assumption is the integral relation between the macroscopic and the microscopic free energy as advocated by Carol et al. (2001) and Kuhl et al. (2001). Thereby, the micro plane laws are chosen such that the macroscopic Clausius-Duhem inequality is fully satisfied. This theoretical framework is considered to derive both elastic and elastoplastic micro polar micro plane models. Numerical predictions of the uniaxial tensile and simple shear tests in plane strain conditions obtained with a micro polar micro plane elastoplastic model are also presented and contrasted to the corresponding predictions of the classical micro polar elastoplastic model. <![CDATA[Evaluation of some methods for lower bound determination in the transition region of ferritic steels]]> Fracture toughness characterization of ferritic steels in the ductile to brittle transition region is problematic due to the observed scatter. This scatter makes not possible to obtain a single toughness value, although some statistical methods presented in the literature allow to manage it and indeed to have a lower bound value. The ability of some of these proposals to give a technological lower bound value from experimental sets of data was studied. Data from literature and our research were used for this purpose. One hundred random combinations of reduced number of elements from each data set were taken, repeating this procedure for different number of elements. The lower bound value dependence on sample size and the minimum number of specimens needed for a technological lower bound value determination were studied. The SPRÖDZON Method seems to give the best estimation of a technological lower bound LB. <![CDATA[Control of pusher furnaces for steel slab reheating using a numerical model]]> Steel slabs are reheated in pusher-type furnaces up to a temperature of 1200ºC in the steel sheet manufacturing process. In this article we describe a control system that uses an on-line numerical model to calculate the furnace setpoints in order to improve the heating quality. Examples of actual furnace operation with and without the system are presented to show the improvements that are obtained handling typical non-stationary situations. <![CDATA[A new correlation for the specific heat of metals, metal oxides and metal fluorides as a function of temperature]]> The objective of this work is to find a suitable correlation that best fits the specific heat of metals, metal oxides and metal fluorides as a function of temperature. It was found that a multilinear regression model of the form C P = aTb e cTe d/T has the lowest deviation from experimental data compared to other correlations including a 4th to 6th-order polynomial regression model. The coefficient of determination, R², was very close to unity in most cases and the average of the absolute relative errors, AARE, was less than 5% for the specific heat of most of the systems studied. The overall AARE was about 1.8% for metals and 3% for metal oxides and metal fluorides, which is within the experimental error. <![CDATA[Frequency domain approach to Hopf bifurcation for van der Pol equation with distributed delay]]> The van der Pol equation with a distributed time delay and a strong kernel is analyzed. Its linear stability is investigated by employing the generalized Nyquist stability and Routh-Hurwitz criteria. Moreover, local asymptotic stability conditions are also derived in the case of the strong kernel. By using the mean time delay as a bifurcation parameter, the model is found to undergo a sequence of Hopf bifurcations. The direction and the stability criteria of the bifurcating periodic solutions are obtained by the graphical Hopf bifurcation theory. Some numerical simulation examples for justifying the theoretical analysis are also given. <![CDATA[Hydrodynamic analysis of the performance of stern flaps in a semi-displacement hull]]> The effects of stern flaps in the hydrodynamic performance of a semi-displacement hull were evaluated by systematic testing of scaled models in a towing tank. Chord length, span and flap angle were chosen as the main variable parameters to obtain six different stern flap configurations. The semi-displacement hull in conjunction with each stern flap was tested in order to assess the influence of each flap in resistance and running trim. The bare hull, i.e. without stern flap, was also tested to provide a suitable base for flap performance comparison. Main characteristics of the stern flaps were chosen according to existent recommendations provided in the relevant technical literature. The results showed good agreement with these recommendations; however, it was found that larger values of chord lengths and negative flap angles could possibly have a better performance on this craft. <![CDATA[Desorption of ethyl acetate from modified clays by supercritical carbon dioxide]]> The supercritical regeneration with carbon dioxide of modified and unmodified clays was experimentally studied after their use to adsorb ethyl acetate from aqueous solutions. Two quaternary amine modifiers (tetramethyl ammonium chlorine / TMA+ and hexadecyltrimethylammonium bromide/ HDTMA+) were used. The desorption of ethyl acetate adsorbed over the clays was performed with carbon dioxide at temperatures ranging from 301 K to 333 K and pressures ranging from 69.0 bar to 413.8 bar. The regeneration capacity was almost coincidental and high pressure was more favourable to regeneration. The effect of pressure and temperature was characterised under different conditions (gas, liquid and supercritical) and the supercritical has shown to be the best. A crossover effect was observed. The experimental data was fitted to a simple model, being the best results corresponding to desorption with carbon dioxide in its supercritical region.