Torres Díaz, Isaac G.

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    Impacts of low land use on a tropical montane cloud forest under a changing coastal climate
    (2007) Torres Díaz, Isaac G.; Raj Pandya, R. Vikram; College of Engineering; Ramírez, Nazario; Venkataraman, Nellore S.; Department of Mechanical Engineering; Harmsen, Eric
    Tropical Montane Cloud Forests (TMCF) are a primary source of fresh water in tropical locations and are highly sensitive to climate changes. Climatic analysis for El Yunque Rain Forest, a TMCF, located in North Eastern of the Island of Puerto Rico 40km south-east of the coastal city of San Juan, reflects changes in the regional meteorology reflected by increasing surface air temperatures in certain regions within this forest. This TMCF is the main water resource of San Juan, one of the largest cities in the Caribbean, and it is surrounded by increasing urban sprawl in the lower elevations; these changes in the low lands have affected atmospheric and surface variables such as sensible heat flux, surface albedo and surface roughness, and the overall energy budget within the forest. It is also hypothesized that increases in sea surface temperatures are also influencing the climate of the forest. The focus of the present research is to quantify the impacts of changes in land use close to coastal TMCFs, characterized by the case of El Yunque in the north-eastern coast of Puerto Rico during the dry season. A climatological and numerical analysis is presented to characterize these land use processes under a changing coastal climate. The research makes use of a high resolution visible imagery from the NASA ATLAS sensor to characterize the current land use conditions of the area. Surface parameters such as albedo and land classes were introduced into a Mesoscale Model RAMS (Regional Atmospheric Modeling System). The atmospheric model was calibrated favorably against a high density network of surface temperature sensors located in and around the TMCF. The coupled and decoupled effects of land use changes and global warming (GW represented by SSTs) are investigated in detail by organizing an ensemble of simulation runs that include reconstructed past land-use, present land use, reconstructed average past atmospheric variables and present climate conditions. Results indicate significant impacts due to GW effects with increasing SSTs, which are highly reflected on a TMCF environment, generating variations in its sensitive variables such as cloud base, cloud cover, air temperature and precipitation. Contrary, Land use changes generate local affects, which are less pronounced than GW, generating variations in physical variables close to the land. The variables that most influence the TMCF fresh water productivity are the GW effects.
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    Ferrofluid flow under the influence of rotating magnetic fields
    (2012-05) Torres Díaz, Isaac G.; Rinaldi, Carlos; College of Engineering; Acevedo, Aldo; Córdoba, Ubaldo; Venkataraman, Nellore S.; Department of Chemical Engineering; Souto, Fernando
    This work was centered in the understanding both the phenomenological experimental behavior and theoretical description of the flow of a ferrofluid under a rotating magnetic field. The ferrofluid flow under a uniform rotating magnetic field was first reported by Moskowitz and Rosensweig in 1967. As a first contibution, a bi-dimensional axisymmetric analysis of the ferrofluid flow in an annular/cylindrical geometry was considered that takes into account the non-uniformities in the rotating magnetic field produced by non-idealities in the multipole stator winding, where the spin-up flow that was previously analyzed results as a special case for two pole stator winding, where it was proved that the non-uniformities in the magnetic field cannot produce any ferrofluid flow without the presence of the couple stress and a non negligible value of the spin viscosity. Furthermore, in other contribution we consider a systematic analysis of the ferrofluid flow in a spherical cavity under the influence of a uniform rotating magnetic field generated by coupled spherical coils, commonly called fluxball. Results show qualitative agreement between the spin diffusion theory and the velocity measurements obtained by the ultrasound velocity profile technique. The main contribution in the thesis is to provide experimental measurements using a dilute ferrofluid with magnetically blocket nanoparticles under the influence of a uniform rotating magnetic field generated for a two pole stator winding. These measurements in cylindrical and annular geometries have good quantitative agreement with the spin diffusion theory, and provides experimental evidence of a non-negligible value of the spin viscosity that results 3.65 × 10−8 kg m s−1 for water based ferrofluid WBF-1 and 6.85 × 10−8 kg m s−1 for WBF-2 ferrofluid, which are 2 orders of magnitude larger than the predicted value for Chaves et al. (2008) for a magnetite commercial ferrofluid EMG900, and 13 orders of magnitude larger than the predicted value from dimensional analysis by Feng et al. (2006). These experiments show the importance in these ferrofluid flows of the couple stress and the spin viscosity for their analytical description. However, torque measurements in the cylindrical geometry are overpredicted, in one order of magnitude, by the spin diffusion theory. Further velocity measurements and torque experiments for dilute and semidilute ferrofluids are presented for a wide range of Langevin parameter and dimensionless time, which provides information about this phenomenon for future analysis.