Bernardo Figueroa-Espinoza

Clustering in high Re monodispersed bubbly flows

Experiments were conducted to determine the amount of clustering that occurs in bubbly flows for which the liquid motion can be described, with a certain degree of accuracy, using potential flow theory. A Hele-Shaw-type channel was used in which bubble overlap was avoided. Direct video image analysis was performed to calculate bubbles properties and identify cluster formation. Despite the significant wall influence of this configuration, it was found that the bubbles do form aggregates with a statistical horizontal tendency. The flow structure was also analyzed using the radial probability distribution, giving indications that support the clustering hypothesis.

The effect of confinement on the motion of a single clean bubble

The effect of confining a gas bubble between two parallel walls was investigated for the inertia-dominated regime characterized by high Reynolds and low Weber numbers. Single bubble experiments were performed with non-polar liquids such that the bubble surface could be considered clean; hence, shear free. The drag coefficient was found to be the result of two main effects: the Reynolds number and the confinement. The total drag could be written as the product of the corresponding unconfined drag, which depended mainly on the Reynolds number, and a function F ( s )=1 + κ s 3 . The confinement parameter s was defined as the ratio of the bubble radius to the gap width. The value of the constant κ depended on the way in which the bubbles moved within the gap, which was found to be either in a rectilinear (κ≈8) or oscillatory trajectory (κ≈80). For Re Re > 70, oscillations were observed in the bubble trajectory. In all cases, the oscillation occurred in a zigzag manner. Near the transition the bubbles oscillated but did not reach the walls; for larger Reynolds numbers, the bubbles collided repeatedly with the walls as they ascended. The instability, which is different from the well-known unconfined path instability, resulted from the reversal of sign of the wall-induced lift force: for low Reynolds number, the walls have a stabilizing effect because of the repulsive nature of the lift force between the walls and the bubble, while for high Reynolds number the lift is attractive and trajectories become unstable. Considering a model for the lift force of a bubble moving near a wall, the conditions for the transition were identified. A reasonable agreement between the model and experiments was found.

On the wind power potential in the northwest of the Yucatan Peninsula in Mexico

Wind power density, vertical velocity profiles, and other wind characteristics were established using a 51 m meteorological mast located very close to the shoreline on the northwest of the Yucatan peninsula in the Gulf of Mexico. A comparative study of the wind power density was carried out using information obtained between September 2010 and September 2011. The wind speed probability density function was found to be bimodal due to sea-land breezes, a characteristic that becomes less evident as the vertical distance to the ground increases. The distinction between these two wind regimes was used to fit the Weibull-Weibull curve using a linear least-squares criterion in the parameters. In addition, numerical simulations from a mesoscale model are in close agreement with measurements above z = 50 m (z is the vertical distance to the ground). This result suggests that some mesoscale simulations may serve as a preliminary wind energy assessment tool in coastal zones with extended low-lying areas.

A note on the onset of recirculation in a 2D Couette flow over a wavy bottom

Laminar Couette flow over a fixed wavy surface was studied with direct numerical simulation in a 2D periodic numerical domain. The mesh was generated by a conformal transformation that sets horizontal flow at the top of the domain, where a constant velocity boundary condition is given. The bottom of the domain is a wavy sinusoidal surface of wave slope 2πa/λ. The combined effect of bottom shape, inertia, and viscosity was explored using different Reynolds numbers (Re) and two dimensionless parameters in terms of channel width h, wavelength λ, and the amplitude of the wavy bottom a. Even if the Reynolds number was large, the simulations were not perturbed so the regime was always laminar. However, a recirculation appeared at the vicinity of the trough. The horizontal location of the eddy center was reported as a function of 2πa/λ and Reλ/h. The conditions for the onset of this recirculation were studied and compared with results from the literature. Two regimes can be clearly identified from the numerical results; a viscous regime with a weak dependence between 2πa/λ and Reλ/h for small Reynolds numbers and an inertial regime with an exponential dependence between 2πa/λ and Reλ/h for large Reynolds numbers, which presents an approximate slope of −1/3. Almost all results collapse in one single curve that characterizes the phenomenon (with the exception of some points where the flow is confined due to a large λ/h ratio).

Analysis of a direct-drive wind turbine with Axial-Flux Permanent-Magnet Generator operating in close-shore conditions

Despite new efforts to improve wind turbine (WT) reliability are constantly made in every subsystem of the Megawatt large WTs, small wind turbines (SWT) lack of reliability and Conditioning Monitoring (CM), probably due to economic considerations. Here, an analysis of field test data for a self-made SWT with a direct drive, slot-less, concentrated winding, Axial Flux Permanent-Magnet Generator (AFPMG) is presented. A broken blade fault event is also analyzed and a simple condition monitoring algorithm using the dc bus signals is proposed. This analysis shows that the mechanical nature of the described fault makes a slow sampling rate algorithm suitable for CM systems for SWTs.

On the ecohydrology of the Yucatan Peninsula: Evapotranspiration and carbon intake dynamics across an eco-climatic gradient

Laboratorio de Ingeniería y Procesos Costeros, Instituto de Ingeniería, Universidad Nacional Autónoma de Mexico, Sisal, Mexico Department of Geological Sciences, University of Texas at El Paso, El Paso, Texas Departamento de Ciencias del Agua y Medio Ambiente, Instituto Tecnológico de Sonora, Obregon, Mexico Laboratorio Nacional de Resiliencia Costera (LANRESC), Sisal, Mexico Correspondence Bernardo Figueroa‐Espinoza, Laboratorio de Ingeniería y Procesos Costeros, Instituto de Ingeniería, Universidad Nacional Autónoma de México, Av. Colón No. 503‐F x Av. Reforma y 62, Col. Centro, Mérida, Yucatán, C.P. 97000, México. Email: bfigueroae@iingen.unam.mx

Agitation in a Liquid Fluidized Bed

The random motion of mono-dispersed particles in a liquid fluidized bed was measured and processed from video recordings, using a refractive index matching method. 3D trajectories of coloured particles have been collected in a wide range of solid fraction, and statistical quantities have been derived in the range of high particle Reynolds number (O(10) < Re p < O(103)) and intermediate Stokes number (O(1) < St < O(10)). The evolution of the particle velocity variance as a function of solid fraction has been determined for different concentrations.