Humberto Chaves

Dynamic processes occurring during the spreading of thin liquid films produced by drop impact on hot walls

Abstract When liquid drops impinge on a wall heated up above the boiling temperature of the liquid, bubbling and convection are observed in the emerging film which spreads along the wall. Experimental investigations of these processes and of the connected evaporation of the liquid are aimed at an improved understanding of the underlying mechanisms important for many technical applications. Typical Weber numbers of the impinging drops were varied between 50 and 700, the temperature of the polished aluminium wall between 70°C and 400°C, i.e., above the boiling temperature of ethanol used in the experiments. The growth rate of bubbles, their maximum diameter and their collapse as well as the formation and destruction of convection cells in the liquid film were recorded and analysed.

Visualizing Flow Partitioning in a Model of the Upper Human Lung Airways

The convective transport of fluid within the human upper airways is investigated in a transparent model of the tracheobronchial tree. Oscillatory flow through the branching network with six generations was studied at varying Reynolds numbers between 400 and 2600 and Womersley numbers from 5.5 to 12.3 in the trachea representing clinical conditions during high frequency oscillatory ventilation. The flow partitioning within the model was visualized using advection of neutrally buoyant tracer particles, which were illuminated by short light pulses and recorded by a high speed camera. Integration of the particle locations for a large number of cycles provides the probability distribution of particles passing certain branches within the bifurcating network, and thus, the dispersion of particles in the airways. The results show the different characteristics of flow partitioning at varying Womersley and Reynolds numbers.

3D Flow in the Venom Channel of a Spitting Cobra: Do the Ridges in the Fangs Act as Fluid Guide Vanes?

The spitting cobra Naja pallida can eject its venom towards an offender from a distance of up to two meters. The aim of this study was to understand the mechanisms responsible for the relatively large distance covered by the venom jet although the venom channel is only of micro-scale. Therefore, we analysed factors that influence secondary flow and pressure drop in the venom channel, which include the physical-chemical properties of venom liquid and the morphology of the venom channel. The cobra venom showed shear-reducing properties and the venom channel had paired ridges that span from the last third of the channel to its distal end, terminating laterally and in close proximity to the discharge orifice. To analyze the functional significance of these ridges we generated a numerical and an experimental model of the venom channel. Computational fluid dynamics (CFD) and Particle-Image Velocimetry (PIV) revealed that the paired interior ridges shape the flow structure upstream of the sharp 90° bend at the distal end. The occurrence of secondary flow structures resembling Dean-type vortical structures in the venom channel can be observed, which induce additional pressure loss. Comparing a venom channel featuring ridges with an identical channel featuring no ridges, one can observe a reduction of pressure loss of about 30%. Therefore it is concluded that the function of the ridges is similar to guide vanes used by engineers to reduce pressure loss in curved flow channels.

Channelling optics for high quality imaging of sensory hair.

A long distance microscope (LDM) is extended by a lens and aperture array. This newly formed channelling LDM is superior in high quality, high-speed imaging of large field of views (FOV). It allows imaging the same FOV like a conventional LDM, but at improved magnification. The optical design is evaluated by calculations with the ray tracing code ZEMAX. High-speed imaging of a 2 × 2 mm(2) FOV is realized at 3.000 frames per second and 1 μm per pixel image resolution. In combination with flow sensitive hair the optics forms a wall shear stress sensor. The optics images the direct vicinity of twenty-one flow sensitive hair distributed in a quadratic array. The hair consists of identical micro-pillars that are 20 μm in diameter, 390 μm in length and made from polydimethylsiloxane (PDMS). Sensor validation is conducted in the transition region of a wall jet in air. The wall shear stress is calculated from optically measured micro-pillar tip deflections. 2D wall shear stress distributions are obtained with currently highest spatiotemporal resolution. The footprint of coherent vortical structures far away from the wall is recovered in the Fourier spectrum of wall shear stress fluctuations. High energetic patterns of 2D wall shear stress distributions are identified by proper orthogonal decomposition (POD).

Investigation of the Gas‐Liquid Flow in a Stopper‐Rod Controlled SEN

The behaviour of two-phase gas-liquid flows in a stopper-rod controlled submerged entry nozzle (SEN) is investigated in water model experiments. The observed two-phase flow patterns can be classified into either bubble coring or bubbly slug. The scaling of the two-phase flows by means of similarity parameters is discussed. In the experiments, it is found that the liquid flow rates depend strongly on the two-phase flow patterns. Additionally, the influence of swirl on the flow patterns is investigated in detail. It is shown that swirl has a marked impact on the transition from bubble coring to bubbly slug. Finally, an estimation of the two-phase argon-steel flow patterns in industrialscale SEN flows is given.

Transport at Air-Liquid Bridges under High-Frequency Ventilation

In diseased lungs airway closure can occur due to the formation of liquid bridges. These can be caused e.g. by surface tension-driven instabilities. The airway closure leads to a blockage of gas exchange in the deeper part of the lung which in severe cases requires to apply mechanical ventilation and recruitment maneuvers. High-frequency ventilation is refered therein as a proper way to enhance mass transport and keep the lung open. The present paper discusses the transport near the air-liquid interface under oscillatory excitation. A rigid tube model partially filled with liquid representing the airway blockage is used. An oscillatory flow with varying frequencies and amplitudes is applied with the aim to investigate the conditions for liquid break up and drop formation at the interface. It was found in high-frequency oscillation that near the interface a convective mass transport is generated due to secondary streaming. Above a critical value of excitation amplitudes for constant frequencies, the interface becomes unstable and drop formation starts. It can be assumed that despite the physical blockage effect in the presence of liquid bridges, high-frequency ventilation induces enhanced mass exchange across the interface and may help to break-up the liquid bridges.