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.

In vitro experimental investigation of voice production.

The process of human phonation involves a complex interaction between the physical domains of structural dynamics, fluid flow, and acoustic sound production and radiation. Given the high degree of nonlinearity of these processes, even small anatomical or physiological disturbances can significantly affect the voice signal. In the worst cases, patients can lose their voice and hence the normal mode of speech communication. To improve medical therapies and surgical techniques it is very important to understand better the physics of the human phonation process. Due to the limited experimental access to the human larynx, alternative strategies, including artificial vocal folds, have been developed. The following review gives an overview of experimental investigations of artificial vocal folds within the last 30 years. The models are sorted into three groups: static models, externally driven models, and self-oscillating models. The focus is on the different models of the human vocal folds and on the ways in which they have been applied.

Experimental flow study of modeled regular and irregular glottal closure types.

The present study shows the results of visualization experiments of the jet formation through a dynamic model of the human vocal folds. The model consists of two counter-rotating, 3D-shaped driven cams covered with a stretched silicone membrane. The 3D contours of the cams are a result of an optimized mapping of observed characteristic clinical vocal fold motions. The experiments are performed by using cams which produce the convex, triangular, rectangular, and concave or sand-glass regular glottis closure types. Two irregular cases are investigated: one of the convex cams is statically closed or opened. These cases cause an oscillating jet which attaches to the ventricular folds and appears to change the part of the aero-acoustic sound spectrum induced by vortices.

Three-dimensional nature of the glottal jet.

The factors contributing to human voice production are not yet fully understood. Even normal human phonation with a symmetric glottal opening area is still the subject of extensive investigation. Among others, it has already been shown that fluid dynamics has a strong influence on the vocal process. The full characterization of the glottal jet has not been accomplished yet. Time-resolved measurement and visualization of the three-dimensional (3D) flow downstream the human vocal folds are difficult if not impossible to perform in vivo. Therefore, it is common to use mechanical and numerical models with a simplified shape and motion profile of the vocal folds. In this article, further results regarding the 3D flow structure obtained in a 3:1 up-scaled dynamic glottis model (cam model) in a water circuit are given, extending earlier work [M. Triep et al. (2005). Exp. Fluids 39, 232-245]. The model mimics the temporal variation in the 3D contour of the glottal gap while water flow reduces the characteristic frequencies by the order of 1/140. The unsteady flow processes downstream of the vocal folds are visualized in slow motion and analyzed in detail via particle imaging techniques. The visualization results show complex 3D flow behavior of lengthwise jet contraction and axis switching. In addition, the time-dependent flow rate during the phonatory oscillation cycle is measured in detail. It is shown that the pressure loss is decreased in the presence of a second constriction downstream of the glottis in form of ventricular folds and it is observed that for this case the jet is stabilized in the divergent phase of the cycle.

Investigation of the washout effect in a magnetically driven axial blood pump.

For a long-term implementation of the magnetically driven CircuLite blood pump system, it is extremely important to be able to ensure a minimum washout flow in order to avoid dangerous stagnation regions in the gap between the impeller and the motor casing as well as near the pivot-axle area at the holes in the impeller's hub. In general, stagnation zones are prone to thrombus formation. Here, the optimal impeller/motor gap width will be determined and the washout flow for different working conditions will be quantitatively calculated. The driving force for this secondary flow is mainly the strong pressure difference between both ends of the gap. Computational fluid dynamics (CFD) and digital particle image velocimetry (DPIV) will be used for this analysis.

Computational fluid dynamics and digital particle image velocimetry study of the flow through an optimized micro-axial blood pump.

A detailed knowledge of the flow field in a blood pump is indispensable in order to increase the efficiency of the pump and to reduce the shear-induced hemolysis. Thus, three different impeller designs were developed and tested by means of computational fluid dynamics (CFD) and digital particle image velocimetry (DPIV). The results show a good agreement of CFD and DPIV data. An optimization of the impeller could be achieved by following the concept of turbulent drag reduction for the axisymmetric center body.