Computational analysis of airflow dynamics for predicting collapsible sites in the upper airways: a preliminary study

The present study aimed to detail the relationship between the flow and structure characteristics of the upper airways and airway collapsibility in obstructive sleep apnea. Using a computational approach, we performed simulations of the flow and structure of the upper airways in two patients having different facial morphologies: retruding and protruding jaws, respectively. First, transient flow simulation was performed using a prescribed volume flow rate to observe flow characteristics within upper airways with an unsteady effect. In the retruding jaw, the maximum magnitude of velocity and pressure drop with velocity shear and vortical motion was observed at the oropharyngeal level. In contrast, in the protruding jaw, the overall magnitude of velocity and pressure was relatively small. To identify the cause of the pressure drop in the retruding jaw, pressure gradient components induced by flow were examined. Of note, vortical motion was highly associated with pressure drop. Structure simulation was performed to observe the deformation and collapsibility of soft tissue around the upper airways using the surface pressure obtained from the flow simulation. At peak flow rate, the soft tissue of the retruding jaw was highly expanded, and a collapse was observed at the oropharyngeal and epiglottis levels. NEW & NOTEWORTHY Aerodynamic characteristics have been reported to correlate with airway occlusion. However, a detailed mechanism of the phenomenon within the upper airways and its impact on airway collapsibility remain poorly understood. This study provides in silico results for aerodynamic characteristics, such as vortical structure, pressure drop, and exact location of the obstruction using a computational approach. Large deformation of soft tissue was observed in the retruding jaw, suggesting that it is responsible for obstructive sleep apnea.

High-speed visualization of vortical cavitation using synchrotron radiation

High-speed X-ray phase-contrast imaging of the cavitating flow developing within an axisymmetric throttle orifice has been conducted using high-flux synchrotron radiation. A white X-ray beam with energy of 6 keV was utilized to visualize the highly turbulent flow at 67 890 frames per second with an exposure time of 347 ns. The working medium employed was commercial diesel fuel at flow conditions characterized by Reynolds and cavitation numbers in the range of 18 000–35 500 and 1.6–7.7, respectively. Appropriate post-processing of the obtained side-view radiographs enabled the detailed illustration of the interface topology of the arising vortical cavity. In addition, the visualization temporal and spatial resolution allowed the correlation of the prevailing flow conditions to the periodicity of cavitation onset and collapse, to the magnitude of the underlying vortical motion, as well as to the local turbulence intensity.

Flow conditions in the grooves of a Low-Consistency refiner

The flow pattern in the grooves plays a major role for the homogeneity of refining as well as for the transfer and loading of fiber flocs in refining position on the bar edges. However, it is an area where very little information is available. In the present study, flow conditions in the grooves in a Low-Consistency (LC) - disc refiner were studied both experimentally and numerically. The experimental study involved high-speed imaging through a 3 cm peephole into a commercial refiner. The Computational Fluid Dynamics (CFD) simulation focused on the flow condition in a radial groove, considering both Newtonian and non-Newtonian flows. Flow conditions for stator and rotor grooves were modeled along the groove at different angular speeds and pressure differences over the refiner. Both the experimental and the modeling results show a dual flow pattern in the grooves; a rotational/spiral movement at the top of the groove and a flow in the direction of the groove at the bottom, which to the authors knowledge has not been reported in literature. The strong vortical motion at the top of the grooves observed both for the rotor and the stator are believed to be important for placing the fibers onto the bar edges and to induce shear forces in such a way that the fibers get treated. Moreover, a large sensitivity to suspension properties in terms of the development of flow pattern was detected.

Simulating SPH Fluid with Multi-Level Vorticity

The vortical motion of fluid in various scales is one of the most important elements in capturing the vivid realism of turbulent water. However, it is still challenging to resolve mul-ti-level vorticity effectively. This paper presents a novel hybrid method that combines a smoothed particle hydrodynamics (SPH) system with multiple Eulerian grids to reproduce the multi-level vorticity. In our hybrid method, the SPH system is responsible for resolving flow velocity while the multiple grids support the SPH system in efficiently detecting and computing the multi-level vor-ticity field.

Negatively buoyant projectiles – from weak fountains to heavy vortices

An experimental investigation to establish the maximum rise height zm attained by a finite volume of fluid forced impulsively vertically upwards against its buoyancy into quiescent surroundings of uniform density is described. In the absence of a density contrast, the release propagates as a vortex ring and the vertical trajectory is limited by viscous effects. On increasing the source density of the release, gravitational effects limit the trajectory and a maximum rise height zm is reached. For these negatively buoyant releases, the dependence of zm on the length L of the column of ejected fluid, nozzle diameter D (= 2r0), dispensing time and source reduced gravity is determined by injecting saline solution into a fresh-water environment. For 3.4 ≲ L/D ≲ 9.0, zm/r0 is shown to scale on the source parameter η = Fr(L/D), a product of the source Froude number Fr and the aspect ratio L/D for the finite-volume release. Our results show that the morphology of the cap that develops above the source and the vortical motion induced within are sensitively dependent on the source conditions. Moreover, three rise-height regimes are identified: ‘weak-fountain-transition’, ‘vorticity-development’ and ‘forced-release’ regimes, each with a distinct morphology and dependence of dimensionless rise height on η.

Laminar flow around corners triggers the formation of biofilm streamers

Bacterial biofilms have an enormous impact on medicine, industry and ecology. These microbial communities are generally considered to adhere to surfaces or interfaces. Nevertheless, suspended filamentous biofilms, or streamers, are frequently observed in natural ecosystems where they play crucial roles by enhancing transport of nutrients and retention of suspended particles. Recent studies in streamside flumes and laboratory flow cells have hypothesized a link with a turbulent flow environment. However, the coupling between the hydrodynamics and complex biofilm structures remains poorly understood. Here, we report the formation of biofilm streamers suspended in the middle plane of curved microchannels under conditions of laminar flow. Experiments with different mutant strains allow us to identify a link between the accumulation of extracellular matrix and the development of these structures. Numerical simulations of the flow in curved channels highlight the presence of a secondary vortical motion in the proximity of the corners, which suggests an underlying hydrodynamic mechanism responsible for the formation of the streamers. Our findings should be relevant to the design of all liquid-carrying systems where biofilms are potentially present and provide new insights on the origins of microbial streamers in natural and industrial environments.