Thomas Rösgen

Three-Dimensional Convective Alveolar Flow Induced by Rhythmic Breathing Motion of the Pulmonary Acinus

Low Reynolds number flows (Re<1) in the human pulmonary acinus are often difficult to assess due to the submillimeter dimensions and accessibility of the region. In the present computational study, we simulated three-dimensional alveolar flows in an alveolated duct at each generation of the pulmonary acinar tree using recent morphometric data. Rhythmic lung expansion and contraction motion was modeled using moving wall boundary conditions to simulate realistic sedentary tidal breathing. The resulting alveolar flow patterns are largely time independent and governed by the ratio of the alveolar to ductal flow rates, Qa/Qd. This ratio depends uniquely on geometrical configuration such that alveolar flow patterns may be entirely determined by the location of the alveoli along the acinar tree. Although flows within alveoli travel very slowly relative to those in acinar ducts, 0.021%<or=Ua/Ud<or=9.1%, they may exhibit complex patterns linked to the three-dimensional nature of the flow and confirm findings from earlier three-dimensional simulations. Such patterns are largely determined by the interplay between recirculation in the cavity induced by ductal shear flow over the alveolar opening and radial flows induced by wall displacement. Furthermore, alveolar flow patterns under rhythmic wall motion contrast sharply with results obtained in a rigid alveolus, further confirming the importance of including inherent wall motion to understand realistic acinar flow phenomena. The present findings may give further insight into the role of convective alveolar flows in determining aerosol kinematics and deposition in the pulmonary acinus.

Heat transfer variations of bicycle helmets

Abstract Bicycle helmets exhibit complex structures so as to combine impact protection with ventilation. A quantitative experimental measure of the state of the art and variations therein is a first step towards establishing principles of bicycle helmet ventilation. A thermal headform mounted in a climate-regulated wind tunnel was used to study the ventilation efficiency of 24 bicycle helmets at two wind speeds. Flow visualization in a water tunnel with a second headform demonstrated the flow patterns involved. The influence of design details such as channel length and vent placement was studied, as well as the impact of hair. Differences in heat transfer among the helmets of up to 30% (scalp) and 10% (face) were observed, with the nude headform showing the highest values. On occasion, a negative role of some vents for forced convection was demonstrated. A weak correlation was found between the projected vent cross-section and heat transfer variations when changing the head tilt angle. A simple analytical model is introduced that facilitates the understanding of forced convection phenomena. A weak correlation between exposed scalp area and heat transfer was deduced. Adding a wig reduces the heat transfer by approximately a factor of 8 in the scalp region and up to one-third for the rest of the head for a selection of the best ventilated helmets. The results suggest that there is significant optimization potential within the basic helmet structure represented in modern bicycle helmets.

Low-coherence interferometric tip-clearance probe

We propose an all-fiber, self-calibrating, economical probe that is capable of near-real-time, single-port, simultaneous blade-to-blade tip-clearance measurements with submillimeter accuracy (typically < 100 microm, absolute) in the first stages of a gas turbine. Our probe relies on the interference between backreflected light from the blade tips during the 1-micros blade passage time and a frequency-shifted reference with variable time delay, making use of a low-coherence light source. A single optical fiber of arbitrary length connects the self-contained optics and electronics to the turbine.

Optical Density Visualization and Abel Reconstruction of Vortex Rings Using Background-Oriented Schlieren

These figures represent a sequence of visualizations of CO2-loaded vortex rings generated at the orifice opening of a piston-cylinder apparatus (diameter of orifice opening is 7 cm; Re = 36’000; ratio of the piston stroke length to diameter is 0.5 and field of view is 15 x 20 cm). Qualitative Schlieren visualizations are obtained using a Background Oriented Schlieren (BOS) technique (Fig. 1) and a vector map of the gradients of the refractive index is extracted using a PIV algorithm (Fig. 2). The projected density field (Fig. 3) is then obtained by integrating the measured gradient field. Finally, an Abel inverse transform is implemented to reconstruct the true radial vortex ring profiles for enhanced visualization of flow structures such as the recirculating spiral roll-ups and trailing wakes (Fig. 4). Fig. 1. Schlieren visualization: absolute difference between phase and reference are displayed. Fig. 2. Vector map of the refractive index gradient (scale in arbitrary units).

Visualization of respiratory flows from 3D reconstructed alveolar airspaces using X-ray tomographic microscopy

A deeper knowledge of the three-dimensional (3D) structure of the pulmonary acinus has direct applications in studies on acinar fluid dynamics and aerosol kinematics. To date, however, acinar flow simulations have been often based on geometrical models inspired by morphometrical studies; limitations in the spatial resolution of lung imaging techniques have prevented the simulation of acinar flows using 3D reconstructions of such small structures. In the present study, we use high-resolution, synchrotron radiation-based X-ray tomographic microscopy (SRXTM) images of the pulmonary acinus of a mouse to reconstruct 3D alveolar airspaces and conduct computational fluid dynamic (CFD) simulations mimicking rhythmic breathing motion. Respiratory airflows and Lagrangian (massless) particle tracking are visualized in two examples of acinar geometries with varying size and complexity, representative of terminal sacculi including their alveoli. The present CFD simulations open the path towards future acinar flow and aerosol deposition studies in complete and anatomically realistic multi-generation acinar trees using reconstructed 3D SRXTM geometries.Graphical Abstract

In vitro model of a semicircular canal: Design and validation of the model and its use for the study of canalithiasis

We present an experimental model for a semicircular canal with canalithiasis. Canalithiasis is a pathological condition where free-floating particles disturb the flow field in the semicircular canals. It may lead to a specific form of vertigo known as BPPV or top-shelf vertigo. A careful scaling of the physical and geometrical parameters allows us to study the mechanics of this disease on an enlarged model of a single semicircular canal with laser vibrometry and video particle tracking. Early results confirm the proper operation of the model canal and support the current theories on the mechanisms of BPPV.

Development of a quantitative flow visualization tool for applications in industrial wind tunnels

The development of a measurement system to visualize, classify (based on topological features) and quantify complex flows in large scale wind tunnel experiments is described. A new approach is sought where the topological features of the flow, e.g. stream lines, separation and reattachment regions, stagnation points and vortex lines are extracted directly and preferably visualized in realtime in a virtual wind tunnel environment. The system is based on a three dimensional particle tracking method (3D-PTV) using a stereo arrangement of 2 CCD cameras. A frame rate of 120 frames/s allows measurements at high flow velocities. For the 3D-PTV method an approach is taken where the tracer particles are recorded such that consecutive frames form continuous path lines. The 3-dimensional positions and shapes of the particle path lines are reconstructed by means of the epipolar constraint and the stereo camera model. The particle path segments which contain both velocity and topological information are then analysed to extract the relevant information. Neutrally buoyant helium bubbles are used as tracer particles. Matching the density of the ambient air, the bubbles are ideal flow tracers in this respect.

Acoustic Streaming Visualization in Elastic Spherical Cavities

Flow visualizations are presented for acoustic streaming occurring inside spherical elastic cavities oscillating in an acoustic field. Streaming flows are visualized using Particle Image Velocimetry (PIV) and results are observed for a range of values of a dimensionless frequency parameter,M=120–306. Over the frequency range investigated, streaming flow fields remain steady at a given value ofM. The magnitude of the flows circulating inside the cavity remains small (<1 mm/s) and follows a non-linear dependency with respect to the acoustic power of the sound wave. The present boundary-driven cavity flows may enhance particle fluid transport mechanisms, leading ultimately to potential fluid mixing applications.

Sound wave channelling in near-critical sulfur hexafluoride (SF6)

Strong density and speed of sound gradients exist in fluids near their liquid-vapor critical point under gravity. The speed of sound has an increasingly sharp minimum and acoustic waves are channelled within a layer of fluid. Geometrical acoustic calculations are presented for different isothermal fluid columns of sulfur hexafluoride (SF6) under gravity using a semiempirical crossover equation of state. More than 40% of the emitted acoustic energy is channelled within a 20 mm high duct at 1 mK above the critical temperature. It is shown how, by changes in temperature, frequency, and gravitational strength, the governing length scales (wavelength, radius of ray curvature, and correlation length of the critical density fluctuations) can be varied. Near-critical fluids allow table-top sound channel experiments.

Fluid Flow Visualization by Three-dimensionally Reconstructed Tracer Path Lines

AbstactThe development of a measurement system for the visualization, topological classification and quantitative analysis of complex flows in large-scale wind tunnel experiments is described. A new approach is sought whereby the topological features of the flow, e.g. stream lines, separation and reattachment regions, stagnation points and vortex lines are extracted directly and are preferably visualized in real-time in a virtual wind tunnel environment. The system is based on a stereo arrangement of two CCD cameras. A frame rate of 120 f/s allows measurements at high flow velocities. Helium filled soap bubbles are used as tracer particles. The present paper describes a simple camera calibration procedure for large measurement environments and examines the problem of fast and accurate reconstruction of path lines in three dimensions, which will enable true three-dimensional and time-resolved fluid flow visualization. Experimentally obtained visualization results for a free-stream flow, flow around a circular plate and flow over a delta wing are presented.