Visualization of low Reynolds boundary-driven cavity flows in thin liquid shells

Classic examples of low-Reynolds recirculating cavity flows are typically generated from lid-driven boundary motion at a solid-fluid interface, or alternatively may result from shear flow over cavity openings. Here, we are interested in an original family of boundary-driven cavity flows occurring, in contrast to classic setups, at fluid-fluid interfaces. Particle image velocimetry (PIV) is used to investigate the structure of internal convective flows observed in thin liquid shells. Under the specific configuration investigated, the soap bubble's liquid shell is in fact in motion and exhibits sporadic local "bursts”. These bursts induce transient flow motion within the cavity of order Re ∼ O(1). The combination of PIV and proper orthogonal decomposition (POD) is used to extract dominant flow structures present within bubble cavities. Next, we show that thermally induced Marangoni flows in the liquid shell can lead to forced, (quasi) steady-state, internal recirculating flows. The present findings illustrate a novel example of low-Reynolds boundary-driven cavity flows. Graphical Abstrac

Non-contact boundary layer profiler using low-coherence self-referencing velocimetry

A spatially self-referencing velocimetry system based on low-coherence interferometry has been developed. The measurement technique is contactless and relies on the interference between back-reflected light from an arbitrary reference surface and seeding particles in the flow. The measurement location and the flow velocity are measured relative to the reference surface's location and velocity, respectively. Scanning of the measurement location along the beam direction does not require mechanical movement of the sensor head. The reference surface (which can move or vibrate relative to the sensor head) can be either an external object or the surface of a body over which measurements are to be performed. The absolute spatial accuracy and the spatial resolution only depend on the coherence length of the light source (tens of microns for a superluminescent diode). The prototype is an all-fiber assembly. An optical fiber of arbitrary length connects the self-contained optical and electronics setup to the sensor head. Proof-of-principle measurements in water (Taylor-Couette flow) and in air (Blasius boundary layer) are reported in this pape

Time-Resolved Micro PIV in the Pivoting Area of the Triflo Mechanical Heart Valve

The Lapeyre-Triflo FURTIVA valve aims at combining the favorable hemodynamics of bioprosthetic heart valves with the durability of mechanical heart valves (MHVs). The pivoting region of MHVs is hemodynamically of special interest as it may be a region of high shear stresses, combined with areas of flow stagnation. Here, platelets can be activated and may form a thrombus which in the most severe case can compromise leaflet mobility. In this study we set up an experiment to replicate the pulsatile flow in the aortic root and to study the flow in the pivoting region under physiological hemodynamic conditions (CO = 4.5 L/min / CO = 3.0 L/min, f = 60 BPM). It was found that the flow velocity in the pivoting region could reach values close to that of the bulk flow during systole. At the onset of diastole the three valve leaflets closed in a very synchronous manner within an average closing time of 55 ms which is much slower than what has been measured for traditional bileaflet MHVs. Hot spots for elevated viscous shear stresses were found at the flanges of the housing and the tips of the leaflet ears. Systolic VSS was maximal during mid-systole and reached levels of up to 40 Pa

Optimal subpixel interpolation in particle image velocimetry

ISSN:0723-4864ISSN:1432-111

Second Sound Scattering in Superfluid Helium

Focusing cavities are used to study the scattering of second sound in liquid helium II. The special geometries reduce wall interference effects and allow measurements in very small test volumes. In a first experiment, a double elliptical cavity is used to focus a second sound wave onto a small wire target. A thin film bolometer measures the side scattered wave component. The agreement with a theoretical estimate is reasonable, although some problems arise from the small measurement volume and associated alignment requirements. A second cavity is based on confocal parabolas, thus enabling the use of large planar sensors. A cylindrical heater produces again a focused second sound wave. Three sensors monitor the transmitted wave component as well as the side scatter in two different directions. The side looking sensors have very high sensitivities due to their large size and resistance. Specially developed cryogenic amplifiers are used to match them to the signal cables. In one case, a second auxiliary heater is used to set up a strong counterflow in the focal region. The second sound wave then scatters from the induced fluid disturbances. Attempts to observe scattering from quantized vortex lines in the rotating parabolic cavity ultimately did not succeed, although a theoretical estimate seems to indicate a basic feasibility.</p

Deep learning based instance segmentation of particle streaks and tufts

3D particle streak velocimetry (3D-PSV) and surface flow visualization using tufts both require the detection of curve segments, particle streaks or tufts, in images. We propose the use of deep learning based instance segmentation neural networks Mask region-based convolutional neural network (R-CNN) and Cascade Mask R-CNN, trained on fully synthetic data, to accurately identify, segment, and classify streaks and tufts. For 3D-PSV, we use the segmented masks and detected streak endpoints to volumetrically reconstruct flows even when the imaged streaks partly overlap or intersect. In addition, we use Mask R-CNN to segment images of tufts and classify the detected tufts according to their range of motion, thus automating the detection of regions of separated flow while at the same time providing accurate segmentation masks. Finally, we show a successful synthetic-to-real transfer by training only on synthetic data and successfully evaluating real data. The synthetic data generation is particularly suitable for the two presented applications, as the experimental images consist of simple geometric curves or a superposition of curves. Therefore, the proposed networks provide a general framework for instance detection, keypoint detection and classification that can be fine-tuned to the specific experimental application and imaging parameters using synthetic data.ISSN:0957-0233ISSN:1361-650

Global Doppler frequency shift detection with near-resonant interferometry

ISSN:0723-4864ISSN:1432-111

An Internet of Things Sensor Array for Spatially and Temporally Resolved Indoor Climate Measurements

The COVID-19 pandemic has emphasized the need for infection risk analysis and assessment of ventilation systems in indoor environments based on air quality criteria. In this context, simulations and direct measurements of CO2 concentrations as a proxy for exhaled air can help to shed light on potential aerosol pathways. While the former typically lack accurate boundary conditions as well as spatially and temporally resolved validation data, currently existing measurement systems often probe rooms in non-ideal, single locations. Addressing both of these issues, a large and flexible wireless array of 50 embedded sensor units is presented that provides indoor climate metrics with configurable spatial and temporal resolutions at a sensor response time of 20 s. Augmented by an anchorless self-localization capability, three-dimensional air quality maps are reconstructed up to a mean 3D Euclidean error of 0.21 m. Driven by resolution, ease of use, and fault tolerance requirements, the system has proven itself in day-to-day use at ETH Zurich, where topologically differing auditoria (at-grade, sloped) were investigated under real occupancy conditions. The corresponding results indicate significant spatial and temporal variations in the indoor climate rendering large sensor arrays essential for accurate room assessments. Even in well-ventilated auditoria, cleanout time constants exceeded 30 min.ISSN:1424-822

Flow in near-critical fluids induced by shock and expansion waves

ISSN:1432-2153ISSN:0938-128