3D-LIF Experiments in an Open Wet Clutch by means of Defocusing PTV

Defocusing particle tracking velocimetry (DPTV) is applied to sub-millimetre rotor-stator gap flows on a test rig of an open wet clutch model. A recently proposed in-situ calibration approach (Fuchs et al., 2016) is modified to account for both the rotating no-slip condition and the grooves of the clutch lamella. Fluorescent particles are used to suppress the reflection issues at the walls and the groove patterns. The results demonstrate that DPTV is capable to derive sufficiently accurate velocity information in the smooth Couette-like gap region. Moreover, the experiments uncover formerly unknown flow properties inside the lamella grooves. The results provide evidence that the application of DPTV is a promising means to achieve a deeper insight into cause-effect relations between the flow fields inside an open wet clutch and the resulting adverse drag-torque characteristics

Interferometric Particle Imaging for Particle Sizing in the Front-, Side-, and Back-Scatter Region

Interferometric particle imaging is a widely used optical measuring technique for the sizing of poly-dispersed spherical particles like droplets and bubbles. In its conventional approach, the method is limited to forward-scattering angles and therefore, requiring a second optical access, restricting the range of possible applications. In the present work, this limitation of the scattering angle is addressed, showing that also other scattering angles, especially in the back-scatter region are applicable, expanding the technique to applications with only a single optical access. A general method for the identification of suitable scattering angles both for droplets and bubbles is proposed. The visibility criterion for interference patterns from particles is generalized and possible glare point parings and their separation in the forward-, side- and back-scatter regimes are discussed for droplets and bubbles. Due to being the most popular examples, different scattering angles are proposed for water droplets and air bubbles in water. In the last part, the method is validated on a bubble sizing experiment.Comment: 16 pages, 10 figure

Strömungsmechanische Bewertung der Umströmung des Gesichtsschildes PlexyCap der Firma Hans Keim Kunststoffe GmbH

Die Erstellung dieses Berichts wurde von der Firma Hans Keim Kunststoffe GmbH in Auftrag gegeben, um das firmeneigene, CE-zertifizierte Produkt PlexyCap1,2 als Gesichtsschild hinsichtlich seiner strömungsmechanischen Wirksamkeit zur Minimierung der Tröpfchen- und Aerosolausbreitung vor dem Hintergrund einer Ansteckungsgefahr durch das Virus SARS-CoV-2 zu bewerten. Es sei ausdrücklich darauf hingewiesen, dass alle getroffenen Aussagen und abgeleiteten Schlussfolgerungen in diesem Dokument auf rein kontinuumsmechanischer Grundlage ohne jederart medizinischer und/oder virologischer Expertise basieren. Als Bewertungsgrundlage dienen Hochgeschwindigkeitsaufnahmen einer aerosolangereicherten Hustenwolke, die sowohl von außen aus verschiedenen Richtungen als auch von innen auf das Schild appliziert wurde. Das Bildmaterial wurde von der Firma iLA_5150 GmbH erzeugt und dem ISTM als Bewertungsgrundlage zur Verfügung gestellt

Laser-Optical Shear-Flow Analysis across the Annular Gap of a Simplified Displacement Compressor Model

The present experimental feasibility study testifies the two flow measurement techniques Defocusing Particle Tracking Velocimetry (DPTV) and Interferometric Particle Imaging (IPI) for their applicability to measure the two-phase flow of thin (sub-millimeter) annular rotor-stator gaps such as occur across for the leakage flow e.g. in the housing gap of oil-injected rotary positive displacement ompressors (RPDC). To provide unrestriced optical access to the annular gap and in turn eliminate secondary effects, a simplified displacement compressor model has been developed and fabricated from perspex. The proof-of-concept results of both experimental campaigns (DPTV & IPI) are discussed and avenues for future efforts towards a straight-forward and accurate applicability of either method are elaborated

Spatio-temporal reconstruction of drop impact dynamics by means of color-coded glare points and deep learning

The present work introduces a deep learning approach for the three-dimensional reconstruction of the spatio-temporal dynamics of the gas-liquid interface in two-phase flows on the basis of monocular images obtained via optical measurement techniques. The dynamics of liquid droplets impacting onto structured solid substrates are captured through high-speed imaging in an extended shadowgraphy setup with additional reflective glare points from lateral light sources that encode further three-dimensional information of the gas-liquid interface in the images. A neural network is learned for the physically correct reconstruction of the droplet dynamics on a labelled dataset generated by synthetic image rendering on the basis of gas-liquid interface shapes obtained from direct numerical simulation. The employment of synthetic image rendering allows for the efficient generation of training data and circumvents the introduction of errors resulting from the inherent discrepancy of the droplet shapes between experiment and simulation. The accurate reconstruction of the gas-liquid interface during droplet impingement on the basis of images obtained in the experiment demonstrates the practicality of the presented approach based on neural networks and synthetic training data generation. The introduction of glare points from lateral light sources in the experiments is shown to improve the reconstruction accuracy, which indicates that the neural network learns to leverage the additional three-dimensional information encoded in the images for a more accurate depth estimation. Furthermore, the physically reasonable reconstruction of unknown gas-liquid interface shapes indicates that the neural network learned a versatile model of the involved two-phase flow phenomena during droplet impingement

The influence of edge undulation on vortex formation for low-aspect-ratio propulsors

Experiments to study the effect of edge undulation on vortex formation have been conducted on impulsively accelerated plates. Abstractions of propulsors found in nature are produced by imprinting undulatory features with varying wavelengths onto the circumferential vortex-forming edge of circular plates. The effects of the small-scale disturbances introduced by these modifications are accessed by means of force measurements and time-resolved particle image velocimetry. Investigations of four different geometries at two different Reynolds numbers reveal an insensitivity of the flow towards length scales smaller than or similar to the thickness of the feeding shear layer. However, the instabilities in the shear layer and the coherence of the vortex wake are influenced when the wavelength of the undulation exceeds the shear-layer thickness by a significant margin. This results in a force augmentation due to enhanced entrainment into the turbulent vortex core, and thus an associated faster vortex growth rate. Yet, contrary to prior expectations, the time of vortex pinch-off remains constant for all edge modifications. The cause–effect relationship behind the stability of the vortex wake is further investigated. While for small edge undulations a turbulent transition of the vortex core results in vortex pinch-off, for larger edge undulations the turbulent vortex core is found to be fed constantly with additional circulation from the shear layer