Vortex Formation and Decay: The Scaling of Vortex-Wall Interaction

Das Ziel der vorliegenden Arbeit ist es den Einfluss von Turbulenz und im speziellen der Reynoldszahl ($Re$) auf die Prozesse der Wirbelformation und des Wirbelzerfalls zu untersuchen. Inspiriert von Wirbeln, die bei der Fortbewegung von schwimmenden und fliegenden Tieren auftreten, werden drei charakteristische Merkmale identifiziert, die durch das Zusammenspiel eines Wirbels mit einem beschleunigten Festkörper (z.B. einem Flügel oder einer Flosse) entstehen: eine gekrümmte freie Scherschicht, der Wirbelkern und die Grenzschicht zwischen dem Wirbel und dem beschleunigten Körper. Im Rahmen der Arbeit werden mehrere Experimente und Simulationen vorgestellt, welche diese Merkmale der Wirbel in vereinfachten Konfigurationen reproduzieren und von anderen Effekten isolieren. Dies ermöglicht es den Einfluss der Turbulenz auf besagte Wirbelmerkmale zu quantifizieren. Zunächst wird die Interaktion eines generischen Wirbels mit einer Wand betrachtet. Hierzu wird ein Zylinder, welcher ein Fluid in Starrkörperrotation enthält, schlagartig angehalten. Das Abklingen und der Zerfall der des Starrkörperwirbels wird mithilfe einer direkten numerischen Simulation (DNS) für Reynoldszahlen im Bereich $Re \leq 2.8 \cdot 10^4$ analysiert. Fünf Stufen des Wirbelzerfalls können aufgrund der zugrundeliegenden Strukturen der Strömung charakterisiert werden. Darüber hinaus liefern die Ergebnisse der DNS empirische Skalierungsgesetze, die verschiedene Stufen den Wirbelzerfalls beschreiben. Die Skalierungsgesetze werden anschließend anhand von zwei experimentellen Kampagnen im moderaten ($Re\leq 5.6\cdot 10^5$) und hohen ($Re\leq 4\cdot 10^6$) Reynoldszahlbereich validiert. Anschließend wird die gekrümmte Scherschicht und der Wirbelkern genauer betrachtet. In Experimenten in einem großskaligen Schleppkanal wird eine runde Platte aus der Ruhe beschleunigt. Hierbei liefern Experimente in einem großen Reynoldszahlbereich Erkenntnisse über die Auswirkungen von kleinskaligen Strukturen auf die Wirbelformation. Weiterhin wird die Turbulenz im Wirbel durch Modifikationen der umlaufenden Plattenkante beeinflusst. Inspiriert von in der Natur auftretenden Flossenformen werden wellenartige Strukturen aufgeprägt, welche Strukturen im Bereich ihrer eigenen Wellenlänge in die Strömung einbringen. Sind diese Wellenlängen größer als die Dicke der gekrümmten Scherschicht, wird das Wirbelwachstum beeinflusst und damit die Kraft, die auf die Platte wirkt, modifiziert

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

Force estimates in turbulent vortex wakes of accelerating propulsors: The effects of edge undulation on vortex formation

The effects of edge undulation on separated flows and vortex formation are investigated with various geometries of accelerated, low aspect-ratio propulsors. In addition to force measurements, multi-camera planar particle image velocimetry (PIV) is applied for a large field of view of approximately 1.9 m × 0.3 m. Edge undulations with a wavelength λ, that is significantly larger than the thickness δ of the separated shear layer are identified to enhance the propulsion force during the stable vortex growth and before the vortex detaches. Edge undulation leads to a more turbulent vortex core and a faster vortex growth. The application of different approaches to recover the acting forces from PIV data lead to the conclusion, that the three-dimensionality of the turbulent vortex wake leads to both: a loss of out-of-plane vorticity and an underestimation of the kinetic energy in the vortex

Reynolds-number scaling of a vorticity-Annihilating boundary layer

To mimic the unsteady vortex–wall interaction of animal propulsion in a canonical test case, a vorticity-annihilating boundary layer was examined through the spin-down of a vortex from solid-body rotation. A cylindrical, water-filled tank was rapidly stopped, and the decay of the vortex from solid-body rotation was observed by means of planar and stereo particle image velocimetry. High Reynolds-number (Re) measurements were achieved by combining a large-scale facility (diameter, D=13 m) with a novel approach to reduce end-wall effects. The influence of the boundary-layer formation at the tank\u27s bottom wall was minimised by introducing a saturated salt-water layer. The experimental efforts have allowed us to assess the Re dependency of the laminar–turbulent transition of the vorticity-annihilating side-wall boundary layer at scales similar to large cetaceans. The scaling of the transition mechanism and its onset time were found to agree with predictions from linear stability analysis. Furthermore, the growth rate of the curved turbulent boundary layer was also in good agreement with an empirical scaling formulated in the literature for much smaller Re. Eventually, the scaling of vorticity annihilation was addressed. The earlier onset of transition at high Re compensates for the reduced effects of viscosity, leading to similar vorticity annihilation rates during the early stages of the spin-down for a wide Re range

Boundary Layer Characterization during Impulsive Spin-up and Spin-down Motions of High-Reynolds Number Rotating Flow

International audienceVarious planar and stereo PIV measurements have been conducted for high-Re in a rotating tank during spin-up and spin-down with the purpose to characterize the side-wall boundary-layer development and corresponding vorticity distributions. The effect of Ekman pumping was successfully suppressed with an additional saltwater layer. An algorithm was developed to identify distinct patterns such as Görtler vortices and transition. Despite a linearly stable character, the spin-up showed transition turbulent for high Reynolds numbers. It is hypothesized that shear-layer instabilities trigger the transition process for high velocities combined with extremely small wall curvature

Load estimation in unsteady flows from sparse pressure measurements: Application of transition networks to experimental data

Inspired by biological swimming and flying with distributed sensing, we propose a data-driven approach for load estimation that relies on complex networks. We exploit sparse, real-time pressure inputs, combined with pre-trained transition networks, to estimate aerodynamic loads in unsteady and highly-separated flows. The transition networks contain the aerodynamic states of the system as nodes along with the underlying dynamics as links. A weighted average-based (WAB) strategy is proposed and tested on realistic experimental data on the flow around an accelerating elliptical plate at various angles-of-attack. Aerodynamic loads are then estimated for angles of attack cases not included in the training dataset so as to simulate the estimation process. An optimization process is also included to account for the system's temporal dynamics. Performance and limitations of the WAB approach are discussed, showing that transition networks can represent a versatile and effective data-driven tool for real-time signal estimation using sparse and noisy signals (such as surface pressure) in realistic flows

A novel single-camera approach to large-scale, three-dimensional particle tracking based on glare-point spacing

Abstract: A novel method for three-dimensional particle tracking velocimetry (PTV) is proposed that enables flow measurements in large volumes [V= O(10 m3)] using a single camera. Flow is seeded with centimeter-sized soap bubbles, when combined with suitable illumination, produce multiple glare points. The spacing between the two brightest glare points for each bubble is then utilized to reconstruct its depth. While the use of large soap bubbles comes at the expense of non-Stokesian behaviour, the excellent ray optics allow for large volume illumination when coupled with for instance pulsed LED banks. Possible error sources and the out-of-plane accuracy are discussed before the feasibility of the method is tested in an industrial-scale wind tunnel facility (test section of cross section 9.1 m × 9.1 m). In particular, the vortical structure in the wake of a 30%-scale tractor-trailer model at a 9 ∘ yaw angle is captured in a 4.0 m × 1.5 m × 1.5 m measurement volume. Long tracks of up to 80 time steps are extracted in three-dimensional space via a single perspective. The successful proof-of-concept confirms the potential of the novel approach for three-dimensional measurements in volumes of industrial scale. Graphic abstract: [Figure not available: see fulltext.]Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Aerodynamic