Christian Willert

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

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Joachim Klinner

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

J. Kompenhans

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

Markus Raffel

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

Guido Stockhausen

Nanjing University of Science and Technology

Christian J. Kähler

Universität der Bundeswehr München

Steven T. Wereley

Purdue University West Lafayette

Manfred Beversdorff

Nanjing University of Science and Technology

Michael Schroll

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

Julio Soria

Monash University

Melanie Voges

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

Cooperative Institutions

Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR)

143

Centre National de la Recherche Scientifique

Linde (Germany)

Office National d'Études et de Recherches Aérospatiales

Technical University of Munich

Monash University

Universität der Bundeswehr München

RWTH Aachen University

Dienstleistungszentrum Ländlicher Raum

Technische Universität Berlin

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Measurements of the Near-Wall Region of Pipe Flowat High Reynolds Numbers

Christian Willert Joachim Klinner Julio Soria Omid Amili Michael Eisfelder

Im Rahmen einer durch EuHIT (www.euhit.org) geforderten Messkampagne wurde im Rohrwindkanal CICLoPE der Uni Bologna die PIV-Profilmesstechnik eingesetzt, um die turbulente Rohrstromung mit hoher Orts- und Zeitauflosung zu erfassen. Die gekrummte Geometrie des Rohres mit 900 mm Durchmesser (110 m Lange) und eine begrenzte Zuganglichkeit durch runde Blindstopfen erforderte die Verwendung eines Umlenkspiegels, um das Wand-normale Geschwindigkeitsprofil zu erfassen. Messreihen mit bis zu 200,000 Bildpaaren bei bis zu 10kHz Aufnahmefrequenz liefern konvergierte Statistiken inkl. hoherer Momente, Frequenzspektren, Zwei-Punkt-Korrelationen und Haufigkeitsverteilungen (Joint-PDFs) der kanonischen Stromung und erweitern bisherige Ergebnisse aus der „Super-Pipe“ der Uni Princeton bei ahnlichen Reynoldszahlen.

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Planar measurements of CO concentrations in flames at atmospheric and elevated pressure by laser-induced fluorescence

Applied Physics B (2019)

Lena Voigt Johannes Heinze Metin Korkmaz Klaus Peter Geigle Christian Willert

Planar laser-induced fluorescence

10.1007/s00340-019-7181-4

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High-Speed Particle Image Velocimetry for the Efficient Measurement of Turbulence Statistics - Can PIV replace the hotwire?

Christian Willert

A high frame rate camera and a continuous wave laser are used to capture long particle image sequences exceeding 100,000 consecutive frames at framing frequencies up to 50 kHz. The electronic shutter of the high-speed CMOS camera is reduced to 5\mu s to prevent excessive particle image streaking. The combination of large image number and high frame rate is possible by limiting the field of view to a narrow strip to primarily capture a profile of velocity data and derived quantities, such as vorticity and higher order statistics. Multi-frame PIV processing algorithms are employed to improve the dynamic range of recovered PIV data. The recovered data is temporally well resolved and provides sufficient samples for statistical convergence of the fluctuating velocity components. The measurement technique is demonstrated on a developing turbulent duct flow inside a small wind tunnel with Re_\delta = 4800 and Re_\theta = 490 as well as on a turbulent boundary layer in the 1m wind tunnel of DLR-Gottingen with Re_\delta = 34000 and Re_\theta = 4000. By means of single-line correlation it is further possible to extract the near wall velocity profile in the viscous sub-layer, both time-averaged as well as instantaneous, which can then be used to estimate the wall shear rate and with it the shear stress \tau_w and friction velocity u_\tau. Finally initial results of the ongoing measurement campaign on an adverse pressure gradient turbulent boundary layer are presented.

Frame rate

Streak

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Pressure, temperature, and three-component velocity fields by filtered Rayleigh scattering velocimetry

Optics Letters (2017)

Ulrich Doll Guido Stockhausen Christian Willert

The filtered Rayleigh scattering (FRS) technique, extended by the method of frequency scanning, is a powerful tool to characterize thermodynamic, as well as aerodynamic, properties of technical flows. In this Letter, we report on the first application of an FRS velocimeter, which is capable of acquiring time-averaged planar pressure, temperature, and three-component velocity distributions simultaneously. The method is validated by characterizing the near-field, as well as the far-field, of a turbulent jet.

Velocimetry

10.1364/ol.42.003773

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Dynamic Wall Shear Stress Measurement using Event-based 3D Particle Tracking

arXiv (Cornell University) (2024)

Christian Willert Joachim Klinner

We describe the implementation of a 3d Lagrangian particle tracking (LPT) system based on event-based vision (EBV) and demonstrate its application for the near-wall characterization of a turbulent boundary layer (TBL) in air. The viscous sublayer of the TBL is illuminated by a thin light sheet that grazes the surface of a thin glass window inserted into the wind tunnel wall. The data simultaneously captured by three synchronized event-cameras is used to reconstruct the 3d particle tracks within 400 $\mu$m of the wall on a field of view of 12.0 mm x 7.5 mm. The velocity and position of particles within the viscous sublayer permit the estimation of the local vector of the unsteady wall shear stress (WSS) under the assumption of linearity between particle velocity and WSS. Thereby, time-evolving maps of the unsteady WSS and higher order statistics are obtained that are in agreement with DNS data at matching Reynolds number. Near-wall particle acceleration provide the rate of change of the WSS which exhibits fully symmetric log-normal superstatistics. Two-point correlations of the randomly spaced WSS data are obtained by a bin-averaging approach and reveal information on the spacing of near-wall streaks. The employed compact EBV hardware coupled with suited LPT tracking algorithms provide data quality on par with currently used, considerably more expensive, high-speed framing cameras.

Particle (ecology)

Tracking (education)

10.48550/arxiv.2409.01757

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RANS-Turbulenzmodellierung für subsonische druckinduzierte Strömungsablösung

Tobias Knopp Matteo Novara Daniel Schanz Andreas Schröder Christian Willert

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ONERA-DLR Common Research Project "Cube": Comparison between planar PIV and CFD results

Quentin Gallas Anne Gilliot Jean-Claude Monnier Jérôme Delva Claude Geiler

The CUBE CRP is dedicated to the generation of an experimental data base of the flow around a 3D obstacle (here a cube) for validation of advanced CFD simulation tools. It is a 30 months research activity program and has started beginning of 2015. Planar PIV, planar time-resolved PIV and tomographic PIV were recorded for two hydrodynamic configurations: laminar and turbulent boundary layers in interaction with the cube at varying free stream conditions.

Cube (algebra)

Data cube

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Advanced image analysis for particle image velocimetry. Part I: Processing individual PIV recordings

Christian Willert

The article provides an overview of existing (digital) image evaluation methods for particle image velocimetry (PIV), which were developed in the past 20 years, and demonstrates their respective performance using actual PIV image recordings. The algorithms share the common goal of faithfully extracting the displacement field encoded by the motion of imaged particles from one image recording to the next using various implementations of mainly correlation-based pattern matching. Modern algorithms make use of multi-resolution coarse-to-fine processing strategies augmented with iterative, full image distortion in a predictor-corrector sense to increase both the spatial resolution and velocity dynamic range obtainable with PIV. The article reviews various processing schemes, required image interpolation methods as well as correlation signal enhancing image spatial or spectral filtering.

Interpolation

Distortion (music)

Image scaling

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Performance and accuracy investigations of two Doppler global velocimetry systems applied in parallel

Measurement Science and Technology (2007)

Christian Willert Guido Stockhausen Joachim Klinner C. Lempereur P. Barricau

Two Doppler global velocimetry systems were applied in parallel to assess their performance in wind tunnel environments. Both DGV systems were mounted on a common traverse surrounding the glass-walled 1.4 × 1.8 m2 test section of the wind tunnel. The traverse normally supports a three-component forward-scatter laser Doppler velocimetry system. The reproducible tip-vortex flow field generated by the blunt tip of an airfoil was chosen for this investigation and was precisely surveyed by LDA just prior to the DGV measurements. Both DGV systems shared the same continuous wave laser light source, laser frequency monitoring and fibre optic light sheet delivery system. The principal differences between the DGV implementations are with regard to the imaging configuration. One configuration relied on a single camera view that observed three successively operated light sheets. In the second configuration, three camera views simultaneously observed a single light sheet using a four-branch fibre imaging bundle. The imaging bundle system had all three viewpoints in a forward scattering arrangement which increased the scattering efficiency but reduced the frequency shift sensitivity. Since all three light sheet observation components were acquired onto the same image frame, acquisition times could be reduced to a minimum. On the other hand, the triple light sheet–single camera system observed two light sheets in forward scatter and one light sheet in backscatter. Although three separate images had to be recorded in succession, the image quality, spatial resolution and signal-to-noise ratio were superior to the imaging bundle system. Comparison of the DGV data with LDV measurements shows very good agreement to within 1–2 m s−1. The remaining discrepancy has a variety of causes, some are related to the reduced resolving power of the fibre imaging bundle system (graininess, smoothing), exact localization of the receiver head with respect to the scene, laser frequency drift or background influences. The extensive data base available allows a detailed study of the influence of each of these factors.

Velocimetry

Aliasing

Structured Light

10.1088/0957-0233/18/8/027

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Study of Thermal Convection and Couette Flows

Christoph Böhm Christian Willert H. Richard

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