Investigation of turbulent shear flows with high resolution PIV methods

The separated wake flow of a generic space launcher model was investigated experimentally at a very high Reynolds number and a transonic Mach number in order to analyze the physical phenomena that lead to buffeting and to generate reliable validation data for computational fluid dynamics. Due to the large dynamic range of scales and velocities in turbulent shear flows, it was necessary to improve the state-of-the-art PIV techniques. A fundamental investigation of the spatial resolution limit identified the ideal measurement setup and the best evaluation approach to obtain the desired results of the wake flow for the investigated space launcher model. Additionally, a new method was developed to estimate the velocities’ probability density function directly from the PIV correlation function. This approach allows for the direct determination of the Reynolds normal and shear stress without significant spatial low-pass filtering, compared to established methods, and results in significantly improved resolution and accuracy. Furthermore, individual vortices in the separated shear layer were detected in instantaneous velocity fields to investigate the formation and motion of turbulent structures. A statistical analysis of the detected vortices reveals their spatial distribution with respect to their size and swirling strength. The combination of the developed evaluation techniques yields a broad overview of the flow phenomena in the space launcher model’s wake and results in a data basis that may serve for the validation of new numerical tools. Beyond that, the improved estimation of Reynolds stresses and the theoretical considerations regarding the spatial resolution are of fundamental importance for many other experiments.Die abgelöste Strömung im Nachlauf eines generischen Trägerraketenmodells wurde experimentell bei einer sehr hohen Reynolds-Zahl und einer transsonischen Machzahl untersucht, um die physikalischen Phänomene, die zum Buffeting führen, aufzuklären und verlässliche Validierungsdaten für numerische Strömungssimulationen zu generieren. Aufgrund der großen Dynamik der verschiedenen Skalen und der Geschwindigkeit in turbulenten Scherschichtströmungen war im Rahmen der Arbeit eine Weiterentwicklung der etablierten PIV-Messtechnik erforderlich. Eine grundlegende Untersuchung der räumlichen Auflösungsgrenze identifizierte eine ideale Messanordnung und die beste Auswertetechnik, um die gewünschten Ergebnisse über die Nachlaufströmung des untersuchten Trägerraketenmodells zu erhalten. Außerdem wurde ein neues Verfahren entwickelt, um die Wahrscheinlichkeitsdichtefunktion der Geschwindigkeit direkt aus der PIV-Korrelationsfunktion abzuschätzen. Dieser Ansatz ermöglicht die direkte Bestimmung der Reynolds- Normal- und Schubspannung ohne nennenswerte räumliche Tiefpassfilterung gegenüber etablierten Auswertemethoden und führt zu einer wesentlich verbesserten Auflösung und Genauigkeit. Weiterhin wurden einzelne Wirbel in der abgelösten Scherschicht in momentanen Geschwindigkeitsfeldern detektiert, um die Entstehung und Bewegung turbulenter Strukturen zu analysieren. Eine statistische Analyse der detektierten Wirbel zeigt deren räumliche Verteilung in Abhängigkeit von der Wirbelgröße und -stärke. Die kombinierte Anwendung der entwickelten Auswerteverfahren liefert einen breiten Überblick über die Strömungsvorgänge im Nachlauf des Trägerraketenmodells und liefert eine Datenbasis, die für die Validierung neuer numerischer Werkzeuge eingesetzt werden kann. Darüber hinaus sind die verbesserte Abschätzung der Reynoldsspannungen und die theoretischen Überlegungen zur räumliche Auflösung von grundlegender Bedeutung für viele andere Experimente

Photoleitende Antennen zur Mehrkanaldetektion ultrakurzer Terahertzpulse

Der Terahertz-Spektralbereich ist sowohl für Grundlagenforschung als auch für industrielle Anwendungen, Sicherheitstechnik und sogar Medizintechnik von großem Interesse. Um die Terahertz-Messtechnik weiträumig etablieren zu können, ist eine signifikante Senkung der Messdauer erforderlich. Diese Arbeit umfasst Untersuchungen zur Reduzierung der Messzeit in bildgebenden THz-Systemen. Hierzu werden photoleitende Antennen für ein Mehrkanalsystem entwickelt, die viele Punkte einer Probe in möglichst kurzer Zeit vermessen können. Analytische Rechnungen zur Detektionsfunktion , numerischen Simulationen sowie vergleichende Experimente mit Interdigitalantennen, Oberflächenemitter und Dipolantennen sind Gegenstand dieser Diplomarbeit

Characterizing the turbulent drag properties of rough surfaces with a Taylor--Couette setup

Wall-roughness induces extra drag in wall-bounded turbulent flows. Mapping any given roughness geometry to its fluid dynamic behaviour has been hampered by the lack of accurate and direct measurements of skin-friction drag. Here the Taylor-Couette (TC) system provides an opportunity as it is a closed system and allows to directly and reliably measure the skin-friction. However, the wall-curvature potentially complicates the connection between the wall friction and the wall roughness characteristics. Here we investigate the effects of a hydrodynamically fully rough surface on highly turbulent, inner cylinder rotating, TC flow. We find that the effects of a hydrodynamically fully rough surface on TC turbulence, where the roughness height k is three orders of magnitude smaller than the Obukhov curvature length Lc (which characterizes the effects of curvature on the turbulent flow, see Berghout et al. arXiv: 2003.03294, 2020), are similar to those effects of a fully rough surface on a flat plate turbulent boundary layer (BL). Hence, the value of the equivalent sand grain height ks, that characterizes the drag properties of a rough surface, is similar to those found for comparable sandpaper surfaces in a flat plate BL. Next, we obtain the dependence of the torque (skin-friction drag) on the Reynolds number for given wall roughness, characterized by ks, and find agreement with the experimental results within 5 percent. Our findings demonstrate that global torque measurements in the TC facility are well suited to reliably deduce wall drag properties for any rough surface.Comment: 18 pages, 13 figure

Time-resolved image analysis for turbulent flows Conference paper

International audienceClassical Particle Image Velocimetry (PIV) uses two representations of the particle image distribution to determine the displacement of the particle image pattern by spatial cross-correlation. The accuracy and the robustness are however limited by the fact that only two representations at t and t +Δt are present. Thus, only a first order approximation of the velocity can be estimated. To enhance the precision in estimating the flow velocity, multi-pulse or multi-frame techniques were already investigated in the early days of PIV as summarized by Adrian (1991) and Hain and Kähler (2007). Today with the increasing power of high repetition rate lasers and enhanced sensitivity of the digital cameras it is possible to have a time-resolved sampling of even aerodynamically relevant flows, were the particles are much smaller than in water flows. The easiest sampling scheme is the equidistant temporal sampling of the particle distribution such that a robust displacement estimation between successive frames (1+2, 2+3, 3+4, ...) is possible. This so called TR-PIV does not only provide the possibility to follow the evolution of flow structures, but offers the ability to strengthen the data processing by using information from more than two frames (e.g. Hain and Kähler, 2007). Within the AFDAR-project (Advanced Flow Diagnostics for Aeronautical Research funded by the European Union) different approaches to evaluat time-resolved image series were developed by the different groups. The current contribution focuses on the comparison of the algorithms that were developed within the AFDAR project by the partners of the consortium. To verify and validate the performance of the different algorithms a short image sequence of an experiment on the flow over periodic hills (ERCOFTAC test case 81) was provided to all partners and evaluated with the current version of the algorithms

Experimental evidence of near-wall reverse flow events in a zero pressure gradient turbulent boundary layer

This study reports on experimentally observed rare near-wall reverse flow events in a fully developed turbulent flat plate boundary layer at zero pressure gradient with Reynolds numbers between Re_\theta \approx 2500 and Re_\theta \approx 8000 (Re_\tau = 800-2400). The reverse flow events are captured using high magnification particle image velocimetry sequences with record lengths varying from 50 000 to 126 000 samples. Time resolved particle image sequences allow singular reverse flow events to be followed over several time steps whereas long records of nearly statistically independent samples provide a variety of single snapshots at a higher spatial resolution. The probability of occurrence lies in the order of 0.012-0.018% which matches predictions from direct numerical simulations (DNS). The typical size of the reverse flow bubble is about 30 wall units in length and 5 wall units in height which agrees well with similar observations made in existing DNS data

Multi-PIV Measurements of an Adverse Pressure Gradient Turbulent Boundary Layer

We report on a multi-national measurement campaign aimed at providing highly resolved flow field data of a turbulent boundary layer subjected to an adverse pressure gradient (APG). In the case of APGs the structure and dynamics of large scale turbulent flow structures along with their significance on the statistical properties of the flow is not well understood. Hence the fundamental aim was to resolve and characterise the large-scale coherent structures in an APG boundary layer flow. In addition to large-field-of-view PIV measurements using 16 sCMOS cameras along a 3.5m length, stereoscopic PIV measurements were performed at specific locations in order to also resolve the span-wise velocity statistics. Long-distance, high-speed micro-PIV measurements provided near wall statistics at selected locations including the time-resolved wall shear stress. The measurements were performed in the boundary layer wind tunnel of the Laboratoire de Mécanique de Lille (LML) and funded by EuHIT (www.euhit.org)

Non-Standard Errors

In statistics, samples are drawn from a population in a data-generating process (DGP). Standard errors measure the uncertainty in estimates of population parameters. In science, evidence is generated to test hypotheses in an evidence-generating process (EGP). We claim that EGP variation across researchers adds uncertainty: Non-standard errors (NSEs). We study NSEs by letting 164 teams test the same hypotheses on the same data. NSEs turn out to be sizable, but smaller for better reproducible or higher rated research. Adding peer-review stages reduces NSEs. We further find that this type of uncertainty is underestimated by participants