TY - JOUR A1 - Strangfeld, Christoph A1 - Müller-Vahl, H. F. A1 - Nayeri, C. N. A1 - Paschereit, C. O. A1 - Greenblatt, D. T1 - Airfoil synchronous surging and pitching N2 - Combined surging and pitching of an airfoil at the identical frequency (i.e. synchronously), at four different phase differences, was investigated theoretically and experimentally. The most general unsteady theoretical formulation was adopted to calculate the lift coefficient, and then extended to explicitly compute the unsteady bound vortex sheet. This was used for comparison with experiments and facilitated the computation of both Joukowsky and impulsive-pressure lift contributions. Experiments were performed using a symmetric 18 % thick airfoil in an unsteady wind tunnel at an average Reynolds number of 3.0*10^5 , with a free-stream oscillation amplitude of 51 %, an angle-of-attack range of 2° +- 2° and a reduced frequency of 0.097. In general, excellent correspondence was observed between theory and experiment, representing the first direct experimental validation of the general theory. It was shown, both theoretically and experimentally, that the lift coefficient was not accurately represented by independent superposition of surging and pitching effects, due to variations in the instantaneous effective reduced frequency not accounted for during pure pitching. Deviations from theory, observed at angle-of-attack phase leads of 90° and 180°, were attributed to bursting of separation bubbles during the early stages of the acceleration phase. The largest deviations occurred when the impulsive-pressure lift contribution was small relative to the Joukowsky contribution, because the latter was most affected by bubble bursting. Bubble bursting resulted in large form-drag oscillations that occurred at identical phase angles within the oscillation cycle, irrespective of the phase difference between surging and pitching, as well as in the absence of pitching. KW - Unsteady aerodynamics KW - Dynamic stall KW - Separation bubble KW - Bubble bursting PY - 2025 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-630068 DO - https://doi.org/10.1017/jfm.2025.220 SN - 1469-7645 VL - 1009 SP - 1 EP - 33 PB - Cambridge University Press (CUP) AN - OPUS4-63006 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Greenblatt, D. A1 - Müller-Vahl, H. A1 - Strangfeld, Christoph T1 - Laminar separation bubble bursting in a surging stream N2 - The effect of high-amplitude harmonic surging on airfoil laminar separation bubbles, at small angles of attack, was investigated experimentally in a dedicated surging-flow wind tunnel. A generalized pressure coefficient was developed that accounts for local static pressure variations due to surging. This critical generalization facilitated direct comparisons between surging and quasisteady pressure coefficients, and thus unsteady effects could be distinguished from Reynolds number effects. A momentum-integral boundary layer analysis was implemented to determine movement of the bubble separation point, and movement of the transition point was extracted from experimental surface pressure coefficients. The most significant finding was that bubble bursting occurs, counterintuitively, during early imposition of the favorable temporal pressure gradient, because the favorable pressure gradient rapidly drives the bubble aft, rendering it unable to reattach. This surge-induced dynamic stall mechanism resulted in large lift and form-drag coefficient oscillations. Furthermore, failure to implement the generalized pressure coefficient definition resulted in temporal form-drag coefficient errors of up to 400 counts. KW - Aerodynamics KW - Boundary layer receptivity, stability & separation KW - Boundary layers PY - 2023 DO - https://doi.org/10.1103/PhysRevFluids.8.L012102 SN - 2469-990X VL - 8 IS - 1 SP - 1 EP - 11 PB - American Physical Society AN - OPUS4-56860 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Müller-Vahl, H. F. A1 - Strangfeld, Christoph A1 - Nayeri, C. N. A1 - Paschereit, C. O. A1 - Greenblatt, D. T1 - Dynamic Stall Under Combined Pitching and Surging N2 - Dynamic stall often occurs under conditions of simultaneous unsteady pitching and surging (e.g., rotorcraft and wind turbines), butmanymodels employ a dimensionless time base that implicitly assumes that surging is superimposed, in a quasi-steady manner, on dynamic pitching. An unsteady wind tunnel was used to examine this assumption, where a technique was developed to quantify the unsteady effects of surging on a pitching NACA 0018 airfoil. The technique involved performing multiple harmonic pitching experiments under nominally steady freestream conditions that bracketed a corresponding 50% surging amplitude (1.25 ⋅ 105 ≤ Re ≤ 3.75 ⋅ 105). By interpolating these data, unsteady-pitching/quasi-steady-surging data sets were constructed and compared with de facto synchronous pitch and surging experiments, thereby isolating the unsteady effects of surging on a pitching airfoil. Both large and small poststall maximum angles of attack (αs + 5° and αs + 15°) were considered at multiple pitch-surge phase differences. During deep dynamic stall (αs � 15°), with large-scale separation, surging was seen to have a secondary effect on the unsteady aerodynamics. However, at small poststall maximum angles of attack (αs + 5°), either light or deep dynamic stall behavior was observed depending upon the pitch-surge phase difference. This was attributed to Reynolds number history effects, exemplified by boundary-layer transition, and thus it can be referred to as “transitional” dynamic stall. KW - Dynamic stall KW - Angle of attack oscillations KW - Free stream velocity osciallations PY - 2020 DO - https://doi.org/10.2514/1.J059153 VL - 58 IS - 12 SP - 5134 EP - 5145 PB - American Institute of Aeronautics and Astronautics AN - OPUS4-51037 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Müller, R. A1 - Boenke, T. A1 - Dörfler, S. A1 - Abendroth, T. A1 - Härtel, P. A1 - Althues, H. A1 - Kaskel, S. A1 - Kardjilov, N. A1 - Markötter, Henning A1 - Sintschuk, Michael A1 - Hilger, A. A1 - Manke, I. A1 - Risse, S. T1 - Multimodal Operando Analysis of Lithium Sulfur Multilayer Pouch Cells: An In-Depth Investigation on Cell Component Design and Performance N2 - This study presents an innovative operando analysis of lithium-sulfur (Li/S) multilayer pouch cells, employing a combination of lab-source and synchrotron x-ray imaging to investigate sulfur crystallite dissolution and lithium dendrite formation. By integrating advanced X-ray imaging, impedance spectroscopy, and simultaneous monitoring of temperature and pressure, the research uncovers critical insights into the behavior of active and inactive cell components. The analysis reveals significant degradation increments, primarily driven by side product accumulation and the deterioration of lithium microstructures, which contribute to performance loss over cycling. Additionally, temperature distribution analysis shows a strong correlation between joule heating, polarization resistance, and the observed endothermic processes during crystallization. These findings provide a comprehensive understanding of the mechanistic processes within industrially relevant pouch cells, highlighting opportunities for optimizing Li/S cell designs and advancing high-energy-density battery systems for commercial applications. KW - Current collector perforation KW - Impedance spectroscopy KW - Lthium sulfur batteries KW - Multilayer pouch cells KW - X-ray imaging PY - 2025 DO - https://doi.org/10.1002/aenm.202404256 SP - 1 EP - 12 PB - Wiley VHC-Verlag AN - OPUS4-62436 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Liens, A. A1 - Reveron, H. A1 - Douillard, T. A1 - Blanchard, N. A1 - Lughi, V. A1 - Sergo, V. A1 - Laquai, René A1 - Müller, Bernd R. A1 - Bruno, Giovanni A1 - Schomer, S. A1 - Fürderer, T. A1 - Adolfsson, E. A1 - Courtois, N. A1 - Swain, M. A1 - Chevalier, J. T1 - Phase transformation induces plasticity with negligible damage in ceria-stabilized zirconia-based ceramics N2 - Ceramics and their composites are in general brittle materials because they are predominantly made up of ionic and covalent bonds that avoid dislocation motion at room temperature. However, a remarkable ductile behavior has been observed on newly developed 11 mol.% ceria-stabilized zirconia (11Ce-TZP) composite containing fine alumina (8 vol.% Al2O3) and elongated strontium hexa-aluminate (8 vol.% SrAl12O19) grains. The as-synthesized composite also has shown full resistance to Low Temperature Degradation (LTD), relatively high strength and exceptionally high Weibull modulus, allowing its use in a broader range of biomedical applications. In this study, to deepen the understanding of plastic deformation in Ce-TZP based composites that could soon be used for manufacturing dental implants, different mechanical tests were applied on the material, followed by complete microstructural characterization. Distinct from pure Ce-TZP material or other zirconia-based ceramics developed in the past, the material here studied can be permanently strained without affecting the Young modulus, indicating that the ductile response of tested samples cannot be associated to damage occurrence. This ductility is related to the stress-induced tetragonal to monoclinic (t-m) zirconia phase transformation, analogue to Transformation-Induced Plasticity (TRIP) steels, where retained austenite is transformed to martensite. The aim of this study is to corroborate if the observed plasticity can be associated exclusively to the zirconia t-m phase transformation, or also to microcraking induced by the transformation. The t-m transformed-zones produced after bending and biaxial tests were examined by X-ray refraction and SEM/TEM coupled with Raman. The results revealed that the observed elastic-plastic behavior occurs without extensive microcracking, confirming a purely elastic-plastic behavior driven by the phase transformation (absence of damage). KW - Zirconia KW - Ceria KW - Ceramic matrix composite KW - Plasticity KW - Phase transformation KW - X-Ray Refraction PY - 2020 UR - http://www.sciencedirect.com/science/article/pii/S1359645419307177 DO - https://doi.org/10.1016/j.actamat.2019.10.046 SN - 1359-6454 VL - 183 SP - 261 EP - 273 PB - Elsevier Ltd. AN - OPUS4-49740 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Reveron, H. A1 - Serrano-Munoz, Itziar A1 - Kupsch, Andreas A1 - Müller, Bernd R. A1 - Chevalier, J. A1 - Bruno, Giovanni T1 - Transformation-induced plasticity in zirconia during tensile loading: A combined microscopy and synchrotron X-ray refraction study N2 - The stress-induced tetragonal to monoclinic (t-m) zirconia phase transformation can provide a certain degree of plasticity to Ceria-stabilized (Ce-TZP) zirconia-based composites. Characterizing and monitoring this phase transition on a millimeter-size range, within the bulk and in-situ remains a challenge. In this work, the mechanical behavior of Ce-TZP based composite was studied in tension, combining microscopy and synchrotron Xray refraction techniques. In contrast with microscopy methods, which only provide surface information, X-ray refraction radiography (SXRR) allowed the visualization of all the transformation bands, over the entire length and thickness of tested specimens, opening up new avenues for in-situ stress-induced t-m transformation studies. KW - Zirconia KW - Ceria KW - Composite KW - Phase transformation KW - Plasticity KW - Synchrotron X-ray refraction PY - 2024 DO - https://doi.org/10.1016/j.matlet.2024.136445 SN - 0167-577X SN - 1873-4979 VL - 366 SP - 1 EP - 4 PB - Elsevier Ltd. CY - Amsterdam AN - OPUS4-59977 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Schob, D. A1 - Sagradov, I. A1 - Roszak, R. A1 - Sparr, H. A1 - Franke, R. A1 - Ziegenhorn, M. A1 - Kupsch, Andreas A1 - Léonard, Fabien A1 - Müller, Bernd R. A1 - Bruno, Giovanni T1 - Experimental determination and numerical simulation of material and damage behaviour of 3D printed polyamide 12 under cyclic loading N2 - The material and damage behaviour of additively manufactured polyamide 12 under cyclic loading was characterized by cyclic tests and microstructure analysis by using microscopy, X-ray refraction, and computed tomography. The results were used to determine parameters for the viscoplastic material model by Chaboche and a damage model by Gurson-Tvergaard-Needleman. The temperature was monitored during the experiments and the self-heating effect was observed. By including this effect, a higher accuracy could be achieved with the results of mechanical experiments. KW - 3D printing Polyamide 12 KW - Chaboche model KW - GTN model KW - Material and damage behaviour KW - X-ray refraction KW - Computed tomography PY - 2020 DO - https://doi.org/10.1016/j.engfracmech.2019.106841 SN - 0013-7944 VL - 229 SP - 106841-1 EP - 106841-13 PB - Elsevier Ltd. CY - Amsterdam AN - OPUS4-50557 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Kupsch, Andreas A1 - Schob, D. A1 - Léonard, Fabien A1 - Müller, Bernd R. A1 - Sagradov, I. A1 - Roszak, R. A1 - Sparr, H. A1 - Franke, R. A1 - Ziegenhorn, M. A1 - Bruno, Giovanni T1 - Bestimmung der Mikrostruktur und Simulation des Schädigungsverhaltens von lasergesintertem Polyamid 12 unter quasistatischer Zugbelastung N2 - Um das Material- und Schädigungsverhalten von additiv gefertigtem Polyamid 12 (PA12) unter quasistatischer Belastung zu charakterisieren, wurden mechanische Tests und Röntgenverfahren zur Bestimmung der Mikrostruktur eingesetzt. Die Proben wurden nach dem Prinzip des Selektiven Lasersinterns (SLS) hergestellt. Unter quasistatischer Belastung mit Haltezeiten ergab sich ein viskoplastisches Materialverhalten. Im Zugversuch wurde eine maximale Zugfestigkeit von 40.6 MPa und eine Bruchdehnung von 7.4% beobachtet. Mittels Röntgenrefraktion wurde eine Erhöhung von inneren Oberflächen beobachtet, die senkrecht zur Zugrichtung orientiert sind. Die Analyse der Gesamtporosität aus Computertomographie-Messungen ergab keine Änderung infolge der Zugbelastung. Jedoch wurde eine bimodale Porengrößenverteilung und eine steigende Sphärizität festgestellt. Das Materialverhalten wurde mit dem Chaboche-Modell simuliert und ergab eine sehr gute Übereinstimmung mit den experimentellen Ergebnissen. Allerdings gestattet dieses Modell nicht, das Schädigungsverhalten abzubilden. Daher wurde zur Simulation des Schädigungsverhaltens das Modell gemäß dem Ansatz von Gurson, Tvergaard und Needleman unter Berücksichtigung der mikrostrukturellen Parameter erweitert. Der Schwerpunkt des Beitrags liegt auf den Röntgenverfahren zur experimentellen Bestimmung der Mikrostruktur. T2 - DGZfP-Jahrestagung 2021 CY - Online meeting DA - 10.05.2021 KW - Additive Fertigung (AM) KW - Polyamid 12 KW - Röntgenrefraktion KW - Computertomographie KW - Numerische Simulation PY - 2021 UR - https://nbn-resolving.org/urn:nbn:de:kobv:b43-526833 UR - https://jahrestagung.dgzfp.de/Portals/jt2021/bb/P16.pdf SP - 1 EP - 8 PB - Deutsche Gesellschaft für Zerstörungsfreie Prüfung (DGZfP) CY - Berlin AN - OPUS4-52683 LA - deu AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -