TY - GEN A1 - Harlander, Uwe A1 - Sukhanovskii, Andrei A1 - Abide, Stéphane A1 - Borcia, Ion-Dan A1 - Popova, Elene A1 - Rodda, Costanza A1 - Vasiliev, Andrei A1 - Vincze, Miklos T1 - New Laboratory Experiments to Study the Large-Scale Circulation and Climate Dynamics T2 - Atmosphere N2 - The large-scale flows of the oceans and the atmosphere are driven by a non-uniform surface heating over latitude, and rotation. For many years scientists try to understand these flows by doing laboratory experiments. In the present paper we discuss two rather new laboratory experiments designed to study certain aspects of the atmospheric circulation. One of the experiments, the differentially heated rotating annulus at the Brandenburg University of Technology (BTU) Cottbus, has a cooled inner cylinder and a heated outer wall. However, the structure of the atmospheric meridional circulation motivates a variation of this “classical” design. In the second experiment described, operational at the Institute of Continuous Media Mechanics (ICMM) in Perm, heating and cooling is performed at different vertical levels that resembles more the atmospheric situation. Recent results of both experiments are presented and discussed. Differences and consistencies are highlighted. Though many issues are still open we conclude that both setups have their merits. The variation with heating and cooling at different levels might be more suited to study processes in the transition zone between pure rotating convection and the zone of westerly winds. On the other hand, the simpler boundary conditions of the BTU experiment make this experiment easier to control. Y1 - 2023 UR - https://www.mdpi.com/2073-4433/14/5/836 U6 - https://doi.org/10.3390/atmos14050836 VL - 14 IS - 5 ER - TY - GEN A1 - Maltese Meletti de Oliveira, Gabriel A1 - Abide, Stéphane A1 - Viazzo, Stephane A1 - Harlander, Uwe T1 - A parameter study of strato-rotational low-frequency modulations: impacts on momentum transfer and energy distribution T2 - Philosophical transactions of the Royal Society : Series A, Mathematical, physical and engineering sciences Y1 - 2023 U6 - https://doi.org/10.1098/rsta.2022.0297 SN - 1364-503X SN - 1471-2962 VL - 381 IS - 2246 ER - TY - GEN A1 - Merbold, Sebastian A1 - Hamede, Mohammed Hussein A1 - Froitzheim, Andreas A1 - Egbers, Christoph T1 - Flow regimes in a very wide-gap Taylo-Couette flow with counter rotating cylinders T2 - Philosophical transactions of the Royal Society : Series A, Mathematical, physical and engineering sciences Y1 - 2023 U6 - https://doi.org/10.1098/rsta.2022.0113 SN - 1364-503X SN - 1471-2962 VL - 381 IS - 2246 ER - TY - GEN A1 - Hamede, Mohammed Hussein A1 - Merbold, Sebastian A1 - Egbers, Christoph T1 - Experimental methods for investigating the formation of flow patterns in a very wide gap Taylor-Couette flow ($\eta=0.1$) T2 - tm - Technisches Messen Y1 - 2023 U6 - https://doi.org/10.1515/teme-2022-0107 SN - 2196-7113 VL - 90 IS - 5 SP - 332 EP - 339 ER - TY - GEN A1 - Merbold, Sebastian A1 - Hasanuzzaman, Gazi A1 - Buchwald, Tom A1 - Schunk, Christoph A1 - Schmeling, Daniel A1 - Volkmann, André A1 - Brinkema, Robert A1 - Hampel, Uwe A1 - Schröder, Andreas A1 - Egbers, Christoph T1 - Reference experiment on aerosol particle transport for dynamic situations T2 - tm - Technisches Messen Y1 - 2023 U6 - https://doi.org/10.1515/teme-2022-0118 SN - 2196-7113 VL - 90 IS - 5 ER - TY - GEN A1 - Gaillard, Yann A1 - Szabo, Peter A1 - Egbers, Christoph T1 - AtmoFlow: Thermo-electrohydrodynamic convection in the thermally driven spherical shell with differential rotation T2 - EGU General Assembly 2023 Y1 - 2023 U6 - https://doi.org/10.5194/egusphere-egu23-1841 ER - TY - GEN A1 - Buchwald, Tom A1 - Hasanuzzaman, Gazi A1 - Merbold, Sebastian A1 - Schanz, Daniel A1 - Egbers, Christoph A1 - Schröder, Andreas T1 - Large-scale flow field and aerosol particle transport investigations in a classroom using 2D-Shake-The-Box Lagrangian Particle Tracking T2 - Heliyon Y1 - 2023 U6 - https://doi.org/10.1016/j.heliyon.2023.e22826 SN - 2405-8440 VL - 9 IS - 12 ER - TY - GEN A1 - Hasanuzzaman, Gazi A1 - Egbers, Christoph T1 - Application of machine learning for sustainable aviation : the role of friction drag and flow control in turbulent boundary layer flows T2 - The 2nd International Conference on Advancing Sustainable Futures (ICASF 2024) Y1 - 2024 U6 - https://doi.org/10.13140/RG.2.2.31060.92807 SP - 1 EP - 8 ER - TY - THES A1 - Hamede, Mohammed Hussein Haytham T1 - The turbulent very wide-gap Taylor-Couette flow : experimental investigation Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:co1-opus4-64456 ER - TY - THES A1 - Shahirpour, Amir T1 - A characteristic dynamic mode decomposition to detect transport-dominated large-scale coherent structures in turbulent wall-bounded flows Y1 - 2024 U6 - https://doi.org/10.26127/BTUOpen-6958 ER - TY - GEN A1 - Simon, Kühne A1 - Yaraslau, Sliavin A1 - Szabo, Peter A1 - Carter, R. A1 - Krebs, Andreas A1 - Egbers, Christoph T1 - Simultaneous particle image velocimetry and background oriented schlieren measurements of convective flows T2 - Experimentelle Strömungsmechanik, Hrsg.: B. Ruck et al. Y1 - 2024 UR - https://www.researchgate.net/publication/384774435_Simultaneous_Particle_Image_Velocimetry_and_Background_Oriented_Schlieren_measurements_of_convective_flows SN - 978-3-9816764-4-0 PB - German Association for Laser Anemometry 26.8 GALA e.V. CY - Karlsruhe ER - TY - GEN A1 - Kühne, Simon A1 - Sliavin, Yaraslau A1 - Szabo, Peter A1 - Carter, Richard A1 - Krebs, Andreas A1 - Egbers, Christoph T1 - Phase-demodulation of interferograms of thermo-electrohydrodynamic convection in a differentially heated cylindrical annulus T2 - Experimentelle Strömungsmechanik : 31. Fachtagung, 3.-5. September 2024, Berlin / Herausgeber: C. Rauh, B. Ruck, A. Leder Y1 - 2024 UR - https://www.researchgate.net/publication/384772995_Phase-demodulation_of_interferograms_of_thermo-electrohydrodynamic_convection_in_a_differentially_heated_cylindrical_annulus SN - 978-3-9816764-4-0 SP - 7.1. EP - 7.8 PB - German Association for Laser Anemometry 26.8 GALA e.V. CY - Karlsruhe ER - TY - GEN A1 - Vincze, Miklos A1 - Hancock, Cathrine A1 - Harlander, Uwe A1 - Rodda, Costanza A1 - Speer, Kevin T1 - Extreme temperature fluctuations in laboratory models of the mid-latitude atmospheric circulation T2 - Scientific reports Y1 - 2023 U6 - https://doi.org/10.1038/s41598-023-47724-2 VL - 13 (2023) ER - TY - GEN A1 - Agaoglou, M. A1 - García-Garrido, V. J. A1 - Harlander, U. A1 - Mancho, A. M. T1 - Building transport models from baroclinic wave experimental data T2 - Physics of Fluids Y1 - 2024 U6 - https://doi.org/10.1063/5.0179875 VL - 36 (2024) IS - 1 SP - 1 EP - 16 PB - AIP Publishing ER - TY - GEN A1 - Harlander, U. A1 - Schön, F.-T. A1 - Borcia, I. D. A1 - Richter, S. A1 - Borcia, R. A1 - Bestehorn, M. T1 - Resonant water-waves in ducts with different geometries: forced KdV solutions T2 - European Journal of Mechanics - B/Fluids Y1 - 2024 U6 - https://doi.org/10.1016/j.euromechflu.2024.03.008 SN - 0997-7546 VL - 106 IS - July–August 2024 SP - 107 EP - 115 ER - TY - GEN A1 - Borcia, I. D. A1 - Bestehorn, M. A1 - Borcia, R. A1 - Schön, F.-T. A1 - Harlander, U. A1 - Richter, S. T1 - Mean flow generated by asymmetric periodic excitation in an annular channel T2 - The European Physical Journal Special Topics Y1 - 2024 U6 - https://doi.org/10.1140/epjs/s11734-024-01181-8 VL - 233 (2024) SP - 1665 EP - 1672 PB - Springer ER - TY - GEN A1 - Schön, F.-T. A1 - Borcia, I. D. A1 - Harlander, U. A1 - Borcia, R. A1 - Richter, S. A1 - Bestehorn, M. T1 - Resonant surface waves in an oscillating periodic tank with a submerged hill T2 - Journal of Fluid Mechanics Y1 - 2024 U6 - https://doi.org/10.1017/jfm.2024.885 VL - 999 (2024) SP - 1 EP - 18 ER - TY - GEN A1 - Harlander, Uwe A1 - Kurgansky, Michael V. A1 - Speer, Kevin A1 - Vincze, Miklos T1 - Baroclinic instability from an experimental perspective T2 - Comptes Rendus Physique Y1 - 2024 U6 - https://doi.org/10.5802/crphys.198 VL - 25 (2024) SP - 1 EP - 48 ER - TY - GEN A1 - Shahirpour, Amir A1 - Egbers, Christoph A1 - Sesterhenn, Jörn T1 - Detection of energetic low dimensional subspaces in spatio-temporal space in turbulent pipe flow T2 - Springer Nature Y1 - 2025 U6 - https://doi.org/10.1007/s10494-024-00600-z IS - Volume 114 SP - 1017 EP - 1041 ER - TY - THES A1 - Schön, Franz-Theo T1 - Transport and waves in parametrically excited fluid layers N2 - The transport and waves in parametrically excited fluid layers play a significant role in an understanding of non-linear surface wave phenomena and tidal resonances. In this thesis, we study resonant waves occurring in a circular channel with various obstacles under external oscillatory excitation. Typically, such sloshing experiments are conducted in rectangular, straight channels. The external excitation is implemented using a rotating table on which the entire experiment, including measurement equipment, is mounted. The excitation is either sinusoidal or ratched motion. The obstacles include a fully blocking barrier and a symmetric or asymmetric hill. The channel circumference is 4.76 m, with water depths ranging from 2 cm to 6 cm. Wave displacements within the channel are measured using 17 ultrasonic sensors equidistantly distributed along half of the channel. Particle Image Velocimetry (PIV) is employed to measure the flow. We also consider a simplified numerical model capable of reproducing the experimental results. This model is based on a long-wave approximation and vertical integration using a profile function (Kármán-Pohlhausen approach). Additionally, we use a wave attractor model to quantitatively explain the development of resonances. These resonances are distributed in bands of the excitation frequency around the linear eigenfrequency. The experimental wave attractor and numerical results are consistent with each other. The waves observed within these resonant frequency bands appear as undular bores or solitary waves. In the fully blocking case, bands of constructive and destructive interference are observed, while in the presence of hills, all eigenfrequencies exhibit resonances of varying intensity. These non-linear wave phenomena are characterized by strong transport properties, which can be studied here due to the fact that the circular channel is not fully blocked. The ratched excitation generated asymmetric wave fields, which also induced asymmetric transport in the channel, leading to the emergence of a mean flow in the channel. A similar mean channel flow is observed for the asymmetric hill; however, wave-induced transport played a lesser role in this case. This is attributed to a large separation vortex on the steep side of the hill, which created a valve effect that rectified part of the oscillatory flow. These results are of interest not only for engineering applications but also for the understanding of tidal flows over seabed topography. N2 - Der Transport und die Wellen in parametrisch angeregten Flüssigkeitsschichten spielt eine große Rolle im Verständnis von nichtlinearen Oberflächenwellenphänomenen und Gezeitenresonanzen. In dieser Arbeit werden resonante Wellen untersucht, die in einem kreisförmigen Kanal mit verschiedenen Hindernissen auftreten, wenn dieser einer externen oszillatorischen Anregung ausgesetzt wird. Normalerweise werden solche Schwapp-Experimente in rechteckigen, geraden Kanälen durchgeführt. Die externe Anregung wird durch einen Rotationstisch realisiert, auf dem sich das gesamte Experiment inklusive Messtechnik befindet. Die Anregung erfolgt entweder sinusoidal oder sägezahnförmig. Die Hindernisse sind eine völlig verschließende Barriere sowie ein symmetrischer oder asymmetrischer Berg. Der mittlere Kanalumfang hat eine Gesamtlänge von 4,76 m, während Wasserhöhen von 2 cm bis 6 cm untersucht werden. Als Messmethode für die Auslenkung der im Kanal befindlichen Wellen werden 17 Ultraschallsensoren verwendet, welche äquidistant über die Hälfte des Kanals verteilt sind. Zur Messung der Strömung wird Particle Image Velocimetry verwendet. Wir betrachten in dieser Arbeit ebenfalls ein vereinfachtes numerisches Modell, das in der Lage ist, die experimentellen Ergebnisse zu reproduzieren. Dieses Modell basiert auf einer Langwellennäherung und der vertikalen Integration über eine Profilfunktion (Kármán-Pohlhausen-Ansatz). Ein Wellenattraktor-Modell beschreibt die Entwicklung von Resonanzen qualitativ, die in Bändern der Anregungsfrequenz um die lineare Eigenfrequenz herum angeordnet sind. Die experimentellen Wellenattraktor- und numerischen Ergebnisse sind konsistent miteinander. Die Wellen, die innerhalb dieser resonanten Frequenzbänder beobachtet werden, treten als gewellter Schwall (undular bore) oder Einzelwelle (solitary wave) auf. Im vollständig blockierenden Fall werden Bänder konstruktiver und destruktiver Interferenz beobachtet, während im Fall der Berge alle Eigenfrequenzen Resonanzen unterschiedlicher Stärke aufweisen. Diese nichtlinearen Wellenphänomene zeichnen sich durch starke Transporteigenschaften aus, die wir hier aufgrund der Kreisförmigkeit des nicht komplett verschlossenen Kanals studieren können. Die sägezahnförmige Anregung erzeugt asymmetrische Wellenfelder, die ebenfalls einen asymmetrischen Transport im Kanal bewirkt, der zum Erscheinen einer Kanalmittelströmung führt. Eine ähnliche Kanalmittelströmung wird für den asymmetrischen Berg beobachtet, allerdings spielt der Transport durch Wellen hier eine untergeordnete Rolle. Dies liegt daran, dass eine große Ablöseblase an der steilen Bergseite einen Ventileffekt erzeugte, die einen Teil der oszillierenden Strömung gleichrichtete. Diese Ergebnisse sind nicht nur für ingenieurtechnische Anwendungen von Interesse, sondern auch für das Verständnis von Gezeitenströmen über Bodenformen. Y1 - 2025 U6 - https://doi.org/10.26127/BTUOpen-6995 PB - Brandenburgische Technische Universität CY - Cottbus ; Senftenberg ER - TY - GEN A1 - Hasanuzzaman, Gazi A1 - Buchwald, Tom A1 - Schunk, Christoph A1 - Schröder, Andreas A1 - Egbers, Christoph A1 - Hampel, Uwe T1 - DATIV—remote enhancement of smart aerosol measurement system using raspberry Pi-based distributed sensors T2 - Sensors N2 - Enclosed public spaces are hotspots for airborne disease transmission. To measure and maintain indoor air quality in terms of airborne transmission, an open source, low cost and distributed array of particulate matter sensors was developed and named Dynamic Aerosol Transport for Indoor Ventilation, or DATIV, system. This system can use multiple particulate matter sensors (PMSs) simultaneously and can be remotely controlled using a Raspberry Pi-based operating system. The data acquisition system can be easily operated using the GUI within any common browser installed on a remote device such as a PC or smartphone with a corresponding IP address. The software architecture and validation measurements are presented together with possible future developments. KW - Aerosol transport, indoor ventilation, sensors, measurement, aerodynamics Y1 - 2024 UR - https://www.mdpi.com/1424-8220/24/13/4314 U6 - https://doi.org/10.3390/s24134314 VL - 24 SP - 1 EP - 12 ER - TY - GEN A1 - Schön, Franz-Theo A1 - Harlander, Uwe A1 - Borcia, Ion Dan A1 - Borcia, Rodica A1 - Bestehorn, Michael T1 - Mean fluid transport in an oscillating circular channel with asymmetric forcing T2 - Water waves : an interdisciplinary journal N2 - We investigate surface waves in an oscillating circular channel with local topography. The focus is on spatially or temporally breaking this dynamic system’s symmetry. Asymmetrical wave dynamics and a mean flux excitation are detected to varying degrees, depending on the two input parameters, fluid depth and the tank’s oscillation frequency. The fluid resonates around multiples of the fundamental eigenfrequency of the channel. The development of solitary wave-trains (undular bores) is observed in these resonance bands. A particle image velocimetry system measures the velocity field in the vertical plane of the free surface flow. Moreover, we are using 17 evenly distributed ultrasonic sensors to measure the surface displacement. This makes it possible to find out how strongly the mean flux depends on the resonance frequencies and to study the influence of the surface waves on the symmetry breaking. A numerical long-wave model helps to isolate the various factors influencing the mean flux. Y1 - 2025 U6 - https://doi.org/10.1007/s42286-025-00121-w SN - 2523-3688 VL - 2025 SP - 1 EP - 21 PB - Birkhäuser, part of Springer Nature CY - Basel ER - TY - GEN A1 - Meletti, Gabriel A1 - Abide, Stéphane A1 - Harlander, Uwe A1 - Raspo, Isabelle A1 - Stéphane Viazzo, Stéphane T1 - On the influence of the heat transfer at the free surface of a thermally driven rotating annulus T2 - Physics of fluids N2 - Experiments on rotating annuli that are differentially heated in the radial direction have largely contributed to a better understanding of baroclinic instabilities. This configuration creates waves at a laboratory scale that are related to atmospheric circulations. Pioneer studies in baroclinic tanks have shown that experiments with low aspect ratios are more suitable to reproduce small-scale inertia gravity waves, but these tanks have a larger free surface, which leads to higher interactions with their surrounding environment. Considering the heat transferred through the free surface, the present work investigates its impacts on the baroclinic instability using direct numerical simulations (DNS). Y1 - 2025 U6 - https://doi.org/10.1063/5.0248712 SN - 1089-7666 VL - 37 SP - 1 EP - 16 PB - AIP Publishing CY - College Park, MD ER - TY - GEN A1 - Szabo, Peter A1 - Egbers, Christoph T1 - Dielectrophoretic-driven thermoelectrohydrodynamic convection in a dielectric fluid layer induced by an inhomogeneous external electric field T2 - Physical review E Y1 - 2025 U6 - https://doi.org/10.1103/PhysRevE.111.045105 SN - 2470-0053 VL - 111 SP - 1 EP - 9 PB - American Physical Society CY - College Park, MD ER - TY - GEN A1 - Travnikov, Vadim A1 - Szabo, Peter A1 - Gaillard, Yann A1 - Egbers, Christoph T1 - Centrifugally-driven spherical gap convection with polar angle-dependent boundary condition : can the Nusselt number fall below unity? T2 - Physics of fluids Y1 - 2025 U6 - https://doi.org/10.1063/5.0281082 SN - 1089-7666 VL - 37 SP - 1 EP - 15 PB - American Institute of Physics CY - New York ER - TY - GEN A1 - Barzantny, Marcel A1 - Hamede, Mohammed Hussein A1 - Majchrzyk, Michał A1 - Merbold, Sebastian A1 - Egbers, Christoph A1 - Kostowski, Wojciech T1 - Experimental investigation of the flow characteristics driving the Ranque–Hilsch phenomenon T2 - International journal of heat and mass transfer N2 - The Ranque–Hilsch vortex tube (RHVT) is a device that separates a pressurized inlet stream into two decompressed streams of different temperature, flowing to the so-called hot and cold outlets. In this study, the flow structures within the vortex tube were examined qualitatively. The examination considered both compressible and incompressible fluids, using pressurized air and water as working fluids. A parametric study was conducted, in which the fluid inlet pressure and the vortex tube length were varied. Three tubes, with the same diameter but differing lengths (100, 180, and 240 mm) were utilized. The flow inside the tube was investigated using a flow visualization technique, which was employed in a variety of configurations and setups contingent on the specific fluid conditions under examination. The visualization process required the use of aerosol injection in the case of air, and kalliroscope particles in the case of water investigation. The research enabled the visualization of the flow structure within the vortex tube, thereby significantly advancing the comprehension of the underlying physical processes. The findings of the experimental research demonstrated the existence of phenomena of considerable scientific value. The internal vortex and its spatial and temporal structure observed in the RHVT were consistent with literature data. This was achieved despite the so-far established consensus that this type of research is challenging and not entirely reliable. In the course of water-based investigation, the cavitation phenomenon was observed in the vicinity of the internal vortex. This discovery is likely to be the first of its kind and may contribute significantly to the advancement of research on the Ranque–Hilsch phenomenon. KW - Ranque–Hilsch phenomenon KW - Vortex tube KW - Flow visualization KW - Cavitation Y1 - 2025 U6 - https://doi.org/10.1016/j.ijheatmasstransfer.2025.127543 SN - 0017-9310 VL - 253 SP - 1 EP - 14 PB - Elsevier BV CY - Amsterdam ER - TY - THES A1 - Haun, Peter T1 - Modelling of thermo-electro hydrodynamic (TEHD) convection N2 - In Thermo-Electro Hydrodynamics (TEHD), an electric field is applied to a fluid within a heated domain to induce thermal convection. The fluid and the electric field must meet specific conditions to establish a dielectrophoretic force that acts as a buoyancy force on the fluid. This buoyancy force is utilised in experiments to replicate gravitational buoyancy, explore resulting flow structures, or develop heat transfer systems without moving parts. In this study, the electric force field acting on a dielectric fluid in a capacitor is derived from the Maxwell equations and coupled with the Navier-Stokes equation for fluid motion. Furthermore, an Open Source Field Operation and Manipulation (OpenFOAM) solver is extended to incorporate TEHD momentum and energy-contributing terms. In a dimensional analysis, dimensionless parameters are derived and tested. Therefore, parameter studies in 2D approximations of planar and cylindrical geometries are done. Additionally, the 2D investigations are utilised to study the behaviour of heat transfer and boundary layer properties, and some scaling laws are derived. Finally, 3D spherical shell microgravity experiments are analysed and linked to the results of 3D numerical analysis. The results verify the derived methods, which are expanded and applied to the upcoming space experiment, AtmoFlow. Y1 - 2025 SN - 978-3-689-52666-5 U6 - https://doi.org/10.61061/ISBN_9783689526665 PB - Cuvillier Verlag CY - Göttingen ET - 1st edition ER - TY - GEN A1 - Durst, Franz A1 - Zanoun, El-Sayed T1 - Laminar pipe flow instability : a theoretical-experimental perspective T2 - Fluids N2 - This paper revisits the theoretically predicted inherent stability of fully developed laminar pipe flow, which remains unconfirmed by experimental evidence. A recently developed theory of pipe-flow stability/instability addresses the gap between experimental observations and classical theoretical predictions by accounting for a parallel secondary flow through the pipe’s roughness layer that accompanies the main stream. This secondary flow alters the near-wall velocity profile in the rough-wall region, creating an inflection point that promotes shear-driven instabilities and triggers the laminar-to-turbulent transition. A stability factor 𝑆=𝐷𝑐/𝐷 is introduced, where D is the nominal pipe diameter and 𝐷𝑐 refers to the critical pipe diameter. The pipe flow remains laminar and stable for 𝑆>1.0, and becomes unstable for 𝑆<1. Various experimental findings are theoretically derived, and the laminar-to-turbulent transition is identified at 𝑆=1.0. Particular attention is paid to the dependence of flow transition on both pipe diameter and pipe length. Rather than relying on a critical Reynolds number 𝑅𝑒𝑐, this study proposes the critical pipe diameter 𝐷𝑐 as the key parameter governing the laminar pipe flow instability, where 𝑅𝑒𝑐 refers here to the condition-dependent threshold at which laminar pipe flow becomes unstable and transition to turbulence occurs. The present analysis further suggests that instability arises only if the pipe length L exceeds a critical threshold 𝐿𝑐, that is, 𝐿>𝐿𝑐. The theoretical treatment presented provides deeper physical insights into the onset of laminar pipe flow instability including the phenomenon of reverse transition. It also distinguishes between natural and forced flow transitions, providing a refined understanding of the transition process. Finally, suggestions for future experimental work are made to further validate or challenge this new theoretical perspective on pipe flow instability. Y1 - 2025 U6 - https://doi.org/10.3390/fluids10080216 SN - 2311-5521 VL - 10 IS - 8 SP - 1 EP - 21 PB - MDPI CY - Basel ER - TY - GEN A1 - Zanoun, El-Sayed A1 - Bauer, Christian A1 - Wagner, Claus A1 - Durst, Franz A1 - Egbers, Christoph A1 - Bellani, Gabriele A1 - Talamelli, Alessandro T1 - Cross-validation of numerical and experimental data in turbulent pipe flow with new scaling correlations T2 - Journal of turbulence N2 - The dependence of turbulence statistics and wall friction on Reynolds number in fully developed turbulent pipe flow remains a fundamental subject in fluid mechanics. This paper cross-validates experimental and numerical results, focusing on the scaling of turbulence statistics at the pipe centerline and across the inner-outer flow region. Pipe flow experiments were reviewed for friction Reynolds numbers 810≤Re𝜏≤55×103, where Re𝜏=𝑢𝜏⁢𝑅/𝜈, 𝑢𝜏 is the wall friction velocity, 𝑅 the pipe radius, and 𝜈 the kinematic viscosity. Complementary DNS data for 180≤Re𝜏≤2880 provide detailed insight into near-wall turbulence. A novel friction correlation, Re𝜏=0.048Re0.923 c is introduced, predicting pipe-wall friction across a wide range of Re𝑐 with accuracy better than ±2.06%, where Re𝑐 is the Reynolds number based on the centerline streamwise mean velocity component 𝑈zc. This correlation enables reliable friction estimates from centerline single-point measurements or DNS data without requiring near-wall or streamwise pressure-gradient information and is validated by consistent agreement with both experiments and DNS. The monotonic decrease in centerline turbulence intensity ⟨𝑢′𝑧2⟩1/2/𝑈zc with increasing Re𝑐 is explained using the streamwise mean momentum equation. Finally, azimuthal spatial filtering of DNS data highlights the limitations of hot-wire resolution near the wall. For Re𝜏≥2880, higher-order experimental statistics agree well with DNS for 𝑦+≥30 and into the logarithmic region, with both datasets equally well described by logarithmic or power-law correlations, while near-wall discrepancies remain due to resolution limits. KW - Pipe flow KW - Turbulence statistics KW - DNS KW - Experiments Y1 - 2025 U6 - https://doi.org/10.1080/14685248.2025.2560314 SP - 1 EP - 22 PB - Taylor & Francis CY - London ER - TY - GEN A1 - Hamede, Mohammed Hussein A1 - Sliavin, Yaraslau A1 - Motuz, Vasyl A1 - Egbers, Christoph T1 - The effect of flow initial conditions and geometry on the saturation of the thermo-electrohydrodynamic instability within microgravity conditions T2 - Physics of fluids N2 - The present study investigates the thermo-electrohydrodynamic-induced convection in a dielectric fluid confined between two concentric differentially heated cylinders under microgravity conditions through experimental means. The experiments were conducted during parabolic flight campaigns. In the present study, our primary focus was on the saturation of perturbations induced by thermo-electrohydrodynamic instability. The effect of initial flow conditions on perturbation growth rates is examined when entering the microgravity phase. Furthermore, a series of experimental setups with varied lengths and aspect ratios were considered to investigate the impact of geometry on flow behavior. The present study quantitatively investigated the flow using particle image velocimetry in the meridional and radial–azimuthal planes. Furthermore, direct measurements were taken of the global response of the flow, in this case heat transfer, using a heat flux sensor attached to the outer cylinder wall. KW - Heat transfer KW - Electrostatics KW - Microgravity KW - Dielectric materials KW - Fluid instabilities KW - Fluid dynamics KW - Flow instabilities KW - Flow visualization Y1 - 2025 U6 - https://doi.org/10.1063/5.0287547 VL - 37 IS - 9 SP - 1 EP - 10 PB - AIP Publishing CY - College Park, Maryland ER - TY - GEN A1 - Kriese, Maximilian A1 - Lockan, Michael A1 - Schaffrath, Robert A1 - Stathopoulos, Panagiotis T1 - Quasi-static simulation of a start-up procedure for a multistage centrifugal compression system T2 - Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics & Instrumentation; Cycle Innovations; Education; Electric Power N2 - In order to achieve the European Union’s climate protection targets by 2050, energy use in the private and industrial sectors must be switched from using fossil fuels to renewable energy. For industry, this might require that the production processes itself needed to be changed or new technologies to manufacture in a sustainable way needed to be introduced. One example of the latter mentioned are high-temperature heat pumps (HTHP), which will be capable to generate sustainable process heat up to 300°C e.g. for the pulp and paper or food industry. There are currently only a few HTHPs commercially available that can reach temperatures above 150°C. In order to overcome the technological challenges to reach 300°C, two HTHP prototypes are currently developed at the Institute of Low-Carbon Industrial Processes of the German Aerospace Centre (DLR). One of them is the pilot plant ZiRa which is based on the reversed Rankine process and uses steam as its working medium. Herein three centrifugal compressors are applied to achieve a saturated steam temperature of 200°C at the sink. The safe speed-up of these three compressors, titled as the start-up procedure, from the standstill of each compressors up to their respective nominal speed, is modelled in the flow chart software EBSILON Professional. In addition to the compressors, all necessary components, i.e. the intercoolers, the fittings and valves, the pipes, the separators and the bypasses are modelled so that all parameters influencing the thermodynamic inlet state of the respective compressor are considered. The start-up of the multi-stage compression system of the pilot plan ZiRa is shown as a quasi-static time series. As a first step, the speed increments are defined. Afterwards, absolute pressures between 1.25 and 2 bar upstream of the first compressor and their effect on the operation of the subsequent compressor stages, are analyzed. Therefore, the mass flow rates and rotational speeds are specified manually at first so that an executable, editable model is created. Based on these results, the operation of the compressors are optimized in order to ensure a surge margin of 15 % and to avoid choke in each operating point. KW - High temperature heat pump KW - Compressor startup KW - Reverse Rankine cycle KW - Process simulation Y1 - 2025 SN - 978-0-7918-8880-3 U6 - https://doi.org/10.1115/GT2025-151935 PB - American Society of Mechanical Engineers CY - New York, NY ER - TY - GEN A1 - Vankelekom, Christophe A1 - Stathopoulos, Panagiotis A1 - Demeyer, Frederiek A1 - De Paepe, Ward ED - Vankelekom, Christophe T1 - Thermodynamic performance assessment of recuperated Brayton cycle high-temperature heat pumps for combined heat and cold roduction T2 - Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics & Instrumentation; Cycle Innovations; Education; Electric Power N2 - Producing heat at high temperatures for industrial processes like chemistry, food, or paper is still achieved using fossil fuels. To reach net-zero CO2 emissions by 2050, as desired by the European Union, technology driven by renewable energy must emerge to electrify these sectors. For this reason, high-temperature heat pumps, based on a Reversed Brayton Cycle working with air, offer a promising solution. Indeed, nowadays, most high-temperature heat pumps are vapor compression cycles but they face several issues when a higher temperature is necessary (200 °C or more). The selection of refrigerant is challenging (high compression ratio and temperature limitation), and it has a non-negligible environmental impact. Nevertheless, the Reversed Brayton cycle still suffers from a low Coefficient Of Performance (COP) that could be improved by valorizing the cold heat flux produced in the cycle. In the literature, few works are related to this combined use of heating and cooling for sub-MWth applications, which is very important for the food industry (cooking and freezing). With this work, we aim to fill this gap by proposing an analysis of the achievable potential based on the 2nd law of thermodynamics using a generic model in Aspen Plus. Using a heat sink of 250 °C and 300 °C and a heat source of −10 °C, the potential of different cycles were analyzed by varying the secondary inlet temperature of the heat sink from 100 °C to 200 °C. For low inlet temperatures, the heating and total COP can reach a value of up to 1.3 and 1.66 respectively with an exergetic efficiency up to 54%. The COP decreases when the secondary inlet temperature of the heat sink increases while the exergy efficiency stays almost constant. A further increase does not allow to provide cooling anymore for lower outlet heat sink temperature. KW - High-temperature heat pumps KW - Heating and cooling KW - Reversed Brayton cycle KW - Exergy analysis Y1 - 2025 SN - 978-0-7918-8880-3 U6 - https://doi.org/10.1115/GT2025-153988 PB - American Society of Mechanical Engineers CY - New York, NY ER - TY - GEN A1 - Oehler, Johannes A1 - Yücel, Fatma Cansu A1 - Stathopoulos, Panagiotis T1 - Experimental performance analysis of a 50 kW Brayton turbomachine heat pump demonstrator T2 - Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics & Instrumentation; Cycle Innovations; Education; Electric Power N2 - The high-temperature heat pump “CoBra” is the first demonstrator of a closed-loop Brayton heat pump realizing sink temperatures above 150 °C. The experimental results can proof the feasibility of a turbomachine-driven Brayton heat pump delivering heat and cold simultaneously. The German Aerospace Center (DLR) developed the Cottbus Brayton cycle heat pump “CoBra”, a closed-loop Brayton heat pump utilizing dry air as the working medium. The CoBra is fully operational and has been used for experiments since spring 2024. This work evaluates the steady-state performance of the CoBra heat pump demonstrator experimentally for the first time. In the current setup, the heat pump achieves a heating capacity of up to 55 kW and can supply process heat up to 170°C with a coefficient of performance (COP) of 1.39 using air at TIII = 15 °C as sole heat source. The heat pump’s operational envelope and performance in terms of supplied thermal power, supply temperature and efficiency is analyzed for different operating conditions. Increasing compressor speed raises both heating power and supply temperatures. Using recuperation raises both supply temperature and COP. Fluid inventory control is experimentally confirmed to be a promising control strategy for closed-loop Brayton machines. It widens the operational envelope towards part load operation at constant supply temperatures while maintaining constant efficiency. Operating points in the compressor map remain on the same operating line, thus compressor stability is not impacted by this control method. KW - High-temperature heat pump KW - Post-fossile industry KW - Experimental analysis KW - Fluid inventory control Y1 - 2025 SN - 978-0-7918-8880-3 U6 - https://doi.org/10.1115/GT2025-152419 PB - American Society of Mechanical Engineers CY - New York, NY ER - TY - GEN A1 - Kabat, Nancy A1 - Jende, Enrico A1 - Yücel, Fatma Cansu A1 - Stathopoulos, Panagiotis T1 - Thermodynamic analysis of a novel high-temperature heat pump cycle configuration based on the reversed Brayton cycle T2 - International journal of sustainable energy N2 - High-temperature heat pumps offer a promising approach to deliver CO$_2$-neutral process heat by upgrading waste heat with electricity from renewable resources. In this work, a heat pump configuration including multiple heat sinks and sources is presented and compared to the recuperated Brayton cycle. An exergy analysis is conducted by using numerical simulations to provide process heat at 250°C and process cooling at -30°C simultaneously. The results show, that a multi-stage compression leads to an improved overall performance for simultaneous heating and cooling. A parametric study presents the influence of the isentropic efficiency, heat exchanger effectiveness and heat sink and source inlet temperature on the second law efficiency. This study serves as a basis to gain a better understanding on the physics of the novel cycle, highlighting the potentials of improving the efficiency of Brayton heat pumps with cycle layouts adaptation and identify the most important components for minimizing exergy losses. Y1 - 2025 SN - 1478-6451 VL - 44 IS - 1 SP - 1 EP - 37 PB - Taylor & Francis CY - Abingdon, Oxon ER - TY - GEN A1 - Tran, A. Phong A1 - Stathopoulos, Panagiotis T1 - Dynamic simulation and experimental validation of a high-temperature Brayton heat pump T2 - Applied thermal engineering N2 - The decarbonization of industrial process heating will require widespread adoption of high-temperature heat pumps. Brayton cycle heat pumps are capable of providing heat at temperatures that currently cannot be achieved by conventional vapor-compression cycle heat pumps. However, significant challenges remain in adapting these systems to industrial applications, particularly with regard to operational safety, control strategies, and flexibility in response to varying operational conditions. This study presents a dynamic model of a closed-loop Brayton cycle heat pump capable of producing temperatures of 250 °C and higher, validated using experimental data. The physics-based model implemented in Modelica captures key thermodynamic processes and system dynamics, including thermal inertia and volume dynamics. An optimization-based method is used to calibrate model parameters, minimizing the error between measured and simulated data. Given the significant impact of the compressor on overall heat pump performance, a novel calibration method is introduced to adjust an existing compressor map using limited measurement data. This approach ensures that the compressor behavior is represented with sufficient accuracy, smoothness, and numerical robustness. The calibrated model achieves mean-normalized root mean squared errors (NRMSE) ranging from 0.12 % to 1.46 % for temperatures, pressures, and mass flow rates. The model is applied to examine the system’s start-up and deceleration sequences, offering insights into compressor stability and heat exchanger temperature profiles. These results demonstrate the model’s utility for control design, performance evaluation, and stability analysis. KW - High temperature heat pump KW - Brayton cycle KW - Dynamic simulation KW - Modelica KW - Process heat Y1 - 2025 U6 - https://doi.org/10.1016/j.applthermaleng.2025.126536 SN - 1359-4311 VL - 274, Part B SP - 1 EP - 13 PB - Elsevier BV CY - Amsterdam ER - TY - GEN A1 - Schaffrath, Robert A1 - Stathopoulos, Panagiotis A1 - Schmitz, Andreas A1 - Nicke, Eberhard T1 - Multistage turbomachinery optimization for high-temperature heat pumps with the reverse rankine cycle T2 - Journal of turbomachinery N2 - The electrification of process heat generation will be a key to achieving carbon neutrality in the coming decades. One of the most promising approaches is to replace conventional heat supply systems with high-temperature heat pumps (HTHPs). A promising heat pump concept is based on the reverse Rankine cycle that uses water as its working fluid. By using turbomachinery for the compression process in this cycle, the performance of the HTHP can be increased compared to the volumetric displacement systems, like screw or piston compressors. Although the design of the compressor geometry can be done sequentially in relation to the HTHP cycle design, better results can be obtained by an approach that integrates turbomachinery and the thermodynamic cycle design. Against this background, an automated optimization method for a reverse Rankine HTHP with two radial turbo-compressors in series is presented. In contrast to the current state of the art, the presented novel optimization approach uses 3D computational fluid dynamics data to calculate the compressor’s performance. Furthermore, the integration of low-fidelity compressor specific reduced-order models are used to accelerate the gradient-free optimization process by a CO-Kriging surrogate model. The advantages of the novel approach are justified by comparing the numerical effort and the final values of the optimization objectives. KW - Reverse Rankine cycle KW - Turbomachinery optimization KW - Centrifugal compressor KW - Centrifugal compressors and pumps KW - Compressor Y1 - 2025 U6 - https://doi.org/10.1115/1.4068480 SN - 1528-8900 VL - 147 IS - 111003 SP - 111003-1 EP - 111003-12 PB - ASME CY - New York, NY ER - TY - GEN A1 - Abu Khass, Omar A1 - Cristofaro, Marco A1 - Kopparthy, Saketh Bharadwaj A1 - Klöppel, Steffen A1 - Nicke, Eberhard A1 - Stathopoulos, Panagiotis T1 - Numerical investigation of two-phase water ejectors for high-temperature heat pumps : insights into flow behavior and shock wave dynamics T2 - Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics & Instrumentation; Cycle Innovations; Education; Electric Power N2 - Two-phase water ejectors can serve as a secondary steam compression mechanism in high-temperature heat pump (HTHP) systems. By integrating an ejector, high-pressure water can be combined with hot steam from the compressor, achieving simultaneous cooling and pressure increase. This integration offers the potential to reduce both the power and the number of stages required to obtain the specified compression. However, the complex flow behavior within two-phase water ejectors, especially under high-pressure, high-temperature conditions, remains insufficiently explored in the literature. This study addresses this gap by conducting a detailed numerical investigation of two-phase flow and shock wave behavior using the compressible two-phase mixture approach in computational fluid dynamics (CFD) commercial solver Ansys Fluent. The ejector geometry and critical operating points were derived from a prior study using one-dimensional (1D) analysis, which provided the design and operational conditions used in the current simulations. The ejector was modeled in two-dimensional (2D) axisymmetric configurations. The Lee model, in conjunction with the water saturation curve, was applied to capture non-equilibrium mass transfer processes, including evaporation and condensation. During the simulations, the presence of a liquid-vapor mixture may cause the speed of sound to drop, making the flow locally supersonic. Initial simulations revealed a series of shock waves in the mixing section, which elevated the mixture flow pressure to the designated set back-pressure value, thereby achieving an ejector pressure ratio of approximately 1.3. These findings provide crucial insights into how thermodynamic conditions influence two-phase flow behavior, particularly in the generation and intensity of shock waves. This work advances the modeling procedures for two-phase ejectors and enhances the understanding of the physical phenomena occurring within two-phase water ejectors designed for HTHP applications. KW - Two-phase ejector KW - High-temperature heat pump KW - Multiphase CFD simulation KW - Steam compression Y1 - 2025 SN - 978-0-7918-8880-3 U6 - https://doi.org/10.1115/GT2025-151706 PB - American Society of Mechanical Engineers CY - New York, NY ER - TY - GEN A1 - Kabat, Nancy A1 - Oehler, Johannes A1 - Stathopoulos, Panagiotis T1 - Experimental exergy analysis of a high-temperature Brayton heat pump T2 - Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics & Instrumentation; Cycle Innovations; Education; Electric Power N2 - High-temperature heat pumps (HTHP) are a promising technology to provide emission free process heat at high temperatures. HTHPs available on the market can provide process heat up to 150°C. However, many industrial processes, especially in the food industry, require higher temperatures, so the industrial sector remains one of the largest emitters of greenhouse gases due to the burning of fossil fuels such as coal, natural gas and oil. To advance the decarbonization of industry towards climate neutrality and drive forward the development of HTHPs for the provision of industrial process heat, challenges such as the temperature level to be achieved while maintaining high efficiency must be addressed to ensure applicability for the end user. Despite the knowledge of the relevance of heat pumps for the decarbonization of industry, there are only a few experimental heat pumps that work with high temperature lifts at sink temperatures above 150°C. This paper shows first experimental results of a HTHP demonstrator based on the reversed Brayton cycle with air as working medium for the simultaneous provision of process heat above 150°C and process cooling below 0°C. The design of the heat pump and the experimental test procedure are explained, followed by the execution of an exergy analysis to evaluate the efficiency of the system and the components, taking into account exergy losses in order to identify optimization potential. The HTHP demonstrator called “CoBra” at the Institute of Low-Carbon Industrial Processes of the German Aerospace Center (DLR) in Cottbus consists of two radial compressors, one turbine and three shell and tube heat exchangers. One of the heat exchangers is used as a recuperator. Dry ambient air is used as the working medium in the primary and secondary cycles. The tests are performed without and with recuperation reaching heat sink temperatures of 158°C and 168°C and heat source temperatures of 11°C and −3°C. A temperature lift of 130 K and 134 K is achieved and COPs of 1.37 and 1.4, respectively. The exergy analysis indicates the compressor to be the component with the highest optimization potential with a relative irreversibility of around 50%. The high-temperature heat exchanger has the highest exergy efficiency of all components with around 75%. The advantages of using a recuperator are confirmed by the increase in the overall heat pump performance. The present study provides new insights into the actual behavior of a heat pump that simultaneously provides process heat above 150 °C and process cooling below 0 °C, while also demonstrating the feasibility of such a system. This work contributes to the benefits and feasibility of Brayton heat pumps for industrial applications. KW - High-temperature heat pump KW - Brayton cycle KW - Recuperation KW - Exergy analysis Y1 - 2025 SN - 978-0-7918-8880-3 U6 - https://doi.org/10.1115/GT2025-153648 PB - American Society of Mechanical Engineers CY - New York, NY ER - TY - GEN A1 - Kabat, Nancy A1 - Jende, Enrico A1 - Yücel, Fatma Cansu A1 - Stathopoulos, Panagiotis T1 - Thermodynamic analysis of a novel high-temperature heat pump cycle with inter cooled compression and reheated expansion based on the reversed Brayton cycle T2 - International journal of sustainable energy N2 - High temperature heat pumps offer a promising approach to deliver CO2-neutral process heat by upgrading waste heat with electricity from renewable resources. In this work, a heat pump cycle including inter cooled compression and reheated expansion is presented and compared to the simple Brayton cycle. Numerical simulations are used to investigate the provision of process heat at 250°C and process cooling at -30°C. An exergy analysis and parameter study is carried out. The results show an improved performance of the complex cycle as a result of more efficient temperature profiles improving the exergy utilisation. While previous studies often consider idealised process variants or are focused on individual purposes, the present work contributes to the analysis and evaluation of complex cycles with multi-stage process control while simultaneously supplying heat and cold. A deeper understanding of the thermodynamic interactions provides a reliable basis for the energy-related design of systems with multiple temperature levels. KW - High-temperature heat pump KW - Brayton cycle KW - Inter cooled compression KW - Reheated expansion KW - Exergy analysis KW - Parametric study Y1 - 2025 U6 - https://doi.org/10.1080/14786451.2025.2515452 SN - 1478-6451 VL - 44 IS - 1 SP - 1 EP - 37 PB - Taylor & Francis CY - Abingdon, Oxon ER - TY - GEN A1 - Dalal, Varshil Sujal A1 - Klöppel, Steffen A1 - Stathopoulos, Panagiotis T1 - CO2 heat pumps in the drying industry : identifying the break-even point between transcritical and supercritical operation T2 - 9th European Drying Conference N2 - In the drying industry, energy-efficient heat recovery systems are essential for optimizing thermal processes and reducing operational costs. High-temperature heat pumps, particularly CO2-based systems, offer promising potential due to their favourable thermodynamic properties and environmental benefits. Drying processes typically operate at temperatures ranging from 60°C to 220°C, with hot air commonly used as the medium for heat transfer. Transcritical and supercritical CO2 heat pumps each have unique advantages depending on the temperature and pressure conditions of the process. This research investigates the performance characteristics of trans-critical and supercritical CO2 heat pumps in industrial drying applications, while identifying the break-even point between these two configurations, focusing on key parameters such as system efficiency and operational conditions. Thermodynamic modelling and performance simulations are employed to assess how variations in source temperature, system design, and operating conditions affect the choice between transcritical and Supercritical operation. Understanding this break-even point is crucial for optimizing CO2 heat pump systems for drying processes, ensuring that the system operates at peak efficiency while meeting the thermal demands of industrial drying. By examining these configurations in-depth, the study aims to contribute valuable insights into the potential for CO2 heat pumps to enhance energy efficiency, reduce environmental impact, and support more sustainable practices in the drying industry. Y1 - 2025 UR - https://elib.dlr.de/215301/ ER -