@misc{HarlanderSukhanovskiiAbideetal., author = {Harlander, Uwe and Sukhanovskii, Andrei and Abide, St{\´e}phane and Borcia, Ion-Dan and Popova, Elene and Rodda, Costanza and Vasiliev, Andrei and Vincze, Miklos}, title = {New Laboratory Experiments to Study the Large-Scale Circulation and Climate Dynamics}, series = {Atmosphere}, volume = {14}, journal = {Atmosphere}, number = {5}, doi = {10.3390/atmos14050836}, pages = {19}, abstract = {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.}, language = {en} } @misc{MalteseMelettideOliveiraAbideViazzoetal., author = {Maltese Meletti de Oliveira, Gabriel and Abide, St{\´e}phane and Viazzo, Stephane and Harlander, Uwe}, title = {A parameter study of strato-rotational low-frequency modulations: impacts on momentum transfer and energy distribution}, series = {Philosophical transactions of the Royal Society : Series A, Mathematical, physical and engineering sciences}, volume = {381}, journal = {Philosophical transactions of the Royal Society : Series A, Mathematical, physical and engineering sciences}, number = {2246}, issn = {1364-503X}, doi = {10.1098/rsta.2022.0297}, language = {en} } @misc{MerboldHamedeFroitzheimetal., author = {Merbold, Sebastian and Hamede, Mohammed Hussein and Froitzheim, Andreas and Egbers, Christoph}, title = {Flow regimes in a very wide-gap Taylo-Couette flow with counter rotating cylinders}, series = {Philosophical transactions of the Royal Society : Series A, Mathematical, physical and engineering sciences}, volume = {381}, journal = {Philosophical transactions of the Royal Society : Series A, Mathematical, physical and engineering sciences}, number = {2246}, issn = {1364-503X}, doi = {10.1098/rsta.2022.0113}, language = {en} } @misc{HamedeMerboldEgbers, author = {Hamede, Mohammed Hussein and Merbold, Sebastian and Egbers, Christoph}, title = {Experimental methods for investigating the formation of flow patterns in a very wide gap Taylor-Couette flow (\$\eta=0.1\$)}, series = {tm - Technisches Messen}, volume = {90}, journal = {tm - Technisches Messen}, number = {5}, issn = {2196-7113}, doi = {10.1515/teme-2022-0107}, pages = {332 -- 339}, language = {en} } @misc{MerboldHasanuzzamanBuchwaldetal., author = {Merbold, Sebastian and Hasanuzzaman, Gazi and Buchwald, Tom and Schunk, Christoph and Schmeling, Daniel and Volkmann, Andr{\´e} and Brinkema, Robert and Hampel, Uwe and Schr{\"o}der, Andreas and Egbers, Christoph}, title = {Reference experiment on aerosol particle transport for dynamic situations}, series = {tm - Technisches Messen}, volume = {90}, journal = {tm - Technisches Messen}, number = {5}, issn = {2196-7113}, doi = {10.1515/teme-2022-0118}, pages = {13}, language = {en} } @misc{GaillardSzaboEgbers, author = {Gaillard, Yann and Szabo, Peter and Egbers, Christoph}, title = {AtmoFlow: Thermo-electrohydrodynamic convection in the thermally driven spherical shell with differential rotation}, series = {EGU General Assembly 2023}, journal = {EGU General Assembly 2023}, doi = {10.5194/egusphere-egu23-1841}, language = {en} } @misc{BuchwaldHasanuzzamanMerboldetal., author = {Buchwald, Tom and Hasanuzzaman, Gazi and Merbold, Sebastian and Schanz, Daniel and Egbers, Christoph and Schr{\"o}der, Andreas}, title = {Large-scale flow field and aerosol particle transport investigations in a classroom using 2D-Shake-The-Box Lagrangian Particle Tracking}, series = {Heliyon}, volume = {9}, journal = {Heliyon}, number = {12}, issn = {2405-8440}, doi = {10.1016/j.heliyon.2023.e22826}, language = {en} } @misc{HasanuzzamanEgbers, author = {Hasanuzzaman, Gazi and Egbers, Christoph}, title = {Application of machine learning for sustainable aviation : the role of friction drag and flow control in turbulent boundary layer flows}, series = {The 2nd International Conference on Advancing Sustainable Futures (ICASF 2024)}, journal = {The 2nd International Conference on Advancing Sustainable Futures (ICASF 2024)}, doi = {10.13140/RG.2.2.31060.92807}, pages = {1 -- 8}, language = {en} } @phdthesis{Hamede, author = {Hamede, Mohammed Hussein Haytham}, title = {The turbulent very wide-gap Taylor-Couette flow : experimental investigation}, doi = {10.26127/BTUOpen-6445}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-64456}, language = {en} } @phdthesis{Shahirpour, author = {Shahirpour, Amir}, title = {A characteristic dynamic mode decomposition to detect transport-dominated large-scale coherent structures in turbulent wall-bounded flows}, doi = {10.26127/BTUOpen-6958}, pages = {xxxv, 157}, language = {en} } @misc{SimonYaraslauSzaboetal., author = {Simon, K{\"u}hne and Yaraslau, Sliavin and Szabo, Peter and Carter, R. and Krebs, Andreas and Egbers, Christoph}, title = {Simultaneous particle image velocimetry and background oriented schlieren measurements of convective flows}, series = {Experimentelle Str{\"o}mungsmechanik, Hrsg.: B. Ruck et al.}, journal = {Experimentelle Str{\"o}mungsmechanik, Hrsg.: B. Ruck et al.}, publisher = {German Association for Laser Anemometry 26.8 GALA e.V.}, address = {Karlsruhe}, isbn = {978-3-9816764-4-0}, language = {en} } @misc{KuehneSliavinSzaboetal., author = {K{\"u}hne, Simon and Sliavin, Yaraslau and Szabo, Peter and Carter, Richard and Krebs, Andreas and Egbers, Christoph}, title = {Phase-demodulation of interferograms of thermo-electrohydrodynamic convection in a differentially heated cylindrical annulus}, series = {Experimentelle Str{\"o}mungsmechanik : 31. Fachtagung, 3.-5. September 2024, Berlin / Herausgeber: C. Rauh, B. Ruck, A. Leder}, journal = {Experimentelle Str{\"o}mungsmechanik : 31. Fachtagung, 3.-5. September 2024, Berlin / Herausgeber: C. Rauh, B. Ruck, A. Leder}, publisher = {German Association for Laser Anemometry 26.8 GALA e.V.}, address = {Karlsruhe}, isbn = {978-3-9816764-4-0}, pages = {7.1. -- 7.8}, language = {en} } @misc{VinczeHancockHarlanderetal., author = {Vincze, Miklos and Hancock, Cathrine and Harlander, Uwe and Rodda, Costanza and Speer, Kevin}, title = {Extreme temperature fluctuations in laboratory models of the mid-latitude atmospheric circulation}, series = {Scientific reports}, volume = {13 (2023)}, journal = {Scientific reports}, doi = {10.1038/s41598-023-47724-2}, pages = {13}, language = {en} } @misc{AgaoglouGarciaGarridoHarlanderetal., author = {Agaoglou, M. and Garc{\´i}a-Garrido, V. J. and Harlander, U. and Mancho, A. M.}, title = {Building transport models from baroclinic wave experimental data}, series = {Physics of Fluids}, volume = {36 (2024)}, journal = {Physics of Fluids}, number = {1}, publisher = {AIP Publishing}, doi = {10.1063/5.0179875}, pages = {1 -- 16}, language = {en} } @misc{HarlanderSchoenBorciaetal., author = {Harlander, U. and Sch{\"o}n, F.-T. and Borcia, I. D. and Richter, S. and Borcia, R. and Bestehorn, M.}, title = {Resonant water-waves in ducts with different geometries: forced KdV solutions}, series = {European Journal of Mechanics - B/Fluids}, volume = {106}, journal = {European Journal of Mechanics - B/Fluids}, number = {July-August 2024}, issn = {0997-7546}, doi = {10.1016/j.euromechflu.2024.03.008}, pages = {107 -- 115}, language = {en} } @misc{BorciaBestehornBorciaetal., author = {Borcia, I. D. and Bestehorn, M. and Borcia, R. and Sch{\"o}n, F.-T. and Harlander, U. and Richter, S.}, title = {Mean flow generated by asymmetric periodic excitation in an annular channel}, series = {The European Physical Journal Special Topics}, volume = {233 (2024)}, journal = {The European Physical Journal Special Topics}, publisher = {Springer}, doi = {10.1140/epjs/s11734-024-01181-8}, pages = {1665 -- 1672}, language = {en} } @misc{SchoenBorciaHarlanderetal., author = {Sch{\"o}n, F.-T. and Borcia, I. D. and Harlander, U. and Borcia, R. and Richter, S. and Bestehorn, M.}, title = {Resonant surface waves in an oscillating periodic tank with a submerged hill}, series = {Journal of Fluid Mechanics}, volume = {999 (2024)}, journal = {Journal of Fluid Mechanics}, doi = {10.1017/jfm.2024.885}, pages = {1 -- 18}, language = {en} } @misc{HarlanderKurganskySpeeretal., author = {Harlander, Uwe and Kurgansky, Michael V. and Speer, Kevin and Vincze, Miklos}, title = {Baroclinic instability from an experimental perspective}, series = {Comptes Rendus Physique}, volume = {25 (2024)}, journal = {Comptes Rendus Physique}, doi = {10.5802/crphys.198}, pages = {1 -- 48}, language = {en} } @misc{ShahirpourEgbersSesterhenn, author = {Shahirpour, Amir and Egbers, Christoph and Sesterhenn, J{\"o}rn}, title = {Detection of energetic low dimensional subspaces in spatio-temporal space in turbulent pipe flow}, series = {Springer Nature}, journal = {Springer Nature}, number = {Volume 114}, doi = {10.1007/s10494-024-00600-z}, pages = {1017 -- 1041}, language = {en} } @phdthesis{Schoen, author = {Sch{\"o}n, Franz-Theo}, title = {Transport and waves in parametrically excited fluid layers}, publisher = {Brandenburgische Technische Universit{\"a}t}, address = {Cottbus ; Senftenberg}, doi = {10.26127/BTUOpen-6995}, pages = {xvi, 133}, abstract = {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 (K{\´a}rm{\´a}n-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.}, language = {en} } @misc{HasanuzzamanBuchwaldSchunketal., author = {Hasanuzzaman, Gazi and Buchwald, Tom and Schunk, Christoph and Schr{\"o}der, Andreas and Egbers, Christoph and Hampel, Uwe}, title = {DATIV—remote enhancement of smart aerosol measurement system using raspberry Pi-based distributed sensors}, series = {Sensors}, volume = {24}, journal = {Sensors}, doi = {10.3390/s24134314}, pages = {1 -- 12}, abstract = {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.}, language = {en} } @misc{SchoenHarlanderBorciaetal., author = {Sch{\"o}n, Franz-Theo and Harlander, Uwe and Borcia, Ion Dan and Borcia, Rodica and Bestehorn, Michael}, title = {Mean fluid transport in an oscillating circular channel with asymmetric forcing}, series = {Water waves : an interdisciplinary journal}, volume = {2025}, journal = {Water waves : an interdisciplinary journal}, publisher = {Birkh{\"a}user, part of Springer Nature}, address = {Basel}, issn = {2523-3688}, doi = {10.1007/s42286-025-00121-w}, pages = {1 -- 21}, abstract = {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.}, language = {en} } @misc{MelettiAbideHarlanderetal., author = {Meletti, Gabriel and Abide, St{\´e}phane and Harlander, Uwe and Raspo, Isabelle and St{\´e}phane Viazzo, St{\´e}phane}, title = {On the influence of the heat transfer at the free surface of a thermally driven rotating annulus}, series = {Physics of fluids}, volume = {37}, journal = {Physics of fluids}, publisher = {AIP Publishing}, address = {College Park, MD}, issn = {1089-7666}, doi = {10.1063/5.0248712}, pages = {1 -- 16}, abstract = {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).}, language = {en} } @misc{SzaboEgbers, author = {Szabo, Peter and Egbers, Christoph}, title = {Dielectrophoretic-driven thermoelectrohydrodynamic convection in a dielectric fluid layer induced by an inhomogeneous external electric field}, series = {Physical review E}, volume = {111}, journal = {Physical review E}, publisher = {American Physical Society}, address = {College Park, MD}, issn = {2470-0053}, doi = {10.1103/PhysRevE.111.045105}, pages = {1 -- 9}, language = {en} } @misc{TravnikovSzaboGaillardetal., author = {Travnikov, Vadim and Szabo, Peter and Gaillard, Yann and Egbers, Christoph}, title = {Centrifugally-driven spherical gap convection with polar angle-dependent boundary condition : can the Nusselt number fall below unity?}, series = {Physics of fluids}, volume = {37}, journal = {Physics of fluids}, publisher = {American Institute of Physics}, address = {New York}, issn = {1089-7666}, doi = {10.1063/5.0281082}, pages = {1 -- 15}, language = {en} } @misc{BarzantnyHamedeMajchrzyketal., author = {Barzantny, Marcel and Hamede, Mohammed Hussein and Majchrzyk, Michał and Merbold, Sebastian and Egbers, Christoph and Kostowski, Wojciech}, title = {Experimental investigation of the flow characteristics driving the Ranque-Hilsch phenomenon}, series = {International journal of heat and mass transfer}, volume = {253}, journal = {International journal of heat and mass transfer}, publisher = {Elsevier BV}, address = {Amsterdam}, issn = {0017-9310}, doi = {10.1016/j.ijheatmasstransfer.2025.127543}, pages = {1 -- 14}, abstract = {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.}, language = {en} } @phdthesis{Haun, author = {Haun, Peter}, title = {Modelling of thermo-electro hydrodynamic (TEHD) convection}, edition = {1st edition}, publisher = {Cuvillier Verlag}, address = {G{\"o}ttingen}, isbn = {978-3-689-52666-5}, doi = {10.61061/ISBN_9783689526665}, pages = {XXIV, 369}, abstract = {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.}, language = {en} } @misc{DurstZanoun, author = {Durst, Franz and Zanoun, El-Sayed}, title = {Laminar pipe flow instability : a theoretical-experimental perspective}, series = {Fluids}, volume = {10}, journal = {Fluids}, number = {8}, publisher = {MDPI}, address = {Basel}, issn = {2311-5521}, doi = {10.3390/fluids10080216}, pages = {1 -- 21}, abstract = {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.}, language = {en} } @misc{ZanounBauerWagneretal., author = {Zanoun, El-Sayed and Bauer, Christian and Wagner, Claus and Durst, Franz and Egbers, Christoph and Bellani, Gabriele and Talamelli, Alessandro}, title = {Cross-validation of numerical and experimental data in turbulent pipe flow with new scaling correlations}, series = {Journal of turbulence}, journal = {Journal of turbulence}, publisher = {Taylor \& Francis}, address = {London}, doi = {10.1080/14685248.2025.2560314}, pages = {1 -- 22}, abstract = {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.}, language = {en} } @misc{HamedeSliavinMotuzetal., author = {Hamede, Mohammed Hussein and Sliavin, Yaraslau and Motuz, Vasyl and Egbers, Christoph}, title = {The effect of flow initial conditions and geometry on the saturation of the thermo-electrohydrodynamic instability within microgravity conditions}, series = {Physics of fluids}, volume = {37}, journal = {Physics of fluids}, number = {9}, publisher = {AIP Publishing}, address = {College Park, Maryland}, doi = {10.1063/5.0287547}, pages = {1 -- 10}, abstract = {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.}, language = {en} } @misc{KrieseLockanSchaffrathetal., author = {Kriese, Maximilian and Lockan, Michael and Schaffrath, Robert and Stathopoulos, Panagiotis}, title = {Quasi-static simulation of a start-up procedure for a multistage centrifugal compression system}, series = {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}, journal = {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}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-151935}, pages = {12}, abstract = {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.}, language = {en} } @misc{VankelekomStathopoulosDemeyeretal., author = {Vankelekom, Christophe and Stathopoulos, Panagiotis and Demeyer, Frederiek and De Paepe, Ward}, title = {Thermodynamic performance assessment of recuperated Brayton cycle high-temperature heat pumps for combined heat and cold roduction}, series = {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}, journal = {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}, editor = {Vankelekom, Christophe}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-153988}, pages = {14}, abstract = {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.}, language = {en} } @misc{OehlerYuecelStathopoulos, author = {Oehler, Johannes and Y{\"u}cel, Fatma Cansu and Stathopoulos, Panagiotis}, title = {Experimental performance analysis of a 50 kW Brayton turbomachine heat pump demonstrator}, series = {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}, journal = {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}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-152419}, pages = {10}, abstract = {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.}, language = {en} } @misc{KabatJendeYueceletal., author = {Kabat, Nancy and Jende, Enrico and Y{\"u}cel, Fatma Cansu and Stathopoulos, Panagiotis}, title = {Thermodynamic analysis of a novel high-temperature heat pump cycle configuration based on the reversed Brayton cycle}, series = {International journal of sustainable energy}, volume = {44}, journal = {International journal of sustainable energy}, number = {1}, publisher = {Taylor \& Francis}, address = {Abingdon, Oxon}, issn = {1478-6451}, pages = {1 -- 37}, abstract = {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.}, language = {en} } @misc{TranStathopoulos, author = {Tran, A. Phong and Stathopoulos, Panagiotis}, title = {Dynamic simulation and experimental validation of a high-temperature Brayton heat pump}, series = {Applied thermal engineering}, volume = {274, Part B}, journal = {Applied thermal engineering}, publisher = {Elsevier BV}, address = {Amsterdam}, issn = {1359-4311}, doi = {10.1016/j.applthermaleng.2025.126536}, pages = {1 -- 13}, abstract = {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.}, language = {en} } @misc{SchaffrathStathopoulosSchmitzetal., author = {Schaffrath, Robert and Stathopoulos, Panagiotis and Schmitz, Andreas and Nicke, Eberhard}, title = {Multistage turbomachinery optimization for high-temperature heat pumps with the reverse rankine cycle}, series = {Journal of turbomachinery}, volume = {147}, journal = {Journal of turbomachinery}, number = {111003}, publisher = {ASME}, address = {New York, NY}, issn = {1528-8900}, doi = {10.1115/1.4068480}, pages = {111003-1 -- 111003-12}, abstract = {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.}, language = {en} } @misc{AbuKhassCristofaroKopparthyetal., author = {Abu Khass, Omar and Cristofaro, Marco and Kopparthy, Saketh Bharadwaj and Kl{\"o}ppel, Steffen and Nicke, Eberhard and Stathopoulos, Panagiotis}, title = {Numerical investigation of two-phase water ejectors for high-temperature heat pumps : insights into flow behavior and shock wave dynamics}, series = {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}, journal = {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}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-151706}, pages = {11}, abstract = {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.}, language = {en} } @misc{KabatOehlerStathopoulos, author = {Kabat, Nancy and Oehler, Johannes and Stathopoulos, Panagiotis}, title = {Experimental exergy analysis of a high-temperature Brayton heat pump}, series = {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}, journal = {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}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-153648}, pages = {16}, abstract = {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.}, language = {en} } @misc{KabatJendeYueceletal., author = {Kabat, Nancy and Jende, Enrico and Y{\"u}cel, Fatma Cansu and Stathopoulos, Panagiotis}, title = {Thermodynamic analysis of a novel high-temperature heat pump cycle with inter cooled compression and reheated expansion based on the reversed Brayton cycle}, series = {International journal of sustainable energy}, volume = {44}, journal = {International journal of sustainable energy}, number = {1}, publisher = {Taylor \& Francis}, address = {Abingdon, Oxon}, issn = {1478-6451}, doi = {10.1080/14786451.2025.2515452}, pages = {1 -- 37}, abstract = {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.}, language = {en} } @misc{DalalKloeppelStathopoulos, author = {Dalal, Varshil Sujal and Kl{\"o}ppel, Steffen and Stathopoulos, Panagiotis}, title = {CO2 heat pumps in the drying industry : identifying the break-even point between transcritical and supercritical operation}, series = {9th European Drying Conference}, journal = {9th European Drying Conference}, abstract = {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.}, language = {en} }