Publications

@article{Chu:2021fr,
title = {Transport of turbulence across permeable interface in a turbulent channel flow: Interface-resolved direct numerical simulation},
author = {Xu Chu and Wenkang Wang and Guang Yang and Alexandros Terzis and Rainer Helmig and Bernhard Weigand},
url = {https://doi.org/10.1007/s11242-020-01506-w},
doi = {10.1007/s11242-020-01506-w},
year = {2021},
date = {2021-01-01},
journal = {Transport in Porous Media},
volume = {136},
number = {1},
pages = {165--189},
abstract = {Turbulence transportation across permeable interfaces is investigated using direct numerical simulation, and the connection between the turbulent surface flow and the pore flow is explored. The porous media domain is constructed with an in-line arranged circular cylinder array. The effects of Reynolds number and porosity are also investigated by comparing cases with two Reynolds numbers (Re=3000,6000) and two porosities (phi=0.5,0.8). It was found that the change of porosity leads to the variation of flow motions near the interface region, which further affect turbulence transportation below the interface. The turbulent kinetic energy (TKE) budget shows that turbulent diffusion and pressure transportation work as energy sink and source alternatively, which suggests a possible route for turbulence transferring into porous region. Further analysis on the spectral TKE budget reveals the role of modes of different wavelengths. A major finding is that mean convection not only affects the distribution of TKE in spatial space, but also in scale space. The permeability of the wall also have an major impact on the occurrence ratio between blow and suction events as well as their corresponding flow structures, which can be related to the change of the Kármán constant of the mean velocity profile.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2020jn,
title = {High-frequency water vapor sorption cycling using fluidization of metal-organic frameworks},
author = {Alexandros Terzis and Ashwin Ramachandran and Kecheng Wang and Mehdi Asheghi and Kenneth E Goodson and Juan G Santiago},
url = {https://doi.org/10.1016/j.xcrp.2020.100057},
doi = {10.1016/j.xcrp.2020.100057},
year = {2020},
date = {2020-12-25},
journal = {Cell Reports Physical Science},
volume = {1},
number = {5},
pages = {100057},
abstract = {The productivity of continuously cycled atmospheric water harvesting methods using metal-organic frameworks (MOFs) has been limited by a lack of scalable designs and robust MOF form factors compatible with rapid heat and mass transport. Explored here is the fluidization of MOF-801 powder in its native particulate form as a water vapor sorption unit. Fluidization results in a very high sorbent-air interface area and small distances over which mass diffusion must occur. This arrangement enables adsorption and desorption cycling with periods of 26 and 36 min at, respectively, 18% and 39% relative humidity (RH) with ∼80% of MOF-801 uptake capacity. This results in dynamic steady-state operation water vapor harvesting rates of 0.33 L/h per kilogram of MOF at 18% RH (40 cycles per day at 22°C) and 0.52 L/h per kilogram of MOF at 39% RH (55 cycles per day at 23.5°C). The measurements may have direct application to water harvesting systems.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2020fs,
title = {Simultaneous optical and infrared thermal imaging of isotachophoresis},
author = {Alexandros Terzis and Ashwin Ramachandran and Jinliang Kang and Juan G Santiago},
url = {https://doi.org/10.1016/j.aca.2020.07.014},
doi = {10.1016/j.aca.2020.07.014},
year = {2020},
date = {2020-11-17},
journal = {Analytica Chimica Acta},
volume = {1131},
pages = {9--17},
abstract = {Joule heating in isotachophoresis (ITP) can limit minimum assay times and efforts to scale up processed sample volumes. Despite its significance, the dynamics of Joule heating on spatiotemporal temperature fields in ITP systems have not been investigated. We here present novel measurements of spatiotemporal temperature and electromigration fields in ITP. To achieve this, we obtain simultaneous and registered optical and infrared thermal images of the ITP process. We conduct a series of experiments at constant current operation and vary the leading electrolyte concentration to study and highlight the importance of buffer-dependent ionic conductivity on the resulted temperature rise. The measurements demonstrate a substantial increase of temperature in the adjusted trailing electrolyte region, and the propagation of a thermal wave in the ITP channel with a velocity equal to that of the electromigration front. We present scaling of the experimental data that indicates the dependence of front velocity and temperature rise on current density and ionic conductivity. The current study has direct application to the design and optimization of scaled-up ITP systems and the validation of numerical models of Joule heating.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Yang:2020bz,
title = {Droplet mobilization at the walls of a microfluidic channel},
author = {Guang Yang and Xu Chu and Visakh Vaikuntanathan and Shanshan Wang and Jingyi Wu and Bernhard Weigand and Alexandros Terzis},
url = {https://doi.org/10.1063/1.5139308},
doi = {10.1063/1.5139308},
year = {2020},
date = {2020-01-01},
journal = {Physics of Fluids},
volume = {32},
number = {1},
pages = {012004},
abstract = {The mechanism of dynamic wetting and the fluid dynamics during the onset of droplet mobilization driven by a microchannel flow are not clearly understood. In this work, we use microparticle tracking velocimetry to visualize the velocity distribution inside the droplet both prior to and during mobilization. Time-averaged and instantaneous velocity vectors are determined using fluorescent microscopy for various capillary numbers. A circulating flow exists inside the droplet at a subcritical capillary number, in which case the droplet is pinned to the channel walls. When the capillary number exceeds a critical value, droplet mobilization occurs, and this process can be divided into two stages. In the first stage, the location of the internal circulation vortex center moves to the rear of the droplet and the droplet deforms, but the contact lines at the top walls remain fixed. In the second stage, the droplet rolls along the solid wall, with fixed contact angles keeping the vortex center in the rear part of the droplet. The critical capillary number for the droplet mobilization is larger for the droplet fluid with a larger viscosity. A force-balance model of the droplet, considering the effect of fluid properties, is formulated to explain the experimental trends of advancing and receding contact angles with the capillary number. Numerical simulations on internal circulations for the pinned droplet indicate that the reversed flow rate, when normalized by the inlet flow rate and the kinematic viscosity ratio of the wetting and nonwetting phases, is independent of the capillary number and the droplet composition.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Weishaupt:2020ek,
title = {A hybrid-dimensional coupled pore-network/free-flow model including pore-scale slip and Its application to a micromodel experiment},
author = {Kilian Weishaupt and Ioannis Zarikos and Alexandros Terzis and Guang Yang and Bernd Flemisch and Matthijs D A de Winter and Rainer Helmig},
url = {https://doi.org/10.1007/s11242-020-01477-y},
doi = {10.1007/s11242-020-01477-y},
year = {2020},
date = {2020-01-01},
journal = {Transport in Porous Media},
volume = {135},
number = {1},
pages = {243--270},
abstract = {Modeling coupled systems of free flow adjacent to a porous medium by means of fully resolved Navier–Stokes equations is limited by the immense computational cost and is thus only feasible for relatively small domains. Coupled, hybrid-dimensional models can be much more efficient by simplifying the porous domain, e.g., in terms of a pore-network model. In this work, we present a coupled pore-network/free-flow model taking into account pore-scale slip at the local interfaces between free flow and the pores. We consider two-dimensional and three-dimensional setups and show that our proposed slip condition can significantly increase the coupled model’s accuracy: compared to fully resolved equidimensional numerical reference solutions, the normalized errors for velocity are reduced by a factor of more than five, depending on the flow configuration. A pore-scale slip parameter Bp required by the slip condition was determined numerically in a preprocessing step. We found a linear scaling behavior of Bp with the size of the interface pore body for three-dimensional and two-dimensional domains. The slip condition can thus be applied without incurring any run-time cost. In the last section of this work, we used the coupled model to recalculate a microfluidic experiment where we additionally exploited the flat structure of the micromodel which permits the use of a quasi-3D free-flow model. The extended coupled model is accurate and efficient.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2019bs,
title = {Microscopic velocity field measurements inside a regular porous medium adjacent to a low Reynolds number channel flow},
author = {Alexandros Terzis and Ioannis Zarikos and Kilian Weishaupt and Guang Yang and Xu Chu and Rainer Helmig and Bernhard Weigand},
url = {https://doi.org/10.1063/1.5092169},
doi = {10.1063/1.5092169},
year = {2019},
date = {2019-12-31},
journal = {Physics of Fluids},
volume = {31},
number = {4},
pages = {042001},
abstract = {This study examines experimentally the hydrodynamic interaction between a regular porous medium and an adjacent free-flow channel at low Reynolds numbers (Re < 1). The porous medium consists of evenly spaced micro-structured rectangular pillars arranged in a uniform pattern, while the free-flow channel features a rectangular cross-sectional area. The overall arrangement comprises a polydimethylsiloxane microfluidic model where distilled water, doped with fluorescent particles, is the examined fluid. Using micro-particle image velocimetry, single-phase quantitative velocity measurements are carried out at the pore scale to reveal the microscopic characteristics of the flow for such a coupled system. Interfacial velocity-slip and stress-jump coefficients are also evaluated with a volume-averaging method based on the Beavers-Joseph and Ochoa-Tapia-Whitaker models, respectively. The results show that, from a microscopic point of view, parallel flow at the interface is not obtained due to the periodically generated U-shaped flow profile between the interface pillars. However, the interface coefficients show no sensitivity to moderate flow angles. The highly resolved experimental information obtained in this study can also be used for the validation of numerical models providing a unique dataset for free-flow and porous media coupled systems.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Yang:2019jm,
title = {Internal flow patterns of a droplet pinned to the hydrophobic surfaces of a confined microchannel using micro-PIV and VOF simulations},
author = {Guang Yang and Alexandros Terzis and Ioannis Zarikos and Majid S Hassanizadeh and Bernhard Weigand and Rainer Helmig},
url = {https://doi.org/10.1016/j.cej.2019.03.191},
doi = {10.1016/j.cej.2019.03.191},
year = {2019},
date = {2019-11-06},
journal = {Chemical Engineering Journal},
volume = {370},
pages = {444--454},
abstract = {We present both experimental results and numerical simulations of the fluid dynamics of a droplet pinned to the hydrophobic surfaces of a confined microfluidic channel, as a result of contact angle hysteresis. Internal circulations in the droplet are observed and quantified using micro-particle image velocimetry (micro-PIV). As the channel inlet velocity increases, the difference between the contact angles at the front and the rear part of the contact line is also increased, while the equilibrium Young’s contact angle remains essentially constant. Numerical simulations based on a Volume-Of-Fluid (VOF) method combined with a Laplacian filter for the phase function are also performed to consider contact angle hysteresis effects. Major quantities from the simulations, including the velocity distribution inside the droplet, the contact angles, and the vortex structures, show good agreement with experimental results. In addition, force balance models of the pinned droplet have been built for various inlet conditions, indicating that the adhesion force at the side walls and the blockage of the droplet have significant effects on the liquid motion within the droplet. The recirculation flow rate inside the droplet is found to vary linearly with the Capillary number.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Yang:2019dw,
title = {Impact of a linear array of hydrophilic and superhydrophobic spheres on a deep water pool},
author = {Guang Yang and Visakh Vaikuntanathan and Alexandros Terzis and Xin Cheng and Bernhard Weigand and Rainer Helmig},
url = {https://doi.org/10.3390/colloids3010029},
doi = {10.3390/colloids3010029},
year = {2019},
date = {2019-01-01},
journal = {Colloids and Interfaces},
volume = {3},
number = {1},
abstract = {The impact of solid bodies on the free surface of liquid pools is involved in many practical applications—such as bullets and air-to-sea anti-torpedo defense systems, or the water entry of athletes in water sports—aimed at improving the performance through a control of cavity dynamics. This work reports an experimental investigation of the impact of a linear array of hydrophilic (H) and superhydrophobic (SH) spheres on a deep water pool. The array consisted of ten magnetic spheres, with different permutations of H and SH spheres. Using high speed shadowgraphy, we captured the underwater kinematics of the array for different permutations of H and SH spheres. In particular, we observed the evolution or absence of an air cavity attached to the array as a function of the position of the H and SH spheres. The position of the first SH sphere from the leading edge of the array (ZSH) emerged as a key parameter that alters the characteristics of cavity evolution. The appearance and pinch-off characteristics of a wake cavity behind the trailing edge were governed by the wetting properties of the leading and trailing surfaces of the array. The position of the first SH surface, as well as the wetting characteristics of the leading and trailing surfaces, are potential control parameters to alter underwater cavity evolution during solid surface impact on deep water pools.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Yang:2019ie,
title = {On the Beavers-Joseph interface condition for non-parallel coupled channel flow over a porous structure at high Reynolds numbers},
author = {Guang Yang and Edward Coltman and Kilian Weishaupt and Alexandros Terzis and Rainer Helmig and Bernhard Weigand},
url = {https://doi.org/10.1007/s11242-019-01255-5},
doi = {10.1007/s11242-019-01255-5},
year = {2019},
date = {2019-01-01},
journal = {Transport in Porous Media},
volume = {77},
number = {6},
pages = {1--27},
abstract = {A channel flow coupled with a transversal stream through porous structures is investigated numerically in this study. Velocity profiles are obtained on the pore scale and averaged to the macroscale in order to evaluate the validity of the Beavers–Joseph interface condition. For this purpose, different ratios between the velocity at the inlet of the channel and the velocity at the base of the porous structure are considered. The effects of Reynolds number, velocity ratio, and geometrical arrangement of the porous structure on the Beavers–Joseph constant are then examined. A critical velocity ratio is found where, for lower ratios, the interface momentum transfer is mainly affected by the channel flow and, for higher ratios, the flow in the porous structure governs. The uniformity of the Beavers–Joseph constant at the interface is found to decrease with increasing velocity ratios. The present results have then been compared with simulations from a coupled Navier–Stokes/Darcy–Forchheimer model.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2019hz,
title = {Splashing characteristics of diesel exhaust fluid (AdBlue) droplets impacting on urea-water solution films},
author = {Alexandros Terzis and Maximilian Kirsch and Visakh Vaikuntanathan and Anne Geppert and Grazia Lamanna and Bernhard Weigand},
url = {https://doi.org/10.1016/j.expthermflusci.2018.11.002},
doi = {10.1016/j.expthermflusci.2018.11.002},
year = {2019},
date = {2019-01-01},
journal = {Experimental Thermal and Fluid Science},
volume = {102},
pages = {152--162},
abstract = {The rapid implementation of Selective-Catalytic-Reduction (SCR) technologies into light passenger and commercial vehicles, led to the omission of fundamental research creating several reliability issues that are largely related to the research field of droplet dynamics. In this study, the splashing behaviour of an AdBlue droplet impacting onto thin urea-water solution films is experimentally investigated over a range of impact parameters. In particular, the crown-type splashing threshold, the number of fingers and the characteristics of the ejected secondary droplets are evaluated for various drop impact velocities, wall-film thicknesses and urea concentrations in the liquid film. The results show that impact parameters that are able to enhance the energy dissipation in the wall-film, e.g. film thickness and viscosity, influence negatively the intensity of splashing. On the other hand, as the droplet kinetic energy increases or the wall-film thickness decreases, more energy is available to intensify the splashing outcome, and consequently, the upward ejected secondary droplet volume. The obtained trends are correlated in simple empirical expressions providing a remarkable industrial design tool, and they are also compared to the state-of-the-art literature of single- and binary-droplet/wall-film interactions aiming to draw generalised theories and paradigms that will support the connection between SCR applications and academic research.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Yang:2018ki,
title = {Numerical simulation of turbulent flow and heat transfer in a three-dimensional channel coupled with flow through porous structures},
author = {Guang Yang and Bernhard Weigand and Alexandros Terzis and Kilian Weishaupt and Rainer Helmig},
url = {https://doi.org/10.1007/s11242-017-0995-9},
doi = {10.1007/s11242-017-0995-9},
year = {2018},
date = {2018-01-01},
journal = {Transport in Porous Media},
volume = {122},
number = {1},
pages = {145--167},
abstract = {This study investigates numerically the turbulent flow and heat transfer characteristics of a T-junction mixing, where a porous media flow is vertically discharged in a 3D fully developed channel flow. The fluid equations for the porous medium are solved in a pore structure level using an Speziale, Sarkar and Gatski turbulence model and validated with open literature data. Overall, two types of porous structures, consisted of square pores, are investigated over a wide range of Reynolds numbers: an in-line and a staggered pore structure arrangement. The flow patterns, including the reattachment length in the channel, the velocity field inside the porous medium as well as the fluctuation velocity at the interface, are found to be strongly affected by the velocity ratio between the transversely interacting flow streams. In addition, the heat transfer examination of the flow domain reveals that the temperature distribution in the porous structure is more uniform for the staggered array. The local heat transfer distributions inside the porous structure are also studied, and the general heat transfer rates are correlated in terms of area-averaged Nusselt number accounting for the effects of Reynolds number, velocity ratio as well as the geometrical arrangement of the porous structures.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2018ke,
title = {Characterisation of acid-base surface free energy components of urea-water solutions},
author = {Alexandros Terzis and Elmar Sauer and Guang Yang and Joachim Groß and Bernhard Weigand},
url = {https://doi.org/10.1016/j.colsurfa.2017.11.068},
doi = {10.1016/j.colsurfa.2017.11.068},
year = {2018},
date = {2018-01-01},
journal = {Colloids and Surfaces A: Physicochemical and Engineering Aspects},
volume = {538},
pages = {774--780},
abstract = {The objective of this study is to determine the Lifshitz–van der Waals/Acid–Base (LW/AB) surface energy components of urea–water-solutions (UWS) for different urea mass fractions. The surface energy parameters are evaluated by ring tensiometry and contact angle measurements of sessile drops placed onto pre-determined solid substrates. Therefore, the energetic characteristics of UWS are evaluated in relation to probe liquids. The results indicate that aqueous solutions of urea become less polar with increasing urea mass fraction while their overall surface tension is also increased. This is attributed to a significant grow of the Lifshitz–van der Waals surface energy component that compensates the reduction of the polar part. In addition, aqueous solutions of urea are characterised by a significant electron donor capacity compared to pure water while their electron acceptor parameter is reduced. Subsequently, the ratio of electron donor over electron acceptor capacity is continuously reduced with increasing urea concentration. The enhancement of electron-donicity is also reflected to the pH of the solutions while the overall trends are independent from the selection of acid-to-base ratio for pure water. The above findings are related to physicochemical aspects based on molecular and intermolecular interactions.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Schmid:2018bx,
title = {Crystallization of urea from an evaporative aqueous solution sessile droplet at sub-boiling temperatures and surfaces with different wettability},
author = {Julian Schmid and Alexandros Terzis and Ioannis Zarikos and Norbert Roth and Bernhard Weigand},
url = {https://doi.org/10.1016/j.expthermflusci.2017.10.008},
doi = {10.1016/j.expthermflusci.2017.10.008},
year = {2018},
date = {2018-01-01},
journal = {Experimental Thermal and Fluid Science},
volume = {91},
pages = {80--88},
abstract = {The injection of urea-water-solution sprays in the exhaust pipe of modern diesel engines eliminates NOx emissions in a very great extent. However, as water evaporates from the solution, urea is crystallized and causes wall-deposit formations hindering the performance of selective-catalytic-reaction. In this study, the crystallization of urea from an evaporative aqueous solution droplet placed on a heated wall is experimentally investigated, aiming to understand macroscopically the morphology of crystal growth at various conditions. Using optical and thermal imaging, urea crystallization patterns are examined at sub-boiling temperatures and substrates with different wettability. In all cases, the macroscopic initiation of crystal growth starts at the solid-liquid interface when urea concentration has reached supersaturated conditions. The experiments indicate two different crystallization modes depending on surface temperature and wettability as well as a significant heat release at the solidification front due the exothermic character of the process.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2018kea,
title = {A temperature-based diagnostic approach for paper-based microfluidics},
author = {Alexandros Terzis and Guang Yang and Ioannis Zarikos and Emmanuel Elizalde and Bernhard Weigand and Anestis I Kalfas and Xianting Ding},
url = {https://doi.org/10.1007/s10404-018-2054-4},
doi = {10.1007/s10404-018-2054-4},
year = {2018},
date = {2018-01-01},
journal = {Microfluidics and Nanofluidics},
volume = {22},
number = {35},
abstract = {We present the potential of a quantitative temperature-based diagnostic approach for paper-based microfluidics, extending the work of Terzis et al. (J Colloid Interface Sci 504:751–757, 2017) which demonstrated a significant heat release at the liquid front during capillary-driven flows in cellulosic materials. Here, we investigate the applicability of biological fluids to provide a temperature rise at the imbibition front, and successfully demonstrate a monotonic trend between the level of local temperature rise and the concentration of specific analytes. In addition, effects of paper thickness and width are also examined.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Zarikos:2018ej,
title = {Velocity distributions in trapped and mobilized non-wetting phase ganglia in porous media},
author = {Ioannis Zarikos and Alexandros Terzis and S Majid Hassanizadeh and Bernhard Weigand},
url = {https://doi.org/10.1038/s41598-018-31639-4},
doi = {10.1038/s41598-018-31639-4},
year = {2018},
date = {2018-01-01},
journal = {Scientific Reports},
volume = {8},
number = {1},
pages = {13228},
abstract = {Understanding the mobilisation of trapped globules of non-wetting phase during two-phase flow has been the aim of numerous studies. However, the driving forces for the mobilisation of the trapped phases are still not well understood. Also, there is little information about what happens within a globule before, at the onset and during mobilization. In this work, we used micro-particle tracking velocimetry in a micro-fluidic model in order to visualise the velocity distributions inside the trapped phase globules prior and during mobilisation. Therefore, time-averaged and instantaneous velocity vectors have been determined using fluorescent microscopy. As a porous medium, we used a polydimethylsiloxane (PDMS) micro-model with a well-defined pore structure, where drainage and imbibition experiments were conducted. Three different geometries of trapped non-wetting globules, namely droplets, blobs and ganglia were investigated. We observed internal circulations inside the trapped phase globules, leading to the formation of vortices. The direction of circulating flow within a globule is dictated by the drag force exerted on it by the flowing wetting phase. This is illustrated by calculating and analyzing the drag force (per unit area) along fluid-fluid interfaces. In the case of droplets and blobs, only one vortex is formed. The flow field within a ganglion is much more complex and more vortices can be formed. The circulation velocities are largest at the fluid-fluid interfaces, along which the wetting phase flows and decreases towards the middle of the globule. The circulation velocities increased proportionally with the increase of wetting phase average velocity (or capillary number). The vortices remain stable as long as the globules are trapped, start to change at the onset of mobilization and disappear during the movement of globules. They reappear when the globules get stranded. Droplets are less prone to mobilization; blobs get mobilised in whole; while ganglia may get ruptured and get mobilised only partially.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Sauer:2018il,
title = {Prediction of contact angles and density profiles of sessile droplets using classical density functional theory based on the PCP-SAFT equation of state},
author = {Elmar Sauer and Alexandros Terzis and Marc Theiss and Bernhard Weigand and Joachim Groß},
url = {https://doi.org/10.1021/acs.langmuir.8b01985},
doi = {10.1021/acs.langmuir.8b01985},
year = {2018},
date = {2018-01-01},
journal = {Langmuir},
volume = {34},
number = {42},
pages = {12519--12531},
abstract = {This study demonstrates the capability of the density functional theory (DFT) formalism to predict contact angles and density profiles of model fluids and of real substances in good quantitative agreement with molecular simulations and experimental data. The DFT problem is written in cylindrical coordinates, and the solid–fluid interactions are defined as external potentials toward the fluid phase. Monte Carlo (MC) molecular simulations are conducted in order to assess the density profiles resulting from the Helmholtz energy functional used in the DFT formalism. Good quantitative agreement between DFT predictions and MC results for Lennard-Jones and ethane nanodroplets is observed, both for density profiles and for contact angles. That comparison suggests, first, that the Helmholtz energy functional proposed in a previous study [Sauer, E.; Gross, J. Ind. Eng. Chem. Res. 56, 2017, 4119−4135] is suitable for three-phase contact lines and, second, that Lagrange multipliers can be used to constrain the number of molecules, similar to a canonical ensemble. Experiments of sessile droplets on solid surfaces are performed to assess whether a real solid with its microscopic roughness can be described through a simple model potential. Comparison of DFT results to experimental data is done for a Teflon surface because Teflon can be regarded as a substrate exhibiting only attractive interactions of van der Waals type. It is shown that the real solid can be described as a perfectly planar solid with effective solvent-to-solid interactions, defined through a single adjustable parameter for the solid. Subsequent predictions for the contact angle of eight solvents, including polar components such as water, are found in very good agreement to experimental data using simple Berthelot–Lorentz combining rules. For the eight investigated solvents, we find mean absolute deviations of 3.77°.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2017fv,
title = {Heat release at the wetting front during capillary filling of cellulosic micro-substrates},
author = {Alexandros Terzis and Eleftheria Roumeli and Kilian Weishaupt and Stefan Brack and Hamed Aslannejad and Joachim Groß and S Majid Hassanizadeh and Rainer Helmig and Bernhard Weigand},
url = {https://doi.org/10.1016/j.jcis.2017.06.027},
doi = {10.1016/j.jcis.2017.06.027},
year = {2017},
date = {2017-12-30},
journal = {Journal of Colloid and Interface Science},
volume = {504},
pages = {751--757},
abstract = {Spontaneous imbibition in cellulosic materials is an expanding field of research due to the direct applicability in paper-based microfluidics. Here, we show experimentally, using simultaneous thermal and optical imaging that the temperature at the wetting front during capillary filling of paper is temporarily increased, even if the imbibed fluid and the cellulosic substrate are initially at isothermal conditions. Several liquids and two types of filter paper, characterised by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, were investigated demonstrating a significant temperature rise at the wetting front that cannot be neglected form the process. The temperature rise is found to be related to the energetics of imbibition compounds, including acid-base contributions, that result in electrostatic attractions as the liquid molecules are adhered on the fiber surfaces upon capillary contact.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Aslannejad:2017ky,
title = {Occurrence of temperature spikes at a wetting front during spontaneous imbibition},
author = {Hamed Aslannejad and Alexandros Terzis and Majid S Hassanizadeh and Bernhard Weigand},
url = {https://doi.org/10.1038/s41598-017-07528-7},
doi = {10.1038/s41598-017-07528-7},
year = {2017},
date = {2017-01-01},
journal = {Scientific Reports},
volume = {7},
number = {1},
pages = {7268},
abstract = {It is reported that temperature rises at wetting front during water infiltration into soil. The temperature goes back to the background value after passage of water front. Different explanations have been provided for source of energy causing temperature spike. Some have contributed it to heat of condensation released due to condensation of vapor on “dry” solid surface. Some other stated that the heat of wetting or heat of adsorption is responsible for the temperature rise. In this research, we revisited this issue. First, we provide a comprehensive review about occurrence of temperature spike at a wetting front. Then, we report about experiments we performed on the rise of water in dry paper. Using infrared and optical imaging techniques, we could monitor temperature changes in time and space. For all samples maximum temperature rise occurred at the wetting front. The magnitude of temperature spike depended on paper material, thickness, and liquid composition. It was larger for cellulose-fiber-based paper than for plastic-based paper. For a given paper type, thicker samples showed a larger temperature spike. Adding salt to the water caused reduction of temperature spike. It was concluded that replacement of air-solid interface with water-solid interface releases energy, which causes temperature rise.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Geppert:2017gh,
title = {A benchmark study for the crown-type splashing dynamics of one- and two-component droplet wall–film interactions},
author = {Anne Geppert and Alexandros Terzis and Grazia Lamanna and Marco Marengo and Bernhard Weigand},
url = {https://doi.org/10.1007/s00348-017-2447-2},
doi = {10.1007/s00348-017-2447-2},
year = {2017},
date = {2017-01-01},
journal = {Experiments in Fluids},
volume = {58},
number = {12},
pages = {172},
abstract = {The present paper investigates experimentally the impact dynamics of crown-type splashing for miscible two- and one-component droplet wall–film interactions over a range of Weber numbers and dimensionless film thicknesses. The splashing outcome is parametrised in terms of a set of quantifiable parameters, such as crown height, top and base diameter, wall inclination, number of fingers, and secondary droplet properties. The results show that the outcome of a splashing event is not affected by the choice of similar or dissimilar fluids, provided the dimensionless film thickness is larger than 0.1. Below this threshold, distinctive features of two-component interactions appear, such as hole formation and crown bottom breakdown. The observation of different crown shapes (e.g. V-shaped, cylindrical, and truncated-cone) confirms that vorticity production induces changes in the crown wall inclination, thus affecting the evolution of the crown height and top diameter. The evolution of the crown base diameter, instead, is mainly dependent on the relative importance of liquid inertia and viscous losses in the wall-film. The maximum number of liquid fingers decreases with increasing wall, film thickness, due to the enhanced attenuation of the effect of surface properties on the fingering process. The formation of secondary droplets is also affected by changes in the crown wall inclination. In particular, for truncated-cone shapes the occurrence of crown rim contraction induces a large scatter in the secondary droplet properties. Consequently, empirical models for the maximum number and mean diameter of the secondary droplets are derived for V-shaped crowns, as observed for the hexadecane-Hyspin interactions.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2016jo,
title = {On the correspondence between flow structures and convective heat transfer augmentation for multiple jet impingement},
author = {Alexandros Terzis},
url = {https://doi.org/10.1007/s00348-016-2232-7},
doi = {10.1007/s00348-016-2232-7},
year = {2016},
date = {2016-12-31},
journal = {Experiments in Fluids},
volume = {57},
number = {9},
pages = {146},
abstract = {The correspondence between local fluid flow structures and convective heat transfer is a fundamental aspect that is not yet fully understood for multiple jet impingement. Therefore, flow field and heat transfer experiments are separately performed investigating mutual–jet interactions exposed in a self-gained crossflow. The measurements are taken in two narrow impingement channels with different cross-sectional areas and a single exit design. Hence, a gradually increased crossflow momentum is developed from the spent air of the upstream jets. Particle image velocimetry (PIV) and liquid crystal thermography (LCT) are used in order to investigate the aerothermal characteristics of the channel with high spatial resolution. The PIV measurements are taken at planes normal to the target wall and along the centreline of the jets, providing quantitative flow visualisation of jet and crossflow interactions. Spatially resolved heat transfer coefficient distributions on the target plate are evaluated with transient techniques and a multi-layer of thermochromic liquid crystals. The results are analysed aiming to provide a better understanding about the impact of near-wall flow structures on the convective heat transfer augmentation for these complex flow phenomena.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Schulz:2016hd,
title = {On the effects of coating thickness in transient heat transfer experiments using thermochromic liquid crystals},
author = {Sebastian Schulz and Stefan Brack and Alexandros Terzis and Jens von Wolfersdorf and Peter Ott},
url = {https://doi.org/10.1016/j.expthermflusci.2015.08.011},
doi = {10.1016/j.expthermflusci.2015.08.011},
year = {2016},
date = {2016-01-01},
journal = {Experimental Thermal and Fluid Science},
volume = {70},
pages = {196--207},
abstract = {Transient heat transfer experiments typically employ thermochromic liquid crystals to temporally map surface temperatures. The desired heat transfer coefficient is then calculated from the solution of Fourier’s 1D transient heat conduction equation which is set to model the wall temperature at the solid–fluid interface. However, the experimental conditions do not always justify this assumption due to occurring layers of additional paint shielding the actual liquid crystal from the immediate exposure to the working fluid. The disregard of these additional layers with respect to their thicknesses in the evaluation process produces biased heat transfer results. In order to systematically assess the effect of coating thickness on the evaluated heat transfer, the present investigation reports on the application of three different liquid crystal types in layers in transient experiments. These were conducted for two different flow regimes using separate test facilities, i.e. a flow over a tetrahedra-shaped vortex generator and jet flows from an in-line row of orifices within a low aspect ratio impingement channel. Reynolds numbers of 100,000 and 50,000 based on hydraulic and jet orifice diameter were investigated, respectively. Upon consideration of the actual liquid crystals’ coating thicknesses from measurements, the investigations show that disregarding the layer thicknesses can lead to a significant underestimation of the resulting heat transfer, particularly for large thicknesses. By taking into account the respective coating thicknesses the experimental discrepancies could be reduced from to less than , accomplishing high data redundancy.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2016cd,
title = {Improved accuracy in jet impingement heat transfer experiments considering the layer thicknesses of a triple thermochromic liquid crystal coating},
author = {Alexandros Terzis and Stavros Bontitsopoulos and Peter Ott and Jens von Wolfersdorf and Anestis I Kalfas},
url = {https://doi.org/10.1115/1.4031786},
doi = {10.1115/1.4031786},
year = {2016},
date = {2016-01-01},
journal = {Journal of Turbomachinery},
volume = {138},
number = {2},
pages = {021003},
abstract = {This paper examines the applicability of a triple layer of thermochromic liquid crystals (TLCs) for the determination of local heat transfer coefficients using the transient liquid crystal (LC) technique. The experiments were carried out in a narrow impingement channel, typically used for turbine blade cooling applications. Three types of narrow bandwidth LCs (1 °C range) of 35 °C, 38 °C, and 41 °C were individually painted on the target plate of the cooling cavity and the overall paint thickness was accurately determined with an integral coating thickness gauge. The 1D transient heat conduction equation is then implicitly solved for each individual TLC layer on its realistic depth on the painted surface. Local heat transfer coefficients are therefore calculated three times for the same location in the flow improving the measurement accuracy, especially at regions where the LC detection times are too short (stagnation points) or too long (wall-jet regions). The results indicate that if multiple LC layers are used and the paint thickness is not considered, the heat transfer coefficients can be significantly underestimated.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2016kq,
title = {Aerothermal investigation of a single row divergent narrow impingement channel by particle image velocimetry (PIV) and liquid crystal thermography},
author = {Alexandros Terzis and Christoforos Skourides and Peter Ott and Jens von Wolfersdorf and Bernhard Weigand},
url = {https://doi.org/10.1115/1.4032328},
doi = {10.1115/1.4032328},
year = {2016},
date = {2016-01-01},
journal = {Journal of Turbomachinery},
volume = {138},
number = {5},
pages = {051003},
abstract = {Integrally cast turbine airfoils with wall-integrated cooling cavities are greatly applicable in modern turbines providing enhanced heat exchange capabilities compared to conventional cooling passages. In such arrangements, narrow impingement channels can be formed where the generated crossflow is an important design parameter for the achievement of the desired cooling efficiency. In this study, a regulation of the generated crossflow for a narrow impingement channel consisting of a single row of five inline jets is obtained by varying the width of the channel in the streamwise direction. A divergent impingement channel is therefore investigated and compared to a uniform channel of the same open area ratio. Flow field and wall heat transfer experiments are carried out at engine representative Reynolds numbers using particle image velocimetry (PIV) and liquid crystal thermography (LCT). The PIV measurements are taken at planes normal to the target wall along the centerline for each individual jet, providing quantitative flow visualization of jet and crossflow interactions. The heat transfer distributions on the target plate of the channels are evaluated with transient techniques and a multilayer of liquid crystals (LCs). Effects of channel divergence are investigated combining both the heat transfer and flow field measurements. The applicability of existing heat transfer correlations for uniform jet arrays to divergent geometries is also discussed.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2015hf,
title = {Effect of varying jet diameter on the heat transfer distributions of narrow impingement channels},
author = {Alexandros Terzis and Peter Ott and Magali Cochet and Jens von Wolfersdorf and Bernhard Weigand},
url = {https://doi.org/10.1115/1.4028294},
doi = {10.1115/1.4028294},
year = {2015},
date = {2015-01-01},
journal = {Journal of Turbomachinery},
volume = {137},
number = {2},
pages = {021004},
abstract = {The development of integrally cast turbine airfoils allows the production of narrow impingement channels in a double-wall configuration, where the coolant is practically injected within the wall of the airfoil providing increased heat transfer capabilities. This study examines the cooling performance of narrow impingement channels with varying jet diameters using a single exit design in an attempt to regulate the generated crossflow. The channel consists of a single row of five inline jets tested at two different channel heights and over a range of engine representative Reynolds numbers. Detailed heat transfer coefficient distributions are evaluated over the complete interior surfaces of the channel using the transient liquid crystal technique. Additionally, local jet discharge coefficients are determined by probe traversing measurements for each individual jet. A 10%-increasing and a 10%-decreasing jet diameter pattern are compared with a baseline geometry of uniform jet size distribution, indicating a considerable effect of varying jet diameter on the heat transfer level and the development of the generated crossflow.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Ullmer:2015jp,
title = {Impact of ns-DBD plasma actuation on the boundary layer transition using convective heat transfer measurements},
author = {Dirk Ullmer and Philip Peschke and Alexandros Terzis and Peter Ott and Bernhard Weigand},
url = {https://doi.org/10.1088/0022-3727/48/36/365203},
doi = {10.1088/0022-3727/48/36/365203},
year = {2015},
date = {2015-01-01},
journal = {Journal of Physics D: Applied Physics},
volume = {48},
number = {36},
pages = {365203},
abstract = {This paper demonstrates that the impact of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators on the structure of the boundary layer can be investigated using quantitative convective heat transfer measurements. For the experiments, the flow over a flat plate with a C4 leading edge thickness distribution was examined at low speed incompressible flow (6.6–11.5 m/s). An ns-DBD plasma actuator was mounted 5 mm downstream of the leading edge and several experiments were conducted giving particular emphasis on the effect of actuation frequency and the freestream velocity. Local heat transfer distributions were measured using the transient liquid crystal technique with and without plasma activated. As a result, any effect of plasma on the structure of the boundary layer is interpreted by local heat transfer coefficient distributions which are compared with laminar and turbulent boundary layer correlations. The heat transfer results, which are also confirmed by hot-wire measurements, show the considerable effect of the actuation frequency on the location of the transition point elucidating that liquid crystal thermography is a promising method for investigating plasma-flow interactions very close to the wall. Additionally, the hot-wire measurements indicate possible velocity oscillations in the near wall flow due to plasma activation.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2015bw,
title = {A method to visualise near wall fluid flow patterns using locally resolved heat transfer experiments},
author = {Alexandros Terzis and Jens von Wolfersdorf and Bernhard Weigand and Peter Ott},
url = {https://doi.org/10.1016/j.expthermflusci.2014.09.009},
doi = {10.1016/j.expthermflusci.2014.09.009},
year = {2015},
date = {2015-01-01},
journal = {Experimental Thermal and Fluid Science},
volume = {60},
pages = {223--230},
abstract = {The present study demonstrates an alternative approach for describing fluid flow characteristics very close to the wall, using locally resolved convective heat transfer experiments. Heat transfer coefficients on the base surface and around a surface mounted vortex generator of delta-wing shape design, are evaluated with the transient liquid crystal measurement technique and over a range of freestream velocities. Therefore, the local values of exponent m in the equation, Nu=C Re^m, which is directly linked to the structure of the boundary layer, can be determined over the complete heat transfer area. The local distributions of exponent m are then directly compared to the footprint of the flow obtained with typical oil and dye surface flow visualisation. The results indicate that a more appropriate interpretation of the flow structures very close to the wall is possible by analysing the spatial variation of exponent m, which approximates better the flow pattern compared to the heat transfer coefficients. As a result, fluid flow topologies can be directly evaluated from the heat transfer experiments since the distributions of oil-flow visualisation and exponent m are qualitatively similar.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Llucia:2015cv,
title = {Heat transfer characteristics of high crossflow impingement channels: Effect of number of holes},
author = {Santiago Llucià and Alexandros Terzis and Peter Ott and Magali Cochet},
url = {https://doi.org/10.1177/0957650915594074},
doi = {10.1177/0957650915594074},
year = {2015},
date = {2015-01-01},
journal = {Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy},
volume = {229},
number = {5},
pages = {560--568},
abstract = {In modern turbine airfoils, narrow impingement cooling channels can be formed in a double-wall configuration. In these wall-integrated cooling cavities, the generated crossflow is one of the most important design factors, and hence, the number of impingement holes included in a channel. This study examines experimentally the influence of the number of impingement holes on the heat transfer characteristics of narrow impingement channels. The channels consist of two rows of jets where the number of holes in the axial direction is varied from 5 to 10, maintaining the same jet plate open area. Local heat transfer coefficient distributions are obtained for all channel interior walls using the transient liquid crystal technique and over a range of Reynolds numbers (20,300–41,500). The results show an important heat transfer degradation at higher open areas and a small influence of the number of holes at upstream channel positions.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2014jg,
title = {Hole staggering effect on the cooling performance of narrow impingement channels using the transient liquid crystal technique},
author = {Alexandros Terzis and Guillaume Wagner and Jens von Wolfersdorf and Peter Ott and Bernhard Weigand},
url = {https://doi.org/10.1115/1.4027250},
doi = {10.1115/1.4027250},
year = {2014},
date = {2014-01-01},
journal = {Journal of Heat Transfer},
volume = {136},
number = {7},
pages = {071701},
abstract = {This study examines experimentally the cooling performance of narrow impingement channels as could be cast-in in modern turbine airfoils. Full surface heat transfer coefficients are evaluated for the target plate and the sidewalls of the channels using the transient liquid crystal technique. Several narrow impingement channel geometries, consisting of a single row of five cooling holes, have been investigated composing a test matrix of nine different models. The experimental data are analyzed by means of various post-processing procedures aiming to clarify and quantify the effect of cooling hole offset position from the channel centerline on the local and average heat transfer coefficients and over a range of Reynolds numbers (11,100–86,000). The results indicated a noticeable effect of the jet pattern on the distribution of convection coefficients as well as similarities with conventional multi-jet impingement cooling systems.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Wyssmann:2014bd,
title = {A comparative study of the local heat transfer distributions around various surface mounted obstacles},
author = {Robert Wyssmann and Dirk Ullmer and Alexandros Terzis and Peter Ott},
url = {https://doi.org/10.1007/s11630-014-0692-8 },
doi = {10.1007/s11630-014-0692-8 },
year = {2014},
date = {2014-01-01},
journal = {Journal of Thermal Science},
volume = {23},
number = {2},
pages = {169--176},
abstract = {In many engineering applications, heat transfer enhancement techniques are of vital importance in order to ensure reliable thermal designs of convective heat transfer applications. This study examines experimentally the heat transfer characteristics on the base plate around various surface mounted obstacles. Local convection coefficients are evaluated in the vicinity of each individual protruding body with great spatial resolution using the transient liquid crystal technique. Five different obstacles of constant height-to-hydraulic diameter ratio (∼1.3) are considered. These include: a cylinder, a square, a triangle, a diamond and a vortex generator of delta wing shape design. The experiments were carried out over a range of freestream Reynolds numbers, based on the hydraulic diameter of each obstacle, varying from 4,000 to 13,000. The results indicate a negligible effect of the flow speed on the heat transfer topological structure and a considerable effect of the obstacle geometry on the level and distribution of heat transfer enhancement.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2014ha,
title = {Detailed heat transfer distributions of narrow impingement channels for cast-in turbine airfoils},
author = {Alexandros Terzis and Peter Ott and Jens von Wolfersdorf and Bernhard Weigand and Magali Cochet},
url = {https://doi.org/10.1115/1.4027679},
doi = {10.1115/1.4027679},
year = {2014},
date = {2014-01-01},
journal = {Journal of Turbomachinery},
volume = {136},
number = {9},
pages = {091011},
abstract = {The current capabilities of the foundry industry allow the production of integrally cast turbine airfoils. Impingement cooling effectiveness can be then further increased due to the manufacturing feasibility of narrow impingement cavities in a double-wall configuration. This study examines experimentally, using the transient liquid crystal technique, the cooling performance of narrow cavities consisting of a single row of five impingement holes. Heat transfer coefficient distributions are obtained for all channel interior surfaces over a range of engine realistic Reynolds numbers varying between 10,900 and 85,900. Effects of streamwise jet-to-jet spacing (X/D), channel width (Y/D), jet-to-target plate distance (Z/D), and jet offset position (Δy∕D) from the channel centerline are investigated composing a test matrix of 22 different geometries. Additionally, the target plate and sidewalls heat transfer rates are successfully correlated within the experimental uncertainties providing an empirical heat transfer model for narrow impingement channels. The results indicate similarities with multijet impingement configurations; however, the achievable heat transfer level is about 20% lower compared to periodic multijet impingement correlations found in open literature.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Caggese:2013ds,
title = {Experimental and numerical investigation of a fully confined impingement round jet},
author = {Oriana Caggese and Gabriel Gnaegi and Gweneal Hannema and Alexandros Terzis and Peter Ott},
url = {https://doi.org/10.1016/j.ijheatmasstransfer.2013.06.043},
doi = {10.1016/j.ijheatmasstransfer.2013.06.043},
year = {2013},
date = {2013-01-01},
journal = {International Journal of Heat and Mass Transfer},
volume = {65},
pages = {873--882},
abstract = {The heat transfer characteristics of a fully confined impingement jet are experimentally and numerically evaluated. Full surface heat transfer coefficient distributions are obtained for the target and impingement plate of the model using the transient liquid crystal technique and a commercial CFD solver. The confined box consists of a single round jet impinging over a flat surface at relatively low jet-to-target plate distances, varied between 0.5 and 1.5 jet diameters. The impingement geometry is blocked from the three sides, and therefore, the air of the jet is forced to exit the model in a single direction resulting in a fully confined configuration. Experiments were carried out over a range of Reynolds varying between 16,500 and 41,800. The experimental data is compared to the numerical simulations aiming to quantify the degree of accuracy to which the heat transfer rates can be predicted.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Fechter:2013fd,
title = {Experimental and numerical investigation of narrow impingement cooling channels},
author = {Stefan Fechter and Alexandros Terzis and Peter Ott and Bernhard Weigand and Jens von Wolfersdorf and Magali Cochet},
url = {https://doi.org/10.1016/j.ijheatmasstransfer.2013.09.003},
doi = {10.1016/j.ijheatmasstransfer.2013.09.003},
year = {2013},
date = {2013-01-01},
journal = {International Journal of Heat and Mass Transfer},
volume = {67},
pages = {1208--1219},
abstract = {Impingement cooling effectiveness of gas turbine vanes and blades can be further increased due to the manufacturing feasibility of integrally cast airfoils which can provide narrow impingement cooling cavities. This study examines experimentally using the transient liquid crystal technique, and numerically using a commercial CFD package, the heat transfer characteristics of narrow impingement channels over their complete heat transfer area. The baseline configuration consists of a narrow impingement channel with a single row of five impingement jets. Effects of channel height (Z/D) and impingement hole offset position from the channel centerline (Δy/D) are investigated over a range of engine representative Reynolds numbers (10,000–40,000) based on the jet diameter. The CFD simulations are compared to the experiments aiming to quantify the degree of accuracy to which the local and averaged heat transfer rates can be predicted. The results are analysed by various post-processing procedures and compared to existing multi-array impingement cooling correlations.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2012ms,
title = {Thermocouple thermal inertia effects on impingement heat transfer experiments using the transient liquid crystal technique},
author = {Alexandros Terzis and Jens von Wolfersdorf and Bernhard Weigand and Peter Ott},
url = {http://dx.doi.org/10.1088/0957-0233/23/11/115303},
year = {2012},
date = {2012-01-01},
journal = {Measurement Science and Technology},
volume = {23},
number = {11},
pages = {115303},
abstract = {The transient liquid crystal technique is widely used for impingement heat transfer experiments. Additionally, due to the difficulty of producing pure temperature steps in the flow, many authors assumed the fluid temperature evolution as a series of step changes using Duhamel's superposition theorem. However, for small impingement configurations where the jets are fed from the same plenum chamber, and hence flow velocities are relatively small, thermal inertia of commercial thermocouples causes a delay, lagging from the real plenum temperature history. This paper investigates thermal inertia characteristics of thermocouples and their effect on the calculation of impingement heat transfer coefficient. Several thermocouples with exposed junction and different wire diameter were considered over a range of plenum flow conditions typically found in impingement heat transfer experiments. The effect of thermocouple time constant on the evaluation of the heat transfer rate was investigated in a narrow channel consisting of five inline impingement jets. The results indicated a significant effect of thermocouple response on the stagnation point region heat transfer, while lower local heat transfer rates are negligibly affected as liquid crystal signals appear later in time and the driving gas temperature history has a smaller influence on the evaluated data.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2012ur,
title = {Swirl Jets and Crossflow Interactions at Low Velocity Ratios},
author = {Alexandros Terzis and Charilaos Kazakos and Anestis I Kalfas and Pavlos K Zachos and Peter Ott},
url = {https://infoscience.epfl.ch/record/164506?ln=en},
year = {2012},
date = {2012-01-01},
journal = {Journal of Mechanics Engineering and Automation},
volume = {2},
number = {4},
pages = {256--266},
abstract = {Swirl jets in crosflow are investigated experimentally and numerically. The main difference with non-swirl cases is an asymmetry of the dominant kidney vortex and a slight distortion of the jet trace downstream of the injection hole. Jet rotation, at relatively low swirl numbers (S=0.1-0.4) is investigated by a computational tool while the numerical results are analyzed by means of various post-processing procedures aiming to clarify and quantify the impact of jet swirl on the ballistic behavior of a jet in crossflow. In general, swirl introduces an asymmetry in the flow domain and prevents the penetration of the jet into the crossflow. The rotation of the jet results in an imparity of the two parts of the main kidney vortex which is no longer symmetric to the axial centerline plane. Higher swirl numbers destruct the dominant kidney shape vortex which is transformed into a comma shape vortex, rotating close to the wall.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2012bj,
title = {Heat transfer and performance characteristics of axial cooling fans with downstream guide vanes},
author = { Alexandros Terzis and Ioannis Stylianou and Anestis I Kalfas and Peter Ott},
url = {https://doi.org/10.1007/s11630-012-0531-8},
doi = {10.1007/s11630-012-0531-8},
year = {2012},
date = {2012-01-01},
journal = {Journal of Thermal Science},
volume = {21},
number = {2},
pages = {162--171},
abstract = {This study examines experimentally the effect of stators on the performance and heat transfer characteristics of small axial cooling fans. A single fan impeller, followed by nine stator blades in the case of a complete stage, was used for all the experimental configurations. Performance measurements were carried out in a constant speed stage performance test rig while the transient liquid crystal technique was used for the heat transfer measurements. Full surface heat transfer coefficient distributions were obtained by recording the temperature history of liquid crystals on a target plate. The experimental data indicated that the results are highly affected by the flow conditions at the fan outlet. Stators can be beneficial in terms of pressure drop and efficiency, and thus more economical operation, as well as, in the local heat transfer distribution at the wake of the stator blades if the fan is installed very close to the cooling object. However, as the separation distance increases, enhanced heat transfer rate in the order of 25% is observed in the case of the fan impeller.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2011hv,
title = {On the applicability of oil and dye flow visualization technique during the design phase and operation of experimental rigs},
author = { Alexandros Terzis and Pavlos Zachos and Bernard Charnley and Vassilios Pachidis and Anestis I Kalfas},
url = {https://doi.org/10.1615/JFlowVisImageProc.2011002885},
year = {2011},
date = {2011-01-01},
journal = {Journal of Flow Visualization and Image Processing},
volume = {18},
number = {3},
pages = {199--214},
abstract = {In this paper, an oil and dye flow visualization technique applied in two different cases of experimental testing is described, namely a single fan in crossflow and the wake of a compressor cascade. In both cases, the mixture of paint was prepared using a highly volatile light mineral or heavy machine oil together with very fine pigments of titanium dioxide (TiO2) or fluorescein sodium in various colors. After the preparation of the mixture, a homogeneous thin film was applied onto the whole investigated surface by painting with a soft brush. The air stream which flows over the surface of the plate modifies the concentration and the homogeneity of the oil film according to the flow conditions very close to the wall. The film was dried by the airflow and photographed for further consideration and field observation. A number of successful as well as unsuccessful flow visualization experiments are presented hereafter. The objective of this work is to provide further information and some general guidelines about mixture concentration at low-and high-speed incompressible flows and to discuss the applicability of this method of visualization technique in complex turbomachinery flows.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}

@article{Terzis:2011do,
title = {Effects of Swirl Velocities From Fan Assemblies Mounted on Lifting Surfaces},
author = { Alexandros Terzis and Charilaos Kazakos and Nikolaos Papadopoulos and Anestis I Kalfas and Pavlos K Zachos and Pericles Pilidis},
url = {https://doi.org/10.1115/1.4002099},
year = {2011},
date = {2011-01-01},
volume = {133},
number = {3},
pages = {031702},
abstract = {The penetration of a jet of fluid into a traversal moving stream is a basic configuration of a wide range of engineering applications, such as film cooling and V/STOL aircrafts. This investigation examines experimentally the effect of blowing ratio of fans in crossflow, and numerically, the effect of the swirl velocity of jets in crossflow, downstream of the injection hole. The experimental results indicated an agreement with typically straight jets in crossflow (no vorticity), illustrating that the trace of the jet, remains close to the wall and subsequently enhance cooling at low blowing ratios in the case of turbine blade applications. However, the rotation of the jet results in an imparity between the two parts of the counter rotating vortex pair and as a consequence, the injected fluid not only bends in the direction of the main stream but also diverts in the direction of the rotation in order to conserve its angular momentum. The induction of the swirl velocity on the injected jet destructs one of the two parts of the kidney vortex, which entrains fluid from the crossflow to the jet promoting the mixing between the two fluids while the trace of a swirled jet remains closer to the wall downstream of the injection hole. Finally, the use of contrarotating jet or fan configurations reduces the wall shear stress in a very great extent, leading to better thermal protection of turbine blades, as well as cancels out the yaw torques of each fan separately, resulting in better flight control of typical lift surface.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}