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Diez Sanhueza, R.G. (author), Smit, S.H.H.J. (author), Peeters, J.W.R. (author), Pecnik, R. (author)This paper presents a machine learning methodology to improve the predictions of traditional RANS turbulence models in channel flows subject to strong variations in their thermophysical properties. The developed formulation contains several improvements over the existing Field Inversion Machine Learning (FIML) frameworks described in the...journal article 2023
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Nederstigt, P. (author), Pecnik, R. (author)Isentropic processes in thermodynamics are fundamental to our understanding of numerous physical phenomena across different scientific and engineering fields. They provide a theoretical reference case for the evaluation of real thermodynamic processes and observations. Yet, as analytical relations for isentropic transformations in gas dynamics...journal article 2023
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Bugeat, B. (author), Boldini, P.C. (author), Pecnik, R. (author)Ren et al. (2019) recently studied the stability of the boundary layer flow over a flat plate for supercritical CO2. While only one unstable mode usually exists for boundary layer flows, the authors found an additional unstable mode, whose origin has so far not been identified. In the present work, we carry out a stability analysis in the...conference paper 2022
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Agostinho, Luewton L.F. (author), Pecnik, R. (author), Woisetschläger, Jakob (author), de Kroon, Esther (author), Şişcanu, Nicolae (author), van de Griend, Maarten V. (author), Loiskandl, Willibald (author), Fuchs, Elmar C. (author)Aeration is one of the most cost intensive steps in water and wastewater treatment due to the large energy requirement for the creation of large surfaces for sufficient gas exchange as well as for providing efficient liquid transport in order to exchange saturated liquid elements at the surface with unsaturated ones from the bulk. In this...journal article 2022
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Silvestri, S. (author), Pecnik, R. (author)We present direct numerical simulations of developing turbulent channel flows subjected to thermal expansion or contraction downstream of a heated or cooled wall. Using different constitutive relations for viscosity we analyse the response of variable property flows to streamwise acceleration/deceleration by separating the effect of...journal article 2021
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Hirai, Ryo (author), Pecnik, R. (author), Kawai, Soshi (author)In supersonic turbulent boundary layers over isothermal walls, we investigate how the wall heat flux affects turbulent statistics and velocity scaling laws. To distinguish local Reynolds number and compressibility effects, we consider a conventional ideal gas with Sutherland's law and a fluid for which the dynamic viscosity is proportional to...journal article 2021
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Otero Rodriguez, G.J. (author), Smit, S.H.H.J. (author), Pecnik, R. (author)Organic Rankine cycle (ORC) systems are a readily available technology to convert thermal energy from renewable- and waste heat sources into electricity. However, their thermal performance is relatively low due to the low temperature of the available heat sources, but more importantly, due to the low efficiency of the employed expander....journal article 2021
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Fuchs, Elmar C. (author), Woisetschläger, Jakob (author), Wexler, Adam D. (author), Pecnik, R. (author), Vitiello, Giuseppe (author)A horizontal electrohydrodynamic (EHD) liquid bridge (also known as a “floating water bridge”) is a phenomenon that forms when high voltage DC (kV·cm<sup>-1</sup>) is applied to pure water in two separate beakers. The bridge, a free-floating connection between the beakers, acts as a cylindrical lens and refracts light. Using an...journal article 2021
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Silvestri, S. (author), Roekaerts, D.J.E.M. (author), Pecnik, R. (author)The present work investigates the modeling of turbulent heat transfer in flows where radiative and convective heat transfer are coupled. In high temperature radiatively participating flows, radiation is the most relevant heat transfer mechanism and, due to its non-locality, it causes counter intuitive interactions with the turbulent...journal article 2021
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Schöffer, S.I. (author), Klein, S.A. (author), Aravind, P.V. (author), Pecnik, R. (author)New technologies are being developed to produce electricity cleaner and more efficient. Promising technologies among these are the solid oxide fuel cell and the supercritical carbon dioxide Brayton cycle. This study investigates the potential of integrating both technologies. The solid oxide fuel cell is known as a potentially clean and...journal article 2021
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Ren, J. (author), Fu, Song (author), Pecnik, R. (author)The objective of this work is to investigate linear modal and algebraic instability in Poiseuille flows with fluids close to their vapour-liquid critical point. Close to this critical point, the ideal gas assumption does not hold and large non-ideal fluid behaviours occur. As a representative non-ideal fluid, we consider supercritical carbon...journal article 2019
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Silvestri, S. (author), Pecnik, R. (author)We implemented a fast Reciprocal Monte Carlo algorithm to accurately solve radiative heat transfer in turbulent flows of non-grey participating media that can be coupled to fully resolved turbulent flows, namely to Direct Numerical Simulation (DNS). The spectrally varying absorption coefficient is treated in a narrow-band fashion with a...journal article 2019
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Ren, J. (author), Marxen, Olaf (author), Pecnik, R. (author)We investigate the hydrodynamic stability of compressible boundary layers over adiabatic walls with fluids at supercritical pressure in the proximity of the Widom line (also known as the pseudo-critical line). Depending on the free-stream temperature and the Eckert number that determines the viscous heating, the boundary-layer temperature...journal article 2019
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Anand, N. (author), Vitale, S. (author), Pini, M. (author), Otero Rodriguez, G.J. (author), Pecnik, R. (author)The stator vane of high-temperature Organic Rankine Cycle radial-inflow turbines operates under severe expansion ratios and the associated fluid-dynamic losses account for nearly two-third of the total losses generated inside the machine. The efficiency of the machine can strongly benefit from specialized high-fidelity design methods able to...journal article 2018
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Silvestri, S. (author), Patel, A. (author), Roekaerts, D.J.E.M. (author), Pecnik, R. (author)The present work consists of an investigation of the turbulence radiation interaction (TRI) in a radiative turbulent channel flow of grey gas bounded by isothermal hot and cold walls. The optical thickness of the channel is varied from 0.1 to 10 to observe different regimes of TRI. A high-resolution finite volume method for radiative heat...journal article 2018
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Otero Rodriguez, G.J. (author), Patel, A. (author), Diez Sanhueza, Rafael (author), Pecnik, R. (author)This paper presents a novel methodology for improving eddy viscosity models in predicting wall-bounded turbulent flows with strong gradients in the thermo-physical properties. Common turbulence models for solving the Reynolds-averaged Navier–Stokes equations do not correctly account for variations in transport properties, such as density and...journal article 2018
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Mehrali, M. (author), Ghatkesar, M.K. (author), Pecnik, R. (author)The wide-spread adoption of solar thermal absorbers is currently hampered by their low absorption efficiencies and their high capital cost. As a result, a number of initiatives, including direct absorption solar collectors (DASC), are currently underway to improve the absorber efficiencies. In this regard, this study focused on application of...journal article 2018
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Rinaldi, E. (author), Patel, A. (author), Schlatter, Philipp (author), Pecnik, R. (author)We investigate the stability of streaks in the buffer layer of turbulent channel flows with temperature-dependent density and viscosity by means of linear theory. The adopted framework consists of an extended set of the Orr-Sommerfeld-Squire equations that accounts for density and viscosity nonuniformity in the direction normal to the walls....journal article 2017
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Patel, A. (author), Pecnik, R. (author), Peeters, J.W.R. (author), Boersma, B.J. (author), Hickel, S. (author), Moghadam, M.E. (author)We investigate the effectiveness of the semi-local Reynolds number Re τ to parametrize wall-bounded flows with strong density, ρ, and viscosity, µ, gradients. Several cases are considered, namely, volumetrically heated low-Mach-number turbulent channel flows, a simultaneously heated and cooled flow with CO2 at supercritical pressure, and heated and...book chapter 2017
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Pecnik, R. (author), Patel, A. (author)We derive an alternative formulation of the turbulent kinetic energy equation for flows with strong near-wall density and viscosity gradients. The derivation is based on a scaling transformation of the Navier-Stokes equations using semi-local quantities. A budget analysis of the semi-locally scaled turbulent kinetic energy equation shows that...journal article 2017
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