The turbulent flow of a fluid carrying trace amounts of a condensable species through a different... more The turbulent flow of a fluid carrying trace amounts of a condensable species through a differentially cooled vertical channel geometry is simulated using singlephase direct numerical simulations. The release of latent heat during condensation is modeled by interdependent temperature and vapor concentration source terms governing the relation between the removal of excess vapor from the system and the associated local increase in fluid temperature. A coupling between condensation and turbulence is implemented via solutal and thermal buoyancy. When compared to simulations of an identical system without phase transition modeling, the modifications of the subcooled boundary layer due to the transient and highly localized release of latent heat could be observed. A separate analysis of fluid before and after phase transition events shows a clear increase in post-interaction streak spacing, with the release of latent heat during condensation events opposing the cooling effect of the channel wall and the associated damping of turbulence.
The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed ... more The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed convection airflow are analyzed and discussed. To access local temperature and velocity fields in airflows, we present a combination of stereoscopic particle image velocimetry and particle image thermometry. The obtained flow fields make it possible to determine the local convective heat fluxes, thus giving insight into the dynamics of plumes and Taylor-Görtler-like vortices. The evaluated mean local heat fluxes further reveal that the main convection roll transports a substantial amount of heat along the cooling plate and back to the heated bottom plate. Yet, the associated mean turbulent heat fluxes remain positive as they are dominated by the correlation of the temperature and the vertical velocity component. More specifically, a statistical analysis of the local heat flux distribution reveals that Taylor-Görtler-like vortices lead to more skewed distributions of the turbulent convective heat fluxes than plumes.
Temperature and humidity measurements are conducted in mixed convective humid-air duct flow with ... more Temperature and humidity measurements are conducted in mixed convective humid-air duct flow with condensation. The latent and total heat transfer during the experiment are determined through the thermal balance for inlet temperatures from 27.5 • C to 35.5 • C, relative humidities from 30% to 55% and at four Reynolds numbers (2000-8000). The experimental results are compared with a heat transfer model from the literature. Adjusted in terms of the geometry and surface properties, the model shows partial agreement for the cases with forced convection but has to be further adjusted regarding the influence of thermal convection.
The dispersion of aerosols originating from one source, the 'index' passenger, within the cabin o... more The dispersion of aerosols originating from one source, the 'index' passenger, within the cabin of the aircraft Do728 is studied experimentally using an aerosol-exhaling thermal manikin and in Reynolds-averaged Navier-Stokes simulations (RANS). The overall aim of the present study is the experimental determination of the aerosol spreading for the state-of-the-art mixing ventilation (MV) and to evaluate the potential of alternative ventilation concepts for controlling the aerosol spreading in RANS. For MV, the experiments showed that the ratio of inhaled to exhaled aerosol particles drops below 0.06% (volume ratio) for distances larger than two seat rows from the source. However, within a single row, the observed ratio is higher. Further, the dispersion is much weaker for a standing than for a seated index passenger. High air exchange rates and a wellguided flow prevent a dispersion of the aerosols in high concentrations over larger distances. Additionally, the positive effect of a mask and an increased air flow rate, and especially their combination are shown. In the complementary conducted RANS, the advantages of floor-based cabin displacement ventilation (CDV) which is alternative ventilation concept to MV, regarding spreading lengths and the dwell time of the aerosols in the cabin were determined. The obtained results also underline the importance of the flow field for the aerosol dispersion. Further, additional unsteady RANS (URANS) simulations of the short-term process of the initial aerosol cloud formation highlighted that the momentum decay of the breathing and the evaporation processes take place within a few seconds only.
Reconfiguration events in turbulent mixed convection, i.e. the superposition of thermal and force... more Reconfiguration events in turbulent mixed convection, i.e. the superposition of thermal and forced flow contributions, at the two different Richardson numbers Ri = 1.5 and Ri = 3.7 and similar Rayleigh numbers of Ra ≈ 10 8 are investigated with tomographic particle image velocimetry in combination with local temperature measurements. For both cases, the three-dimensional velocity fields reflect diagonally aligned large-scale circulations (LSC) switching their alignment by rotating their axes around a pivot located at the centre of the LSC, while the temperatures perform a translation movement of the structures in agreement with earlier temperature-based investigations. For the high Ri case, the switching process of the observed spontaneous reconfigurations is induced by a reversing thermal flow contribution while the forced flow contribution is constant. Furthermore, it is shown that a secondary roll structure, which drives the reconfiguration process in Rayleigh-Bénard convection, also exists in mixed convection. However, in the latter, the flow reversals are triggered by different structures which accumulate and release their kinetic energy according to a proper orthogonal decomposition analysis. In contrast, for the low Ri case, the structure formation during continuous reconfigurations is governed by a Taylor-or Görtler-type instability. This means that the forced convection substantially affects the reconfiguration mechanism of these structures. Therefore, the reconfigurations cannot be described by a simple superposition of structures associated with the two flow contributions as for the high Ri.
Results of a direct numerical simulation (DNS) of turbulent sudden pipe expansion flow at a Reyno... more Results of a direct numerical simulation (DNS) of turbulent sudden pipe expansion flow at a Reynolds number based on upstream centerline velocity and diameter of 6950 are presented. They are obtained with a central finite-difference scheme of second order accuracy in space and time. It is shown that the scheme is well suited for DNS as long as the smallest turbulent scales are properly resolved. One-dimensional spatial sharp cut-off filtering of the data is performed in order to assess the energy transfer between resolved and unresolved motions in large-eddy simulations (LES). Backward transfer of kinetic energy from subgrid-scale (SGS) to grid-scale turbulent motions is found in the mean which cannot be described by simple eddy-viscosity type SGS models.
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2014
Aerodynamic noise generated by low Mach number flow through a pipe with two differently sized dia... more Aerodynamic noise generated by low Mach number flow through a pipe with two differently sized diaphragm orifices is investigated both theoretically as well as numerically. Such a flow may be considered as a simple model of an aircraft climate control system. The theory is based on an acoustic analogy introduced by Mohring and Obermeier in the seventies. They identified by means of the so-called Green’s vector function acoustic sources in unsteady flows as the unsteady motion of vorticity “vortices are the voice of flows”. In addition, the unsteady flow through the pipe, the aerodynamic sources, and the spectra of sound are evaluated numerically. Here effects of different numerical algorithms (PISO—PIMPLE) and of different numerical boundary conditions at the outlet of the pipe (advective, convective—wave transmissive) are identified and discussed.
A novel method for assimilating and extending measured turbulent Rayleigh-Bénard convection data ... more A novel method for assimilating and extending measured turbulent Rayleigh-Bénard convection data is presented, which relies on the fractional step method also used to solve the incompressible Navier-Stokes equation in direct numerical simulations. Our approach is used to make measured tomographic particle image velocimetry (tomo PIV) fields divergence-free and to extract temperature fields. Comparing the time average of the extracted temperature fields with the temporally averaged temperature field, measured using particle image thermometry in a subdomain of the flow geometry, shows that extracted fields correlate well with measured fields with a correlation coefficient of C T T = 0.84 . Additionally, extracted temperature fields as well as divergence-free velocity fields serve as initial fields for subsequent direct numerical simulations with and without feedback which generate small-scale turbulence initially absent in the experimental data. Although the tomo PIV data set was spatially under-resolved and did not include any information on the boundary layers, the here-proposed method successfully generates velocity and temperature fields featuring small-scale turbulence and thermal as well as kinetic boundary layers, without disturbing the large-scale circulation contained in the original experimental data significantly. The latter is underpinned by high vertical and horizontal velocity correlation coefficients-computed from velocity fields averaged in time and horizontal x-direction obtained from the measurement and from the simulation without feedback-of C vṽ = 0.92 and C w w = 0.91 representing the large-scale structure. For simulations with feedback, the generated velocity fields resemble the experimental data increasingly well for higher feedback gain values, whereas the temperature fluctuation intensity deviates noticeably from the values obtained from a direct numerical simulation without feedback for gain values ≥ 1 . Thus, a feedback gain of = 0.1 was found optimal with correlation coefficients of C vṽ = 0.96 and C w w = 0.95 as well as a realistic temperature fluctuation intensity profile. The xt-averaged temperature fields obtained from the direct numerical simulations with and without feedback correlate somewhat less with the extracted temperature field ( C T T ≈ 0.6 ), which is presumably caused by spatially under-resolved and temporally oscillating initial tomo PIV fields reflected by the extracted temperature field.
In this paper a numerical approach for the prediction of the unsteady aerosol formation and trans... more In this paper a numerical approach for the prediction of the unsteady aerosol formation and transmission process in a train cabin is presented. For that purposes models to simulate unsteady flows including the transient behaviour of two-phase atomization process and thermal air flow are employed. Results of aerosol distribution for coughing, speaking and breathing (with and without mask) in specified train cabin compartment are discussed. The dispersion of the exhaled droplets was analysed for a double cough, 10 s talking and continuous breathing of one source passenger. The results obtained show that the dispersion of aerosol particles in the cabin after coughing is two times deeper than when speaking, 2.5 times deeper than when free breathing and 17 times deeper than when breathing with a mask. Further, the results revealed that 2 minutes after the end of the coughing, only about 6% of active aerosol particles remain in the compartment and relatively clean air is again in the cabin.
Proceeding of Seventh International Symposium on Turbulence and Shear Flow Phenomena, 2011
We perform direct numerical simulations (DNS) of turbulent Rayleigh-Bénard convection coupled wit... more We perform direct numerical simulations (DNS) of turbulent Rayleigh-Bénard convection coupled with surface-tosurface radiation in a rectangular enclosure filled with air to investigate whether this interaction influences the heat transfer, temperature distribution and the flow structures. To do so, horizontal solid plates with finite conductivity are employed for the considered Rayleigh-Bénard cell. Such boundary conditions allow local variations of the temperature at the hot and cold interfaces due to their interaction with the fluid and surface radiation. In order to investigate the maximum effect of those boundary conditions, both interfaces are treated as a blackbody (ε = 1) and the cell is filled with a radiatively non-participating fluid (Prandtl number Pr=0.7). The effects of radiation for highly conducting plates are shown and compared to the case where radiation is neglected. It is found that due to highly conducting plates the mean temperature at the interfaces changes only 0.04% from the one of the infinite conductive plates. Furthermore, we observe that due to surface-to-surface radiation coupled with highly conducting plates, the mean temperature at the interfaces changes 0.1% at the interfaces and 0.2% in the bulk. It is shown that the temperature at the hot interface tends to decrease due to the radiative heat loss while the temperature at the cold interface slightly increases. Apart from that, we observe small changes in the temperature distribution at the interfaces due to surfaceto-surface radiation. We notice that the highest temperatures at the top interface appear in the middle and the values steadily decrease towards the edges. Additionally, we observe a small drop of the convective Nusselt number and little variations of the temperature distribution at the interfaces. Finally, it is shown that all mentioned variations caused by heat radiation between interfaces are too small to visibly change the large scale flow structures when highly conducting plates are employed. It is also shown that in the non-radiation case of poorly conducting plates the heat transfer and the temperature variations at the interfaces are influenced significantly.
Large Eddy Simulations (LES) of turbulent flows around a segment of the FX-79-W151 airfoil have b... more Large Eddy Simulations (LES) of turbulent flows around a segment of the FX-79-W151 airfoil have been performed for different Reynolds numbers between Re = 5•10 3 and Re = 10 5 and the angle of attack 12 • using a Spectral Element Method (SEM). The results of the LES for Re = 5 • 10 3 and Re = 5 • 10 4 are compared. The turbulence statistics obtained in the LES reveal laminar and turbulent flow separation regions for the lower and higher Reynolds numbers, respectively.
The high-frequency oscillation ventilation (HFOV) is a mechanical respiration technique which unl... more The high-frequency oscillation ventilation (HFOV) is a mechanical respiration technique which unlikely produces any cyclic alveolar recollapse and recruitment. Hence, HFOV was proposed to reduce ventilator associated lung injury (VALI) and might advance from being a rescue proceeding to being an accepted alternative ventilation strategy for patients suffering from acute respiratory distress syndrom (ARDS) . But optimising HFOV certainly requires more detailed knowledge of the governing transport mechanisms in the lung, which are rather complex and yet not fully understood . Within the framework of the Protective Artificial Respiration project (PAR), supported by the German Research Foundation (DFG), we aim to analyse the air flow under the conditions of spontaneous breathing and artificial respiration.
Notes on numerical fluid mechanics and multidisciplinary design, 2006
Direct numerical simulation (DNS) of incompressible turbulent pipe flow was carried out on unstru... more Direct numerical simulation (DNS) of incompressible turbulent pipe flow was carried out on unstructured grids for a Reynolds number based on the friction velocity and the pipe diameter of Rcr = 360 using the spectral/hp element method (SEM) by Karniadakis and Sherwin [3]. The main objective was to investigate the computational aspects of this DNS with respect to accuracy, CPU time and memory requirements. DNS results of evaluated statistical moments of up to fourth order agree well with data from the literature. A conducted performance study reveals the computational requirements for DNS of turbulent flows using this SEM.
The pattern formation of a Poiseuille flow in a rectangular duct, which is heated from below, is ... more The pattern formation of a Poiseuille flow in a rectangular duct, which is heated from below, is studied experimentally for the Reynolds number Re = 200 within the Rayleigh number range 4 × 10 8 ≤ Ra ≤ 7 × 10 8 . The channel has an aspect ratio of Γ yz = width : height = 25 : 2. As working fluid, water (Pr ≈ 6) is used. In order to study the influence of buoyancy on the forced flow, velocity fields are measured plane parallel to the heated bottom plate by means of Particle Image Velocimetry (PIV). Furthermore, the measured data is analyzed concerning the flow structure formation as a function of the temperature difference between the inflow and the bottom of the duct. In this connection, two different mechanisms can be distinguished, which cause the transition from a laminar to a turbulent flow: a successively progressing transition on the one hand and, above a certain Archimedes number Ar, an abrupt transition.
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2011
Page 1. Magnetic Resonance Imaging and Computational Fluid Dynamics of High Frequency Oscillatory... more Page 1. Magnetic Resonance Imaging and Computational Fluid Dynamics of High Frequency Oscillatory Ventilation (HFOV) Alexander-Wigbert K. Scholz, Lars Krenkel, Maxim Terekhov, Janet Friedrich, Julien Rivoire, Rainer ...
The turbulent flow of a fluid carrying trace amounts of a condensable species through a different... more The turbulent flow of a fluid carrying trace amounts of a condensable species through a differentially cooled vertical channel geometry is simulated using singlephase direct numerical simulations. The release of latent heat during condensation is modeled by interdependent temperature and vapor concentration source terms governing the relation between the removal of excess vapor from the system and the associated local increase in fluid temperature. A coupling between condensation and turbulence is implemented via solutal and thermal buoyancy. When compared to simulations of an identical system without phase transition modeling, the modifications of the subcooled boundary layer due to the transient and highly localized release of latent heat could be observed. A separate analysis of fluid before and after phase transition events shows a clear increase in post-interaction streak spacing, with the release of latent heat during condensation events opposing the cooling effect of the channel wall and the associated damping of turbulence.
The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed ... more The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed convection airflow are analyzed and discussed. To access local temperature and velocity fields in airflows, we present a combination of stereoscopic particle image velocimetry and particle image thermometry. The obtained flow fields make it possible to determine the local convective heat fluxes, thus giving insight into the dynamics of plumes and Taylor-Görtler-like vortices. The evaluated mean local heat fluxes further reveal that the main convection roll transports a substantial amount of heat along the cooling plate and back to the heated bottom plate. Yet, the associated mean turbulent heat fluxes remain positive as they are dominated by the correlation of the temperature and the vertical velocity component. More specifically, a statistical analysis of the local heat flux distribution reveals that Taylor-Görtler-like vortices lead to more skewed distributions of the turbulent convective heat fluxes than plumes.
Temperature and humidity measurements are conducted in mixed convective humid-air duct flow with ... more Temperature and humidity measurements are conducted in mixed convective humid-air duct flow with condensation. The latent and total heat transfer during the experiment are determined through the thermal balance for inlet temperatures from 27.5 • C to 35.5 • C, relative humidities from 30% to 55% and at four Reynolds numbers (2000-8000). The experimental results are compared with a heat transfer model from the literature. Adjusted in terms of the geometry and surface properties, the model shows partial agreement for the cases with forced convection but has to be further adjusted regarding the influence of thermal convection.
The dispersion of aerosols originating from one source, the 'index' passenger, within the cabin o... more The dispersion of aerosols originating from one source, the 'index' passenger, within the cabin of the aircraft Do728 is studied experimentally using an aerosol-exhaling thermal manikin and in Reynolds-averaged Navier-Stokes simulations (RANS). The overall aim of the present study is the experimental determination of the aerosol spreading for the state-of-the-art mixing ventilation (MV) and to evaluate the potential of alternative ventilation concepts for controlling the aerosol spreading in RANS. For MV, the experiments showed that the ratio of inhaled to exhaled aerosol particles drops below 0.06% (volume ratio) for distances larger than two seat rows from the source. However, within a single row, the observed ratio is higher. Further, the dispersion is much weaker for a standing than for a seated index passenger. High air exchange rates and a wellguided flow prevent a dispersion of the aerosols in high concentrations over larger distances. Additionally, the positive effect of a mask and an increased air flow rate, and especially their combination are shown. In the complementary conducted RANS, the advantages of floor-based cabin displacement ventilation (CDV) which is alternative ventilation concept to MV, regarding spreading lengths and the dwell time of the aerosols in the cabin were determined. The obtained results also underline the importance of the flow field for the aerosol dispersion. Further, additional unsteady RANS (URANS) simulations of the short-term process of the initial aerosol cloud formation highlighted that the momentum decay of the breathing and the evaporation processes take place within a few seconds only.
Reconfiguration events in turbulent mixed convection, i.e. the superposition of thermal and force... more Reconfiguration events in turbulent mixed convection, i.e. the superposition of thermal and forced flow contributions, at the two different Richardson numbers Ri = 1.5 and Ri = 3.7 and similar Rayleigh numbers of Ra ≈ 10 8 are investigated with tomographic particle image velocimetry in combination with local temperature measurements. For both cases, the three-dimensional velocity fields reflect diagonally aligned large-scale circulations (LSC) switching their alignment by rotating their axes around a pivot located at the centre of the LSC, while the temperatures perform a translation movement of the structures in agreement with earlier temperature-based investigations. For the high Ri case, the switching process of the observed spontaneous reconfigurations is induced by a reversing thermal flow contribution while the forced flow contribution is constant. Furthermore, it is shown that a secondary roll structure, which drives the reconfiguration process in Rayleigh-Bénard convection, also exists in mixed convection. However, in the latter, the flow reversals are triggered by different structures which accumulate and release their kinetic energy according to a proper orthogonal decomposition analysis. In contrast, for the low Ri case, the structure formation during continuous reconfigurations is governed by a Taylor-or Görtler-type instability. This means that the forced convection substantially affects the reconfiguration mechanism of these structures. Therefore, the reconfigurations cannot be described by a simple superposition of structures associated with the two flow contributions as for the high Ri.
Results of a direct numerical simulation (DNS) of turbulent sudden pipe expansion flow at a Reyno... more Results of a direct numerical simulation (DNS) of turbulent sudden pipe expansion flow at a Reynolds number based on upstream centerline velocity and diameter of 6950 are presented. They are obtained with a central finite-difference scheme of second order accuracy in space and time. It is shown that the scheme is well suited for DNS as long as the smallest turbulent scales are properly resolved. One-dimensional spatial sharp cut-off filtering of the data is performed in order to assess the energy transfer between resolved and unresolved motions in large-eddy simulations (LES). Backward transfer of kinetic energy from subgrid-scale (SGS) to grid-scale turbulent motions is found in the mean which cannot be described by simple eddy-viscosity type SGS models.
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2014
Aerodynamic noise generated by low Mach number flow through a pipe with two differently sized dia... more Aerodynamic noise generated by low Mach number flow through a pipe with two differently sized diaphragm orifices is investigated both theoretically as well as numerically. Such a flow may be considered as a simple model of an aircraft climate control system. The theory is based on an acoustic analogy introduced by Mohring and Obermeier in the seventies. They identified by means of the so-called Green’s vector function acoustic sources in unsteady flows as the unsteady motion of vorticity “vortices are the voice of flows”. In addition, the unsteady flow through the pipe, the aerodynamic sources, and the spectra of sound are evaluated numerically. Here effects of different numerical algorithms (PISO—PIMPLE) and of different numerical boundary conditions at the outlet of the pipe (advective, convective—wave transmissive) are identified and discussed.
A novel method for assimilating and extending measured turbulent Rayleigh-Bénard convection data ... more A novel method for assimilating and extending measured turbulent Rayleigh-Bénard convection data is presented, which relies on the fractional step method also used to solve the incompressible Navier-Stokes equation in direct numerical simulations. Our approach is used to make measured tomographic particle image velocimetry (tomo PIV) fields divergence-free and to extract temperature fields. Comparing the time average of the extracted temperature fields with the temporally averaged temperature field, measured using particle image thermometry in a subdomain of the flow geometry, shows that extracted fields correlate well with measured fields with a correlation coefficient of C T T = 0.84 . Additionally, extracted temperature fields as well as divergence-free velocity fields serve as initial fields for subsequent direct numerical simulations with and without feedback which generate small-scale turbulence initially absent in the experimental data. Although the tomo PIV data set was spatially under-resolved and did not include any information on the boundary layers, the here-proposed method successfully generates velocity and temperature fields featuring small-scale turbulence and thermal as well as kinetic boundary layers, without disturbing the large-scale circulation contained in the original experimental data significantly. The latter is underpinned by high vertical and horizontal velocity correlation coefficients-computed from velocity fields averaged in time and horizontal x-direction obtained from the measurement and from the simulation without feedback-of C vṽ = 0.92 and C w w = 0.91 representing the large-scale structure. For simulations with feedback, the generated velocity fields resemble the experimental data increasingly well for higher feedback gain values, whereas the temperature fluctuation intensity deviates noticeably from the values obtained from a direct numerical simulation without feedback for gain values ≥ 1 . Thus, a feedback gain of = 0.1 was found optimal with correlation coefficients of C vṽ = 0.96 and C w w = 0.95 as well as a realistic temperature fluctuation intensity profile. The xt-averaged temperature fields obtained from the direct numerical simulations with and without feedback correlate somewhat less with the extracted temperature field ( C T T ≈ 0.6 ), which is presumably caused by spatially under-resolved and temporally oscillating initial tomo PIV fields reflected by the extracted temperature field.
In this paper a numerical approach for the prediction of the unsteady aerosol formation and trans... more In this paper a numerical approach for the prediction of the unsteady aerosol formation and transmission process in a train cabin is presented. For that purposes models to simulate unsteady flows including the transient behaviour of two-phase atomization process and thermal air flow are employed. Results of aerosol distribution for coughing, speaking and breathing (with and without mask) in specified train cabin compartment are discussed. The dispersion of the exhaled droplets was analysed for a double cough, 10 s talking and continuous breathing of one source passenger. The results obtained show that the dispersion of aerosol particles in the cabin after coughing is two times deeper than when speaking, 2.5 times deeper than when free breathing and 17 times deeper than when breathing with a mask. Further, the results revealed that 2 minutes after the end of the coughing, only about 6% of active aerosol particles remain in the compartment and relatively clean air is again in the cabin.
Proceeding of Seventh International Symposium on Turbulence and Shear Flow Phenomena, 2011
We perform direct numerical simulations (DNS) of turbulent Rayleigh-Bénard convection coupled wit... more We perform direct numerical simulations (DNS) of turbulent Rayleigh-Bénard convection coupled with surface-tosurface radiation in a rectangular enclosure filled with air to investigate whether this interaction influences the heat transfer, temperature distribution and the flow structures. To do so, horizontal solid plates with finite conductivity are employed for the considered Rayleigh-Bénard cell. Such boundary conditions allow local variations of the temperature at the hot and cold interfaces due to their interaction with the fluid and surface radiation. In order to investigate the maximum effect of those boundary conditions, both interfaces are treated as a blackbody (ε = 1) and the cell is filled with a radiatively non-participating fluid (Prandtl number Pr=0.7). The effects of radiation for highly conducting plates are shown and compared to the case where radiation is neglected. It is found that due to highly conducting plates the mean temperature at the interfaces changes only 0.04% from the one of the infinite conductive plates. Furthermore, we observe that due to surface-to-surface radiation coupled with highly conducting plates, the mean temperature at the interfaces changes 0.1% at the interfaces and 0.2% in the bulk. It is shown that the temperature at the hot interface tends to decrease due to the radiative heat loss while the temperature at the cold interface slightly increases. Apart from that, we observe small changes in the temperature distribution at the interfaces due to surfaceto-surface radiation. We notice that the highest temperatures at the top interface appear in the middle and the values steadily decrease towards the edges. Additionally, we observe a small drop of the convective Nusselt number and little variations of the temperature distribution at the interfaces. Finally, it is shown that all mentioned variations caused by heat radiation between interfaces are too small to visibly change the large scale flow structures when highly conducting plates are employed. It is also shown that in the non-radiation case of poorly conducting plates the heat transfer and the temperature variations at the interfaces are influenced significantly.
Large Eddy Simulations (LES) of turbulent flows around a segment of the FX-79-W151 airfoil have b... more Large Eddy Simulations (LES) of turbulent flows around a segment of the FX-79-W151 airfoil have been performed for different Reynolds numbers between Re = 5•10 3 and Re = 10 5 and the angle of attack 12 • using a Spectral Element Method (SEM). The results of the LES for Re = 5 • 10 3 and Re = 5 • 10 4 are compared. The turbulence statistics obtained in the LES reveal laminar and turbulent flow separation regions for the lower and higher Reynolds numbers, respectively.
The high-frequency oscillation ventilation (HFOV) is a mechanical respiration technique which unl... more The high-frequency oscillation ventilation (HFOV) is a mechanical respiration technique which unlikely produces any cyclic alveolar recollapse and recruitment. Hence, HFOV was proposed to reduce ventilator associated lung injury (VALI) and might advance from being a rescue proceeding to being an accepted alternative ventilation strategy for patients suffering from acute respiratory distress syndrom (ARDS) . But optimising HFOV certainly requires more detailed knowledge of the governing transport mechanisms in the lung, which are rather complex and yet not fully understood . Within the framework of the Protective Artificial Respiration project (PAR), supported by the German Research Foundation (DFG), we aim to analyse the air flow under the conditions of spontaneous breathing and artificial respiration.
Notes on numerical fluid mechanics and multidisciplinary design, 2006
Direct numerical simulation (DNS) of incompressible turbulent pipe flow was carried out on unstru... more Direct numerical simulation (DNS) of incompressible turbulent pipe flow was carried out on unstructured grids for a Reynolds number based on the friction velocity and the pipe diameter of Rcr = 360 using the spectral/hp element method (SEM) by Karniadakis and Sherwin [3]. The main objective was to investigate the computational aspects of this DNS with respect to accuracy, CPU time and memory requirements. DNS results of evaluated statistical moments of up to fourth order agree well with data from the literature. A conducted performance study reveals the computational requirements for DNS of turbulent flows using this SEM.
The pattern formation of a Poiseuille flow in a rectangular duct, which is heated from below, is ... more The pattern formation of a Poiseuille flow in a rectangular duct, which is heated from below, is studied experimentally for the Reynolds number Re = 200 within the Rayleigh number range 4 × 10 8 ≤ Ra ≤ 7 × 10 8 . The channel has an aspect ratio of Γ yz = width : height = 25 : 2. As working fluid, water (Pr ≈ 6) is used. In order to study the influence of buoyancy on the forced flow, velocity fields are measured plane parallel to the heated bottom plate by means of Particle Image Velocimetry (PIV). Furthermore, the measured data is analyzed concerning the flow structure formation as a function of the temperature difference between the inflow and the bottom of the duct. In this connection, two different mechanisms can be distinguished, which cause the transition from a laminar to a turbulent flow: a successively progressing transition on the one hand and, above a certain Archimedes number Ar, an abrupt transition.
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2011
Page 1. Magnetic Resonance Imaging and Computational Fluid Dynamics of High Frequency Oscillatory... more Page 1. Magnetic Resonance Imaging and Computational Fluid Dynamics of High Frequency Oscillatory Ventilation (HFOV) Alexander-Wigbert K. Scholz, Lars Krenkel, Maxim Terekhov, Janet Friedrich, Julien Rivoire, Rainer ...
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