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Abstract 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 $$C_{T\tilde{T}}=0.84$$ 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 $$C_{v\tilde{v}}=0.92$$ C v v ~ = 0.92 and $$C_{w\tilde{w}}=0.91$$ 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 $$\alpha \ge 1$$ α ≥ 1 . Thus, a feedback gain of $$\alpha =0.1$$ α = 0.1 was found optimal with correlation coefficients of $$C_{v\tilde{v}}=0.96$$ C v v ~ = 0.96 and $$C_{w\tilde{w}}=0.95$$ 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 ($$C_{T\tilde{T}}\approx 0.6$$ 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. Graphical abstract

The heat transfer in mixed convective air flows with phase transition is a phenomenon which occurs in nature and a plethora of technical applications. When condensing materials form droplets, condensation leads to an increased heat transfer rate and therefore is advantageous in heat exchangers \cite{Schmidt-1930}, whereas in other technical applications condensation is oftentimes undesirable. For instance, fogging on the windshield or headlights of motor vehicles leads to restrictions when driving and a reduction of optical transparency influences road safety. In addition, a considerable amount of thermal energy is required for defogging, which can have a negative effect on the range of electric cars \cite{WesthoffFAT}. The design and modification of these components with regard to optimizing dehumidification or preventing misting is often based on experience and intuition. Despite the enormous progress of numerical methods in recent years, there is still a lack of reliable and applicable models for the numerical simulation of condensation in general, and in particular of droplet condensation on surfaces. Due to the complexity of the physical processes that determine the heat transport and thus the fogging on the panes, reliable simulations are expensive and time-consuming, and therefore not suitable for industrial applications. Thus, the objective of the present study is to develop a model, using dimensionless numbers, which allows the prediction of the condensation behavior on panes based on a reduced parameter space. In such a configuration the mass transport of the vapor by phase transition, the resulting latent and the sensible heat transfer are determined by the physical processes of convection and diffusion, the boundary conditions and the material properties of the surfaces. In addition, the mass transfer of water vapor changes even with the smallest changes in the boundary conditions. A major challenge of this study is therefore to design and construct an experimental setup with the appropriate measurement technology, which meets the requirements of ensuring defined boundary conditions, reproducibility of the experiments and measurement accuracy. The setup corresponds to a generic replica of a vehicle headlamp.

Ground vehicles are exposed to a vast variety of unsteady aerodynamic phenomena such as gusts. This is why the effect of gusts has been the focus of many studies, e.g. on cars in full scale (Schroeck et al., 2011), trains in full scale (Baker et al., 2004) as well as on simplified car models in wind tunnels (Ferrand, 2014). In order to gain better insight into the flow dynamics, Hemida and Krajnovic (2010) investigated the effect of ´cross winds on a generic train with different nose shapes using large-eddy simulations. Another unsteady effect that has recently gained interest is the multi-stability of the wake behind cars. In this context, Cadot et al. (2016) detected bi-stability in the lift forces acting on full-scale cars during wind tunnel testing. This bi-stability was further investigated in wind tunnel testing on simplified car models by Grandemange et al. (2013) and Pavia et al. (2018). Due to the highly transient behaviour of the flow, the existing numerical methods require revision concerning their capability of coping with the flow dynamics. For such investigations, the flow around a sphere is usually chosen as a setup, as it has been in the focus of aerodynamic research since the very beginning (Prandtl, 1914) and, thus, is the subject of many studies described in the literature. However, the flow around a sphere is an example of a fully separated flow, whose prediction poses a challenge to numerical investigations such as the DES study of Constantinescu and Squires (2004). The highly instationary dynamics in the wake of such a flow are described by hot-wire measurements (Achenbach, 1972) as well as by flow visualisation (Chrust et al., 2013). The latter clearly shows the development of hairpin vortices and the loss of planar symmetry

In collaboration with the University of Stuttgart and the Research Association of Automotive Technology (FAT), a series of on-road [1] and wind tunnel experiments [2] were conducted in order to characterize the onflow conditions for a vehicle driving behind another vehicle under real-world conditions. In conjunction with these experiments a CFD study was undertaken for both the on-road and wind tunnel measurements. This study, the subject of the paper, assesses the ability of two different CFD approaches to reproduce both the observed on-flow conditions as well as the aerodynamic drag of the vehicle. Both sets of CFD calculations are performed within the framework of the OpenFOAM [3] computational library.

Finding a reliable and accurate numerical model to investigate the transient effects of external turbulent flow on vehicle performance is of interest to the automotive industry. Practical applications include the assessment of vehicle wakes and/or over-taking manoeuvres on vehicle aerodynamic forces. On-road measurements are expensive and repeatability of test conditions can be difficult to achieve in practise. Schröck et al. [1] suggests that aerodynamic forces acting on a vehicle are sensitive to side wind conditions and showed that a transferfunction approach could be used to predict the influence on vehicle aerodynamic loading. The current work aims to compliment the work of Schröck et al. by utilizing both experimental and CFD to investigate the influence of time varying on-flow conditions on the drag for a 1:4 scale model vehicle. Outcomes of this work might then be used in development of accurate and cost effective prediction methods.

Single Phase DNS of turbulent flow of humid air along a channel is performed and coupled with a simplified condensation model. Droplet growth is studied using a clustering algorithm to model droplet Agglomeration.

The aim of this study is to detect and visualize the propagating sound waves emanating from a flow around different types of rods. For this purpose, synchronous Particle Image Velocimetry (PIV) and microphone measurements were performed in an aeroacoustic wind tunnel. The velocity fluctuations were recorded not only in the vicinity of the rods but also in several regions between the wake flow field and the far field microphone. By means of the correlation between velocity and pressure fluctuations, coherent structures within and outside the flow could be identified. In particular, the temporal and spatial development of these structures provides an insight into the sound generation mechanism and the transition between the acoustic near and far field. The results are compared with analytical and numerical solutions.

The energy efficiency of rail freight by means of reduced drag can be increased significantly by technical and operational measures. Previous studies have shown that optimizing the car sequence can result in a drag reduction by at least a factor of 2 (Barkan, 2009). Consequently, an aerodynamic optimization of freight trains includes intelligent logistics concepts. Such concepts can be supported by means of providing priority lists for operational measures, resulting from experimental and numerical studies. But the aerodynamic resistance of a freight train with an optimized car sequence is still about 50% higher compared to a passenger train of the same length (Li, 2011). It is clear from this fact that there is a high potential for aerodynamic optimization of the individual wagon in the freight transport sector. As part of the FR8RAIL-Project in the framework of the EU joint undertaking Shift2Rail, an innovative freight wagon will be developed. A look at the existing freight wagon fleet shows that their design gives little consideration to aerodynamics. The entire contour of the existing wagons is determined by the laws of traditional steel construction. UIC (International Union of Railways) compatibility also greatly restricts the options for optimizing drag reduction which is the case for the air flow both underneath and along the sides of the vehicles. In contrast to previous approaches making modification to existing vehicles, a conceptually new approach offers far more options with regard to innovative aerodynamic optimization of single wagons and freight trains. This work needs to ensure the compatibility of the solutions with the logistics systems developed in the framework of the FR8RAIL project, for example by planning a rapid and low-cost assembly of the aerodynamic components. The investigations are performed concomitantly, so the safety-relevant aspects are considered continuously during the aerodynamic design of the freight wagon.

Rheinisch-Westfälische Technische Hochschule Aachen, Diss., 2017; 141 pp., (2017)., The objective of this study was to evaluate the influence of human sperm parameters on fertilization in the context of IVF treatment before and after preparation of the semen sample. For several parameters threshold values were determined. Below those limits no fertilization was observed. Further the relationship between fertilization and the couple´s age, previous partnerships with and without birth of a child, the FSH level and the women´s body mass index were examined. The univariate analysis of the basic semen analysis showed a significant relationship between fertilization and the sperm parameters concentration, total sperm count, motility, long-term motility, morphology, multiple morphological sperm defects, sperm-vitality, chromatin condensation and acrosin activity. The univariate analysis of the native and the prepared semen samples showed significant positive correlations between fertilization and sperm concentration, sperm number and motility. There was a significant relationship between fertilization and the women´s age in the univariate analysis. However, the cycles in the group of the younger patients showed lower fertilization rates than in the group of older patients. The reason for the low fertilization rate was a poorer sperm-quality of the male partners of the younger women. Threshold values of the single parameters were determined in the univariate analysis. A threshold was defined as the value below which no fertilization was documented: before and after preparation of the semen sample 11.4 Mio., Total sperm count native sperm analysis 3.8 Mio., Progressive motility basic sperm analysis 11 %, Progressive motility basic sperm analysis 11 %, Sperm concentration native analysis 7 Mio./ml, Sperm motility after 2 hours 22 %, Sperm vitality test 25 %, Hypo-osmotic swelling test 28 %.Parameters within the significance level p < 0.05 as revealed by univariate analysis, were analysed by a binary logistic regression model. Sperm motility after 2 hours (p = 0.043) and sperm concentration in the native analysis (p = 0.008) had a significant impact on fertilization.Semen analysis is a central component in male fertility testing. However, fertilization depends on many factors. Semen analysis on its own cannot predict the likelihood of fertilization. Although fertile men have better semen parameters on average than infertile men, there is a big overlap between the groups. Therefore it is not easy to clearly distinguish between fertile and infertile patients based on specific values. However, it does increase the probability of fertilization if values are over defined limits., Published by Aachen

Direct Numerical Simulations (DNS) of turbulent channel flow are performed in a computaional domain and for the friction Reynolds number Reτ = 180 realized by Kim with the aim to reduce the skin friction by controlling the near- wall transport processes based on surface modifications. The latter are determined by solving the adjoint Navier-Stokes equations which depend on time-averaged DNS flow fields. It is well known that the wall-normal momentum Transport is organized in sweep and ejection events in the boundary layer which controls skin friction. Bewley et al. used adjoint optimization for suction and blowing in turbulent channel flow. They mitigated the effect of the wall-normal transport by opposing near-wall vertical motions of the fluid with an equal and opposite control velocity at the wall. The aim of this work is to achieve this with wall surface modifications. The idea is further to damp the spanwise fluctuations of streaky structures and thus, to reduce the associated skin friction.

The goals of the present study are the detection, visualization and analysis of the sound propagation process from a strong tonal sound source to the acoustic far-field produced by flow around a square rod. Furthermore, the flow structures related to the aeroacoustic source mechanism are investigated. Velocity measurements were conducted using the particle image velocimetry technique (PIV) in a wind tunnel experiment in anechoic conditions. Simultaneously, the acoustic pressure fluctuations were recorded by microphones in the acoustic far-field. The PIV observation windows were shifted stepwise from the source region to the vicinity of the microphones. This enables to trace the acoustic propagation. The PIV measurement has been performed with a high dynamic range of more than 72 dB to reproduce the acoustic fluctuations through the flow field in observation areas outside the source region. The calculation of the cross-correlation function between the velocity and the pressure fluctuations has been applied which acts like a filter of the velocity fluctuations yielding a proxy variable for the acoustic particle velocity. The temporal evolution of this quantity indicates the propagation of the acoustic perturbations. It can be shown that wave propagation emanates from coherent flow structures in the near-field through the shear-layer of the open jet to the far-field.

This book presents contributions to the 19th biannual symposium of the German Aerospace Aerodynamics Association (STAB) and the German Society for Aeronautics and Astronautics (DGLR). The individual chapters reflect ongoing research conducted by the STAB members in the field of numerical and experimental fluid mechanics and aerodynamics, mainly for (but not limited to) aerospace applications, and cover both nationally and EC-funded projects. Special emphasis is given to collaborative research projects conducted by German scientists and engineers from universities, research-establishments and industries. By addressing a number of cutting-edge applications, together with the relevant physical and mathematics fundamentals, the book provides readers with a comprehensive overview of the current research work in the field. Though the book’s primary emphasis is on the aerospace context, it also addresses further important applications, e.g. in ground transportation and energy. .

Decay and amplification of shear flow turbulence in oscillatory fluid motions is of theoretical interest and practical relevance, since the onset of turbulence can drastically change the transport properties and mixing efficiency. To supplement former theoretical and experimental investigations on the transition to turbulence in Sexl– Womersley (SW) type flows we perform three-dimensional direct numerical simulations (DNS) of oscillatory pipe flows at various Womersley numbers Wo 2 f5; 13; 26; 52g and Reynolds numbers Re 2 f1440; 2880; 5760; 11520g based on the friction velocity u.

For the optimization of modern high-speed trains it is essential to fully understand the instantaneous and time-averaged flow around the train body. Recent numerical studies are mostly based on solutions of the Reynolds Averaged Navier Stokes (RANS) equations to investigate the development of the flow. For the study presented here however Large Eddy Simulation (LES) has been performed to obtain more details regarding the time-dependent aerodynamic loads and more reliable results in general. Since the front of a train is subjected to the largest forces1, the focus of this study was laid on the flow around the train's nose. Therefore a simplified model of a high-speed train has been developed for the first Simulations. Also geometrically more complex train models will be considered. A further simplification of the set-up was to investigate a low Reynolds number flow in order to lower the already quite high computational costs that the application of the LES technique usually implies. Typical Reynolds numbers for investigations of the flow around a modern high-speed train are of the order of 10-15 Mio, based on the free stream velocity and the standard train width (d=3m). This study was conducted with a Reynolds number of 280000, also based on the free stream velocity and, since the model is a scaled version of the original train, the scaled train width (d=3/25m). Simulations have been performed without and with a side-wind leading an angle of attack of 30° to examine cross-wind effects. For subgrid-scale modeling the Smagorinsky model2 in combination with van Driest damping were used. The simulations have been carried out with a second order central differencing method on an unstructured grid with more than 9 Mio (no side-wind, 0°) and 15 Mio (angle of attack: 30°) cells. In Figure 1(a) and (b) the instantaneous velocity field in streamwise diirection is displayed. It shows the turbulent nature of the flow around this bluff body, especially in the wake of the train's head and below the undercarriage. The time-averaged flow behaviour around the train's head can be seen in Figure 2(a) and (b) where the streamlines projected onto the z-plane without and with sidewind component at a 30° angle of attack are presented. In Figure 2(a) two almost symmetrical vortices can be observed in the wake of the train's head. This is characteristic for a bluff body flow. For the 30° angle of attack there is a shift in the two vortices in the wake, since the flow around the body is no longer symmetrical.

We perform direct numerical simulations (DNS) of turbulent Rayleigh-Bénard convection coupled with surface-to-surface 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 surface-to-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.

The present study focuses on the scaling of the main aeroacoustic sound sources of a high speed train. The investigations are carried out by measuring in a wind tunnel by means of microphone array technique. The experiments using a 1:25 Inter City Express 3 model were conducted in two different wind tunnels: The Aeroacoustic Wind tunnel (AWB) of the German Aerospace Center (DLR) in Brunswick, which provides nearly perfect acoustically conditions, and in the Cryogenic wind tunnel (DNW-KKK) of the DNW (German - Dutch wind tunnels) in Cologne, which allows measurements at higher Reynolds numbers. Two different sources of sound with different characteristics are identified from the measurements in the AWB at Reynolds numbers up to Re = 0.46 mio: The aeroacoustic noise from the bogie section is dominant for frequencies f < 6 kHz and can be characterised by cavity mode excitation. The shape of the spectrum is not velocity dependent within the investigated velocity range. The pantograph is the dominant source of sound above $f = 6$ kHz. It is more tonal noise and the frequency peaks show a strong velocity dependence. These two sources of sound demonstrate that it is not possible to formulate one scaling law for all sources of sound of a train. It is essential to treat every single source separately and to t-ake their characteristics into account. In order to investigate the noise generated at the bogies at higher Reynolds numbers up to 3.70 mio a second measurement campaign has been conducted in the cryogenic wind tunnel. By cooling down from T = 300 K to T = 100 K one changes the physical properties of the fluid and the Reynolds number increases by a factor of ~5. Besides, this wind tunnel admits to vary the Mach and Reynolds numbers independently. Drawback of this facility is that it is not optimised for aeroacoustic experiments and reflexions as well as the high background noise level can disturb the results. The Mach number is kept constant and a Reynolds number variation is realised by varying of the temperature. The first bogie shows only a weak Reynolds number dependence. The deviations are within the measurement accuracy. To go more into detail further investigations with an improved setup are planed.