The thermal plume from a human significantly influences indoor air flows, impacting the dispersion of air constituents and consequently affecting indoor air quality. This is also relevant in the transport of respiratory particles, which results in spread of respiratory diseases including COVID-19 caused by SARS-CoV-2. Our focus is on the sitting condition, a common scenario in various ventilated spaces. Prior human thermal plume studies employed predominantly anemometers to measure flow field which lack spatial resolution and detailed flow field. Here, Particle Image Velocimetry (PIV) is utilized to directly visualize and analyze the human thermal plume. No direct comparison of thermal manikin and real human subject is considered in previous studies. Such experiments were performed with a thermal manikin and comparisons were made with a real human. The results show that the thermal plume from manikin without breathing function and the real person have similarities including bi -lobe structure of the flow field. The integral fluxes like volumetric flux, momentum flux, buoyancy force flux, and enthalpy flux were determined and compared. The average volume flux of real person and the thermal manikin was found to be 153 m3/h and 125 m3/h, respectively. The momentum flux was 0.005 N for both the cases. The estimation of enthalpy flux revealed that radiative heat transfer dominates and less than 50 % of the total flux is convected in the human thermal plume. In addition, a zonal simulation model was created and the volumetric flux was determined by simulation and is compared to the measured values.

Accurately determining the wall-shear-stress, τ _w , experimentally is challenging due to small spatial scales and large velocity gradients present in the near-wall region of turbulent flows. To avoid these resolution requirements, several indirect iterative fitting methods, most notably the Clauser chart method, exist for determining τ_w by fitting the mean velocity profile further away from the near-wall region in the log-law layer. These methods often require proper selection of fitting constants, assumptions of a canonical flow state, and other empirical-based generalizations. To reduce the amount of ambiguity, determining the near-wall velocity gradient by assuming a linear relationship between the mean streamwise velocity and wall normal distance in the viscous sublayer can be used. However, this requires an accurate unbiased measurement of the near-wall velocity profile in the region below five viscous spatial units, which can be less than 50 µm for high Reynolds number flows. Therefore, in this study a method for a volumetric defocusing microparticle tracking velocimetry method is presented that is capable of resolving the flow in the viscous sublayer of a turbulent boundary layer up to U_e=44.7 m/s ( Re_θ=27250 ). This method allows for the measurement of the near-wall flow through a single optical access for illumination and imaging and serves as an excellent complement of larger scale measurements that require near-wall information. The τ_w values determined from the defocusing approach were found to be in good agreement values obtained from a simultaneous parallax PTV measurement. Furthermore, analysis of the diagnostic plot and cumulative distribution of measured fluctuations in the near-wall region, showed that both methods are capable of accurately determining mean velocity and fluctuation profiles in the self-similar viscous sublayer region.

The optical measurement technique Defocusing Particle Tracking Velocimetry (Defocusing PTV) is applied to the sub-millimeter gap of an open wet clutch to gain deeper insights into the unknown flow, which causes a significant loss in nowadays automobiles. The present work improves the fundamental understanding of the flow and its contribution to the generated drag torque and the physical process of aeration. A set of governing analytical equations is revealed from in-depth theoretical considerations, which describe the general cause-effect relations of the flow. To gain deeper insights into the unknown intra-groove phenomena Defocusing PTV is successfully applied to (locally) extract precise vortex information and fine resolved wall shear stress values. Metrological insights are generated with the introduction of a new detection strategy and the proven flexibility of Defocusing PTV, which makes comprehensive magnification and location-accuracy studies possible. The work is completed with a flow analysis along the entire radial region of interest, and the consideration of a more complex groove geometry.

Airborne transmission via aerosol particles without close human contact is a possible source of infection with airborne viruses such as SARS-CoV-2 or influenza. Reducing this indirect infection risk, which is mostly present indoors, requires wearing adequate respiratory masks, the inactivation of the viruses with radiation or electric charges, filtering of the room air, or supplying ambient air by means of ventilation systems or open windows. For rooms without heating, ventilation, and air conditioning (HVAC) systems, mobile air cleaners are a possibility for filtering out aerosol particles and therefore lowering the probability of indirect infections. The main questions are as follows: (1) How effectively do mobile air cleaners filter the air in a room? (2) What are the parameters that influence this efficiency? (3) Are there room situations that completely prevent the air cleaner from filtering the air? (4) Does the air cleaner flow make the stay in the room uncomfortable? To answer these questions, particle imaging methods were employed. Particle image velocimetry (PIV) was used to determine the flow field in the proximity of the air cleaner inlet and outlet to assess regions of unpleasant air movements. The filtering efficiency was quantified by means of particle image counting as a measure for the particle concentration at multiple locations in the room simultaneously. Moreover, different room occupancies and room geometries were investigated. Our results confirm that mobile air cleaners are suitable devices for reducing the viral load indoors. Elongated room geometries, e.g., hallways, lead to a reduced filtering efficiency, which needs to be compensated by increasing the volume flow rate of the device or by deploying multiple smaller devices. As compared to an empty room, a room occupied with desks, desk separation walls, and people does not change the filtering efficiency significantly, i.e., the change was less than 10%. Finally, the flow induced by the investigated mobile air cleaner does not reach uncomfortable levels, as by defined room comfort standards under these conditions, while at the same time reaching air exchange rates above 6, a value which is recommended for potentially infectious environments.

Particle-image or particle-tracking velocimetry (PIV/PTV) nonintrusively provides velocity field information, and consequently, the proposition of measurements in a large plane is highly attractive from an experimental standpoint, particularly for experiments conducted in large atmospheric and flight-scale wind tunnels. Physical limits to the size of the useful field-of-view (FOV) that can be achieved with a single camera depend on striking a balance between the physical capabilities of the cameras and the physics of interest in the flow. A typical solution is to stitch together multiple smaller FOVs to achieve the large FOV of interest, presenting a number of challenges, some of which are rooted in the calibration process. For SPIV calibrations, the use of a multi-level target simplifies the calibration process. However, this is problematic for large-FOV measurements, as standardised multi-level targets are typically relatively small and expensive due to the precision-engineering required. As an alternative, LEGO bricks are extremely well-suited to the construction of large, customised multi-level targets due to their high dimensional tolerance and their stackability with high precision. In order to create a prototype two-sided multi-level SPIV target and evaluate whether the LEGO bricks can be used successfully to calibrate a large FOV, the LaVision Type 31 target was chosen as a model. In order to further augment the flexibility offered by the LEGO-based targets, the backing used to mount the baseplates was designed to be modularly reconfigurable. The resultant two-sided multi-level target has an area of approximately 380 x 1150 mm. To evaluate the target, SPIV measurements of the inflow conditions of the Atmospheric Wind Tunnel Munich (AWM) were performed; calibrations were also performed using the Type 31 target for comparison. Analysis of the datasets with both calibration targets shows good agreement in the measurement of the streamwise, out-of-plane component u. However, there is some uncertainty regarding the accuracy of the computation of in-plane components. Due to the high level of agreement in calibration parameters, out-of-plane component, and the qualitative location of flow features, this disagreement is believed to be a fixable issue. These early results indicate that with some refinement, the LEGO-based calibration target can be developed further and used for large-FOV measurements in the future.

Abstract Accurately determining the wall-shear-stress, τw , experimentally is challenging due to small spatial scales and large velocity gradients present in the near-wall region of turbulent flows. To avoid these resolution requirements, several indirect iterative fitting methods, most notably the Clauser chart method, exist for determining τw by fitting the mean velocity profile further away from the near-wall region in the log-law layer. These methods often require proper selection of fitting constants, assumptions of a canonical flow state, and other empirical based generalizations. To reduce the amount of ambiguity, determining the near-wall velocity gradient by assuming a linear relationship between the mean streamwise velocity and wall normal distance in the viscous sublayer can be used. However, this requires an accurate unbiased measurement of the near wall velocity profile in the region below five viscous spatial units, which can be less than 50 μm for high Reynolds number flows. Therefore, in this study a method for a volumetric defocusing micro particle tracking velocimetry method is presented that is capable of resolving the flow in the viscous sublayer of a turbulent boundary layer up to Ue = 44.7 m/s (Reθ = 27250). This method allows for the measurement of the near-wall flow through a single optical access for illumination and imaging and serves as an excellent complement of larger scale measurements that require near-wall information. The τw values determined from defocusing approach was found to be in good agreement values obtained from a simultaneous parallax PTV measurement. Furthermore, analysis of the diagnostic plot and cumulative distribution of measured fluctuations in the near-wall region, showed that both methods are capable of accurately determining mean velocity and fluctuation profiles in the self-similar viscous sublayer region.

In this study we can show that shadowgraphy is a feasible alternative to fluorescence for microscopic particle imaging. Furthermore, using the particle shadow image geometry (diameter for defocusing methods or axis lengths for astigmatism methods), the particle location along the optical axis can be determined, enabling a three-dimensional flow measurement with a single camera. However, the rate of change of the particle shadow image geometry as a function of the distance to the focal plane is one order of magnitude smaller as compared to fluorescence/scattering imaging approaches, such that the measurement depths is effectively increased. This increase is due to the fact that the signal to noise ratio of the relatively small particle shadow images is large, even if the particle is located far away from the focal plane, as the intensity is distributed over a small sensor area. In addition, due to their smaller dimensions, more particle shadow images can be captured on the camera sensor, yielding a larger information density for microscopic particle imaging. A measurement of a micro-channel (channel height: 650 µm) flow showed a good agreement with the ideal parabolic flow profile, proving the viability of this microscopic particle shadow imaging approach as a suitable addition to the existing fluorescence-based methods.

A human thermal plume contributes significantly towards indoor fluid flows. Such flows impact the ventilation system designs like mixing and displacement ventilation. Further, the human thermal plume influences the dispersion of air constituents to and away from the breathing zone. This is also relevant in the transport of respiratory droplets which results in spread of respiratory diseases including COVID-19. One of the general scenarios in many ventilated spaces is the sitting condition which is the focus here. Experiments were performed with a thermal manikin and comparisons were made with a real human. The thermal plume is visualised by Stereoscopic PIV with the visualisation plane located 30 cm above the head parallel to ground. The results presented here show that the thermal plume from a manikin without breathing function have many similarities in the average flow field above the head with the field induced by a real person. The peak velocity observed with the real person is slightly lower than the one of the thermal manikin and the structures of the thermal plumes resemble to some extent in both cases. The real person with breathing or talking revealed a bi-lobe like structure with the secondary lobe due to the breath. The thermal manikin also showed a second lobe which possibly is due to the thermal plume from the legs. Furthermore, the effect due to breathing, speaking, and movement of the real person are analysed. The integral values do not vary with respect to the different situations except for the movement case. The characterizations of these phenomena are important because they cannot be simulated by the manikin, while they affect the extent and strength of the thermal convection flow.

The SARS-CoV-2 pandemic is limiting both the private and public lives of many people around the world. It is now considered certain that SARS-CoV-2 is transmitted via droplets, smear infection, and aerosol particles. While simple masks, spacing, and hand hygiene significantly reduce the risk of infection via the first two routes mentioned, the risk from aerosol particles remains. These small particles move with the air in the room and spread unhindered throughout it. To reduce the risk of infection from viruses present in aerosol particles, the following options exist. First, good respiratory masks can be worn to reduce the viral load in the inhaled air. Another option is to make the viruses harmless (e.g., by UV light). A third option is to reduce the viral load in the room by bringing in virus-free air and moving contaminated air out or cleaning the air in the room. To investigate how well virus load reduction via ventilation works in a real lecture room, measurements were carried out at the Universitat der Bundeswehr Munchen (University of the Federal Armed Forces Munich). The lecture room holds a maximum of approx. 90 people and has a ventilation system as well as 2 windows that can be opened. In the absence of a ventilation system in a comparable room, the effectiveness of a room air cleaner was also investigated.

This paper presents the impact of an axially tilted variable stator vane platform on penny cavity flow and passage flow, with the aid of both optical and pneumatic measurements in an annular cascade wind tunnel as well as steady CFD analyses. Variable stator vanes (VSVs) in axial compressors require a clearance from the endwalls. This means that penny cavities around the vane platform are inevitable. Production and assembly deviations can result in a vane platform which is tilted about the circumferential axis. Due to this deformation, backward facing steps occur on the platform edge. Penny cavity and main flow in geometries with and without platform tilting were compared in an annular cascade wind tunnel, which comprises a single row of 30 VSVs. Detailed particle image velocimetry (PIV) measurements were conducted inside the penny cavity and in the vane passage. Steady pressure and velocity data was obtained by two-dimensional multi-hole pressure probe traverses in the inflow and the outflow. Furthermore, pneumatic measurements were carried out using pressure taps inside the penny cavity. Additionally, oil flow visualization was conducted on the airfoil, hub, and penny cavity surfaces. Steady CFD simulations with boundary conditions, according to the measurements, have been benchmarked against experimental data. The results show that tilting the VSV platform reduces the mass flow into and out of the penny cavity. By decreasing penny cavity leakage, platform tilting also affects the passage flow where it leads to a reduced turbulence level and total pressure loss in the leakage flow region. In summary, the paper demonstrates the influence of penny platform tilting on cavity flow and passage flow and provides new insights into the mechanisms of penny cavity-associated losses.

Wall-roughness induces extra drag in wall-bounded turbulent flows. Mapping any given roughness geometry to its fluid dynamic behaviour has been hampered by the lack of accurate and direct measurements of skin-friction drag. Here the Taylor-Couette (TC) system provides an opportunity as it is a closed system and allows to directly and reliably measure the skin-friction. However, the wall-curvature potentially complicates the connection between the wall friction and the wall roughness characteristics. Here we investigate the effects of a hydrodynamically fully rough surface on highly turbulent, inner cylinder rotating, TC flow. We find that the effects of a hydrodynamically fully rough surface on TC turbulence, where the roughness height k is three orders of magnitude smaller than the Obukhov curvature length Lc (which characterizes the effects of curvature on the turbulent flow, see Berghout et al. arXiv: 2003.03294, 2020), are similar to those effects of a fully rough surface on a flat plate turbulent boundary layer (BL). Hence, the value of the equivalent sand grain height ks, that characterizes the drag properties of a rough surface, is similar to those found for comparable sandpaper surfaces in a flat plate BL. Next, we obtain the dependence of the torque (skin-friction drag) on the Reynolds number for given wall roughness, characterized by ks, and find agreement with the experimental results within 5 percent. Our findings demonstrate that global torque measurements in the TC facility are well suited to reliably deduce wall drag properties for any rough surface.

Current research has shown that SARS-CoV-2 is transmitted via airborne particles. These particles are generated when infected persons exhale and they spread throughout the room, resulting in a high concentration and thus a high risk of infection of non-infected persons. One way to reduce the concentration of particles is to filter them using mobile room air cleaners, which can be easily set up and reduce the concentration of viruses or keep them at a low level. Since many studies are conducted in generic rooms, the question of the cleaning performance of such room air cleaners in real rooms arises. In order to investigate this, measurements of the effectiveness were carried out in a total of 4 different rooms of the “Obermenzinger Gymnasium” (Obermenzinger high school) in Munich. It can be shown that good effects of the room air cleaners are achieved even under realistic conditions. Even Plexiglas screens, which serve as protection against direct infection, have no negative influence.

The volumetric defocusing particle tracking velocimetry (DPTV) approach is applied to measure the flow in the sub-millimeter gap between the disks of a radially grooved open wet clutch. It is shown that DPTV is capable of determining the in-plane velocities with a spatial resolution of $$12\;\upmu \mathrm{m}$$ along the optical axis, which is sufficient to capture the complex and small flow structures in the miniature clutch grooves. A Couette-like velocity profile is identified at sufficient distance from the grooves. Moreover, the evaluation of the volumetric flow information in the rotor-fixed frame of reference uncovers a vortical structure inside the groove, which resembles a cavity roller. This vortex is found to extend well into the gap, such that the gap flow is displaced towards the smooth stator wall. Hence, the wall shear stress at the stator significantly increases in the groove region by up to $$15\%$$ as compared to the ideal linear velocity profile. Midway between the grooves, the wall shear stress is around $$4\%$$ lower than the linear reference. Furthermore, significant amounts of positive radial fluxes are identified inside the groove of the rotor; their counterpart are negative fluxes in the smooth part of the gap. The interaction of the roller in the groove and the resulting manipulation of the velocity profile has a strong impact on the wall shear stress and therefore on the drag torque production. In summary, this DPTV study demonstrates the applicability of such particle imaging approaches to achieve new insights into physical mechanisms of sub-millimeter gap flow scenarios in technical applications. These results help to bring the design- and performance-optimization processes of such devices to a new level.

Overview The future belongs to children and they need education to shape the future with foresight and intention. Children therefore have the right to education, according to Article 29 of the UN Convention on the Rights of the Child [1]. However, professional education is not everything, because children must also experience their strengths and weaknesses together and educate each other to be responsible and considerate people, so that they become socially valuable personalities. Only in this way can they shape the future in a peaceful and humane way. Therefore, attending school is essential. However, children also have the right to protection and care by their parents and the state, because the welfare of the child must also be given priority in accordance with Article 3 of the UN Convention on the Rights of the Child. The question is therefore how schooling in community schools can be realized during the SARS-CoV-2 pandemic without exposing children to an unnecessary risk of infection. It is not only about the children, because if the children are at risk, then so are their parents and grandparents and ultimately society as a whole. There are numerous concepts that promise safety in schools during the pandemic. When selecting concepts, the costs must of course be weighed against the benefits. People rightly expect an efficient use of resources. This means that either the set goal is achieved with the least possible resources or that the available resources are used to achieve the greatest possible approximation to the goal. In addition to the financial resources, however, the long-term consequences for the state, the economy, the population and the environment under the pressure of the pandemic must also be taken into account. Social cohesion and democracy must not be jeopardized either. Various protection concepts are currently under discussion. Often the advantages are overstated and the disadvantages concealed. Furthermore, some arguments are based on assumptions that are not true. The aim of this study is to provide a comparative assessment of the main protection concepts and to demonstrate, with the help of experimental analyses, the extent to which the protection concepts are effective. We will show that a comparatively high level of safety against infection in classrooms can be technically ensured without exposing children to masks. At the same time, the protection concept makes economic sense and the burden on the environment is comparatively low, so that infection prevention and climate protection do not have to be weighed against each other, because infection prevention and climate protection are political and social goals that have to be achieved together.

AbstractIndoor SARS-CoV-2 infections by droplets and aerosols are currently considered to be particularly significant. FFP2/3 respirator masks, which fit tightly and gap free, generally provide very good protection. In public transport, while shopping or in waiting rooms, they are therefore ideally suited to protect against direct and indirect infection. Unfortunately, these masks make it difficult to breathe and can be uncomfortable to wear in the long run. Therefore, these masks should be worn for a maximum of 3 × 75 minutes per day. These masks are therefore hardly suitable for schools or at work. The question therefore arises as to how people in closed rooms can be permanently protected from a SARS-CoV-2 infection. Large safety distances provide both self protection and protection of third parties, but they do not protect against indirect infection if the virus load in the room is high. Mouth and nose covers only offer protection of others against direct infection, but they do not protect the user against indirect infection. The same applies to faceshields and small protective walls. Indirect infections can be effectively prevented by free ventilation with windows or air conditioning systems that supply 100% outside air into the room, provided the air exchange rate is at minimum six times the room volume per hour. However, free ventilation by means of windows is rarely efficient enough, and in winter at the latest, it is no longer possible to open windows without wasting massive amounts of energy and endangering the health and well-being of people. The operation of air conditioning systems is also very energy-intensive during the cold season. Furthermore, most buildings do not have air conditioning systems. The question is therefore, how a largely safe protection against an indirect SARS-CoV-2 infection can be realized in closed rooms without wasting thermal energy and thus valuable resources. Technically, the problem can be solved with mobile disinfection devices or room air cleaners that separate the dangerous aerosol particles or inactivate the viruses by UV radiation or by contact with charge carriers. The potential of these devices is great and, since many German manufacturers produce these devices, they are also available. However, many of the devices offered do not provide effective protection because the volume flow is too small, the separation efficiency of the filters is too low and the performance of the UV and ionization unit is too weak. The Viromed Klinik Akut V 500 disinfection unit appears to meet the performance requirements and therefore the device is analyzed and evaluated in this study for its suitability to protect against SARS-CoV-2 infection.

Accurate prediction of labyrinth seal flows is important for the design and optimisation of turbomachinery. However, the prediction of such flows with RANS turbulence models is still lacking. The identification of modelling deficits and the development of improved turbulence models requires detailed experimental data. Consequently, a new test rig for straight labyrinth seals was built at the Institute for Turbomachinery and Fluid Dynamics which allows for non-intrusive measurements of the three dimensional velocity field in the cavities. Two linear eddy viscosity models and one algebraic Reynolds stress turbulence model have been tested and validated against global parameters, local pressure measurements, and non-intrusive measurements of the velocity field. While some models accurately predict the discharge coefficient, large local errors occurred in the prediction of the wall static pressure in the seal. Although improved predictions were possible by using model extensions, significant errors in the prediction of vortex systems remained in the solution. These were identified with the help of PIV results. All turbulence models struggled to accurately predict the size of separations and the swirl imposed by viscous effects at the rotor surface. Additionally, the expansion of the leakage jet in the outlet cavity is not modelled correctly by the numerical models. This is caused by a wrong prediction of turbulent kinetic energy and, presumably, its rate of dissipation.

The worldwide development of the SARS-CoV-2 infection makes it clear that the pandemic is only just beginning and cannot be stopped. Even if an effective and well-tolerated vaccine were available, it would not be feasible to vaccinate the world population on a large scale to combat the spread of the virus. It is therefore necessary to establish technical solutions to contain the pandemic at least locally. Mouth-and-nose-covers are now generally accepted technical aids to reduce the direct risk of infection when speaking, singing, coughing and sneezing. However, indirect infection via infectious aerosols, which accumulate over time in space, cannot be prevented with simple mouth-and-nose covers or surgical masks [5]. This requires tightly fitting particle-filtering respiratory masks. Alternatively, the aerosol concentration in the room can be reduced by means of filtration or discharged via window ventilation. Ventilation systems that reliably separate aerosol with a diameter smaller than 1 μm are rare. The free ventilation by means of windows is often not efficient and at the latest in winter no longer possible without wasting energy and endangering the health and well-being of people. The question is therefore whether mobile indoor air cleaners are basically suitable for making a meaningful contribution to reducing the risk of infection? To answer this question, a TROTEC TAC V+ indoor air cleaner with a volume flow of up to 1500 m3/h was systematically analysed. The unit has a filter combination that ensures that 99.995% of aerosol with a diameter of 0.1 to 0.3 μm is separated from the room air. The results show that the aerosol concentration in a room with a size of 80 m2 can be reduced to a low level everywhere within a short time. In our opinion, indoor air cleaners with a large volume flow and high-quality filters of class H14 represent a very suitable technical solution to reduce the indirect risk of infection by aerosols in schools, offices, shops, waiting rooms, community and club houses, lounges and dining rooms, etc. However, they can also be used as a support in buildings with air conditioning systems where people stand together (waiting area) and work together or where a lot of aerosol is emitted due to the work load (fitness studio).

With the aim to characterize the near-wall flow structures and their interaction with large-scale motions in the log-law region, time-resolved planar and volumetric flow field measurements were performed in the near-wall and log-law region of an adverse pressure gradient turbulent boundary layer following a zero pressure gradient turbulent boundary layer at a friction Reynolds number $Re_{\unicode[STIX]{x1D70F}}=5000$. Due to the high spatial and temporal resolution of the measurements, it was possible to resolve and identify uniform-momentum zones in the region $z/\unicode[STIX]{x1D6FF}<0.15$ or $z^{+}<350$ and to relate them with well known coherent flow motions near the wall. The space–time results confirm that the turbulent superstructures have a strong impact even on the very near-wall flow motion and also their alternating appearance in time and intensity could be quantified over long time sequences. Using the time record of the velocity field, rare localized separation events appearing in the viscous sublayer were also analysed. By means of volumetric particle tracking velocimetry their three-dimensional topology and dynamics could be resolved. Based on the results, a conceptual model was deduced that explains their rare occurrence, topology and dynamics by means of a complex interaction process between low-momentum turbulent superstructures, near-wall low-speed streaks and tilted longitudinal and spanwise vortices located in the near-wall region.

The inward flow between two parallel, co-rotating disks undergoes a thorough examination by analytical, experimental and numerical means. The analytical approach utilizes the asymptotical truncated series solution provided by Batista (2011) and extends it by a correction for an arbitrary mean tangential velocity at the rotor inlet. Taylor series expansions of the analytical results provide an estimate for the orders of magnitude of velocity components and the polynomial order of their profile shapes. The common assumption of parabolic velocity distributions is only appropriate in the radial direction. In parallel, a unique test rig provides the experimental counterpart of the velocity profiles inside a rotor gap, that is suitably narrow for turbomachinery applications. The optical flow measurements are based on a novel calibration technique and volumetric particle tracking evaluation. Both laminar and turbulent operating conditions are examined. Finally, numerical studies using commercial CFD software provide insight into the flow field inside the test rig rotor where experimental methods fall short and provide an additional means to investigate the effects of the approximations made in the derivation of the analytical results. The velocity distributions acquired by analytical, numerical and experimental means agree well, the asymptotical nature of the analytical solution by Batista (2011) can be observed. The comparison of experimental and numerical results of a turbulent case suggests that the Shear Stress Transport turbulence model reproduces turbulent flow inside the rotor gap appropriately.

Fuchs, Thomas. Dr. Ing, Universität der Bundeswehr München, 07.03.2018. Development and assessment of volumetric particle tracking approaches for the analysis of flows in confined geometries. Professor: Prof. Dr. rer. nat. Christian J. Kähler. Particle-based flow visualization has long been practiced in flow diagnostics to gain insights into the fluid physics. Ludwig Prandtl’s water tunnel experiments of aerofoils are well-known to this day (see Prandtl (1936) and Willert & Kompenhans (2010)). Due to their huge prospects, particle imaging methods have been subject to extensive research, in order to enhance the methods from pure qualitative diagnostics to comprehensive quantitative flow analysis tools. Today, particle imaging techniques along with numerical techniques are the key technologies for capturing three-dimensional (3D) flow fields and adjacent quantities. However, the required multi-camera set-ups are cumbersome, complex, and very costly and therefore limit the applicability. It is the purpose of this research to simplify particle-based 3D flow measurement techniques such that the range of applications can be broadened. A special attention is drawn to challenging measurement environments, with difficult optical access, vibrations, contamination, fluctuating seeding densities, and limited space for equipment. To address the aforementioned requirements, the astigmatism particle tracking velocimetry (APTV) technique, well-established in microfluidics, was developed further, to meet the challenges of macroscopic flow measurements. The APTV method targets applications in compressor, turbine, combustion, and engine research, as well as volumetric flow velocimetry in wind tunnel facilities with high mass flow rates. Furthermore, the three-dimensional particle tracking velocimetry (3D-PTV) approach was refined to meet the specific challenges, which occur in double-pulse systems in a stereoscopic imaging set-up. For densely seeded flows, a combined tomographic 3D-PTV processing approach is introduced targeting at the measurement of complex flow fields with strong velocity gradients, while providing a high spatial resolution without requiring a time-series of recordings. All the mentioned measurement technique developments are based on robust processing procedures and are therefore suitable for flow velocimetry in industrial environments.