The calibration of a multi-camera system for volumetric measurements is a basic requirement of reliable 3D measurements and object tracking. In order to refine the precision of the mapping functions, a new, tomographic reconstruction-based approach is presented. The method is suitable for Volumetric Particle Image Velocimetry (PIV), where small particles, drops or bubbles are illuminated and precise 3D position tracking or velocimetry is applied. The technique is based on the 2D cross-correlation of original images of particles with regions from a back projection of a tomographic reconstruction of the particles. The off-set of the peaks in the correlation maps represent disparities, which are used to correct the mapping functions for each sensor plane in an iterative procedure. For validation and practical applicability of the method, a sensitivity analysis has been performed using a synthetic data set followed by the application of the technique on Tomo-PIV measurements of a jet-flow. The results show that initial large disparities could be corrected to an average of below 0.1 pixels during the refinement steps, which drastically improves reconstruction quality and improves measurement accuracy and reliability.

The pulsating flow through a mechanical three leaflet heart-valve using 3D PTV and 3D PIV methods was studied. An artificial heart valve and a transparent silicone model, which were already in usage for 3D Scanning Stereo PIV measurements, were mounted in a fluid tank. Three high speed cameras were arranged around the hexagonal basin recording the particle movement in a laser illuminated volume including the heart valve and the first two thirds of the aortic root. A typical stroke volume flow rate for juvenile of 35 ml per beat was pumped through the arch, representing the opening and closing of the heart valve within a cycle of 300 ms. At the beginning of the heart cycle a wash-out flow was generated. During the second phase of the systole, the velocity and the core diameter of the flow diminished and retrograde flow occurred. Regions of particles with long residence time in the arch started to grow, when first separation took place. As well a few counterclockwise traces with a helical like structure could be seen. At the end of the cycle, the flow contained multiple, small-scale eddies and counter eddies whose distribution and movements are now chaotic.

For a deeper understanding of the highly unsteady near-wall boundary layer flows in internal combustion (IC) engines, PIV-based flow field measurements close to the inner cylinder and piston walls within transparent engines are required. The herein described flying PIV method in combination with a scanning light-sheet provides time-resolved PIV measurements in a transparent IC engine water analogue in a radial plane 1.5 mm apart from the planar piston crown while the piston is moving. The light-sheet is parallel to the piston surface and moves with the piston thanks to the scanning technique that synchronizes the sheet motion with the non-linear piston motion. A compact high speed camera is positioned within the piston shaft below the transparent piston head and records the particle fields within the illuminated plane in time-resolved manner. The measurements are realized in a water-analogue of a 4-valve engine at 950 rpm engine speed in real situation. Instantaneous pictures are compared to phase-averaged velocity maps and allowed to localize regions of high cycle-to-cycle fluctuations.

This work focusses on the investigation of the unsteady flow in a transparent IC engine during the intake cycle. Special focus is laid on evolution of the flow close to the piston crown while the piston is moving. Therefore we developed a Flying TR-PIV method which allows us to follow the flow evolution in a system of reference fixed with the moving piston and where the light-sheet remains at a constant distance to the piston surface. The measurements are realized in a water-analogue of a 4-valve engine at 1600 rpm engine speed in real situation. The light-sheet is positioned 1mm above the piston crown and moves with the piston thanks to a scanning technique that synchronizes the sheet motion with the piston motion. A compact high-speed camera is positioned within the piston shaft below the transparent piston head and records the particle fields within the illuminated planes parallel to the upper plane of the flat piston. Results are presented for the flow fields above the piston crown that show the strong swirling motion generated in the early phase of intake due to vortex stretching of horseshoe-type vortices generated at the valves. These vortices give reason for cycle-to-cycle variations due to their intense stretching during the intake phase.

Birds are remarkably good flyers and show very special adaptations in their wings for stall delay. The pop-up of some cover feathers during starting and landing gave the idea for the present study to investigate the influence on a wall jet when inserting an array with flaps made of elastomer foil. In a wall jet with Re = 420 a flat plate and two different flap arrays (with a foil thickness of 100 and 200 µm) are measured by a time resolved 3D scanning PIV with 20 laser sheets. 2-dimensional analyses show the forming rolers between the jet flow and the surrounding fluid with a fundamental frequency of 13-14 Hz and the characteristically vortex pairing. By inserting the flap array the jet wallnormal spreading gets intensified and the vortex interaction process results in cooperative formation of larger vortices. The 3-dimensional analyses verify these results and show high 3-dimensional vortical structures which are growing when passing over a flap array. In case of the inserted flap array the vortex pairing process was delayed and accumulation of spanwise vorticity was forced to happen over the first rows of flaps, thus forming the larger structures. Already the used flap array configurations showed a significant impact influence on the jet evolution and the non-linear instabilities. Further investigations will analyze the influence of more parameters as the flap geometry or the distance to the jet flow and nozzle outlet.

The spitting cobra Naja pallida can eject its venom towards an offender from a distance of up to two meters. The aim of this study was to understand the mechanisms responsible for the relatively large distance covered by the venom jet although the venom channel is only of micro-scale. Therefore, we analysed factors that influence secondary flow and pressure drop in the venom channel, which include the physical-chemical properties of venom liquid and the morphology of the venom channel. The cobra venom showed shear-reducing properties and the venom channel had paired ridges that span from the last third of the channel to its distal end, terminating laterally and in close proximity to the discharge orifice. To analyze the functional significance of these ridges we generated a numerical and an experimental model of the venom channel. Computational fluid dynamics (CFD) and Particle-Image Velocimetry (PIV) revealed that the paired interior ridges shape the flow structure upstream of the sharp 90° bend at the distal end. The occurrence of secondary flow structures resembling Dean-type vortical structures in the venom channel can be observed, which induce additional pressure loss. Comparing a venom channel featuring ridges with an identical channel featuring no ridges, one can observe a reduction of pressure loss of about 30%. Therefore it is concluded that the function of the ridges is similar to guide vanes used by engineers to reduce pressure loss in curved flow channels.

This work describes the flow around a sphere with a longitudinal hole inside. Such ring-type particles are useful in many applications like chemical reactors as catalysts (seed particles) to increase the reacting surface in the multi-phase flow. Both heat transfer and the flow structure in and around those catalysts are of interest for fixed bed reactors as well as floating bed systems with solid particles. The focus of this work is the analysis of the 3D flow structure in the borehole and the outer flow around the particle and its influence on particle motion and rotation as well as the change of heat transfer involved herein. Since internal and external flow may induce Lift- and Magnus-forces we expect a complex interaction of particle motion and flow field which is tested in a special flow tank. Refractive index matching is used in combination with Light-Sheet Scanning and 3D Least Squares Matching to obtain the flow field around a silicone sphere with a longitudinal hole. The results show that the flow through the hole imposes pressure forces that lead to particle rotation and drift in a non-trivial way. Thereby, drag and lift-forces as well as torque are changed by an order of magnitude and cannot be represented by semi-empirical equations of the global body shape anymore.

Snapping shrimp use one oversized claw to generate a cavitating high speed water jet for hunting, defence and communication. This work is an experimental investigation about the jet generation. Snapping shrimp (Alpheus-bellulus) were investigated by using an enlarged transparent model reproducing the closure of the snapper claw. Flow inside the model was studied using both High-Speed Particle Image Velocimetry (HS-PIV) and flow visualization. During claw closure a channel-like cavity was formed between the plunger and the socket featuring a nozzle-type contour at the orifice. Closing the mechanism led to the formation of a leading vortex ring with a dimensionless formation number of approximate ΔT*≈4. This indicates that the claw might work at maximum efficiency, i.e. maximum vortex strength was achieved by a minimum of fluid volume ejected. The subsequent vortex cavitation with the formation of an axial reentrant jet is a reasonable explanation for the large penetration depth of the water jet. That snapping shrimp can reach with their claw-induced flow. Within such a cavitation process, an axial reentrant jet is generated in the hollow cylindrical core of the cavitated vortex that pushes the front further downstream and whose length can exceed the initial jet penetration depth by several times.

Vortex pairing is a key mechanism for energy transfer of small to large coherent vortices. This paper describes the three dimensional pairing process under predefined conditions on the basis of 3D Least-Squares-Matching (LSM). Therefore, the bottom edge of a nozzle outlet is aligned horizontally to a wall. The nozzles’ system pressure (and thereby the Reynolds number) as well as the camera position downstream the wake can be varied. Shear layer roll-up is observed close to the nozzle forming typical roller structures, which undergo streamwise instabilities further downstream. With increasing Reynolds number, the rollers start to interact with each other; the instabilities trigger localized vortex pairing. The triggering process is documented in the results which provide insight into the three-dimensional vortex pairing dynamics.

Experiments are carried out in a water analogue 4-stroke internal combustion engine (ICE) with four fully variable intake and outlet valves. Due to the incompressibility of the fluid only the intake stroke is determined. The main test rig, provided by Volkswagen AG, allows the investigation of different opening cycles, equivalent to the different load cases of a real engine in a short period of time. A constant rotational flow structure with its axis perpendicular to the cylinder axis, the so called tumble flow, is of great importance for a reproducible ignition. High-resolution scanning is used to measure a full 3D-field at a high spatiotemporal resolution. A high-speed Nd:Ylf-LASER in combination with a rotating polygon mirror generates 100 parallel and partially overlapping light sheets per volume at 10 kHz. Each light-sheet has a thickness of about 2 mm and an overlap of 75% with the previous and the successive one. The light sheet images are recorded by a Phantom 12.1 running at 10,000 fps with a maximum resolution of 960 x 600 pixels. This leads to an illuminated volume of about 50 mm in scanning direction. This volume can be reconstructed using a stacking technique, which is then analysed by 3D Least Squares Matching (LSM) to retrieve the velocities as well as the velocity gradient matrix. The measurements show a shift of the tumble centre as well as weakened tumble strength during the piston stroke. Hence, the fully developed flow consists of different vortices interacting with the characteristic tumble flow, therefor it is important to investigate fully time resolved 3D data. Different flow characteristics, which lead to a tumble break up, are studied and described in this paper.

Spitting cobras defend themselves by ejecting rapid jets of venom through their fangs towards the face of an offender. To generate these jets, the venom delivery system of spitting cobras has some unique adaptations, such as prominent ridges on the surface of the venom channel. We examined the fluid acceleration mechanisms in three spitting cobra species of the genus Naja. To investigate the liquid-flow through the venom channel we built a three-dimensional 60:1 scale model. First we determined the three-dimensional structure of the channel by using microcomputer tomography. With help of the micro computer tomographical data we then created a negative form out of wax. Finally, silicon was casted around the wax form and the wax removed, resulting in a completely transparent model of the cobra´s venom channel. The physical-chemical properties of the cobra venom were measured by micro rheometry and tensiometry. Thereafter, an artificial fluid with similar properties was generated. Particle image velocimetry (PIV) was performed to visualize the flow of the artificial liquid in the three-dimensional model. Our experiments show how the surface structure of the venom channel determines the liquid flow through the channel and ultimately the form of the liquid jet. Understanding the biological mechanisms of venom ejection helps to enhance industrial processes such as water jet cutting and cleaning as well as injection methods in technical and medical sectors, e.g. liquid microjet dissection in microsurgery.

Goal of this work is to characterize the cylinder scavenging process in a water analogue, backward scavenging, two-stroke internal combustion engine (ICE). The measurements are performed using time resolved 3D scanning particle image velocimetry (3D SPIV). The result is 2C-3D dataset. Stagnation flow is typical for the two-stroke IC engine. It is induced by the transfer and the booster ports, resulting in a complex three dimensional flow. Due to the stagnation flow high frequency fluctuations of the flow direction can occur. In order to maintain the two-stroke IC engine competitive in its different applications it is necessary to obtain a better insight of the three dimensional scavenging process. Therefore, 3D measurement techniques are necessary. Recent developments like tomographic PTV allow the reconstruction of 3D velocity fields. However, a plurality of optical accesses is necessary. Furthermore, the long processing time for data evaluation is a disadvantage of this technique. Hence, it is unsuitable for the two-stroke engine, due to a limited optical access. Instead, a single camera set-up combined with a scanning light sheet is used. This method is also suitable to investigate the cycle to cycle fluctuations. As Bown et al. (2007) showed, it is possible to calculate the third velocity-component from the velocity fields of the scanned planes, using conservation of mass for incompressible flow. The result is a complete three dimensional velocity field (3D 3C) for every scanning cycle of this procedure. Brücker (1997) demonstrated the working principle on IC engines in specific regions of the combustion chamber. With this time resolved measurement technique it is also possible to investigate fluctuations of the flow direction discovered by Hauke et al. (1998). This phenomenon was called flip-flop. The results of the time averaged measurements can qualitatively be compared with experiments in an external driven engine (Britsch 2010). The time averaged flow measurements show a typical tumble motion. Thereby, the flow leaves the transfer and boost ports upwards to the cylinder head, where it moves in direction of the exhaust port and back down. The time resolved three dimensional flow fields are similar to the time averaged results. However, the clearly structured tumble motion of the averaged results is blurred in the time resolved measurements, due to the fluctuations induced by the stagnation flow. Fig. 1 Presentation of the time averaged flow forming the tumble motion shown by iso-surfaces and streamtraces