Real time visualization and analysis of sensory hair arrays using fast image processing and proper orthogonal decomposition

This paper presents an approach both to receiving multiple sensor data from a flow in real time and to analyzing these data in order to characterize the flow condition and, if necessary, control the flow. In order to obtain the data, an optical micro-pillar array acting as distributed wall-shear sensor was developed and interrogated optically with an LDM (long distance microscope). Together, the micro-pillar array and the LDM form a channeling optics, which allows magnified imaging of larger numbers of individual pillars simultaneously. The sensor was tested in a turbulent wall shear stress field under varying conditions (Reynolds number). A frame rate of 3000 fps was used since the higher the temporal resolution is, the more specific flow control strategies might be applied later in realistic application. However, the temporal high resolution would lead to a vast amount of data, which is difficult to analyze in real time. Therefore, a fast image processing algorithm is developed, which detects the tip deflections of the pillars and vectorizes the wall-shear stress field online. The extracted data fields are then broken down into equidistant and overlapping windows in order to guarantee fast POD (proper orthogonal decomposition) modes calculation. The POD is applied to each of these windows and the extracted modes are compared, summarized and collected in a library. Finally, this library is again applied to the flow but under different conditions in order to identify the state of the current flow in real time

4D visualization study of a vortex ring life cycle using modal analyses

In this study, the life cycle of a vortex ring is visualized and simultaneously investigated in a three-dimensional (3D) domain and time resolved for an observer in a reference frame moving with the ring. By traversing the system, the object of interest is captured within the measurement volume during the entire cycle. The 4D (time-resolved 3D) data gained from the experiment are postprocessed by using modal analyses such as proper orthogonal decomposition and dynamic mode decomposition (DMD). The latter is used to reconstruct the vortex dynamics by means of the Q values, based on the most dominant modes in the DMD. These modal analyses allow reconstructing the dominant dynamics of the behavior of the secondary structures and their interaction with the vortex core. The visualization for the vortex ring at ReΓ = 5500 shows the well-known azimuthal instability and its growth at n = 6 in our experiments. As the process of transition further develops, we found a zig-zag-like mesh of tilted secondary structure and finally, helical coil-type vortex ribbons wrapped around the core, which emerge in the late-stage process. It is hypothesized that the initial state of this process is the emergence of a pair of standing helical waves both counterbalancing each other at the beginning and forming the zig-zag pattern. Azimuthal core flow is only weak in this phase. Later in the transition, one of the helical waves starts to take over the other and finally dominates. This results in the helical coil-type vortex structure seen in the reconstructed results, which goes with the increase in axial flow along the core. The hypothesis of coexisting helical waves is drawn from similarities in the transition of attached vortex rings in axisymmetric wakes

4D analysis of flow around a Tumbling Ring-Type Particle using Scanning PIV and 3D Least Squares Matching

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